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Equipment Discussions >> ATM, Optics and DIY Forum

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ed_turco
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Optimized Newts vs APOs
      #5579353 - 12/19/12 05:50 PM

Comments and pictures were shown on a thread concerning Herrig designs and I popped in and went off on a tangent.

Pictures were shown of unobstructed images of a 6" f/10 APO and a 6" f/10 Newt with 16% obstruction. The difference was barely perceptible. In fact, Suiter and I agree that there is no difference at the eyepiece; anyone who says differently is thinking more of what he paid for his telescope than what he is seeing through his telescope in comparison.

Tell you a story: I went on the Refractor Forum (EGads!) and made this offer. Tell me the aperture of your APO and give me time to make an optimized Newt of the same size, with a precision optical window and a 12% obstruction. Special coatings all around. Then, some neutral people would have to be picked to make an assessment as to what was what.

The issue was to be imagery only; not the appearance of either telescope. Perhaps the Stellafane optical judges would be a good choice.

In any case, no matter, win lose or draw, my telescope would be donated to charity.

You know what? No one took the challenge against my 1/20 wave optics. Not one. Not one dared.

I waited 6 months; the offer is withdrawn; why should I put up with all this "mighty APO" BS?


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David PavlichAdministrator
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5579391 - 12/19/12 06:24 PM

The only thing I could think of that might create a difference is the Newt's spider and the diffraction spikes that it creates. That could be eased by installing a curved spider.

About the only way you could see any difference and with my old eyes, I probably couldn't, is under ideal conditions and very dark skies. And how often does that happen?

David


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Astrojensen
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5579400 - 12/19/12 06:29 PM

As I see it, the advantage of an apochromat is not perfect color correction. That is cheaper to get in a reflector, as you point out. No, the advantages of an apochromat are those of the refractor: The eyepiece is at the lower tube end, which means I am sitting in a low chair, when the scope aims at targets high in the sky. With a long-focus newtonian, I am either in a very tall chair or on a ladder. And the refractor is often much better suited for binoviewer observing as well. They also have particularly steady images that make them ideal for double stars.

Many commercial newtonians (dobs) have some serious disadvantages for visual observing, IMO. That is pretty odd, because visual observing is what they excel at. One thing: Most makes are obsessed with low profile focusers to the point that if you use a small eyepiece, like an ortho, you're almost kissing the tube when looking through the eyepiece. It feels like I'm pushing my face into the side of it and I don't particularly like that feeling. By accepting a slightly larger diagonal, one could comfortably get a MUCH larger amount of back focus, so one could use a binoviewer without a barlow and using single eyepieces without kissing the tube, by using an extension tube. This would also lower the eyepiece height a few inches. Trivial, perhaps, but worth mentioning. A secondary obstruction of 25% is considered very small in a Mak-cass, but almost unacceptably huge in a newtonian. Why?

I have used some excellent newtonians and know how superb they can be, but in my opinion, they aren't as ergonomic as a refractor. I am more relaxed at the eyepiece of a refractor than I've been at the eyepiece of the newtonians/dobsonians I've tried.


Clear skies!
Thomas, Denmark


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John Jarosz
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Re: Optimized Newts vs APOs new [Re: Astrojensen]
      #5579425 - 12/19/12 06:47 PM

Actually, there are lots of people that try to pick the fly s**t out of the pepper when it comes to comparing equipment. Once you have any telescope, the main idea is to simply observe the night sky and not worry about defects/aberrations/whatever. Chromatic aberration, coma, collimation (or the lack of it) are what they are for a given scope but you can still observe with it. So observe. it's not realistic to try for perfection in every aspect of doing anything. Relax, enjoy your leisure time activities (no one here does this professionally), there is no need to be uptight about an activity that is supposed to be fun and an escape from the workaday world. Rant off.

Enjoy the end-of-year holidays, no matter where you live or what you do. (Especially if we all get past Friday )

John


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: John Jarosz]
      #5579523 - 12/19/12 08:02 PM

Quote:

So observe. it's not realistic to try for perfection in every aspect of doing anything.




I observe with telescopes of most all sizes and qualities, one night it might be a 70mm F/10 achromat that cost $40 at Walmart, the next, it might be a large Newtonian or a small refractor that cost someone several thousand dollars. They are all fun and all provide satisfying views.

Regardless, some designs offer more perfect views than others. Ed's challenge and discussion specifically addresses the comparison between equal aperture Newtonians and apochromats.

As an observer, I see the advantage of an apochromat is not that it does a great job at one certain type of observation, say splitting double stars or viewing the planets, but rather that it does a great job, within the limitations of it's aperture, at all aspects of observing. One can generally optimize another design, a Newtonian in this example, so that it nearly as competent as an apochromat in one aspect of observation but it will fall down in others.

My own experience is that refractors are optimal for smaller apertures. One reason is because their primary aberration, chromatic, is becomes more problematic as aperture is increased. This is because chromatic aberration essentially scales with the aperture/focal ratio. Refractors are also more efficient with light so in the smaller apertures where light is most at a premium, they are at their best. And their off-axis aberrations can be more easily controlled because there are more optical surfaces.

On the other hand, Newtonians are optimized as larger telescopes for the similar reasons. Unlike chromatic aberration, coma only depends on the focal ratio so one can scale up an 5 inch F/5 Newtonian to larger apertures without having to increase the focal ratio. The field illumination difficulties a small Newtonian faces also decrease as the aperture increases.

So, rather than judging this comparison based solely on the planetary, high magnification views of a Newtonian and an apochromatic refractor, I suggest one consider a building a telescope with the following specifications:

- Aperture: ~4 inches
- Focal ratio: F/5-F/5.5
- Chromatic aberration: essentially Color free
- Coma: Essentially zero
- Field curvature: Essentially zero
- Fully illuminated field: ~ 4 degrees
- Light throughput: 98%
- System Strehl: .98 (includes effect of CO)
- Stable thermally
- Stable collimation

I have done my best to describe as perfect a 4 inch telescope as I can. It excels at both the things a 4 inch telescope does best, that is widefield viewing with pinpoint stars to the edge and its high magnification capabilities are only limited by it's aperture.

Whether one wants a scope with these capabilities, that is an individual question. But if one wants such a scope, in my mind, it must be an apochromatic refractor and in my experience, such scopes are quite wonderful... but then all scopes are quite wonderful, just not quite as perfect.

Jon


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kfrederick
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5579542 - 12/19/12 08:17 PM

Jon My 17 inch off axis Wide Band CHief can do anything a 17 inch APO can do . Ed good post

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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: kfrederick]
      #5579561 - 12/19/12 08:30 PM

Quote:

Jon My 17 inch off axis Wide Band CHief can do anything a 17 inch APO can do . Ed good post




And a 25 inch Obsession can do things a 17 inch off-axis Wide Band Chief cannot do.

But mostly, a TeleVue NP-101 or a Takahashi 106FSQ can do things neither the 25 inch Newt nor the 17 inch Chief nor a 6 inch F/10 Newtonian can do...

And visa versa....

The right scope for the application...

Jon


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ed_turco
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Re: Optimized Newts vs APOs new [Re: David Pavlich]
      #5579575 - 12/19/12 08:37 PM

AH, you weren't reading closely. One of my precision optical windows negates the need for any spider diffraction whatsoever. No need for curved spiders or other cheats to spread the diffraction around.



Ed


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ed_turco
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Re: Optimized Newts vs APOs new [Re: Astrojensen]
      #5579583 - 12/19/12 08:41 PM

Remember, I am talking of image quality only. What ever bothers you or me simply doesn't factor into the equation. Besides, because of the slight taper on a longer focus image cone, there is very little need for the very lowest profile focusser.


Ed


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ed_turco
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Re: Optimized Newts vs APOs new [Re: John Jarosz]
      #5579587 - 12/19/12 08:44 PM

ATM's love to maximize performance in their telescopes as much as observe through them. To optimize is as much or more fun than to observe, though the quality of the observing comes out so much better.

Edited by ed_turco (12/19/12 08:46 PM)


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5579595 - 12/19/12 08:47 PM

Quote:

AH, you weren't reading closely. One of my precision optical windows negates the need for any spider diffraction whatsoever. No need for curved spiders or other cheats to spread the diffraction around.

Ed




There is still diffraction from the secondary but it ain't much. On the other hand, the fully illuminated field of view will be small, the 75% illuminated field will be small. It would be a great planetary scope but not much of a low power scope.

Jon


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tim53
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Re: Optimized Newts vs APOs new [Re: kfrederick]
      #5579598 - 12/19/12 08:49 PM

Long focus newtonians should be renamed "hyperapochromats".

Rant off.

-Tim.


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careysub
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5579606 - 12/19/12 08:55 PM

The ultra wide angle view thing is where small (or large) reflectors cannot compete with small refractors.

An ultra wide angle 2" EP may have a field stop pushing the limit the 2" format supports (44mm or so), say the ES100 25mm, the Nagler 82 deg 31mm, or the ES82 30 mm.

You won't get adequate illumination of that field stop without a secondary that is at least as large, and a 44mm secondary with a CO of not more than 30% would have to be at least 5.87", call it 6". And with a reasonable exit pupil it would have to be F/4.2 (the ES100 25mm), requiring a coma corrector pushing it to F/4.8, or else an F/5.2 outright, with FL of 730-800mm.

OTOH, these EPs can be used with F/4-5 70-100mm objectives, and FLs of only 280-500mm with no CO light or contrast loss at all for extremely wide fields. If used for large nebula observing, with narrowband filters the CA aspect of the short achros disappears.


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: careysub]
      #5579668 - 12/19/12 09:30 PM

Quote:

The ultra wide angle view thing is where small (or large) reflectors cannot compete with small refractors.

An ultra wide angle 2" EP may have a field stop pushing the limit the 2" format supports (44mm or so), say the ES100 25mm, the Nagler 82 deg 31mm, or the ES82 30 mm.

You won't get adequate illumination of that field stop without a secondary that is at least as large, and a 44mm secondary with a CO of not more than 30% would have to be at least 5.87", call it 6". And with a reasonable exit pupil it would have to be F/4.2 (the ES100 25mm), requiring a coma corrector pushing it to F/4.8, or else an F/5.2 outright, with FL of 730-800mm.

OTOH, these EPs can be used with F/4-5 70-100mm objectives, and FLs of only 280-500mm with no CO light or contrast loss at all for extremely wide fields. If used for large nebula observing, with narrowband filters the CA aspect of the short achros disappears.




I have a fair amount of eyepiece time with a 130mm F/5 Newtonian fitted with a 2 inch focuser. It had a 38mm secondary and fitted with a Paracorr and a 31mm Nagler, it would provide a decent 3.2 degree TFoV. By analysis the field was poorly illuminated but at the eyepiece, it was not troublesome, I could see the entire Veil. It was also pretty good at high magnifications.

I also have an 80mm F/5 fitted with a 2 inch focuser. You're right about the narrow band filters, it provides a 6 degree TFoV and the field curvature is not noticeable with an O-III or H-Alpha...

This is drifting off-topic. It's clear an optimized 6 inch F/10 or even a standard 6 inch F/8 Newtonian can be an effective planetary scope though around here one would want an 8 inch or 10 inch. But challenges, accepted or ignored, do not make a telescope...

Jon


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Datapanic
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5579678 - 12/19/12 09:35 PM

I can't find it. Does anyone have the link to that YouTube cartoon about APO vs. Achromatic vs. Dob? The ending has the character asking the other two arguing over APO vs. Achromatic if they would like to look through his dob and they both, simultaneously shout "No!"

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Datapanic
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Re: Optimized Newts vs APOs new [Re: Datapanic]
      #5579692 - 12/19/12 09:42 PM

I found it! It's actually about an APO vs. Questar Funny and probably fitting for what this thread will turn out to.

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kfrederick
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Re: Optimized Newts vs APOs new [Re: Datapanic]
      #5579705 - 12/19/12 09:51 PM

funny

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magic612
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Re: Optimized Newts vs APOs new [Re: tim53]
      #5579708 - 12/19/12 09:52 PM

Quote:

Long focus newtonians should be renamed "hyperapochromats".

Rant off.

-Tim.




I think I'll name my 6" f/10.9 reflector that, Tim!!


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Meep_Esq
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Re: Optimized Newts vs APOs new [Re: kfrederick]
      #5579775 - 12/19/12 10:33 PM

In as much as I own a refractor and am building a newt, I created a spreadsheet to crunch the numbers on things like mirror size, F-ratio, diagonal sizing, the placement of the focal plane, etc. One of my criteria was to try and keep the secondary to the minimum necessary to illuminate the field lens of the average 1 1/4" eyepiece I would expect to use, but to aim for below 18% obstruction (I think I came up with this figure from Sidgwick and a few other sources...) to try and minimise obvious effect due to the central obstruction. Anyhoo - what was interesting was that it was much much harder to meet these criteria with smaller apertures than it was with larger ones. In one iteration I did, a 6" mirror could never meet the specs, and an 8" one had to be about F9 in order to be "optimum" A 12" mirror would be ok a bit over F6, etc. I was generally surprised by this, and it was a fun little experiment. I think it certainly shows that there is a place for refractors in those small to moderate sizes, and after that an optimised reflector makes more sense, performance wise if not $$$ wise :-) I can attach the spreadsheet if anyone is interested

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Pinbout
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5579786 - 12/19/12 10:44 PM

Daniel Mounsey did a comparison already

TEC140 vs. Ceravolo HD145


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StarStuff1
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Re: Optimized Newts vs APOs new [Re: Meep_Esq]
      #5579796 - 12/19/12 10:50 PM

A long f/r newt such as in my avatar can give a similar sized apo a good run for it's performance.

BUT, as Richard Berry said, inch per inch nothing beats an apo. OK, beat me up. (Saying this as an owner of 18 refractors and 4 newts)


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mikey cee
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5579846 - 12/19/12 11:45 PM

Too bad we can't get a likewise consensus on expensive big bucks eyepieces vs plain label orthos and plossls! Mike

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MKV
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5579932 - 12/20/12 12:52 AM Attachment (49 downloads)

Quote:

Pictures were shown of unobstructed images of a 6" f/10 APO and a 6" f/10 Newt with 16% obstruction. The difference was barely perceptible. In fact, Suiter and I agree that there is no difference at the eyepiece...Tell me the aperture of your APO and give me time to make an optimized Newt of the same size, with a precision optical window and a 12% obstruction. Special coatings all around.



That was a fair challenge and, clearly some 30 years overdue!

The picture below shows (in reverse color) PSFs for a 6-inch f/10 configuration with various amounts of central obstruction. The negative image helps visualize the amount of energy in the Airy disc better than against black background. Clearly, some difference exists between these shades of gray, but under real nighttime observing conditions, even under superior seeing, it would be more an act of willful imagination than actual perception to say that this difference would be noticed - even if the images could be observed simultaneously and right next to each other, and a very dark sky.

So, the experiment could never be fully objective, or effective, simply because no one can see these images at once, so close and perfectly still. Maybe one could take photographs of a star (say a Polaris) under high eyepiece projection, using the same eyepiece and exposure time, etc. and then compare the images side by side.

There is no doubt that a well made f/10 Newtonian, as Ed describes, with 12% CO will perform better than an APO simply because it has zero color! At some point a very bright image will reveal chromatic residual even in the best of APOs.

The real difference liles in the fatc that Ed's optic can probably be made with materials costing about $250. A 6-inch f/10 2-element APO objective today sells for more then 10 times that price.

Mladen


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tim53
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5579947 - 12/20/12 01:06 AM

Quote:

Quote:

Long focus newtonians should be renamed "hyperapochromats".

Rant off.

-Tim.




I think I'll name my 6" f/10.9 reflector that, Tim!!




Yep! I call my 6" f/10.3 that!


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MKV
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5580011 - 12/20/12 03:13 AM Attachment (40 downloads)

In 1981, Roland Chirsten came out with his "oiled" triplet apochromat which was a major sensation at Stellafane. This started the "APO fever".

The beauty of the Christen design was that the lens was not only apochromatic (i.e. corrected for four colors simutaneously), but the OG was also perfectly symmetrical. The two outer radii of curvature were equal but opposite, as were the two inner ones. This means they could be matched perfectly by Newotn's interference method, a very elegant solution indeed from the shop production point of view.

In April of 1982, Paul Weissman, an optical engineer at Ferrand Optical in Valhalla, NY, wrote to Sky & Telescope objecting to Christen's original design as under-corrected for color and offered his "correction" as an alternative.

Christen replied shortly thereafter, saying that he "inadvertently" used an old set of figures for the 1981 configuration and then submitted a new set which was better than the original as well as Weissman's but the OG lost its symmetrical elegance Christen made a big deal of in 1981, plus it now introduced a field-limiting coma.

Clearly, a 6-inch well made Newtonian - like the one Ed offered to build for his proposal, i.e. with a 12% central obstruction, and of equivalent focal length - would have easily beaten all three versions of the Christen's f/10.8 triplet.

I wrote a letter to Sky and Telescope, calling attention to this, but it was never published. Christen's APO soon began selling for $2200 or thereabouts, yet it's performance was inferior to that of an equivalent quality, diameter and focal ratio equivalent Newtonian that in 1982 could be built for around $100.

Mladen


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MKV
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5580012 - 12/20/12 03:15 AM Attachment (36 downloads)

And these are gray-scale integrated PSF image simulations at 0.336 deg. off-axis, and normalized at 0.15.

Mladen


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Mirzam
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5580090 - 12/20/12 05:19 AM

I have the optics ready for a 10" f/6.5 newt that will be designed for binoviewing and seated observing. It will be mounted on a driven dobsonian style structure.

So much for the supposed ergonomic advantages of an APO.

JimC


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: Datapanic]
      #5580107 - 12/20/12 05:56 AM

Quote:

I found it! It's actually about an APO vs. Questar Funny and probably fitting for what this thread will turn out to.




It was cute... It is true that often discussions like this boil down to "my telescope is better than your telescope" rather than an objective discussion of the merits of each. There's a lot of emotional baggage...

Me, I got it too but at least I choose my apo refractor for bird watching and use a good sized Newtonian when I want the best views of the planets.

Jon


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Daniel Mounsey
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Re: Optimized Newts vs APOs new [Re: Datapanic]
      #5580244 - 12/20/12 08:53 AM

Quote:

I found it! It's actually about an APO vs. Questar Funny and probably fitting for what this thread will turn out to.




It's 5:30 in the morning and my wife wants to know why I'm laughing so hard. I have to admit, that was pretty funny. We all do get a bit obsessed sometimes.


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Jeff Morgan
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5580303 - 12/20/12 09:44 AM

Quote:

Comments and pictures were shown on a thread concerning Herrig designs and I popped in and went off on a tangent.

Pictures were shown of unobstructed images of a 6" f/10 APO and a 6" f/10 Newt with 16% obstruction. The difference was barely perceptible. In fact, Suiter and I agree that there is no difference at the eyepiece; anyone who says differently is thinking more of what he paid for his telescope than what he is seeing through his telescope in comparison.

Tell you a story: I went on the Refractor Forum (EGads!) and made this offer. Tell me the aperture of your APO and give me time to make an optimized Newt of the same size, with a precision optical window and a 12% obstruction. Special coatings all around. Then, some neutral people would have to be picked to make an assessment as to what was what.

The issue was to be imagery only; not the appearance of either telescope. Perhaps the Stellafane optical judges would be a good choice.

In any case, no matter, win lose or draw, my telescope would be donated to charity.

You know what? No one took the challenge against my 1/20 wave optics. Not one. Not one dared.

I waited 6 months; the offer is withdrawn; why should I put up with all this "mighty APO" BS?




Great post Ed, unfortunately I caught it only about 5 minutes before I have to dash out the door this morning ... but I'll catch up to all the great responses later.

Unfortunately, you won't find too many Mercedes Benz-driving, bottled water-drinking, Nieman Marcus-shopping, Gucci-wearing, APO owning people on the ATM Forum

But seriously, the best advantages of the APO aren't necessarily optical, but mechanical. That being the objective is farther away from the ground where one normally finds a rather sharp temperature gradient and it's detrimental effect on seeing. Sadly, by nature the primary mirror sits right in that gradient.

I'd love to see you go ahead and build that closed-tube Newt and set-up next to the Astro-Physics tent at AstroFest or Stellafane. Perhaps you could invite some of the writers at Sky & Tel to stop by. Now that would create quite a stir!


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: Jeff Morgan]
      #5580330 - 12/20/12 10:00 AM

Quote:


I'd love to see you go ahead and build that closed-tube Newt and set-up next to the Astro-Physics tent at AstroFest or Stellafane. Perhaps you could invite some of the writers at Sky & Tel to stop by. Now that would create quite a stir!




Or you could just read Roland's advice concerning the "Best Planetary Telescope" and build an 8 inch F/8 and be done with it...

What is the Best Planetary Telescope

But the most important question is.... "Is my telescope better than your telescope." Until we answer that, no progress is possible. In general, I think it is reasonable to say that the best planetary telescope is the one that lives where the seeing is the best.

Jon


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ed_turco
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5580423 - 12/20/12 10:51 AM Attachment (38 downloads)

I am now too crippled with rheumatoid arthritis to make another telescope or go to Stellafane. *BLEEP*! That is why Al Hall made his noble effort at the Friday Night Talks to present my Comascope. I don't think he showed it on its "ball mount".

Edited by ed_turco (12/20/12 10:53 AM)


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5580466 - 12/20/12 11:18 AM

Quote:

I am now too crippled with rheumatoid arthritis to make another telescope or go to Stellafane.




Ed:

I am sorry to hear that. Your proposed design sounds like a winner and I am sure it would provide some wonderous planetary views. One of the reasons I have never been interested in a refractor much bigger than 4 inches is that I have a pretty nice RV-6 from the early 70's.

Jon


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magic612
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Re: Optimized Newts vs APOs new [Re: Jeff Morgan]
      #5580500 - 12/20/12 11:34 AM

Quote:

But seriously, the best advantages of the APO aren't necessarily optical, but mechanical. That being the objective is farther away from the ground where one normally finds a rather sharp temperature gradient and it's detrimental effect on seeing. Sadly, by nature the primary mirror sits right in that gradient.




This very issue is one I may address with my 6" f/10.9 Newt by considering a fan pulling air through the tube from the sky, downward at the mirror - so the air would be coming from about the same height that an APO lens would be sitting. Presently, even with the concrete-pier prototype tube it's in at the moment (unflocked, waxed/cardboard interior) with a 17% obstruction and self-built curved vane spider, the images at the eyepiece have so far been quite impressive, and to me, reminded me of view through a quality refractor, because the curved spider produced no straight diffraction spikes. Eventually this will go in a Hastings aluminum tube, with various added things to minimize thermal issues and stabilize the image (the aforementioned fan, etc.).

So far it's run me about $200 or $250 in parts and accessories. I expect to spend another $200 to $400 on it by the time it's done. I certainly can't argue the finer points of optical theory, so I won't try. But I think I'll be pretty darn close to the visual results of similarly sized apo for... what does a 6" APO run these days anyway? (I seriously don't even know.) And besides, I have a daughter going into college next year. So this is the only "apo" in my budget for a long, long time (the next daughter will follow her four years later).

And by the way, this is not an "apo bashing" post. I'm just of the opinion that a quality reflector can be optimized to produce nearly as good views as one for a lot less money. But if others prefer the apo for aesthetic or other mechanical / design reasons, there's nothing wrong with that either. The more the merrier.


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tim53
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5580619 - 12/20/12 12:48 PM

A long focus 6" Newtonian with rotating rings is still small enough to be quite comfortable to use, and a lot easier to move around and set up than a 6" refractor.

Here's my hyperapochromat 6" f/10.3 "Kludge Classic" on a Tak EM-10 mount:



OTA detail. This doesn't even have a small secondary (yet). I think it's a 1 1/4". I have a 3/4", but it's in a 3" f/6 now. I should probably order a couple of 1" diagonals - one for the Kludge and one for my 8" f/9 (under construction).


And here's Mars with it:



-Tim.


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*skyguy*
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5580659 - 12/20/12 01:08 PM Attachment (27 downloads)

Can an optimized Newtonian reflector compete successfully with an APO refractor of similar size? Absolutely ... YES!

I own an optimized reflector ... 6" f/10 (1/8th wave) primary mirror from Edmond Optics, 25mm. secondary mirror (1/20th wave - 16% obstruction), curved vane spider and low profile focuser. It shows very "clean" images with amazing contrast. Not only are planetary views spectacular, but deep-sky objects also benefit greatly from the high contrast. Unfortunately, this scope makes the views from my Criterion RV-6 6" reflector look rather shabby in comparision.


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bremms
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Re: Optimized Newts vs APOs new [Re: *skyguy*]
      #5580685 - 12/20/12 01:21 PM

Cost for cost.. NO CONTEST. 12.5" F8 newt vs a good 6" APO.
My 6" F8 newt is still my best planetary scope My C11 has more resolution but less conrast. 4" F15 Jaegers looks promising at 4" but still has some color. My old 10" F6 was better than a 6" AP nearby on Jupiter. At less than 1/5 the cost. From a cost standpoint not by a long shot. But I'd love a 6" + APO
Really you could build a killer 14-16" newt for the cost of a 6"AP.


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magic612
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Re: Optimized Newts vs APOs new [Re: *skyguy*]
      #5580686 - 12/20/12 01:22 PM

I'd just like to say it is great seeing all these long focal length 6" Newts!

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Pinbout
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Re: Optimized Newts vs APOs new [Re: mikey cee]
      #5580830 - 12/20/12 02:46 PM

Quote:

Too bad we can't get a likewise consensus on expensive big bucks eyepieces vs plain label orthos and plossls! Mike




I used my es14mm 82* against my tv15mm plossl on m15 with my 8inf6. no contest, night and day, I no longer have the es14mm.


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bremms
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5580887 - 12/20/12 03:14 PM



Eyepieces!!! Me I like good simple EP's. TV plossls, UO orthos, Brandons, I have a no name 12.5mm vintage Ortho that is killer. Don't know why it's so good but it works great in all my scopes(I have no scopes under F6)


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pstarr
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Re: Optimized Newts vs APOs new [Re: bremms]
      #5581076 - 12/20/12 05:09 PM

Over on the eyepiece forum, they said it doesn't matter what kind of scope you use. All views will be superior if you use a Brandon eyepiece. Design and optical quality really have no bearing. Well, that's what they said.

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Astrojensen
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Re: Optimized Newts vs APOs new [Re: bremms]
      #5581085 - 12/20/12 05:12 PM

If we factor in the issue of cost, then nothing else comes even close to a newtonian. The price difference between a custom 6" newtonian with the finest mirrors, tube, focuser, etc; and even a low-priced 6" assembly-line apo is ridiculously large.

So why are so many choosing apos and even achromats over newtonians? Clearly the answer lies somewhere else than in theoretical performance. I think there are many answers, but one is that many newtonians made over the years have incredibly poor thermal behaviour, severely limiting their performance under real world conditions. One 6" that I have on loan take three hours to cool down... Before it has cooled, it is little better on the planets than my 80mm refractor. In contrast, my 5" f/14 achromat was almost immediately much more powerful than my 80mm and was utterly fantastic once fully cooled.

But when it had finished cooling, the 6" newtonian was scaringly good. Mars exploded in details. It elongated 0.4" double stars at 600x and showed perfect airy disks. Truly stunning performance.

So why don't I use the 6" all the time? Back then, I knew nothing about thermal design of newtonians, so I quickly gave up on it, due to its long cooldown time and crazy poor ergonomics (I had it on a german equatorial, which is a horror, if you don't have rotating rings. I didn't have rotating rings...). In comparison, the 5" refractor was easy to use, despite being very heavy. Now that I know more, I am thinking about rebuilding the 6". I also need to rebuild the 5" refractor. It is too heavy.

Newtonians can be crazy good, like apos, but they have their own subset of design weaknesses that need to be adressed in a succesful product. In a lot of newtonians of the past, these issues weren't given any attention at all and the result has been that newtonians have been dragging around with an unfair reputation for centuries.

I have been guilty of it myself. It took a 12" dob to really open my eyes to the qualities of the newtonian and that scope has LOTS of issues. The sad thing is that there seems to be no other way to remedy them other than build it oneself.


Clear skies!
Thomas, Denmark


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Dick Parker
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5581195 - 12/20/12 06:02 PM Attachment (35 downloads)

Ed et al -

I have a 6 inch f/12 with a spherical mirror that gives incredible images. Saturn's moons are the smallest cleanest dots with this telescope. Last time I used it for a Messier Marathon I was blown away with it. I don't know why I don't use it more. But I actually plan to. On my list to Santa is a 1 inch secondary. I built it with .75 inch and that is a bit too small. Yup! I'd give it a go with a 6 inch APO. But it is a cheezy no cost thing. No one would give it credit against the nice looking expensive APOs.

My advice would be to not underestimate those long focus Newts

Dick Parker


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MKV
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Re: Optimized Newts vs APOs new [Re: Astrojensen]
      #5581243 - 12/20/12 06:38 PM

@Astrojensen: in my experience no 6-inch would take that long (3 hours!) to cool down. A 6-inch mirror is only 1 inch thick. If you point the tube close to the Zenith, all the heat will quickly escape.

Mladen


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Astrojensen
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5581837 - 12/21/12 03:18 AM

That's what I thought, too, but alas. It really is a thermal pig, right up there with SCTs. I've found a big fan and have rebuild an identical mirror cell I had lying about. Now I just need a new base for the 2" GSO crayford and I'm almost there.


Clear skies!
Thomas, Denmark


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MKV
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Re: Optimized Newts vs APOs new [Re: Astrojensen]
      #5581933 - 12/21/12 06:32 AM

Thomas, I've been an ATM since the 1970's. My first mirror was a 6-inch f/6. I've also had a Cave 6-inch f/8. All mirror blanks in those days were full thickness, no matter what diameter, we didn't use (nor have!) computer fans, and no one ever mentioned a 6-inch "thermal pig".

I have heard the same story about the Houghtons - they never "cool down". Rubbish! I made a 6-inch Houghton in the early 1980's and never had that experience. Maybe it's a generational thing. I don't know, but perhaps in three hours the atmosphere becomes less turbulent too, rather than your 6-inch mirror needing three hours to reach thermal equlibrium. Maybe you are observing on cement rather than grassy area, or over the rooftops.

You are comparing apples and oranges. I never had to cool down my 3-inch Unitron, or my Edmund's 3-inch f/10 Newtonian. A 3-inch aperture is not affected by the atmosphere because of the size of the thermal air cells. Thermal air-cell size is about 4-inches and any aperture above that will be prejudicially affected by the thermal heterogeneity of air to a much greater extent than apertures smaller than thermal air cells.

Any scope over 20 inches in aperture will see this no matter how long it has been "cooling" down unless it's in space.

Mladen


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GlennLeDrew
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5581970 - 12/21/12 07:35 AM

Metal scope tubes are awful things from a thermal standpoint, as they always 'want' to cool well below ambient under a clear sky. This differential maintains air currents for some time.

Mladen,
It's rather simplistic to state that thermal air cells are no smaller than about 4", and so apertures below this limit are not affected. One's sight line through the atmosphere must necessarily contain many cells overlapping, which will present a continuous variation on all scales. I've had 60mm scopes be very badly affected by seeing.


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MKV
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Re: Optimized Newts vs APOs new [Re: GlennLeDrew]
      #5582005 - 12/21/12 08:02 AM

Glenn, Cave Astrola used to sell fiberglass tubes. Parks Optical still does - with end rings. For a 6-inch f/10 it costs $139. I think that might be cheaper than aluminum!

I never used aluminum tubing. My scopes were always fiberglass, wood, or spiral paper. ATMs of my time knew better.

As for apertures smaller than 4" not being affected seriously by air turbulence, that has been well documented. You will notice that I also explained what I meant when I added that larger apertures are affected to "a much greater extent than apertures smaller than thermal air cells."

Of course, anything below 60 degrees will present thicker air mass and greater scintillation. Close to the astronomical horizon you never have air steady enough for observing with anything but the lowest power. Your best bet is always to observe at or near the Zenith.

Mladen


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5582961 - 12/21/12 06:09 PM

Quote:

Glenn, Cave Astrola used to sell fiberglass tubes. Parks Optical still does - with end rings. For a 6-inch f/10 it costs $139. I think that might be cheaper than aluminum!

I never used aluminum tubing. My scopes were always fiberglass, wood, or spiral paper. ATMs of my time knew better.

As for apertures smaller than 4" not being affected seriously by air turbulence, that has been well documented. You will notice that I also explained what I meant when I added that larger apertures are affected to "a much greater extent than apertures smaller than thermal air cells."

Of course, anything below 60 degrees will present thicker air mass and greater scintillation. Close to the astronomical horizon you never have air steady enough for observing with anything but the lowest power. Your best bet is always to observe at or near the Zenith.

Mladen




Unfortunately both Parks and Scope City have closed their doors, only Lumicon remains.

Regarding seeing stability in smaller scopes... If it's unstable it in a large scope, typically I can see it in 4 inch.

Jon


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MKV
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5583013 - 12/21/12 06:48 PM

John Isaacs: The Parks site is still active.

If fiberglass is not available, phenolic is a good substitute, though it may be pricier. Paper tubes also make better tubes than aluminum (aluminum is one of the most heat conductive materials!). Tubes don't have to be round. I thin Texerau's method makes for excellent, lightweight, tubes made out of wood that anyone can afford and make with basic homeowner's tools. Anything but aluminum.

Mladen


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Mark Harry
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5583049 - 12/21/12 07:10 PM

I've used AL tubes for years, and -NEVER- had an issue with 6" aperture, period.
M.


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Mike I. Jones
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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5583097 - 12/21/12 07:34 PM

I have built several Newts and always had good results with aluminum. A layer of 1/8" thick cork, spray painted black and cemented to the ID, removes essentially all thermal boundary layer effects.
Mike


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bremms
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Re: Optimized Newts vs APOs new [Re: Mike I. Jones]
      #5583132 - 12/21/12 08:00 PM

My 6" F8 has an 8" 1/8 wall tube. It's massive and is being replaced with a Protostar tube. 4 lbs vs 16 and better thermal properties. Needs end rings an a stiffener for the spider mounting. Mine takes about 2 hrs to really cool down properly. Turning my scope into a lightweight so it works on one of my Vixen GP mounts.

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Datapanic
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Re: Optimized Newts vs APOs new [Re: bremms]
      #5583182 - 12/21/12 08:48 PM

The Horsetrail Cave has an aluminum tube (original paper/fiberglass tube was destroyed) and I use a nice fan that can be reversed if needed. It performs well with a flock-board interior. Aluminum is excellent for radiating and loosing heat faster than other materials such as painted fiberglass, wood or paper that act as insulators.

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StarStuff1
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Re: Optimized Newts vs APOs new [Re: Datapanic]
      #5583194 - 12/21/12 08:56 PM

--

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MKV
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Re: Optimized Newts vs APOs new [Re: Mike I. Jones]
      #5583228 - 12/21/12 09:27 PM

Quote:

I have built several Newts and always had good results with aluminum. A layer of 1/8" thick cork, spray painted black and cemented to the ID, removes essentially all thermal boundary layer effects.
Mike



Of course! That's like insulating a house, Mike. Parks tubes needed neither paint nor cork, and their price was very competitive. Why buy aluminum?

Mladen


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Mike I. Jones
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5583260 - 12/21/12 09:55 PM

One reason is, I've ordered a few Parks tubes over the decades, and every one I received had a few to several non-trivial cracks in the outer finish. It's not the fault of the tubing, but of shipping and packaging methods, and of the gorillas (mis)handling them. If I lived in the LA area again and could just drive over to Parks and buy one off the shelf, that'd be great. But I live in Texas, and smooth, pristine, dent-free irrigation pipe is available by the mile, and within easy driving distance.

Mike


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tim53
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5583277 - 12/21/12 10:06 PM

I think the biggest problem with fiberglass is its flexibility.

People like Jeff Beish and Don Parker have bee outspoken proponents of aluminum tubes, unpainted on the outside, and of course at least painted black on the inside (paint is an insulator).

I make my tubes out of wood because I like it.

-Tim.


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plyscope
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Re: Optimized Newts vs APOs new [Re: tim53]
      #5583323 - 12/21/12 10:41 PM

I like wood too.

Here is a technical article on cooling for newtonians by Anthony Wesley.

web page


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Daniel Mounsey
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5583542 - 12/22/12 02:48 AM

An optimized Newt would overwhelm any refractor. 12.5" to 14.5" is the sweet spot.

Edited by Daniel Mounsey (12/22/12 10:17 AM)


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MKV
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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5583570 - 12/22/12 03:45 AM

Quote:

An optimized Newt with overwhelm any refractor. 12.5" to 14.5" is the sweet spot.



You make a good point. It's almost too obvious. I would say the same holds for the various unobstructed reflectors as well, unless they come in larger apertures than the usual 4.25 to 8 inches.

Kfrederick has a 17" unobstructed scope. It would be interesting to have it compared simultaneously with another 17 inch of good quality (simple - pick a star or a planet, point both scopes to the same target, two digital cameras of the same make, and two laptops next to each other, freeze the images, save them, stitch them together in Paint, and compare. I am sure there is some freebie grayscale analysis software around).

As for DSO, a 4.25" simply cannot resolve or even see the detail a 12.5" can, even with a 25% central obstruction. A distant star cluster may look like a fuzz ball in a 4.25' or a 6", but in a 12.5" it will be ablaze with fully resolved stars, and will show incomparably more of them!

Mladen


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MKV
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Re: Optimized Newts vs APOs new [Re: tim53]
      #5583575 - 12/22/12 03:58 AM

Quote:

I think the biggest problem with fiberglass is its flexibility.

People like Jeff Beish and Don Parker have bee outspoken proponents of aluminum tubes, unpainted on the outside, and of course at least painted black on the inside (paint is an insulator).

I make my tubes out of wood because I like it.



Tim, paint is an insulator but it's thin, and thin insulation is not very effective. Paper is insulation, but you wouldn't paper your house with a couple of layers! If aluminum is the choice, then cork is probably the best material. I would imagine 1/4 inch thick or even 1/8 would go a long way. It's also important that your tube doesn't end a short distance from where your focuser is. Old fashioned Newotnians had at least the mirror diameter's worth of the tube extension past the eyepiece, so that the heat from your hands and breath doesn't get in the light path.

I really like Texereau's solution - four 2x2 and thin 1/4 inch plywood skin, reinforced where it ought to be with as many layers as needed. Square tubes have many advantages, when it comes to mounting the tube as well as accessories, for example. One thing they don't have is the snob-appeal.

It's everybody's world, to each his own.

Mladen


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kfrederick
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5583749 - 12/22/12 09:14 AM

Miaden One thought /would be to put a fake spider and secondary on the 17 WBChief to see the effect.I think it could be mounted on the primary .

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MKV
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Re: Optimized Newts vs APOs new [Re: kfrederick]
      #5583966 - 12/22/12 11:51 AM

Quote:

Miaden One thought /would be to put a fake spider and secondary on the 17 WBChief to see the effect.I think it could be mounted on the primary .



The important thing is to observe both unobstructed and obstructed images simultaneously. If you look through one scope, then walk away to another and check, this is wrong on so many levels! When you're dealing with "hairline" difference, the samples have to be examined together at the same time.

Again, when there's doubt I defer to the professional community, and I don't see that community using unobstructed reflectors.

Mladen


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Norm Meyer
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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5584142 - 12/22/12 01:54 PM

I have both aluminum and paper (sonotube) I haven't noticed
that much difference. Here in Maine the atmosphere is the
biggest culprit. It is not usually steady enough to detect
any turbulence from the tube. I used my 10" Newt for quite awhile and was disappointed with it's performance until one
night I saw detail on Jupiter I had never seen before it was
amazing. My conclusion was it wasn't my optics it was the
atmospheric conditions. My first commercial scope was a
criterion RV6. I bought it in 1965 and I was expecting a white tube and was a little disappointed when I discovered
they had up graded to a grey phenolic with a krinkle finish.
I quickly got over the
disappointment. Then I lent the scope to a HS student and
his brother knocked the tube over and it broke a large piece
out of it. He tried replacing the tube with one like the
original but Criterion didn't make those anymore. Anyway
I ended up with one of the white ones after all. I always
have trouble deciding what kind of tube to make when building a new scope. Has anyone ever tried one of those bollard covers? they are plastic and come in 7" size which
is good for a 6"mirror. I think they are around $37.95
for a 60" one. No need to paint the outside as they already
have the color molded in.

Norm


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MKV
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Re: Optimized Newts vs APOs new [Re: Norm Meyer]
      #5584205 - 12/22/12 02:36 PM

Quote:

I have both aluminum and paper (sonotube) I haven't noticed that much difference. Here in Maine the atmosphere is the biggest culprit.



That goes without saying. Texereau states the same thing. Large telescopes actually never experience a steady diffraction image unless they have adaptive optics. For smaller ones, that are closer to air-thermal cell size, the tube currents are much more prejudicial. No matter what size, it still makes sense to use material that is not a heat conductor.

Mladen


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paul01
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5584691 - 12/22/12 08:56 PM

Hello there,
I am new to CN but have been scanning the site for some time and like the format so here I am. I have a question for anyone out there who knows the build design for an Orion 110ED premium refractor. I just got one and discovered the focuser assembly that connects at the main tube is spinning quite freely. Not the focuser itself but the whole assembly. Does this assembly twist on similar to the dew sheild cove on the objective lens? I don't want to do any damage trying to tighten unless this is what I should do. Thanks for any information I can get..
Paul


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kfrederick
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Re: Optimized Newts vs APOs new [Re: paul01]
      #5585244 - 12/23/12 09:06 AM

Paul Welcome .You might wont to ask on the refractor forum. If you do not get a reply here

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Re: Optimized Newts vs APOs new [Re: paul01]
      #5585270 - 12/23/12 09:30 AM

check this review Orion 110 ED

its a rotating focuser.

from the review

Quote:

The focuser’s rotating feature works well. I have had two other scopes with inexpensive rotating focusers. On both of those the rotating hubs really needed to be locked down because they had play in them. This focuser fits the adapter very well and I can rotate it with the camera in place without worrying about keeping things squared. In fact recently I finished an imaging run and discovered I’d forgotten to lock the focuser hub. When I checked the data there was no evidence in the images. I can’t compare it to a Feathertouch but the focuser on the 110 ED has been a pleasant surprise




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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5585611 - 12/23/12 01:19 PM

Quote:

I've used AL tubes for years, and -NEVER- had an issue with 6" aperture, period.
M.




If the tube fits too closely, it might cause problems. I use an 8" tube on my 6" reflector designs. Perfect.


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ccaissie
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Re: Optimized Newts vs APOs new [Re: ccaissie]
      #5585624 - 12/23/12 01:29 PM

Just like with guitars...anyone serious about it will have several different types...Archtop, flat top, electric, classical, 12 string and a few other items like mandolins, fiddles and pennywhistles.

Parallel to Newts, Maks, refractors, binoculars, schmidt cameras, antique scopes and toilet-paper-tube/spectacle-lens refractors to amaze and educate the kids.

As the saying goes..It's all good.


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cheapersleeper
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Re: Optimized Newts vs APOs new [Re: ccaissie]
      #5585751 - 12/23/12 02:48 PM

Quote:

Just like with guitars...anyone serious about it will have several different types...Archtop, flat top, electric, classical, 12 string and a few other items like mandolins, fiddles and pennywhistles.

Parallel to Newts, Maks, refractors, binoculars, schmidt cameras, antique scopes and toilet-paper-tube/spectacle-lens refractors to amaze and educate the kids.

As the saying goes..It's all good.




Nonsense. Four electrics, one steel string acoustic and there you go. People have preferences...

Regards,
Brad


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Re: Optimized Newts vs APOs new [Re: cheapersleeper]
      #5586215 - 12/23/12 08:17 PM

A week ago I had been contemplating putting my 6" refractor up to your challenge then last night I took a minute looking at Jupiter through a very good 10"f/8 newt. sigh. I suppose there is room for all kinds of decent scopes in this world.

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Re: Optimized Newts vs APOs new [Re: Sean Cunneen]
      #5586375 - 12/23/12 10:23 PM

Quote:

A week ago I had been contemplating putting my 6" refractor up to your challenge then last night I took a minute looking at Jupiter through a very good 10"f/8 newt. sigh.



That's right. Here is a good article to read to understand why good bigger scopes outperform smaller "perfect" scopes every time.

Mladen


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586405 - 12/23/12 10:39 PM

I believe that the 'challenge' was for equal aperture? Not very fair tossing up a 6" APO against a 10" ...

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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586481 - 12/24/12 12:02 AM

Quote:

I believe that the 'challenge' was for equal aperture? Not very fair tossing up a 6" APO against a 10" ...



Even then, with equal apertures, at 20% obstruction or below, the difference is tenuous (look at Figure 2b), especially if an optical window is used to support the secondary.

Some people are purists and enjoy expensive, apochromatic unobstructed telescopes and they don't need any other reason for it. It's all good.

Neverthless, people should be informed what the science says.

Mladen


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586573 - 12/24/12 01:27 AM

Quote:

I believe that the 'challenge' was for equal aperture? Not very fair tossing up a 6" APO against a 10" ...




It's not fair to the Newt given the cost of the APO...


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586578 - 12/24/12 01:41 AM Attachment (36 downloads)

Using Aberrator(R), I constructed this collection of Jupiter shots starting from 0% obstruction (APO) all the way to 45% (for Newotnians). I intentionally chose a picture that resembles the kind of image reaching the eye rather than the Photoshop(R) art that is commonly used. I think this is a much more realistic portrayal of what an observer can expect to see.

The only thing I did not include are the atmospheric effects at this magnification which would make these images all but stable.

Clearly, the images in the top row are sharper - relatively speaking. You can see minor details that progressively disappear as the central obstruction (CO) continues to increase. But you will also notice that these changes are rather subtle and can be observed if one looks at them simultaneously, and, of course, without any atmosphere involved, rather than running from one scope to another trying to remember the image form the previous one in all its detail.

Now here is the revelation: even with this setup, neatly lined up, you still have to have to look darn close to see the difference.

To make a point, I marked one of those tiny details you can see if you squint, along the equatorial belt, where the clouds make a small dip below the band. This detail is clearly visible in scopes from 0% to 15% obstruction.

It begins to fade progressively at 20% to 30% CO, and is practically invisible with obstructions from 35% to 45%.

Is this really something you'd notice going from one telescope to another, all things considered? Maybe, if you specifically looked for such tiny detail, but it would not be something that would hit you in the face, where you'd say "Wow!".

You be the judge - an APO costing thousands of dollars versus a good quality equivalent aperture and f/ratio Newtonian with an optical window for secondary support or a 3-vane spider, either one is okay, and the cost difference up to 100:1. The quality of image difference - not even close.

Regards,
Mladen

Edited by MKV (12/24/12 01:56 AM)


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Re: Optimized Newts vs APOs new [Re: cheapersleeper]
      #5586583 - 12/24/12 01:45 AM

Quote:

Quote:

I believe that the 'challenge' was for equal aperture? Not very fair tossing up a 6" APO against a 10" ...




It's not fair to the Newt given the cost of the APO...



Good point. For the price of an APO you can afford a Newotnina so big you can use that APO as its finder scope.

Mladen


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Dave O
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586609 - 12/24/12 02:45 AM

Quote:

Quote:

Quote:

I believe that the 'challenge' was for equal aperture? Not very fair tossing up a 6" APO against a 10" ...




It's not fair to the Newt given the cost of the APO...



Good point. For the price of an APO you can afford a Newotnina so big you can use that APO as its finder scope.

Mladen




Again (for those who failed to read the opening post), the challenge was between equal apertures, not equal cost ...


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586680 - 12/24/12 06:07 AM

Quote:

Again (for those who failed to read the opening post), the challenge was between equal apertures, not equal cost ...



Equal apertures were compared two posts above, Dave.

Mladen


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586694 - 12/24/12 06:44 AM

Me personally, I can see a noteable difference from 0 to 15%. All you have to do, is look at the southern half of Jupe, in the "white" belt and the polar cap area.
If you drop down to the 45% pic, this area I mention is for all intents, totally void of any faint but detectable detail; and just a couple smears of smooth shading.
Looking at the obvious knots of swirls that are circled is not the correct way to assess the observation Dave mentioned.
M.


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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5586713 - 12/24/12 07:11 AM Attachment (19 downloads)

Quote:

Me personally, I can see a noteable difference from 0 to 15%. All you have to do, is look at the southern half of Jupe, in the "white" belt and the polar cap area.



You see notable difference between 0% and 15% obstruction? You think you could look through one telescope, then run to the next and immediately tell the difference? I think MTF data would say otherwise. Never mind the atmospheric effects...

Mladen

Edited by MKV (12/24/12 07:15 AM)


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586718 - 12/24/12 07:22 AM

The HST images show big spikes on all stars That is obstruction effects .Just think how much contrast is lost .Real defect easy to see .

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Re: Optimized Newts vs APOs new [Re: kfrederick]
      #5586737 - 12/24/12 07:57 AM

Quote:

The HST images show big spikes on all stars That is obstruction effects .Just think how much contrast is lost .Real defect easy to see .



Kfrederick, we were talking about small telescopes of equal aperture and an optical window (or 3-vane spider) for secondary support. Three-vane spiders actually produce negligibe loss of conrast.

Another thing I wanted to mention is that it takes a central obstruction equal to 33.2% of the clear aperture to produce image degradation equivalent to 1/4 wave wavefront error. Worth remembering.

Mladen


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Dave O
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586741 - 12/24/12 08:00 AM

Quote:

You see notable difference between 0% and 15% obstruction? You think you could look through one telescope, then run to the next and immediately tell the difference? I think MTF data would say otherwise. Never mind the atmospheric effects...

Mladen




My 3 1/2" Questar provides better images visually than these digitized images do ... just saying.


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586751 - 12/24/12 08:15 AM

Quote:

I believe that the 'challenge' was for equal aperture? Not very fair tossing up a 6" APO against a 10" ...




Whether it's fair or not and I think it's fair, it's reality.

Ed's challenge was the most difficult conceivable, equal aperture. Just about any other challenge would be a slam dunk for the Newtonian.

- Equal aperture: close

- Equal dollars: Newtonian

- Equal weight: Newtonian

- ATM built: Newtonian

When I look through the eyepiece, it's the view that counts. My 10 inch run of the mill commercial Dob clearly out performs my 4 inch apo on the planets and double stars, it's a fact of life...

Jon


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586792 - 12/24/12 08:53 AM

If 1/3 CO =1/4 pv then what does 1/6= 1/8?? Use two small lens forget about optical windows spiders Central obstruction and coma corrector Back focus on my 20 inch is 21inches Makes things easy if you need it for a bino or Camera .For two small lens .and a fancy holder.

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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586805 - 12/24/12 09:04 AM

Yes, MVK some folks know how to pay attention to what their eyes can detect/are capable of. Try it sometime, and get away from the simulations at the keyboard. The simulations are not that consistent or reliable. As DaveO mentions, those pics look like they were made in a much smaller scope, fwiw. To me as well.
Not everyone has the same capabilities, though. It's possible yours is substandard??? Ask the optometrist.
M.


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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5586842 - 12/24/12 09:22 AM

I think I need to check my eyes cause everything gets really sharp when I back away from the eyepiece when viewing planetary detail. I can have my eye 2" away from a plossl, I loose the fov but I'm only looking at the planet. I know I'm strange but...

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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586871 - 12/24/12 09:33 AM

Quote:

[My 3 1/2" Questar provides better images visually than these digitized images do ... just saying.



Maybe your atmosphere is better then ours. It's irrelevant what image you use, the central obstruction effect would be the same had I used sharper images. You still have a gradual loss of detail and contrast. I just think using those stacked Photoshop images is not what one sees, and therefore cannot judge.

BTWm, Questar still makes fantastic claims that defy everything we know about optics. In this Questar brochure, which I remember back from the late 70's, the company claims a 3.5 inch clearly resolves license plate number (1/4 inch wide) at a distance of 3.5 km (2.2 miles) over water (!). That's a resolution of less than half an arc second, and theory (Dawes limit = 4.6/D in inches) says 3.5 inch aperture (assuming wavelength of 0.55 um) can resolve no more than 1.3 arc seconds under ideal conditions.

Mladen


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586878 - 12/24/12 09:37 AM

Quote:

Quote:

[My 3 1/2" Questar provides better images visually than these digitized images do ... just saying.



Maybe your atmosphere is better then ours. It's irrelevant what image you use, the central obstruction effect would be the same had I used sharper images. You still have a gradual loss of detail and contrast. I just think using those stacked Photoshop images is not what one sees, and therefore cannot judge.

BTWm, Questar still makes fantastic claims that defy everything we know about optics. In this Questar brochure, which I remember back from the late 70's, the company claims a 3.5 inch clearly resolves license plate number (1/4 inch wide) at a distance of 3.5 km (2.2 miles) over water (!). That's a resolution of less than half an arc second, and theory (Dawes limit = 4.6/D in inches) says 3.5 inch aperture (assuming wavelength of 0.55 um) can resolve no more than 1.3 arc seconds under ideal conditions.

Mladen




Have never looked at license plates. But, I know what I see at the eyepiece, when looking at Jupiter.


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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5586886 - 12/24/12 09:44 AM

Quote:

es, MVK some folks know how to pay attention to what their eyes can detect/are capable of. Try it sometime, and get away from the simulations at the keyboard. The simulations are not that consistent or reliable. As DaveO mentions, those pics look like they were made in a much smaller scope, fwiw. To me as well.
Not everyone has the same capabilities, though. It's possible yours is substandard??? Ask the optometrist.
M.



Nothing new here, just being your usual wonderful self, Mark, right? Dishing our your opinions and now even practicing Internet medicine (got a license?)...

Mladen


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586893 - 12/24/12 09:47 AM

The last planet pic post, I had to back away from the monitor! The ones on pg 4 were fine.
(They do not look like simple enlargements...)
M.
MVK, at what level of power/inch do you need to be barely able to see the airy disc? (least amount needed to just make the disc barely but plainly detectable?)


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586899 - 12/24/12 09:50 AM

Quote:

Have never looked at license plates. But, I know what I see at the eyepiece, when looking at Jupiter.



By I don't know what you see, Dave. I mentioned Fatima in 1917 a couple of pages back, where tens of thousands of people "witnessed" the Sun doing a 'dance' and 'falling' towards the earth; some even reported the earth getting hotter. Too bad the cameras didn't see anything of the sort.

Mladen


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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5586905 - 12/24/12 09:58 AM

Quote:

The last planet pic post, I had to back away from the monitor! The ones on pg 4 were fine.
(They do not look like simple enlargements...)
M.
MVK, at what level of power/inch do you need to be barely able to see the airy disc? (least amount needed to just make the disc barely but plainly detectable?)



I think you've been reading too many conspiracy theories, Mark. The images are from Aberrator, imported into Paint, stitched together unaltered and saved as one picture 1;1 ratio. When you open them in "My Pictures" you can zoom, then copy the screen and put the enlarged image into the Paint and save. Simple enlargement.

As for the Airy disc, I always use a short fl eyepiece, maybe a 6 or a 4 mm. Power/inch will vary with a telescope.

Oh, by the way, as for your disdain for simulations, OSLO is a simulation and you seem to use it all the time.

Mladen

Edited by MKV (12/24/12 10:00 AM)


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586935 - 12/24/12 10:21 AM

Quote:

Quote:

Have never looked at license plates. But, I know what I see at the eyepiece, when looking at Jupiter.



By I don't know what you see, Dave. I mentioned Fatima in 1917 a couple of pages back, where tens of thousands of people "witnessed" the Sun doing a 'dance' and 'falling' towards the earth; some even reported the earth getting hotter. Too bad the cameras didn't see anything of the sort.

Mladen




I have never seen the "Sun Dance"; but I do know that my Questar provides much better images of Jupiter, than the digitized images you have uploaded. Those images appear washed out and smudged compared to what I see at the eyepiece. Call me crazy if you like; but I know what I see.


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5586955 - 12/24/12 10:31 AM

Quote:

I have never seen the "Sun Dance"; but I do know that my Questar provides much better images of Jupiter



Do you have any unretouched (raw) pictures of it?

Mladen


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5586967 - 12/24/12 10:43 AM

The point (which you seem to have missed) is that images can't capture what the eye can see ... it is like taking a picture of a rainbow -- never quite meets what is actually seen in real observation.

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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5587047 - 12/24/12 11:31 AM

As to the Questar claim re. license plate resolution brought up by MlDen. It's not necessary to fully resolve down to the width of the lines making up the individual figures in order to decipher what they are. As long as contrast is sufficient, a fairly smeared image can still be readable.

There is one aspect of the Newtonian secondary which I've not seem addressed, and which I suspect might be behind an effect which has been niggling at my subconscious for many a year. And that is its projection to a slightly farther distance behind the exit pupil than the exit pupil itself. This should be more so the shorter the secondary-to-focus distance and the longer the eyepiece focal length.

What got me thinking about this is the odd behavior I've always noticed whenever my eye moved a little, whereby there seemed to be a slight parallactic shift superimposed on objects in the field. It's very difficult to describe, and the effect is mostly subtle, bit it's definitely a quality of the Newt which detracts from the view for me. It dies seem to be more prominent at larger exit pupils, and is likely related to the relative size of the secondary shadow with respect to my pupil.

It probably would be the case that this behavior would be present to the same degree if the secondary shadow lay precisely in the plane of the image of the more-distant mirror. But it *might* (?) be worth cogitating on the effect of the secondary's shadow being projected a little farther back than the objective aperture...


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5587081 - 12/24/12 11:45 AM

I agree. I am a very scientific/quantitative oriented person, some would say to a fault. However, I do think there is a lot of poorly understood stuff going on between the time the light enters the pupil and the mouth utters "wow", and so I take Aberrator images and other optical modeling results with a grain of salt - they are not modeling the entire system. I am a HUGE reflector fan, but I have to say that putting a high-end APO at one end has resulted in proportionally more "wow" utterances at the other than with reflectors. And, SCTs have rarely produced a "wow". Very unscientific, and totally anecdotal, and I am sure totally unconvincing to those with strongly held opinions, but I have enjoyed the "wow"s nevertheless and intend to continue doing so. BTW, all of my own "wow" producing scopes are homemade reflectors and little refractors.

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Re: Optimized Newts vs APOs new [Re: MKV]
      #5587156 - 12/24/12 12:31 PM

Questar obviously is a better master of optics than most.



It all depends on what your definition of "resolves" is.


Quote:

Quote:

[My 3 1/2" Questar provides better images visually than these digitized images do ... just saying.



Maybe your atmosphere is better then ours. It's irrelevant what image you use, the central obstruction effect would be the same had I used sharper images. You still have a gradual loss of detail and contrast. I just think using those stacked Photoshop images is not what one sees, and therefore cannot judge.

BTWm, Questar still makes fantastic claims that defy everything we know about optics. In this Questar brochure, which I remember back from the late 70's, the company claims a 3.5 inch clearly resolves license plate number (1/4 inch wide) at a distance of 3.5 km (2.2 miles) over water (!). That's a resolution of less than half an arc second, and theory (Dawes limit = 4.6/D in inches) says 3.5 inch aperture (assuming wavelength of 0.55 um) can resolve no more than 1.3 arc seconds under ideal conditions.

Mladen




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Re: Optimized Newts vs APOs new [Re: dpwoos]
      #5587158 - 12/24/12 12:33 PM

You would think glass improvements would have refractors better by now .

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Re: Optimized Newts vs APOs new [Re: kfrederick]
      #5587273 - 12/24/12 01:46 PM

I don't get it. For some reason strange or otherwise I just don't feel the need to "qualify" my scope's performance against other types. I know what type of views I can expect nearly 100% of the time seeing permitting and that's more than OK by me! Knock yourselves out reinforcing each other. Mike

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Re: Optimized Newts vs APOs new [Re: MKV]
      #5587323 - 12/24/12 02:30 PM

That's about 3-6x more power with a 6" F/10 than is necessary for an average individual with 20/20 vision. I rest my case.
*****
There are quite a few accomplished and knowledgeable individuals who have persued unobstructed reflector telescopes for a good reason throughout the last century. And at least 2 Yahoo forums dedicated to them that I'm aware of at present. Must be a reason for all that kind of attention to the subject. There are, however some hurdles as to constructing and making them. They aren't as simple as a long Newt. But doable with someone who has made a good scope before.
Merry Xmas to all,
M.


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Re: Optimized Newts vs APOs new [Re: NHRob]
      #5587342 - 12/24/12 02:42 PM

Quote:

Questar obviously is a better master of optics than most.



It all depends on what your definition of "resolves" is.


Quote:

Quote:

[My 3 1/2" Questar provides better images visually than these digitized images do ... just saying.



Maybe your atmosphere is better then ours. It's irrelevant what image you use, the central obstruction effect would be the same had I used sharper images. You still have a gradual loss of detail and contrast. I just think using those stacked Photoshop images is not what one sees, and therefore cannot judge.

BTWm, Questar still makes fantastic claims that defy everything we know about optics. In this Questar brochure, which I remember back from the late 70's, the company claims a 3.5 inch clearly resolves license plate number (1/4 inch wide) at a distance of 3.5 km (2.2 miles) over water (!). That's a resolution of less than half an arc second, and theory (Dawes limit = 4.6/D in inches) says 3.5 inch aperture (assuming wavelength of 0.55 um) can resolve no more than 1.3 arc seconds under ideal conditions.

Mladen







Don't know what vintage the Questar was but at one point, Cave Optical was making mirrors for Questar. I wouldn't think they did any better on those mirrors than the ones they put in their own scopes. Some Cave mirrors were very good and others, not so much. Making a 3.5" optical surface to a decent standard shouldn't be that difficult.


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ed_turco
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Re: Optimized Newts vs APOs new [Re: pstarr]
      #5587389 - 12/24/12 03:18 PM

I reiterate. With equal apertures, one of my optimized Newts will equal the performance of an APO, period. Honest, neutral observers will not be able to tell the difference. This challenge was never was taken up. Despite all the hoopla over the advantages of an APO and all the optical theory going against my Newt, no APO owner was willing to stake his APO's reputation and price on the line. I had everything to lose here...

The reader can draw his own conclusions.


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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5587431 - 12/24/12 03:47 PM

IMHO the only difference between an optimized 6-in newt and a 6-in APO, both f/10,under ideal similar seeing conditions and no cool down issues would be costs of the scope, mounting (costs, again) and the abilty of the refractor to reach a lower power and thus wider fov.

Again, as the owner of 22 scopes, 18 of them of them being refracters.


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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5587442 - 12/24/12 03:59 PM

Mark Harry wrote:

Quote:

There are quite a few accomplished and knowledgeable individuals who have persued unobstructed reflector telescopes for a good reason throughout the last century. And at least 2 Yahoo forums dedicated to them that I'm aware of at present. Must be a reason for all that kind of attention to the subject. There are, however some hurdles as to constructing and making them. They aren't as simple as a long Newt. But doable with someone who has made a good scope before.
Merry Xmas to all,
M.




Yes.

I was recently asked why make say a 4 1/4" Schief when for probably less investment in time and materials you could have a 6" f10 or f12 Newt, or perhaps an 8".

Initially it was because I had a chance to view through a couple of really good ones and it was an impressive view; and I wanted to challenge myself and have a fine scope that wasn't just like most others.

But after using it for a few years I realized I really enjoy lunar and planetary observing and the Schief's approximately 1/2 degree field can just fit the lunar disk in, as well as a fair number of extended targets.

Being an f27 means I can get about 110X with an ordinary 26mm eyepiece and a really nice 240X with a 12mm ortho and still have plenty of eyerelief--which I need with glasses. On a rare good night I like to think I can push it even further but that's probably just my imagination that the image hasn't degraded.

That and the low viewing position suits me. A 6" f27 Newt would be over 13' long and I'd be on a ladder--which I don't like. Never mind the structure, and mounting.

Construction is more demanding; but certainly do-able by anyone that has made a Newt or is seriously considering one.

Maybe the biggest prior obstacle has been that there were few resources for guidance--which isn't the case now--and the somewhat widespread notion that collimation is very hard--in fact it's fast, simple, and quite a bit easier than for a Newt.

To each their own for whatever reasons.

Having said that, I'm all for knowing the facts about performance and the strong suits and shortcomings of each design.

The Herrig looks pretty neat. I don't have room on my plate now to try one and am a little scared of the alignment requirements...but maybe that wouldn't be an issue if someone made one and worked out any kinks? Since Kevin seems to have conquered that concern with his CHief I nominate him to try a Herrig...

Merry Christmas to all!

Gary


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Re: Optimized Newts vs APOs new [Re: mikey cee]
      #5587461 - 12/24/12 04:13 PM

Quote:

I don't get it. For some reason strange or otherwise I just don't feel the need to "qualify" my scope's performance against other types. I know what type of views I can expect nearly 100% of the time seeing permitting and that's more than OK by me! Knock yourselves out reinforcing each other. Mike






Being a telescope addict with multiple optical configurations represented, it's in my own best interest to stay "astronomically PC", but when it comes to planetary definition, long focus Newts are gods!

Tim


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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5587462 - 12/24/12 04:14 PM

Quote:

I reiterate. With equal apertures, one of my optimized Newts will equal the performance of an APO, period. Honest, neutral observers will not be able to tell the difference. This challenge was never was taken up. Despite all the hoopla over the advantages of an APO and all the optical theory going against my Newt, no APO owner was willing to stake his APO's reputation and price on the line. I had everything to lose here...

The reader can draw his own conclusions.




Are you willing to acknowledge that, in the real world, (almost) all newtonian reflectors suffer from one or more defects (e.g. collimation, thermal, stray light) that can easily explain why many folks have experienced APO superiority? Personally, I am certainly willing to accept that it is possible to build an optimized newt that exhibits none of these defects and so can go toe-to-toe with any APO (or any other design). However, I don't think that I have ever looked through such a newt, including my own. Could I build one? I think that I could. Why haven't I built one? My newts work really, really well and it isn't worth it to me and it isn't why I build newts and I don't think it would be much fun. If somebody else wants to do it or has done it then great - bring it to Stellafane and let's all enjoy it!

I feel really, really good about my newts and the other great newts in our club, and I really, really enjoy observing with excellent APOs (none mine), and I can hold those two positions simultaneously no problem.


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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5587481 - 12/24/12 04:30 PM

Quote:

That's about 3-6x more power with a 6" F/10 than is necessary for an average individual with 20/20 vision. I rest my case.



Maybe you should just rest. FYI, I always used that power because I like it that way.

Mladen


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Re: Optimized Newts vs APOs new [Re: dpwoos]
      #5588360 - 12/25/12 11:32 AM

Quote:

... If somebody else wants to do it or has done it then great - bring it to Stellafane and let's all enjoy it! ....

You've never looked through one of mine. As for me, as I mentioned before, I am now a shut-in with no capabilitities to make any more telescopes. But the challenge was good when I made it.

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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5588919 - 12/25/12 09:21 PM

Ed,
What's your take on the value of windows on newts? Some say they cause more trouble than they're worth.
Thanks
Brian


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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5588983 - 12/25/12 10:19 PM

Quote:

So observe. it's not realistic to try for perfection in every aspect of doing anything.



It might not be realistic to achieve perfection, but to try for it is "fun" and worth while, IMO. Optimizing is enjoyable knowing we're getting the most form our instruments and ourselves. The resulting views are that much better making observing much more enjoyable.

Quote:

ATM's love to maximize performance in their telescopes as much as observe through them. To optimize is as much or more fun than to observe, though the quality of the observing comes out so much better.



I'd agree, especially for those of us bitten with the love for our hobby where we begin picking the fly s*** from the pepper. Learning about and how to do that adds dimension to our hobby: observing.

Quote:

One of my precision optical windows negates the need for any spider diffraction whatsoever.



That's an interesting challenge. The parabola can work wonders on the wavefront. Maybe, to be fair to the APO, you might consider using a spherical "window" and induce some spherical aberration? Maybe toss in an appropriate diffraction grating to generate the tiniest bit of chromatic aberration (optimized for green), too? That may be unnecessarily difficult an inexpensive. Forget it. I dunno, with a super small CO and a very good figure, not sure I would take that challenge, either, especially with a very experienced observer at the eyepiece.


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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5589046 - 12/25/12 11:21 PM

Why a shut-in Ed?

Pete


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Re: Optimized Newts vs APOs new [Re: azure1961p]
      #5589771 - 12/26/12 01:39 PM

First of all, about windows -- They are worth every penny in cost and every hour of time put into them. If you think a window is some random piece of glass thrown in front of Newt, you are in for a disappointment. A true window is optically ground and polished out of optical glass to a high level of perfection, such that it makes no difference when it is placed in front of a Newt, except for a slight shift in focus. And you can throw that d**ned spider away. Why add spider diffraction to a potential perfect image and mess it up with lines of scattered light ?

On the shut-in thing. My Rheumatoid Arthritis got really bad really quick, and a lot of the time I am bedridden.


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MKV
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Re: Optimized Newts vs APOs new [Re: 474747]
      #5590799 - 12/27/12 07:58 AM

First let me say that it is indeed surprising that ever since the "APO fever" bit the ATM community, some 30 years ago, no one has actually done what Ed Turco suggested: compare an APO and a Newtonian with an optical window of the same size and focal ration.

Second it is indeed surprising that no one tried to capitalize on the opportunity to offer such Newtonians commercially as cheaper alternatives to the APOs (perhaps because with APOs the profit margin is a lot higher!).

Optical windows are a lot more work than many realize, which is probably why people like to talk about them but not make them. A 3-vane spider causes minimal diffraction and is a lot cheaper and easier to obtain than an optical window, so naturally people go for what's easier and cheaper.

Making a quality optical window is certainly a major project but not for the industry. From an industrial point of view they are easier than mass-producing Schmidt correctors.

Assuming you decide to make one, say a 6-inches clear aperture, let's not forget that even a perfect 6-inch telescope is just a 6-inch telescope. It may outperform mediocre ones of slightly bigger aperture, but it will not outperform even a mediocre 10-inch. If you don't believe it, look at some globular cluster in a 6-inch and then in an 8 or a 10-inch. Try splitting a 0.5 arc-second double star with a 6-inch. And as for APOs, for the price of an 6-inch APO you can buy a reflector just about big enough to sit in it!

Mladen


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5590905 - 12/27/12 09:57 AM

Quote:

Quote:

Quote:

[My 3 1/2" Questar provides better images visually than these digitized images do ... just saying.



Maybe your atmosphere is better then ours. It's irrelevant what image you use, the central obstruction effect would be the same had I used sharper images. You still have a gradual loss of detail and contrast. I just think using those stacked Photoshop images is not what one sees, and therefore cannot judge.

BTWm, Questar still makes fantastic claims that defy everything we know about optics. In this Questar brochure, which I remember back from the late 70's, the company claims a 3.5 inch clearly resolves license plate number (1/4 inch wide) at a distance of 3.5 km (2.2 miles) over water (!). That's a resolution of less than half an arc second, and theory (Dawes limit = 4.6/D in inches) says 3.5 inch aperture (assuming wavelength of 0.55 um) can resolve no more than 1.3 arc seconds under ideal conditions.

Mladen




Have never looked at license plates. But, I know what I see at the eyepiece, when looking at Jupiter.




According to my calculations this is reasonable. It's possible a Q could resolve a license plate as far away as 2.75 miles. I based this on an article I wrote here on CN a while back.
http://www.cloudynights.com/item.php?item_id=1354


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5590935 - 12/27/12 10:25 AM

Mladen, where I live, my 6" f/5 and f/10.9 reflectors generally will outperform my 10" Dob because the atmosphere overhead introduces enough turbulence that the 10" fares worse most nights. That's not to say it can't; I've had the 10" up to 480x on exceedingly stable nights, in which yes, my average 10" will outperform the optimized 6". It's that old saying in any telescope discussion:

"All things being equal."

As we know, they rarely are equal. An APO has no central obstruction. Then we get into optical windows, and CO, and how big is that CO, and what is thermal cool-down time, and... well, every other argument already discussed in this thread and many others.

I am building an optimized 6" reflector because of several reasons:

1) I can afford it.
2) I can do it with the tools I have.
3) I have room to store it.
4) I don't mind moving it in and out of my garage (weight / bulk).
5) That 1" thick, 6" mirror will cool WAY faster than the 2.5" thick, 10" mirror.

It's that simple. I'd love to have a larger, optimized 8" or 10" or 12" Newt. But I have a daughter going into college next year. I can afford to make the 6" OTA and mount. And neither will be so heavy or bulky that I will be hesitant to bring it out and set it up. Yes, larger aperture will let me see more. But it will cool quickly, offer excellent magnification with long focal length eyepieces which means others who view through it with glasses will have little difficulty seeing an image, and as I have discovered at star parties and outreach events, sometimes "size matters."

And there's the "wow" factor.

Sometimes simply the length of a telescope makes people think the image they are seeing is more impressive than it is, even if we more experienced observers know it is not. And when we talk cost with the person and they realize it is in the realm of affordability (or something they can make themselves), they just might get interested in the hobby too. And the way I see it, the more people we have looking up, the more likely we are to get people pointing their lights down.

To me, that's a win, no matter the aperture.


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Re: Optimized Newts vs APOs new [Re: Darren Drake]
      #5590942 - 12/27/12 10:29 AM

Quote:

According to my calculations this is reasonable. It's possible a Q could resolve a license plate as far away as 2.75 miles



Dawes limit is 4.6/D in arcseconds, where D = aperture diameter in inches. In theory, a perfect 3.5 inch aperture can clearly resolve 1.3 arseconds.

A 0.25 inch wide letter over 2.7 miles is below the ability of a 3.5 inch aperture to resolve.

A quarter inch (6.35 mm) over 2.7 miles (4.32 million mm) forms an angle of 0.0000842 degrees, which is 0.303 arcseconds.

This is what is meant by the Dawes limit:

http://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Airydisks_dawes_sqrt...

Mladen


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Re: Optimized Newts vs APOs new [Re: magic612]
      #5590949 - 12/27/12 10:36 AM

Quote:

It's that old saying in any telescope discussion:

"All things being equal."

As we know, they rarely are equal. An APO has no central obstruction. Then we get into optical windows, and CO, and how big is that CO, and what is thermal cool-down time, and... well, every other argument already discussed in this thread and many others.



Well there is a saying "to each his own".

Some differences are without distinction. Just because we can measure something doesn't mean it's (always) significant.

Theory (MTFs) clealry suggest that small COs are not significant. Some people claim they can see even the smallest CO. Yet no one has dared to take Ed Turco on his challenge!

Mladen


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5590997 - 12/27/12 11:11 AM

Quote:

Quote:

According to my calculations this is reasonable. It's possible a Q could resolve a license plate as far away as 2.75 miles



Dawes limit is 4/6/D in arcseconds, where D = aperture diameter in inches. In theory, a perfect 3.5 inch aperture can clearly resolve 1.3 arseconds.

A 0.25 inch wide letter over 2.7 miles is below the ability of a 3.5 inch aperture to resolve.

A quarter inch (6.35 mm) over 2.7 miles (4.32 million mm) forms an angle of 0.0000842 degrees, which is 0.303 arcseconds.

This is what is meant by the Dawes limit:

http://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Airydisks_dawes_sqrt...

Mladen




Dawes’ limit applies to observations of two point sources (double stars) only.

Extended objects follow different "rules"; and in the case of dark markings, if they are linear (such as a license plate), the Dawes' limit can be exceeded. Experiments by W. H. Pickering indicate that if such a marking is as thick as one-fifth Dawes' limit it can be seen.


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5591023 - 12/27/12 11:30 AM

How big was the plate numbers? some are large .The color of number and plate. The numbers are raised 3d might help. Lighting could help.Looking over water might be more stable air .not sure. .The eye of the observer ,could be a big factor some eagle eyed guy . lots of factors I could count the points on a deer horns over 2 miles away 20 inch f5 newt .

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Re: Optimized Newts vs APOs new [Re: kfrederick]
      #5591036 - 12/27/12 11:36 AM

Looking at the brochure I see that the pic was taken with a 180 or Q7 not a 3.5. This explains why it is so clear but still a Q3.5 could do it.

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Re: Optimized Newts vs APOs new [Re: MKV]
      #5591038 - 12/27/12 11:37 AM

Mladen,

That's all true, depending on what one means by resolution. I've "seen" double stars well below the Sparrow limit in a 6" scope, though not technically resolved with a black space separating them.

The letter might be 0.25" wide, but they are much taller. And they (might) obey the edge spread function, not the point spread function defining Dawes' limit. This is why Cassini can be resolved in modest apertures. You may not be able to "read" the license plate or resolve the letter "E", as you say.

But it wouldn't surprise me if you could "see" something was printed on the plate especially with an obstruction (struggling hard to stay on topic.) So, yea, maybe a bit of marketing in that claim and who is to say it's not accurate? Raleigh?

IME, reducing my CO by about 10% (.37D to .28D optimistically) showed an immediate improvement in stellar and lunar diffraction effects. Basically, bright stars lost the very dim third outer ring and the second was noticeably dimmer (now barely visible at all.) Planetary detail was much less obvious, less conclusive. In fact, I can only surmise there must be some improvement, even if it is so difficult to really tell.

But, can one notice improvement in stellar diffraction from 15% CO to zero? Dunno, maybe not. The first maximum is probably already relatively faint and the central maximum pretty close to EE ~ Strehl. So, if the 6" optimized Newt had a comparable Strehl to the refractor (~0.98 for both), a 15% CO would knock EE down to about 0.95. Then it becomes a test to see the difference between 0.98 (real) and 0.95 (nominal) Strehl. That might be impossible, even if all other things are held constant.

That's the math, anyway.


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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5591071 - 12/27/12 11:56 AM

Dawe's Limit does not apply to lines, folks. You can see stuff on the moon or Saturn that you have no right to see using that limit.

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Re: Optimized Newts vs APOs new [Re: Darren Drake]
      #5591122 - 12/27/12 12:35 PM

Quote:

Looking at the brochure I see that the pic was taken with a 180 or Q7 not a 3.5. This explains why it is so clear but still a Q3.5 could do it.



That ad has been out for more than 30 years now. I remember it from the Questar catalogue from the late '70s and it left the impression the image was taken with the Q3.5 (don't forget this was film photography, not digital!)

Anyway I'd like to see some sort of equation that calculates extended object resolution.

Mladen


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Re: Optimized Newts vs APOs new [Re: Darren Drake]
      #5591153 - 12/27/12 12:49 PM

Darren, maybe you should have used the USAF Test Pattern (rather than ordinary text) in conjunction with the Aberratior.

USAF 1951 Test Pattern


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5591254 - 12/27/12 02:06 PM

Ed,

You have raised a good point optical windows for long focus newts. There are very few long focus newts out there, there are very few of these with optical windows. As far as I know they are not made commercially. It would be neat to see someone start producing them for the ATM market.


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Re: Optimized Newts vs APOs new [Re: gatorengineer]
      #5591260 - 12/27/12 02:15 PM

Is there any good evidence, or even good reasons to believe, that an optical window is a (significant, big, huge) improvement over an open tube and a spider?

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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5591353 - 12/27/12 03:23 PM

Quote:



Dawes’ limit applies to observations of two point sources (double stars) only.

Extended objects follow different "rules"; and in the case of dark markings, if they are linear (such as a license plate), the Dawes' limit can be exceeded. Experiments by W. H. Pickering indicate that if such a marking is as thick as one-fifth Dawes' limit it can be seen.




And especially when it is a known and expected shape, such as letter of the Roman alphabet. The brain can infer the correct response with rather minimal data.


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Re: Optimized Newts vs APOs new [Re: dpwoos]
      #5591548 - 12/27/12 06:00 PM

The difference is significant enough to make that Newt, given sufficient optical accuracy to challenge any APO of equal aperture. Why settle for a halftone image caused by a spider? That is what my challenge was all about.

And by the way, windows are not that tough to make.

Ed


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Re: Optimized Newts vs APOs new [Re: magic612]
      #5592305 - 12/28/12 09:00 AM

"BTWm, Questar still makes fantastic claims that defy everything we know about optics. In this Questar brochure, which I remember back from the late 70's, the company claims a 3.5 inch clearly resolves license plate number (1/4 inch wide) at a distance of 3.5 km (2.2 miles) over water (!). That's a resolution of less than half an arc second, and theory (Dawes limit = 4.6/D in inches) says 3.5 inch aperture (assuming wavelength of 0.55 um) can resolve no more than 1.3 arc seconds under ideal conditions. "

Yes I had that beautiful red jacketed plush Questar catalog when I was a kid. It was Puget Sound on the west coast and I recall you COULD read the license plate numbers on the cars. Resolving that however isn't a function of Dawes...

The Dawes limit is for point sources and it's a standard the good reverend created to find uniformity in resolution perception over differing apertures per observation of that "notch" first appearing. The license plate res is something altogether different. Here we are seeing the results of LINEAR resolution. Typically we can resolve/observe fine lines 10 to 11 times finer than Dawes and the great Harvard refractor at one point resolved a distant human hair at 1/15 the Dawes limit for that 10" scope. Clearly the airy disc patterns are NOT the finest features an aperture will resolve. This is a hot topic among some however as the term "resolved" then comes into question as linear resolution is not the same as true angular resolution and that's a given. But as in the case of those license plates it's nothing to seize at either. If infact the photo of those parked cars was resolved strictly to Dawes and linear resolution was erased the plates would be smears and many things lumpy and ambiguous.

Darren did a nice article supported with revealing images that's revealing.

Pete


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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5592347 - 12/28/12 09:35 AM

Quote:

The difference is significant enough to make that Newt, given sufficient optical accuracy to challenge any APO of equal aperture. Why settle for a halftone image caused by a spider?




Ed, shouldn't that - more accurately (perhaps?) - refer to a straight vane spider? I'm not trying to take anything away from the window. But my 3 vane curved spider makes for unnoticeable diffraction, and having looked through many Newts and refractors, provides a refractor-like view. No "spider spikes" whatsoever.

And it wasn't that tough to make either - a few pieces of brass, some solder and a propane torch.


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Asbytec
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5592367 - 12/28/12 09:56 AM

Pete, I just worked the math, and the Dawes limit at 2.2 miles for a 3" Q is two parked cars separated by their side view mirrors. At that distance over water, you should just be able to tell they are cars. Two SUVs is another story.

Now, thats provided the numbers on the licence plate emit nor reflect no light. Once they do, they become subject to the MTF and low frequency resolution.


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gatorengineer
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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5592393 - 12/28/12 10:20 AM

There is calculational evidence. If you look at a 1.125 secondary in an 8 F8, its roughly 1 square inch of obstruction. When I look at my destiny 3 vane curved the thickness of the vanes approaches the same obstruction area. So I am in a very crude approximation "doubling" the total difraction, but the curved vane simply spreads it around, but it doesnt go away....

In the Northeast, I would need a truly spectacular night of seeing to be able to actually realize the benefit, but I think the benefit is there.


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5592432 - 12/28/12 10:47 AM

Quote:

Yet no one has dared to take Ed Turco on his challenge!




A challenge accepted or ignored does not make a victory. I could venture a challenge to arm wrestle all takers. If no one were to accept my challenge, the wise course of action would be to not claim victory.

Jon


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Pinbout
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5592522 - 12/28/12 11:33 AM

Quote:



Quote:
--------------------------------------------------------------------------------

Yet no one has dared to take Ed Turco on his challenge!


--------------------------------------------------------------------------------



A challenge accepted or ignored does not make a victory. I could venture a challenge to arm wrestle all takers. If no one were to accept my challenge, the wise course of action would be to not claim victory.






without the scope created first, there is no challenge at all cause there is nothing to compete against except for dreams and promises.



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MKV
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5592544 - 12/28/12 11:48 AM

Quote:

Quote:

Yet no one has dared to take Ed Turco on his challenge!




A challenge accepted or ignored does not make a victory. I could venture a challenge to arm wrestle all takers. If no one were to accept my challenge, the wise course of action would be to not claim victory.

Jon



True, but then claims to the superiority of APOs should be mum.

Mladen


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dpwoos
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5592548 - 12/28/12 11:49 AM

For me, it boils down to this. I don't think you would find anyone who makes top-notch APOs who would dispute that a Newtonion Reflector would, if properly optimized, produce views through the eyepiece that are equal to an APO. Whether this requires that an optical window be used to hold the secondary as opposed to a spider is an interesting question, but I bet that it doesn't. On the other hand, there are a bunch of reasons why newts cost a lot less than APOs of the same size, and why folks can make their own, and why APOs greater than 6" aren't readily available at all. These reasons all add up to compromise the performance of newts. Not a huge amount, but enough that folks still spend good money to own APOs.

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MKV
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5592550 - 12/28/12 11:51 AM

Quote:

without the scope created first, there is no challenge at all cause there is nothing to compete against except for dreams and promises.



Oh, come on, Danny, that's just lame, imo. Ed was willing to make one for the challenge.

Mladen


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Pinbout
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5592581 - 12/28/12 12:14 PM

Quote:



Quote:
--------------------------------------------------------------------------------

without the scope created first, there is no challenge at all cause there is nothing to compete against except for dreams and promises.


--------------------------------------------------------------------------------


Oh, come on, Danny, that's just lame, imo. Ed was willing to make one for the challenge.

Mladen




do you want to go there?

He really should make it first. I have no doubt he could. but until you do...

is the challenge about the scopes or is it about him.

Make the ##!@# scope first then present the challenge, you might actually get someone to take that challenge.

...but not without a scope first...and the proof is no one actually took the challenge.


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magic612
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5592652 - 12/28/12 01:00 PM

Why are there no takers? I think basic psychology rules here. A telescope is not a need, it is a want. As such, it become an emotional purchase.

"Do I need a telescope? No. Do I want one? Yes. Do I want this expensive one that costs many thousands of dollars? Yes. Why? Performance, and it is ostensibly 'The Best.'"

So on one hand we have a 'best-performaning" APO refractor telescope, costing many thousands. If a similarly-sized reflector costing far less money is proven to be equal in performance (or of negligible difference), this severely hurts the rationalization that was required to make the many-thousand-of-dollars purchase. It is the equivalent of a souped-up-by-its-owner 1970's era Camaro matching the acceleration of a new Ferrari.

I don't' think even if the reflector with window was made there would be any takers from those who own similarly sized APO's. To me, the math says it would be equal in performance. It is the large cost difference that would hurt the owner of the large APO. "Ouch... I spent HOW MUCH on mine again?" Sure, they might justify it with "Well, the focuser is in a better position" or some mechanically-related rationalization. But performance-wise, why would they put themselves in such a position, knowing in advance what the (likely) result would be?

Unless they are an emotional masochist, they wouldn't. I don't blame them, to be frank about it. I wouldn't put myself in that position either.


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MKV
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5592699 - 12/28/12 01:24 PM

Quote:

do you want to go there? He really should make it first. I have no doubt he could. but until you do...
is the challenge about the scopes or is it about him.



Yes I do want to go there, seeing that perhaps you need to think this trough a little bit more.

Ed would need to know which APO is to be compared, the size, the focal ratio, etc., so he could make a matching Newotnian counterpart. Otherwise you'd have to have someone make a custom-made APO to compare to Ed's Newotnian! Who's going to make an APO just to match his Newotnian?

Any owner of an already existing APO could have accepted Ed's challenge and have Ed make a Newotnian of the same size and focal ratio. But, understandably, no one did, and for a good reason! I'd feel pretty darn stupid if I bought a telescope that costs as much as a good used car only to discover a $250 Newotonian counterpart with an optical window that matches, let alone exceeds, its image quality.

Mladen

Edited by MKV (12/28/12 01:30 PM)


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MKV
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5592705 - 12/28/12 01:32 PM

Quote:

Unless they are an emotional masochist, they wouldn't. I don't blame them, to be frank about it. I wouldn't put myself in that position either.



Spot on!

M


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Darren Drake
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5592712 - 12/28/12 01:36 PM

I know it's not quite apples to apples but Napersky will soon acquire an AP Superplanetary 6 inch and one of the first things I wanna do is compare the Jupiter views to my high quality 8 inch f/6.9 newt. If the seeing is real good I think the results could be most interesting. Of course I will report back....

Edited by Darren Drake (12/29/12 07:54 AM)


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mikey cee
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5592749 - 12/28/12 02:04 PM

Quote:

Quote:

do you want to go there? He really should make it first. I have no doubt he could. but until you do...
is the challenge about the scopes or is it about him.



Yes I do want to go there, seeing that perhaps you need to think this trough a little bit more.

Ed would need to know which APO is to be compared, the size, the focal ratio, etc., so he could make a matching Newotnian counterpart. Otherwise you'd have to have someone make a custom-made APO to compare to Ed's Newotnian! Who's going to make an APO just to match his Newotnian?

Any owner of an already existing APO could have accepted Ed's challenge and have Ed make a Newotnian of the same size and focal ratio. But, understandably, no one did, and for a good reason! I'd feel pretty darn stupid if I bought a telescope that costs as much as a good used car only to discover a $250 Newotonian counterpart with an optical window that matches, let alone exceeds, its image quality.

Mladen


Yeah you and me both! Mike

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kfrederick
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Re: Optimized Newts vs APOs new [Re: mikey cee]
      #5592826 - 12/28/12 02:57 PM

The APO will be better a 6 inch APO will have more light than a 6 newt . The image is formed by refraction so the surfaces need not as smooth . Feet of in focus . But sure for 1/5 the money for the newt .Also having no obstruction is very real even if some of it Might be in the brain .

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Re: Optimized Newts vs APOs new [Re: MKV]
      #5592880 - 12/28/12 03:38 PM Attachment (19 downloads)

I did have a 4" windowed Newt of superlative quality that I recently donated to a youth ward of a certain hospital where I had multiple stays. So the telescope did exist and the challenge went unanswered. The telescope was listed in my signature, so don't anyone tell me that such a telescope didn't exist and that my challenge was a blast of hot air -- please.

Edited by ed_turco (12/28/12 03:55 PM)


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ed_turco
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5592894 - 12/28/12 03:52 PM

At least one and I'll bet two of my students, who have surpassed me in their optical achievements, will carry forth this challenge in the future. One already has a windowed newt of extra superlative quality that he can stop down to the teeny aperture of the challenging APO, fit it with the appropriate diagonal, and once again say, "Bring it On!" But this will be his call. I should live so long to see a taker.

Doesn't it surprise anyone as to how few APOists have actually looked through an excellent Newt, even one that is not optimized? They'll give you all sorts of nonsense for not doing so, spider diffraction, CO, but NOT a look?

I have to doubt the sanity of people who broadcast gleefully about a new 50mm APO that has come out. What are they thinking? Are they thinking at all?

Final word for this posting. It is every teacher's dream that his students surpass him in their achievements. I have been doubly blessed.



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tim53
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5592918 - 12/28/12 04:10 PM

Quote:

A telescope is not a need, it is a want. As such, it become an emotional purchase.




A telescope isn't a need?? I don't know about all ya'all, but I won't survive for long if there isnt at least one long focus newt around here. Like woody Allen might say if he were an amateur astronomer: "that's my second favorite organ!"

Tim


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magic612
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Re: Optimized Newts vs APOs new [Re: tim53]
      #5592960 - 12/28/12 04:49 PM

Quote:

Quote:

A telescope is not a need, it is a want. As such, it become an emotional purchase.




A telescope isn't a need?? I don't know about all ya'all, but I won't survive for long if there isnt at least one long focus newt around here. Like woody Allen might say if he were an amateur astronomer: "that's my second favorite organ!"

Tim




As my mom used to say, if it isn't food, shelter or clothing, it's a want.


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Pinbout
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5592999 - 12/28/12 05:29 PM

Quote:

bought a telescope that costs as much as a good used car only to discover a $250 Newotonian counterpart with an optical window that matches, let alone exceeds, its image quality.





Perhaps you need to recalculate the cost/value of a hand figured windowed newt by a master mirror maker, the optics alone don't cost $250 and because he made them you can't omit the cost.


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gatorengineer
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5593015 - 12/28/12 05:40 PM

I am also going to venture that on a given Northeast Night, you could take a Super Newt, a Super APO, and an 8"F6 GSO special out, and it would be tough to see the difference.

I will also tell you brightness aside there is no difference on the Average Night between my 25"F5 and my 8"F6 Mak Newt....

However, on the Extremely rare night,when I can turn the volume up to 11, and throw the Ethos 3.75 in the 25" then it is worth having every ounce of optical quality one can muster... But those nights are rare, and the cost goes expontential for that quality...

At the end of the day, seeing rules, and its supposed to be fun.....


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Datapanic
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Re: Optimized Newts vs APOs new [Re: Datapanic]
      #5593027 - 12/28/12 05:50 PM

Quote:

I found it! It's actually about an APO vs. Questar Funny and probably fitting for what this thread will turn out to.




So, who won!?


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azure1961p
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Re: Optimized Newts vs APOs new [Re: Datapanic]
      #5593197 - 12/28/12 07:31 PM

What seems to be taking a vacation here is the real physical effects of central obstruction - even 15%. Next are heat plumes off the primary. A window means nothing if you have to focus through a contrast killing thermal boundary layer . It can still be good, even amazing but its not going to match an apo if its still there . Frankly I think the whole optical window thing is getting over blown and not enough attention to treating the thermal issues which is of paramount importance if you want to do this apo versus newt thing.

Pete


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MKV
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5593250 - 12/28/12 08:04 PM Attachment (10 downloads)

Quote:

Perhaps you need to recalculate the cost/value of a hand figured windowed newt by a master mirror maker, the optics alone don't cost $250 and because he made them you can't omit the cost.



If you can make a mirror you can make a window; it's just more work - two sides, and a central hole. No reason for the optics to cost more than $250 in 4 to 6 inch aperture.

But let's even double that and say it'd cost you $500 for optics, is it still worth paying 10 to 20 times more for the same or even inferior performance.

Mladen


Mladen


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Re: Optimized Newts vs APOs new [Re: azure1961p]
      #5593383 - 12/28/12 09:35 PM

Pete,

Did you think


i wouldn't have thought of a small fan?

Ed


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azure1961p
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5593543 - 12/28/12 11:21 PM

LOL ok Ed,

didnt see it mentioned. Im happy as pie - my boundary layer fan finally has proper airflow across the face of the mirror. Just got it in.

I say one thing though that apo or refractor owners seem to have less issue with. Tonight for example Jupiter was way up near zenith and while I was contnually defocusing to check for proper cooling of the primary - evertyime I exhaled or had my hand on the edge of the tubes secondary end, this awful disruptive heat plume would enter the field.
For that Im doing the following...

1. Have a handle or knob of some sort farthjer down the tube so I dont have to grab the scopes open end to move it.

2. It seems odd but maybe somekind of fan intake to carry my breath elsewhere. When the scopes up at that angle my breath trail is like a smokestack. You could say the same for apo users but they arent exhaling inches from there objective opening.

Lower elevations like 45 degrees arent prob lem but 60 degrees and above it seems problematic.

Im all into the thermal thing right now so thats where my scope DIY concentration is.

Pete


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magic612
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Re: Optimized Newts vs APOs new [Re: azure1961p]
      #5593569 - 12/28/12 11:43 PM

Quote:

2. It seems odd but maybe somekind of fan intake to carry my breath elsewhere. When the scopes up at that angle my breath trail is like a smokestack.




This may sound (and likely look) goofy but it is the first thing I thought when I read this: What about using a snorkeling breathing tube? It would shoot your breath back out the opposite direction away from you and the scope.


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cheapersleeper
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5593624 - 12/29/12 12:20 AM

Snorkle? I have done that...

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tim53
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5593627 - 12/29/12 12:21 AM

I focus the image on my computer screen and then sit down at the keyboard while making my observations. My 6" f/10.3 Kludegescope cools down pretty quickly. I've not felt the need for a fan before.

-Tim.


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azure1961p
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5593628 - 12/29/12 12:21 AM

Dead airspace. A snorkel has too much rebreathing of the air in the snorkel itself. You don't notice it in swimming but I'm mindful of proper breathing when observing lest things begin to loOk grainy as a result.

Thanks though!


Pete


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robboski2004
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5593728 - 12/29/12 02:19 AM

Quote:


If you can make a mirror you can make a window; it's just more work - two sides, and a central hole. No reason for the optics to cost more than $250 in 4 to 6 inch aperture.

But let's even double that and say it'd cost you $500 for optics, is it still worth paying 10 to 20 times more for the same or even inferior performance.

Mladen


Mladen




Mladen, your usual understanding of lens designs seems to have deserted you !!

The lens design , Roland Christen published in the Sky and Telescope , was later corrected , due to being over-corrected
for spherical and not optimised for colour.

These glasses are capable of producing, what in 1981, was considered apo correction. i.e green , red , blue within the airy disc.

Toward then end of the article , he states that his and other amateurs were working on lenses for astro-imaging (photography at the time !)
Which is one of the strengths of apo's.
Another would be their relative ease of use and ability to perform year after year for basically as long as their owner requires .
Possibly one area they excel, would be in their capability of producing , fully illuminated fields in wide field eyepieces.

Just some thoughts?
Ian.


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5593814 - 12/29/12 05:18 AM

Quote:

Quote:

Quote:

Yet no one has dared to take Ed Turco on his challenge!




A challenge accepted or ignored does not make a victory. I could venture a challenge to arm wrestle all takers. If no one were to accept my challenge, the wise course of action would be to not claim victory.

Jon



True, but then claims to the superiority of APOs should be mum.

Mladen




Again, a challenge accepted or ignored does not make a victory or a defeat. It does not change reality.

My youngest son weighs 208 lbs, does 120 pushups a day, 120 pullups a day, does reps on the bench of 275lbs and can Bench press 400lbs.

If I challenged him to arm wrestle me and he were to kindly decline, it would not make me any stronger or mean that I were the better arm wrestler.

A 6inch apo is what it is, a 6 inch optimized Newtonian is what it is... A challenge, accepted or ignored does not change that. Both are good, both are different. To me it makes little sense to compare a 6 inch f/10 Newtonian to a 6 inch apo because a decent 10 inch f/6 Newtonian will provide superior views to either one and be far more affordable than the apo.

In my mind, telescopes are all open class, you "run what you brung" and enjoy the views for what they are, regardless of aperture...

I will say this... If I had a 6 inch apo, which I don't... I wouldn't be interested in a challenge because a challenge is competitive with an obvious agenda. On the other hand, if someone wanted to compare the views, I would be happy to make a friendly comparison.

Jon

Edited by Jon Isaacs (12/29/12 05:36 AM)


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Daniel Mounsey
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5593875 - 12/29/12 07:49 AM

Quote:

Using Aberrator(R), I constructed this collection of Jupiter shots starting from 0% obstruction (APO) all the way to 45% (for Newotnians). I intentionally chose a picture that resembles the kind of image reaching the eye rather than the Photoshop(R) art that is commonly used. I think this is a much more realistic portrayal of what an observer can expect to see.




I realize the images were for the purpose of comparing obstructions, but if I saw images that bad, I'd quit observing planets. Why cares about the differences if that's what one should expect to see. What is it that observers really care about? Images or numbers? Seems like the numbers mean more to observers. To me, it just appears to be a numbers game.

I've seen jaw dropping images of Jupiter with telescopes that had 15%, 20%, 25%, 30%, 35% obstructions, but if you see Jupiter images that looked like that with a telescope that had 0% obstruction, why bother. Maybe this explains why there are so many conflicting and confusing reports from others that make absolutely no sense.


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MKV
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5593884 - 12/29/12 08:04 AM

Quote:

Quote:

My youngest son weighs 208 lbs, does 120 pushups a day, 120 pullups a day, does reps on the bench of 275lbs and can Bench press 400lbs. If I challenged him to arm wrestle me and he were to kindly decline, it would not make me any stronger or mean that I were the better arm wrestler...



I think you're mixing apples and oranges. Your son's benchpressing abilities are a fact, and if you claimed the same or superior abilities then his declining the challenge would raise some questions.

There are a lot of (fairy)tales circulating in the ATM community, all based on anecdotal "evidence" and personal impressions. How many times have you read or heard a "testimony" that such and such a scope gave the "best views" (whatever that means) or that it was the "ultimate telescope"?

One of those tales is that APOs are better than Newotnians of equal aperture and focal ratio. How else could anyone convince people to buy a telescope of the same size as another one, but costing 10-20 times more? Because everyone is swearing APOs are "much" better, yet there is no objective data to back it up, nor will anyone submit their APOs to a test.

Ed's challenge was based on the fact that theory disagrees with eyewitness accounts. For that reason it would have been a good thing if someone had accepted his challenge, so that people may know instead of believe other people's impressions of exactly zero scientific value.

I guess you're just one of those people who'd rather believe someone's story than know the facts. That's cool. Why don't you just say so? It's everyone's world. But let's not try to rationalize this issue as "not important".

Mladen


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dpwoos
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5593899 - 12/29/12 08:22 AM

And how does this test, if it occurred, produce anything more than anecdotal evidence? You are kidding yourself if you think that this kind of "test" is going to produce any "facts". Furthermore, who cares enough to engage in such a test? Do you not think that APO owners have had the chance to look through zillions of dobs, SCTs, and other designs at club events and star parties, and even their own other scopes? They have made a choice, and some folks will continue to make that choice, and I don't understand why that bothers anybody else.

Edited by dpwoos (12/29/12 08:24 AM)


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MKV
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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5593910 - 12/29/12 08:31 AM

Quote:

I've seen jaw dropping images of Jupiter with telescopes that had 15%, 20%, 25%, 30%, 35% obstructions, but if you see Jupiter images that looked like that with a telescope that had 0% obstruction, why bother. Maybe this explains why there are so many conflicting and confusing reports from others that make absolutely no sense.



One of the reasons is intellectual dishonesty, reliance on anecdotal tales rather than factually provable claims, ulterior motives (maximizing profit), i.e. promoting business as "hobby" to increase sales, etc.

BTW, Questar is not an unobstructed telescope. And you're absolutely right, such images on astronomical objects would be worthless - except for Photoshop artists and maybe after stacking hundreds of frames. As it turns out 1/4 inch lacense plate numbers are still too small for a Q3.5 even if we're talking about "extended" objects. Questar is an elitist product and makes elitist claims.

How else can you hope to sell a telescope the size of an ordinary finder scope (89 mm clear aperture) for $5K, or a used one on eBay for $3500?

And as for "raw" images, here's an example of a Q3.5 shot of Jupiter in one of archived CN 2005 posts. So much for that.

Mladen


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David PavlichAdministrator
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5593987 - 12/29/12 09:27 AM

Ok, guys...back on topic.

David


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Pinbout
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Re: Optimized Newts vs APOs new [Re: David Pavlich]
      #5594029 - 12/29/12 10:01 AM

I'm still trying to find someone to make a:

-a 6inf10 parabola 1/20~ pv
-a 7in optical window that's 1/20~ pv flat on bothsides and parallel to sub arcminutes with a hole
-and a 1" secondary mirror that's 1/30~ pv

for under $250 cause I would buy those optics right now if someone would make them at that price.

WOW, I could have ostensibly perfect optics in a scope that I'll build, way better than I could ever dream of making. heck with the challenge I would just like the optics with those specs at that price.


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Daniel Mounsey
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Re: Optimized Newts vs APOs new [Re: David Pavlich]
      #5594033 - 12/29/12 10:02 AM

The "optimized" newt is a strong force. I've made side by side comparisons with my high quality 10" f5 Newtonian 10" f12 DK. I even had a 10" stock production 10" f6. I've compared them all to 8" apos and 6" apos. I've seen times when the 10" reflectors were superior on planets and I've seen times where the smaller apos were better on planets. Anyone who makes blanket statements or resorts to numerical figures as an answer is obviously inexperienced, because if they were in fact hands on and experienced in the claims they make, they would know reality does not work that way. If people spent as much time at the eyepiece as they do making numerical claims, they'd learn a thing or two about how telescopes actually perform in reality.

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dpwoos
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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5594060 - 12/29/12 10:13 AM

Thanks - great post.

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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5594187 - 12/29/12 11:20 AM

Quote:

I'm still trying to find someone to make a:

-a 6inf10 parabola 1/20~ pv
-a 7in optical window that's 1/20~ pv flat on bothsides and parallel to sub arcminutes with a hole
-and a 1" secondary mirror that's 1/30~ pv

for under $250 cause I would buy those optics right now if someone would make them at that price.

WOW, I could have ostensibly perfect optics in a scope that I'll build, way better than I could ever dream of making. heck with the challenge I would just like the optics with those specs at that price.



Danny, anyone who would make them just for you would be foolish to sell them for $250! I was talking glass cots for someone who can and is wiling to make them. Ed Turco is certainly one who has the skill. We weren't talking about selling.

Mladen


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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5594205 - 12/29/12 11:33 AM

Quote:

I've made side by side comparisons with my high quality 10" f5 Newtonian 10" f12 DK. I even had a 10" stock production 10" f6. I've compared them all to 8" apos and 6" apos. I've seen times when the 10" reflectors were superior on planets and I've seen times where the smaller apos were better on planets.



I don't think anyone questions these statements but such conclusions are the result of seeing conditions, not telescope characteristics. You would not come to these same conclusions under ideal observing conditions.

Quote:

Anyone who makes blanket statements or resorts to numerical figures as an answer is obviously inexperienced, because if they were in fact hands on and experienced in the claims they make, they would know reality does not work that way.



The reality is that the larger percentage of folks with scopes live in areas where ideal observing conditions are rare or non-existent. Still, they have the right to discuss how a given telescope may function in comparison to another under ideal observing conditions and I think that is really the topic here. (The race car driver is not necessarily qualified to determine the best suspension design although he may well be aware of the end result. A winning team works together to share knowledge.)

Quote:

If people spent as much time at the eyepiece as they do making numerical claims, they'd learn a thing or two about how telescopes actually perform in reality.



Very true. But let's not discount the value that an experienced glass pusher and a qualified mathemetician can bring to the discussion. I for one, have learned a great deal regarding obstructions, Strel, MTF, and numerous other topics, by those who have taken the time to post an explanation behind the theory that I could not otherwise comprehend. I welcome their contribution.

I will probably remain an uncurable skeptic. I enjoy learning about telescopes and their attributes. My scopes and eyepieces are modest. While I don't have a great deal of experience with superb instruments under ideal conditions, I have enjoyed the views I have had when folks have shared their eyepiece time with me. In my experience, Ed's statement that a well made 6" Newt can match a similar sized APO is a fair statement. I have enjoyed following the discussion.

Happy New Year to all,

dan


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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5594213 - 12/29/12 11:35 AM

Quote:

The "optimized" newt is a strong force. I've made side by side comparisons with my high quality 10" f5 Newtonian 10" f12 DK. I even had a 10" stock production 10" f6. I've compared them all to 8" apos and 6" apos. I've seen times when the 10" reflectors were superior on planets and I've seen times where the smaller apos were better on planets. Anyone who makes blanket statements or resorts to numerical figures as an answer is obviously inexperienced, because if they were in fact hands on and experienced in the claims they make, they would know reality does not work that way. If people spent as much time at the eyepiece as they do making numerical claims, they'd learn a thing or two about how telescopes actually perform in reality.



First, it's not clear what you mean by "optimized", but the OP specifically mentioned a Newtonian, same aperture as the APO, and with an optical window so as to eliminate spider vane diffraction effects. IOW, cparing two identical same diameter, same focal ratio, closed tube telescope to one another, except for the central obstruction of 15% or less in the Newotnian.

Second, when you say you have "seen time where the smaller APOs were better on planets", are you saying they were some much better the difference was worth the price difference? Are you aware that a 6-inch APO can cost anywhere from $7,000 to $19,000? Were the views that much better? Just asking.

Of course, to some, that may be pocket change...

As for the disdain that seems so popular for numerical and theoretical, do remember that the numerical and theoretical created these configurations. No one made them by trial and error and anecdotal evidence.

Regards,
Mladen


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Pinbout
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5594218 - 12/29/12 11:37 AM

Quote:

anyone who would make them just for you would be foolish to sell them for $250! I was talking glass cots for someone who can and is wiling to make them. Ed Turco is certainly one who has the skill. We weren't talking about selling.





but everyone is talking about the sale price of an apo compared to material costs of a newt with a >$1k window sale price not even mentioning the sale price of build the scope.

that's not a fair sale price/performance vs material cost/performance comparison.


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Re: Optimized Newts vs APOs new [Re: azure1961p]
      #5594228 - 12/29/12 11:46 AM

Quote:

evertyime I exhaled or had my hand on the edge of the tubes secondary end, this awful disruptive heat plume would enter the field.



Pete, that's why even reflectors can benefit form some extra tube length in front of the eyepiece. And it also helps if the tube is made of something other than aluminum (one of the most heat conductive metals). Try Texereau's wooden tube. I know, it doesn't have the snob appeal, but it works, and is much easier to mount things on it, as well as the tube itself.

Mladen


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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5594246 - 12/29/12 11:56 AM

Quote:

but everyone is talking about the sale price of an apo compared to material costs of a newt with a >$1k window sale price not even mentioning the sale price of build the scope.

that's not a fair sale price/performance vs material cost/performance comparison.



Well, since there are no commercially available windowed Newts, you'd have to make one. And since there are very, very few if any amateur-built apos, we can only compare what is cost-wise.

If you can buy a 6-inch SCT for under $1K, there is no reason a windowed Newt of the same size would cost more. There is so much more to putting together an SCT then a Newt.

If windowed Newts became popular, I am sure, initially, the price would be hiked, but eventually they would end up being competitively priced way under $1K for a 6-inch scope.

Mladen


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tim53
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5594256 - 12/29/12 11:59 AM

Quote:


As for the disdain that seems so popular for numerical and theoretical, do remember that the numerical and theoretical created these configurations. No one made them by trial and error and anecdotal evidence.
Regards,
Mladen




Dawes derived his formula via empirical methods, so yes: trial and error.

Resolution is defined differently for visual observation and imaging applications. Cassini's Division or the Encke Gap in Saturn's rings are often described by amateurs as being "resolved" visually or digitally, even when they are well below the Dawes specification for a particular aperture. A better term would be "detection" - as in the Encke Gap was detected, but not actually resolved (which, in digital imaging, requires that the object subtend at least 3 or 4 pixels in order for it to be identifiable, and depending on the object, contrast, etc.)

-Tim.


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Dave O
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5594270 - 12/29/12 12:06 PM

Quote:

... the OP specifically mentioned a Newtonian, same aperture as the APO, and with an optical window so as to eliminate spider vane diffraction effects. IOW, cparing two identical same diameter, same focal ratio, closed tube telescope to one another, except for the central obstruction of 15% or less in the Newotnian.




Actually, the claim was for a 12% CO ... just trying to keep the facts straight.


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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5594317 - 12/29/12 12:27 PM

Quote:

Anyone who...resorts to numerical figures as an answer is obviously inexperienced, because if they were in fact hands on and experienced in the claims they make, they would know reality does not work that way.




On the contrary, we know reality works that way. That's why we trust science to remove those real world variables that are the root of every disagreement. Like everything else said on this planet from the fiscal cliff to the MTF, nothing is a complete truth when it deals with "jaw dropping" bias. In the real world there are far too many variables, in science those variables are held constant or applied equally across samples. And, importantly, observer "jaw dropping" bias is completely removed.


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Re: Optimized Newts vs APOs new [Re: tim53]
      #5594335 - 12/29/12 12:45 PM

Quote:

Dawes derived his formula via empirical methods, so yes: trial and error.




I said "configurations", referring to telescope designs. And theory says the Newotnian is either equal or better (depending on the apo). Yet the apo of the same diameter costs anywhere from 25 to 80 times more.

Besides, Dawes used micrometric eyepiece with wires to measure the separation and then extrapolated the actual angular separation based on science.

Mladen


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Darren Drake
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5594353 - 12/29/12 12:57 PM

A few years ago Ed Ting did a fascinating and detailed comparo of high end scopes including an apo and a newt (as well as a mak). This was a great read at the time and provides some relevant info for this discussion even though they are not quite equal apertures.

http://www.scopereviews.com/best.html

Edited by Darren Drake (12/29/12 01:00 PM)


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tim53
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5594379 - 12/29/12 01:12 PM

Quote:

Quote:

Dawes derived his formula via empirical methods, so yes: trial and error.




I said "configurations", referring to telescope designs. And theory says the Newotnian is either equal or better (depending on the apo). Yet the apo of the same diameter costs anywhere from 25 to 80 times more.

Besides, Dawes used micrometric eyepiece with wires to measure the separation and then extrapolated the actual angular separation based on science.

Mladen




Empirically. Yes.

-Tim.


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Re: Optimized Newts vs APOs new [Re: tim53]
      #5594388 - 12/29/12 01:15 PM

Forgot to add: No argument here that a) a Newtonian is "equal or better" than an APO, since I am mostly interested in planetary imaging that I can afford in my lifetime. But I do realize that other configurations are better for wide field deep sky (though even then, a short Newt with a Paracorr might give an equal aperture APO a run for it's considerable money).

-Tim.


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Pinbout
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5594475 - 12/29/12 01:54 PM

Quote:

If you can buy a 6-inch SCT for under $1K, there is no reason a windowed Newt of the same size would cost more. There is so much more to putting together an SCT then a Newt.

If windowed Newts became popular, I am sure, initially, the price would be hiked, but eventually they would end up being competitively priced way under $1K for a 6-inch scope.






not to 1/20~pv spec, that would be very expensive.

a 7in window 1/20~pv, parallel to fractional arcmin is probably more than $2k, I was being fair underestimating the cost.

and to be fair the apo should be hand figured to the same spec not something off the shelf.


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siriusandthepup
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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5594499 - 12/29/12 02:07 PM

Well friends, lets summarize. APO refractors are pretty to look at and to look through. They are not cheap. If you can swing one it'll look good and perform well far into the future.

If your budget is small and you just can't bankroll the big APO, take heart you can still have the performance with a few compromises. (compromises = you need a slightly bigger scope - more trouble to handle and mount)

A good rule of thumb is that a Newt with good optics will perform as well as an unobstructed scope that is equivalent to it's primary diameter - secondary diameter. That rule is actually conservative and the Newt will perform better than that, but its a handy rule and easy to remember ex.: 8" scope with 2" diagonal = 6" unobstructed performance.

Real World: At Twin Lakes Star Party a couple years ago I was able to view through a 6" f/15 refractor and an 8" f/6 Dob set up side by side. Jupiter looked great in the refractor. But, truth told, the view in the Starfinder Dob was better. This was not a matter of optical quality. The Refractor had very nice optics and I would have been happy to own it myself. The Dob was upgraded with a 3 vane curved spider and a better diagonal. Nothing fancy or exotic. The owner of the Dob was blessed with an excellent stock unrefigured mass production mirror. Nice scope. It out performed the 6" in both image brightness and fine detail on Jupiter. Either scope would be a pleasure to have.

Real World: I have a 10" f/5 (actually 9.75") Chinese plate glass mirror in a homemade dob which I compared to my 8" f/12 Achromat side by side. The 10" was the superior planetary performer. No - there was nothing wrong with the refractor, except it wasn't big enough. I still have the little Dob.

Real World: The best views I've ever had of Saturn have been in my friends C14's. Gorgeous views of ring plane crossings with a C14 on a stable temperature night are to die for. C14's have a humungous central obstruction. Optical quality is more important than central obstruction. the C14 has about a 4" obstruction. Rule of thumb says 14" - 4" = 10" unobstructed performance. Yes, it works for Schmidt Casses too.

Real World: And just to show I'm not biased (I like all good scopes) - The best view of the Moon was in a 10" f/15 folded refractor with a big chip (approx 2" x 1") out of the edge (sad story previous to my friend buying it) Unbelievable "spider webby" fine detail on the moon down to the resolution limits. That image is still frozen in my brain. The blacked out chip had no meaningful impact on the lens performance. Wish I had had the money to buy that one when my friend got ready to sell it.

MHO: If you don't have mega dollars and would like world class performance, build yourself an optimized newt. and I'm not talking about an optical window either. If you can afford that exotica go buy the refractor. Optical widows scatter light, their AR coatings scatter light, and the dust that you can't keep from settling on the window scatters light. Build your Newt with a 3 vane straight vane spider or a single, double, or 3 vane curved spider. Use the best mirrors you can afford. Do an f/6 or f/7 for a good sized "sweet spot" and then just wait for the good seeing. Enjoy.

It's all good.

So much for my opinion - YMMV.

Edited by siriusandthepup (12/29/12 02:22 PM)


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Daniel Mounsey
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Re: Optimized Newts vs APOs new [Re: dan_h]
      #5594513 - 12/29/12 02:16 PM

Quote:

Quote:

Quote:

I've made side by side comparisons with my high quality 10" f5 Newtonian 10" f12 DK. I even had a 10" stock production 10" f6. I've compared them all to 8" apos and 6" apos. I've seen times when the 10" reflectors were superior on planets and I've seen times where the smaller apos were better on planets.



I don't think anyone questions these statements but such conclusions are the result of seeing conditions, not telescope characteristics. You would not come to these same conclusions under ideal observing conditions.





Nearly everything has to do with telescope characteristics, I'm surprised you think that. The aperture and seeing also depends on the air cells. Once the aperture increases, more often than not, a different optical design is implemented and with it, all the inherent issues and characteristics. So the telescope characteristics play a huge role. Refractors behave completely different than those of most reflective designs. It's the nature of the beast. I have no issues with the OP though. You can see one of our tests here if you like.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4774348/page...


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Daniel Mounsey
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Re: Optimized Newts vs APOs new [Re: dan_h]
      #5594529 - 12/29/12 02:25 PM

Quote:

I for one, have learned a great deal regarding obstructions, Strel, MTF, and numerous other topics, by those who have taken the time to post an explanation behind the theory that I could not otherwise comprehend. I welcome their contribution. I will probably remain an uncurable skeptic.




BTW, More often than not, that's actually the problem. Paralysis by analyses and little if any hands on experience is why so many people in the forums are making ridiculous claims without even realizing it.


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KaStern
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5594539 - 12/29/12 02:31 PM

Hi Ed,

Quote:

You know what? No one took the challenge against my 1/20 wave optics. Not one. Not one dared.




are refractor folks *BLEEP*?

Anyway. I do own a 8"f/6 ATM Newt, somewhat optimized.
The mirrors have been tested with great results.
I compared it to some apochromats.
It performed on par with an 7"f/8 TMB triplet apochromat
when the owner and me compared them on Jupiter and M13.
Seeing was Pickering 6/10 to 8/10

Cheers, Karsten


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MKV
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5594569 - 12/29/12 02:46 PM

Quote:

not to 1/20~pv spec, that would be very expensive.

a 7in window 1/20~pv, parallel to fractional arcmin is probably more than $2k, I was being fair underestimating the cost.

and to be fair the apo should be hand figured to the same spec not something off the shelf.



APOs are mass produced. Doubt they are 1/20 wave optics. AT any rate, even a commercial 1/20 wave windowed Newt would be pocket change compared to a comparably corrected APO of the same size. But the performance would still not be equal to the price difference, if at all.

M


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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5594586 - 12/29/12 02:59 PM

Anecdotal: One of the best views of Jupiter I haver ever had in a 6-in optic was through a long focus newt...on a stick! The guy built a scope with the primary mirror and cell at one end of a 2x4 board and the secondary and focuser at the other end. Single stalk spider.

We have an 8-in f/12 triplet apo at the club's observatory and Jupiter has never looked that good. BUT, seeing conditions are rarely that good there and this scope has cool down issues due to being in a closed observatory.

Another anecdotal: For a few years I had a Meade (gasp!) 5-in f/9 apo. On rare nights it would through up as good an image as that scope on a board did.

At a star party a few years ago I had my C102f with a light blue filter in the eyepiece trained on Jup. A few feet away was a TMB 8-in apo on a massive EQ mount. The views in my scope were much better...until Jupiter got about 45° above the eastern horizon and the big guy had cooled down. Then, no contest.

We have beaten this horse so much on this thread. So much depends on quality of optics, collimation, seeing conditions, cool down...yada, yada. Buy/build what you can afford/like and run what you brung.

Then, just be


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Re: Optimized Newts vs APOs new [Re: StarStuff1]
      #5594692 - 12/29/12 04:22 PM

Relative cost should be allowable since nowhere in the challenge or the ensuing discussion was it stated that the APO could not be an ATM project. The whole idea was two telescopes head to head and no consideration to how they were acquired. As for the challenge itself, it was obviously a reaction to the fact that there is a huge amount of hyperbolic claims regarding the APO refractors.

Regards,
B


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Re: Optimized Newts vs APOs new [Re: cheapersleeper]
      #5594738 - 12/29/12 05:14 PM

I believe (havn't run the numbers yet) that an ellipse of somewhere between 60-80% will net a good 6" F/10 mirror that rates 1/20th pvw or better. About 12 years ago, one of the Stelli judges here got a look at Jupe, and we also saw the "pup" when it was really difficult and hard to separate. He remarked that I should put the secondary on a window, and "do the Newt right". This particular scope was an F/6.3, with a sled focuser for extremely low 'A' distance for focuser, and had a 1.063" secondary (~18% obs). It rated ~ 1/40th wave PVW when I made it. It was a killer. Transits were absolutely splendid, as long as lowish powers weren't used that showed the effects of a curved spider. Nice black dots for shadows, and discs; not fuzzy 'points'(blobs) for the 4 main moons. 8" thin AL tube for insignificant heat issues, etc.
It could have been better. I could have made it F/8, with a 1" secondary for a little less obstruction, but it was pretty -DARN- good with any power from lowest on up as it was. (lowest power with a 32mm Pl seemed to be just under where modest skyglow was noticed)
This seems to fly in the face of what lots (and at least one here) says is impossible to notice. But with folks who have seen really good quality stuff, and know what to look for in the 'proof', debunks all simulations and theoretical postulating. There is a difference. This is where simulations and theory are just that---speculation by inferior technique or uninformed minds. It is where realizing about half the theory and 'carp' written about optics is genuinely false. If this wasn't so, you wouldn't be able to detect fine-line features at half the "dawes limits". Impeccable producers of quality optics such as Zambuto and the likes wouldn't exist, for it would be impossible to "buck the theory".
Well, it seems there's a heck of a waiting list for his ('Z's') optics! Gotta be a reason for that! And a reason for that particular judge that came here and remarked that a window would clinch that scope. (now long gone, after someone offered the bucks for it) Theories and simulations be dayamed- can't hold up to actual performance, often termed as "anecdotal" in this thread. Just that "anecdotal" should be used correctly to berate the correct side of the arguement!!!!!!!!
FWIW,
M.


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EddWen
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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5594764 - 12/29/12 05:35 PM

Amusing musings.

I haven’t seen any objective criteria to be used to define or declare “optical excellence”. How about we set one?

155 mm clear aperture operating @ f7.3
Field to be flat over a 90 mm diameter with no vignetting. Pinpoint stars all the way to the edge of the field. (This allows full use of the field of 6x7 film and the new medium format CCDs coming available).

I have an apo that can do this. Can an optimized Newtonian do this?

If so, all we need is a couple of representative exposures we can all review. Game on ??

Edited by EddWen (12/29/12 05:36 PM)


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Mark Harry
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Re: Optimized Newts vs APOs new [Re: EddWen]
      #5594798 - 12/29/12 05:59 PM

Well, if we did that; there would be an uproar, wouldn't you think?
I have been looking at the flat-field thing... Would 8" med-long achro/dialyte suffice for low-medium powers? That 90mm dia flat field is a tough one--- Don't think I've seen any chip that big. But I'm a visual guy.
(been thinking of redoing the scope I mentioned in about F/7-7.3~)
As to review- you'll have to come here- sorry.
M.


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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5594917 - 12/29/12 07:07 PM

A 90mm unvigneted field in a newt is going to require a secondary bigger than 90mm.

I'm not likely to live long enough to afford a digital camera with a chip that big. And since I mostly image planets with machine vision cameras with 1/3" chips in them, my 8" f/9 won't require a secondary much bigger than an inch. (Sled focuser)

Tim


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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5594919 - 12/29/12 07:09 PM

Quote:

You know what? No one took the challenge against my 1/20 wave optics. Not one. Not one dared.

I waited 6 months; the offer is withdrawn; why should I put up with all this "mighty APO" BS?







Regardless of my love for Newts and Apos alike, I'd be happy to accept this challenge. Unfortunately you're a bit far out. I think a comparison of this level would be quite fun. I have a 6" F-8 doublet apo. The sample is a fine specimen.


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Mark Harry
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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5594928 - 12/29/12 07:13 PM

I'd be happy to look thru either one! Just for the fun; not to prove any point.
M.


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bratislav
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5594942 - 12/29/12 07:19 PM

Quote:


There are a lot of (fairy)tales circulating in the ATM community, all based on anecdotal "evidence" and personal impressions. How many times have you read or heard a "testimony" that such and such a scope gave the "best views" (whatever that means) or that it was the "ultimate telescope"?

One of those tales is that APOs are better than Newotnians of equal aperture and focal ratio. How else could anyone convince people to buy a telescope of the same size as another one, but costing 10-20 times more? Because everyone is swearing APOs are "much" better, yet there is no objective data to back it up, nor will anyone submit their APOs to a test.

Ed's challenge was based on the fact that theory disagrees with eyewitness accounts. For that reason it would have been a good thing if someone had accepted his challenge, so that people may know instead of believe other people's impressions of exactly zero scientific value.





Here we go.

I happen to know an ATM friend who is a master optician. He has made many Newtonians of all sorts of diameteres and obstructions, many zero obstruction systems (his main planetary instrument is 350mm Schiefspiegler), he has made many very advanced designs (including stationary eyepiece Coelostat underground refractor), and in last 10 or so years he started making APOs (doublets and triplets in various sizes, mostly 5, 6 and 8" using Ohara FPL glass). Currently finishing 10" triplet for personal use. He has been a long standing planetary observer (for more than 30 years he headed lunar and planetary section of my local astro society), as well as regular ALPO and BAA contributor. All of his instruments have been made by himself with one sole purpose - give the best possible image.

Unbiased enough for you ?

Do you want to know about his comparison between his unobstructed reflector and APO of same size (both made himself, both sensibly perfect instruments - I have looked through both many times)? He literally used a following phrase :

"Refractor leaves it for dead"

And I happen to fully agree with him.

And before anyone jumps, do an internet search and try to find a more staunch Newtonian defender than yours truly.
There is no more logical, easier or cheaper way to get into a big league planetary instruments than a Newtonian. Visually or (especially) photographically.

But inch for inch, nothing beats or even comes close to a well made APO. Maybe, on paper, their polychromatic Strehl leaves a lot to be desired. But don't forget that our eyes are not wide band devices. Nor they care about theoretical MTF, CTF and EER graphs. They do care about thermal properties of the telescope, heat retention, tube currents, thermal gradients and like.

Just find a good APO, pick a demanding target (say Jupiter) and simply look for yourself. Heck, just look at a bright star and compare in focus diffraction patterns. How many reflectors will show you a first magnitude star with but a single diffraction ring ?
Try it, without prejudice, without bias. You may learn something about theory and reality.

(I do have a few refractors, largest one being a 4" APO solely used for wide angle astrophotography. For observing planets I mainly use 7" (homemade) Mak, 11" SCT (for convenience of binoviewing - I suffer a lot from floaters as I get older) or a 16" (homemade) Newtonian.)


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Re: Optimized Newts vs APOs new [Re: bratislav]
      #5594968 - 12/29/12 07:27 PM

Well my reflectors enjoying one hell of an afterlife lol. I agree with a lot of what you said though Im also a fan of MTF - a growing fan rather. I never bought into an apo equalling my reflector in aperture as the physics is rather rigid no matter how much you tweak the machine. But you kno Im not after apo performance I just work to have my reflector optimized .
I don't know about single ring diffraction patterns though as my 70mm shows them when the stars are bright enough.

I enjoyed your post.

Pete


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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5595122 - 12/29/12 08:48 PM

Thanks, Mark. With my being bedridden with RA a lot of the time, this isn't likely to happen. Funny thing is that I have a partially completed 6" f/8 window scope in totally disarray at the moment as I can't quite get my hands around pieces of the project. But for you, if there is any chance at all . . .


Ed


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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5595125 - 12/29/12 08:51 PM

Quote:

Quote:

I for one, have learned a great deal regarding obstructions, Strel, MTF, and numerous other topics, by those who have taken the time to post an explanation behind the theory that I could not otherwise comprehend. I welcome their contribution. I will probably remain an uncurable skeptic.




BTW, More often than not, that's actually the problem. Paralysis by analyses and little if any hands on experience is why so many people in the forums are making ridiculous claims without even realizing it.




Scientific study of optics is cutting edge knowledge and has been since Newton. It's the best mankind (mankind itself, not a bunch of amateur observers) can say about the topic. It's much more objective than the number of jaws laying around the site. I am sure some of the best minds in the business would disagree their work consists of, "ridiculous claims." In fact scopes are designed using that science. They would rightfully agree that results will vary in the field and explain that induced aberrations and uncontrolled conditions are the cause.

The truth is, all scopes transfer contrast according to the laws of physics acting on the image at every moment in time, that includes seeing, collimation, thermal equilibrium, focus, aberrations, and whatever adverse condition one might care to model. Contrast transfer is exactly what scopes do all the time, and with varying degrees of success all the time.

The essence of this challenge is not about differences of induced aberrations between the samples from one moment to the next nor the beauty of the FOV. It is all about whether or not an experienced observer can notice the difference between a well corrected unobstructed aperture and an equally well corrected obstructed aperture. It boils down to detecting about a 4% difference in the brightness of the rings (affecting resolution.) It might be about the ability of a parabola, the conic of choice for parallel rays, to produce a spherical wavefront with the ability of a spherical system to do the same.

IME, for this to have a prayer of a chance, it will require an unbiased, eagle eyed observer under the best field conditions. And the best chance for success rides on observing a star's diffraction pattern. I doubt anyone will notice such a small a difference on extended object contrast. Not in an hour of observing. It might take weeks of observing Jupiter under real field conditions before any difference can be appreciated, if at all. (Unless of course, conditions favor one over the other, but then we'd be comparing conditions not the scopes.)

What's ironic about this challenge is, even if results were posted, each of us would have to decide who to believe. And the debate would rage on.


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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5595149 - 12/29/12 09:09 PM

Quote:

I'd be happy to accept this challenge. Unfortunately you're a bit far out.




you could bring it to NEAF, but we usually have not so good a lot weather then.


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Re: Optimized Newts vs APOs new [Re: bratislav]
      #5595287 - 12/29/12 11:04 PM

Bratislov,
Nice post.


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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5595368 - 12/30/12 12:06 AM

Norme,

Okay, you raised some valid points, however, since you appear to discuss optics for the most part, I'm not really sure where you stand on this topic. Do you have any experiences you'd like to share?


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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5595372 - 12/30/12 12:10 AM

Quote:

Quote:

I'd be happy to accept this challenge. Unfortunately you're a bit far out.




you could bring it to NEAF, but we usually have not so good a lot weather then.




Unfortunately the seeing at NEAF is pretty pathetic. Might as well be observing with Coke bottles.


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azure1961p
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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5595412 - 12/30/12 12:40 AM






What's ironic about this challenge is, even if results were posted, each of us would have to decide who to believe. And the debate would rage on.




That's exactly what'd happen. And it wouldn't end pretty during the challenge. One party would outright deny what was plain as day while another would guffaw.

No victor would be agreed upon on either side. I think I skirt this argument so well because I'm not trying to build an apo out of a reflector much as you aren't with a Maksutov. There's a lot to be said for optimizing and appreciating ones own instrument for the instrument it is rather than trying to morph a lion into a tiger.

Refractors are beautiful things truly and I've looked through a couple APs and they are great but Im fine where I am.

At any rate , you are correct - this challenge would yield no true victor among the masses and merely each opponent dug in - regardless how right or wrong either was.

I also have seen the comparisons first hand and read of others comparative testimonys. This whole challenge is held up like some heretofore never been tried kind of thing .

Ahhhh moving on...

Pete


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Asbytec
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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5595473 - 12/30/12 02:04 AM

Quote:

Norme,

Okay, you raised some valid points, however, since you appear to discuss optics for the most part, I'm not really sure where you stand on this topic. Do you have any experiences you'd like to share?




Sure. But first, I was just reading one of your reviews of an 8" TMB and a 24" Dob observing Mars. I am truly envious of the views you had that night and entirely respectful of your experience with high end scopes. Your reviews and descriptions seemed perfectly consistent with the science behind it to the best of my ability to understand both the science and your comments. So, no disrespect intended, just defending the science behind it all and the assertion those of us who throw out numbers have little experience under the stars.

My own personal experience is with descent scope, in consistently 8/10 or better seeing, perfectly collimated and cooled to ambient, and focused as perfectly as one can tell. And with a good set of UO Orthos. Seeing does more than simply allow better views, it allows better scrutiny of all induced aberrations and the scope to actually rise toward it's theoretical level of performance. And when it does, those are the jaw dropping moments we all experience especially when real world seeing favors smaller aperture.

I optimized my own scope by reducing the CO by about 9% of its diameter. The first and most apparent improvement was to the stellar diffraction pattern. Light in the rings was immediately reduced just as science said it would be. In fact, the number of rings visible was reduced by 1 on the brightest stars. (In fact the third ring is gone and the second ring is now barely visible on Aldebaran.) Reduced diffraction effects on the moon were also noticeably reduced.

Turning to Jupiter, however, it took a good long time of close scrutiny (sketching) to begin to realize improvement must have occurred. It was not immediately apparent even with that much reduction of the CO. The results simply showed up on the sketches over time. So, if I can extrapolate experience with a 9% reduction in CO by diameter, then it's reasonable to conclude a 4% contrast change induced by a 15% CO will be difficult, depending on the target and the real world conditions you correctly speak to.

This is consistent with the science behind it all. And leads me to believe the challenge is a real one that the most discriminating observer would need pristine conditions to notice much difference according to the math of a top notch optimized reflector. The refractor will be better by the numbers with a peak intensity of 98% compared to ~94% (assuming reasonable Strehl and throughput on both.)

Trick is, can a human see the 4% difference? Dawes set his limit at 5% assuming normal 20/20 vision. If there is any movement in the first ring, however, all bets are off on this challenge ending in other than a tie. And I say that due to experience splitting c Orionis. It was extremely difficult in less than perfect seeing.

Anyway, that's what the numbers say and I am placing my bet this challenge would end in a tie provided Ed puts up a scope with a Strehl comparable to the APO. It sounds like he can and the science seems to back him on it. Because in this case, the only variable will be the CO when all other conditions are constant.


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MKV
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Re: Optimized Newts vs APOs new [Re: bratislav]
      #5595479 - 12/30/12 02:12 AM

Quote:

I happen to know an ATM friend who is a master optician...Do you want to know about his comparison between his unobstructed reflector and APO of same size (both made himself, both sensibly perfect instruments - I have looked through both many times)?...Unbiased enough for you ?



Bratsilav, to me this sounds like:"There once was a prince who lived in a castle..."

There are contributors here who have devoted all their energies to making unobstructed reflectors precisely because in their eyes these systems are superior to anything else - especially APOs.

Why should I believe you, and not them? What do you have to offer that they don't??? After all, they too "have looked through both many times" and have come to a conclusion that's exactly oposite of yours or your anonymous friend's.

Unbiased enough for you? I thought so.

So, experience doesn't settle this issue. Curiously enough, one side was willing to do a controlled comparison of a windowed reflector, 12% central obstruction, and an APO of equivalent aperture and focal ratio. But the other side never dared take up the challenge.

After all, the difference may be small, and that's another good reson for such an experiment: the cost of that difference is huge!

I have yet to read the details of how much better the APO views are and why they are worth the price difference of anywhere form 7 to 19 thousand dollars - in that aperture range.

Edited by MKV (12/30/12 02:23 AM)


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Asbytec
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Re: Optimized Newts vs APOs new [Re: azure1961p]
      #5595503 - 12/30/12 02:46 AM

Quote:

Refractors are beautiful things truly and I've looked through a couple APs and they are great but Im fine where I am.




No doubt in my mind, folks love them for very valid reasons. Some optical geniuses can work wonders with curves and refractive indexes. And, IMO, optical quality does matter per this challenge. A good refractor will have a higher peak intensity than an obstructed scope of the same aperture. Where do we think the rest of the light goes? That how the (1-Ds) approximation came about for lower spacial frequencies (in perfect apertures that do not exist.)

Truth is, a quality APO is a fine scope, no doubt at all. But one cannot confuse aesthetics with performance. Add a little aperture, an excellent Strehl, and a minor CO and it's game over performance wise (provided conditions permit, of course. It can be that way at times.) Aesthetically speaking, well someone else will have to crunch those numbers.

Anyway, its a fascinating challenge full of intrigue and ripe for debate.


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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5595513 - 12/30/12 03:13 AM

I really don't understand the desire to compare APO's against equal size optimized Newts.

An Optimized Newt will come close, but not quite, to an equal size APO. WHO CARES!!

If we consider a 6" APO, I will put it against an 8" optimized Newt. 8" APO? 10" Newt. IF you are a lucky dog with lotsa money and have a 10" APO - fine, we will drag out the optimized 12.5" Newt.

OK now the fight's ON!

Edited by siriusandthepup (12/30/12 03:15 AM)


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bratislav
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5595522 - 12/30/12 03:24 AM

Quote:

Quote:

I happen to know an ATM friend who is a master optician...Do you want to know about his comparison between his unobstructed reflector and APO of same size (both made himself, both sensibly perfect instruments - I have looked through both many times)?



Bratsilav, to me this sounds like:"There once was a prince who lived in a castle..."

There are contributors here who have devoted all their energies to making unobstructed reflectors precisely because in their eyes these systems are superior to anything else - especially APOs.

Why should I believe you, and not them? What do you have to offer that they don't??? After all, they too "have looked through both many times" and have come to a conclusion that's exactly oposite form your or your anonymous friend's.

Convincing enough for you? I thought so.





Mladen,

Let's cut to the chase :

How many top quality APOs of significant aperture (say 6" and larger) have you actually looked through ? In my last 40 years of observing I did, many in fact. And I had many opportunities to do direct comparisons under all sorts of conditions.

How many of those 'devoted' makers of unobstructed reflectors you know that have made an APO of same aperture, and same quality (to have truly unbiased view) ? I have witnessed many 'averted imagination' type shootouts. Having invested a lot of effort, money, time and will into making a scope often (too often) makes you a very biased observer. Unless you own -or even better- have made both instruments, your opinion counts very little in my book.
In other hand I highly respect Barry (not so anonymous ATMer) and his opinion; and even better, I have observed through same instruments myself. In fact I have observed side by side through one of his 6" APOs and my own 7" homemade Mak (so my own bias should be quite strong). Yet, it was quote obvious that APO was delivering much more contrasty, much more stable and much more pleasing image of Jupiter, despite obvious color residuals (APO was f/10, so not much color to see, but it was there).
(before you condemn my Mak, just FYO it measures via Roddier as having Strehl of 0.95+ and will give sharp images -once it is equlibrated- well beyond 50x per inch)

So, why should you believe me ? No reason, no reason at all. We have never met, you have never observed through any of telescopes that I have made (to possibly convince you how critical an observer I am). In fact it is that critical (some would call it an@l) obsession with perfection that drove me to glass pushing and made me notice aberrations in a telescope (ANY telescope!) much more than (often) nice views. It is a curse - I look for, and I always find residuals.

But words are cheap (as proven by this very thread), so I'll leave you to believe whatever you want to believe.

I will just ask you few questions to ponder : why do you think those uber expensive APOs actually exist ? There can't be that many rich fools around who can't see that a reflector can equal an APO for a fraction of price. How come that many not so rich amateurs save for years to be able to afford an APO (I know quite a few)? How come that people like quirky Markus Ludes, who can afford anything, keep using APOs instead of windowed Newtonian (he's got as perfect as it gets 16" windowed Newtoninan made by LOMO , yet he went through all the trouble and $$$ to get a 12" APO made as main observing tool)? How come that people who make world class optics of all types (not 'anonymous' like my friend) - say Roland Christen of AstroPhysics, Yuri Petrunin of TEC and Valery Deryuzhin of Aries share exactly the same view (that, for a given aperture, visually you cannot surpass a quality APO)?

I don't expect you to change your opinion, that to me is quite clear. All I expect is that once you get a chance, have a careful look through a high quality APO and try not to think about how much it costs.


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Asbytec
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Re: Optimized Newts vs APOs new [Re: siriusandthepup]
      #5595529 - 12/30/12 03:37 AM

Quote:

I really don't understand the desire to compare APO's against equal size optimized Newts.

WHO CARES!!



It's like climbing Everest, because it's there. It's fun. And it's an opportunity to open up the story and learn something. For example:

Quote:

"Refractor leaves it for dead"



Yea, your friend's credentials are good enough for me. Okay, so why would that be? Is there a theory behind it or just the reality of it? I suspect the latter, because...

Quote:

Nor they care about theoretical MTF, CTF and EER graphs. They do care about thermal properties of the telescope, heat retention, tube currents, thermal gradients and like.



As Ed alluded to earlier, he would have those variables optimized, too. I am a bit surprised your friend did not. And they really are the same variable, by the way. You forgot focus and collimation, at least. Those have to be good, too. If the scope cannot handle everything it is capable of handling, then it is not optimized. If you allow the initial conditions to vary, the results will vary. No surprise here.

IME, my own scope operates with thermal issues at nil, de-focus at nil, collimation at nil, and seeing minimal. Now, have a peek. Heck, under these conditions, I might just offer a challenge to a 4.5" APO. See who can count the most very small, low contrast white ovals in Jupiter's southern hemisphere. I bagged 5 of them (9 total?), so far, including a faint belt fragment in the NTZ that barely shows on images. That's what happens when you controls those things that can be controlled, much like Daniel's reviews of some very good reflectors.

Quote:

before you condemn my Mak, just FYO it measures via Roddier as having Strehl of 0.95+ and will give sharp images -once it is equlibrated- well beyond 50x per inch.



Pretty much the same here, this is why I love the design despite it's large CO. I'd wager that's the same reason people love refractors. It pleases them to no end. ANd it's no mystery your friend's 6" APO bests your (and my own) well corrected Mak. It should do so, as expected. There is nothing extraordinary that it does.

You make a good point, high end APOs are often very well corrected. That is important. They are essentially textbook, for all intents and purposes they are perfect. And that's what you pay for.

An obstructed scope simply has to be that much better to equal an APO by the numbers. They can get very close, but with an obstruction of any size cannot be "perfect." That's the challenge Ed put up.


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Dave O
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5595578 - 12/30/12 05:01 AM

Quote:

I have yet to read the details of how much better the APO views are and why they are worth the price difference of anywhere form 7 to 19 thousand dollars - in that aperture range.




And perhaps herein lies the problem Mladen ... have you ever looked at Jupiter through a fine APO? If so, and if the view looked the same to you as that in a similar aperture Newtonian, then I agree ... an APO would pretty much be a waste of money for you.

But, I believe that there are some folks who CAN see a difference at the eyepiece between a fine APO and an optimized Newtonian of similar aperture under excellent seeing conditions. The eye is a pretty remarkable detector and not all are created equal. If my views of Jupiter were no better than those images you posted earlier, I certainly would not be wasting my time 'looking' through an eyepiece -- I'd just set up a web cam (or whatever). However, what I see even through a tiny 3 1/2" Questar (yep it is obstructed), is so much better than that flat, washed out, unimpressive image you posted, that I go out and look again, and again. I am not claiming my little Q 3.5 performs better than whatever instrument was used to obtain those images (in fact I am certain it does not) ... what I am saying is those images (or most images for that matter) do not truly capture the view that I see, even through a small telescope.

And no, I do not own an APO ... but would certainly not refuse a chance to view through one.


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5595613 - 12/30/12 06:29 AM

Bratislav made a very fine posting. All theory points to a Polychromatic strehl of an apo that on paper doesn't compare very well with a reflector system. But I've seen enough, even in the cheapa-- achro world that is very intriguing. Must be something justifying refractors as a worthwhile expenditure and why they are sought out.
****
I have been wondering- most reflectors bounce the light through the same general path that can be affected by heat issues twice. Oftentimes open-ended. A refractor, sealed, only allows this to ocurr once. Is it possible that it could be this simple an explanation?
Another aspect- surface polish. If a high standard of polish is routinely achieveable wth accuracy and smoothness standards, a reflector surface will double the errors, and be just a touch fuzzy from scatter. A lens will typically only make half the errors, and I suspect will also reduce the scatter effect a significant amount. A 2- mirror reflector in my estimation, may still retain a significant degree of scatter and error perhaps twice what a good 2 lens achro, or a critical 2 lens apo with exotic glass might have. Food for thought? Hey, I'm just thinking out loud...
M.


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cheapersleeper
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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5595639 - 12/30/12 07:27 AM

Quote:

Quote:

I have yet to read the details of how much better the APO views are and why they are worth the price difference of anywhere form 7 to 19 thousand dollars - in that aperture range.




And perhaps herein lies the problem Mladen ... have you ever looked at Jupiter through a fine APO? If so, and if the view looked the same to you as that in a similar aperture Newtonian, then I agree ... an APO would pretty much be a waste of money for you.

But, I believe that there are some folks who CAN see a difference at the eyepiece between a fine APO and an optimized Newtonian of similar aperture under excellent seeing conditions. The eye is a pretty remarkable detector and not all are created equal. If my views of Jupiter were no better than those images you posted earlier, I certainly would not be wasting my time 'looking' through an eyepiece -- I'd just set up a web cam (or whatever). However, what I see even through a tiny 3 1/2" Questar (yep it is obstructed), is so much better than that flat, washed out, unimpressive image you posted, that I go out and look again, and again. I am not claiming my little Q 3.5 performs better than whatever instrument was used to obtain those images (in fact I am certain it does not) ... what I am saying is those images (or most images for that matter) do not truly capture the view that I see, even through a small telescope.

And no, I do not own an APO ... but would certainly not refuse a chance to view through one.




You continue to refer to those horrible images that Mladen put up as if they have something to do with this discussion. That leads me to ask how many piece of junk run of the mill non-optimized Newts have you observed through? Pretty much every Newt I have had, starting at 6 inches and a fast f5 revealed a much better image than the image that Mladen used.

I also submit that the general run of the mill Newt is much better collimated in this day and age than they were in decades past. We all build our beliefs on the foundations of what we have experience and I think that "observing experience" can actually cause more dogmatic bias in an observer rather than less.

As general comment, not aimed at the post I quoted, I see the usual list of famous guys that use APOs, but even with my limited experience, I recognize people who have skin in the game and/or a possible financial stake in the APO.

Regards,
B


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Re: Optimized Newts vs APOs new [Re: cheapersleeper]
      #5595669 - 12/30/12 08:12 AM

I think that, beyond the whole obstructions issue, there are several other things that might give the apo an edge:

* like Harry mentioned, in a newt the light travels through the tube twice so, and thermal disturbances in the optical path (heat from mirror, boundary layer, etc) have a greater effect on the wavefront
* in an apo, the primary is up in the air, away from the ground, so it tends to cool better and avoid ground thermals
* in a refractor, the optical path starts converging as soon as it leaves the objective. Thus it is moving away from the tube edge and will be less sensitive to tube wall thermals
* in a newt, the optical path also includes the lateral section from the secondary to the focal plane. This also is within the OTA "tube" and exposed to thermal disturbances from thermals with the OTA.
* the diagonal and spider vanes can create thermals within the optical path
* in a refractor, scatter (mostly backscatter) from dirt or surface imperfections tends not to reach the focal plane. In a newtonian the primary's surface backscatter is visible by the focal plane. This is an issue with signal-to-noise and would affect contrast (veiling glare) but not resolution or color performance.
* apos are more virtuous

Now, let us pause to reflect ....

Edited by NHRob (12/30/12 08:14 AM)


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Re: Optimized Newts vs APOs new [Re: cheapersleeper]
      #5595679 - 12/30/12 08:24 AM

I think what's missing here and I just through the conversion on my reflector is a boundary layer fan. Not a rear cooling fan but one that scrubs the surface of the effects of the heat blanket that forever emanates from the mirrors reflective surface. Alan Adler attributed this lensing effect as one of the chief reasons a like sized reflector cannot equal a refractor. Another point is the observers hot breath blowing by the open end of the reflectors tube where as the refractor (usually) has this man made heat plume farther away with more time to equalize and dissipate.

The trouble with theory alone - and I respect the numbers , is that its as if those hypothetical models exist in an airless vacuum void of convection.

The tie or near tie can never happen without thermals addressed properly.

Pete


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azure1961p
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Re: Optimized Newts vs APOs new [Re: NHRob]
      #5595687 - 12/30/12 08:36 AM

Quote:

I think that, beyond the whole obstructions issue, there are several other things that might give the apo an edge:

* like Harry mentioned, in a newt the light travels through the tube twice so, and thermal disturbances in the optical path (heat from mirror, boundary layer, etc) have a greater effect on the wavefront
* in an apo, the primary is up in the air, away from the ground, so it tends to cool better and avoid ground thermals
* in a refractor, the optical path starts converging as soon as it leaves the objective. Thus it is moving away from the tube edge and will be less sensitive to tube wall thermals
* in a newt, the optical path also includes the lateral section from the secondary to the focal plane. This also is within the OTA "tube" and exposed to thermal disturbances from thermals with the OTA.
* the diagonal and spider vanes can create thermals within the optical path
* in a refractor, scatter (mostly backscatter) from dirt or surface imperfections tends not to reach the focal plane. In a newtonian the primary's surface backscatter is visible by the focal plane. This is an issue with signal-to-noise and would affect contrast (veiling glare) but not resolution or color performance.
* apos are more virtuous

Now, let us pause to reflect ....




Excellent points and just to reiterate exhaling hot air even gently by the opening of a reflector particularly when it's pointed near the zenith on a relatively calm not produces a column of visibly warm air right across the face of the aperture. Alas the scope has to deal with this too and it's never ever been addressed. To a degree the horrible seeing caused by guiding the scope with a hand on the tubes open end is also awful. Consider APs 6" super apo :

1. You'll never ever have an observer exhaling hot air 12" from the objective opening
2. You'll also never see anyone with a long enough arm to place his hand on the dew shades edge to aim or guide.

When I FINALLY killed my boundary layer these two transient problems remained and in front of me explained the incurrent flaring as a result. The seeing matters but Im at this point astonished at how poor my reflector fends off my own thermals.

These will be corrected though - easier than a boundary layer.

Pete

Ever reflecting

Edited by azure1961p (12/30/12 08:38 AM)


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Dave O
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Re: Optimized Newts vs APOs new [Re: cheapersleeper]
      #5595773 - 12/30/12 09:56 AM

Quote:

You continue to refer to those horrible images that Mladen put up as if they have something to do with this discussion.




They ARE a part of this discussion ... they come straight from this thread ...

Quote:

That leads me to ask how many piece of junk run of the mill non-optimized Newts have you observed through?




Fewer than you I would guess? Don't own a Newtonian, never have ....

Quote:

Pretty much every Newt I have had, starting at 6 inches and a fast f5 revealed a much better image than the image that Mladen used.




My point exactly ... now, that wasn't so difficult was it?


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5595784 - 12/30/12 10:00 AM



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Re: Optimized Newts vs APOs new [Re: NHRob]
      #5595885 - 12/30/12 10:51 AM

"apos are more virtuous"

NH Rob, I just love it!:)





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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5595925 - 12/30/12 11:16 AM

Final warning. The personal stuff ends here or the thread is locked.

David


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NHRob
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Re: Optimized Newts vs APOs new [Re: David Pavlich]
      #5595955 - 12/30/12 11:33 AM

"Less filling! ...... Tastes great! "



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wh48gs
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Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5595997 - 12/30/12 11:57 AM

Light scatter is potentially significant factor, but not due to the quality of surface itself. Typical polish is very unlikely to generate more than 1/100 wave RMS wavefront error for reflection, and about 1/4 as much for refraction (refractive index n~1.5), or about 1/3 as much for a lens, and half as much for a doublet (assuming error proportional to the square root of the sum of the single surface errors squared). Normally, it is up to several times lower. Taking 1/200 wave RMS for mirror surface gives 0.999 Strehl, so any gain in using lenses here is entirely negligible.

More important is larger-scale roughness, which can easily get to 1/30 wave RMS (roughly 1/10 wave p-v) on mirror surface, resulting in 4-5% contrast loss (i.e. 0.96-0.95 Strehl). For two reflecting surfaces, it is 8-9% contrast loss. With a doublet, this same surface quality would generate only half as large wavefront error as in a single reflecting surface, or 1/60 wave RMS, causing little more than 1% contrast loss.

Another factor is scatter off reflective coatings. Unlike microripple, which is typically in sub-nanometer range, aluminum coating is several hundreds nanometers - roughly a wavelength of green light - thick. Hence variation in the thickness of as little as +/-5% will induce 1/10 wave p-v of roughness. The unevenness in coating deteriorates with age. Measurements of mirror scatter at a Wisconsin observatory showed that newly coated mirrors scatter approx. 0.5% of the light (indicating ~1/90 wave RMS wavefront error), increasing, again roughly, by 0.5% each year (note that the reflectivity is affected significantly less in relative terms).

But what those measurements brought up as the most significant scatter factor is the cleanliness of mirrors. Chemical/dirt deposits can increase scatter from 1-2% to 4-5% per surface in as little as year, which can double in two years.

Putting it all together, combining, say, 0.95 Strehl degradation factor due to larger-scale roughness with 0.98 degradation factor due to 3-year old coating, and 0.95 degradation factor due to a year-old chemical/dirt deposits, gives 0.88-0.89 Strehl degradation factor for a single mirror surface, and down to 0.78 for two surfaces (note that, unlike coatings/dirt, larger-scale roughness does not widely scatter light, but leaves it relatively close to the Airy disc, which is the energy negatively affecting planetary contrast).

At the same time, same-scale roughness would cause half as large RMS wavefront error in a doublet, for 0.985 Strehl degradation factor, transmitting coatings of a lens objective, also causing twice smaller RMS wavefront error (assuming similar degree of thickness variation), would result in 0.995 Strehl degradation factor which, with 0.95 scatter degradation factor due to a year-old chemical/dirt deposit (single surface), would produce 0.93 Strehl degradation factor.

In other words, just due to these mainly inherent differences between the two telescope designs, a doublet comes out with some 16% contrast gain, which can be compared to the effect of 30% linear central obstruction contrast-wise. Actual obstruction in the reflector further increases this gain.

This, of course, is just illustration, and may not be close to the typical real-life scenarios, which can vary widely (more likely toward benefiting lens objective further, than the opposite).

All this is, of course, strictly relevant for the MTF contrast transfer, i.e. continuous extended pattern. For relatively small objects, like planets, widely scattered energy has little effect on the contrast, mainly affecting background brightness. In order to remain within 1 arc minute, where it would affect contrast of planetary details, the average diameter of irregularities due to coatings or dirt/dust should be larger than around 5mm - very unlikely.

Vla


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Mauro Da Lio
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5596007 - 12/30/12 12:01 PM

Quote:

Mladen, where I live, my 6" f/5 and f/10.9 reflectors generally will outperform my 10" Dob because the atmosphere overhead introduces enough turbulence that the 10" fares worse most nights.




I have no doubt you experience that. However the "conclusion" may not be what you think to be. Atmospheric seeig (and I stress "atmospheric") does not invert performance.


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ed_turco
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Re: Optimized Newts vs APOs new [Re: Mauro Da Lio]
      #5596116 - 12/30/12 12:55 PM

We've all be deluged with all sorts of optical theory, when all I was asking for was a gentle comparison by owners, (no anger, no fistfights, and ultimately a handshake ), of an optimized Newt of my own manufacture and an APO of the same size.

As usual sometimes, an issue gets talked to death instead.
There is a possibility, if I can get one of my students, I mentioned, to help, that there would be a 6" f/7.5 (which may be barlowed), available for the challenge. He would share equally in the credit for making the scope. As he would be the ultimate owner of the telescope, the instrument will not be given to charity.

My contention is this. Even to the trained eye, there will be no perceptible difference in the imagery between the two telescopes. Bells and whistles do not count.

No promises yet. I may not be able to attend the convention; my arthritis is terrible, but perhaps my student can be persuaded.

As I said, no promises yet.


Ed


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kfrederick
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5596158 - 12/30/12 01:18 PM

Ed Hope you get feeling better . The important thing for the ATM they can Make a newt . Making a APO is much more work by hand . If I were to say what is the best value as well as easy of use is a 8inch newt.on a dob mount .

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Re: Optimized Newts vs APOs new [Re: cheapersleeper]
      #5596329 - 12/30/12 03:13 PM

Quote:


As general comment, not aimed at the post I quoted, I see the usual list of famous guys that use APOs, but even with my limited experience, I recognize people who have skin in the game and/or a possible financial stake in the APO.

Regards,
B




The theory in Psychology is called Cognitive Dissonance. When the guy setting up next to you is getting the same performance while only committing 5% of the resources you did, my bet is you will experience it.


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hyia
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5596355 - 12/30/12 03:21 PM

Quote:

We've all be deluged with all sorts of optical theory, when all I was asking for was a gentle comparison by owners, (no anger, no fistfights, and ultimately a handshake ), of an optimized Newt of my own manufacture and an APO of the same size.

As usual sometimes, an issue gets talked to death instead.
There is a possibility, if I can get one of my students, I mentioned, to help, that there would be a 6" f/7.5 (which may be barlowed), available for the challenge. He would share equally in the credit for making the scope. As he would be the ultimate owner of the telescope, the instrument will not be given to charity.

My contention is this. Even to the trained eye, there will be no perceptible difference in the imagery between the two telescopes. Bells and whistles do not count.

No promises yet. I may not be able to attend the convention; my arthritis is terrible, but perhaps my student can be persuaded.

As I said, no promises yet.


Ed




Hello Ed,

Actually, I remember seeing your offer and thought of taking you up on it at the time as I live relatively close. I was interested in seeing your design which you've obviously been able to refine over many decades of experience. But, I've been busy and simply don't deal with too many people outside of my "bubble".

Anyways, what I wanted to add is that I don't think there is any significant difference in views between an APO and a *well optimized* newt. The key point, for me, is that the APO does not need to be well optimized. You generaly just take it out, set it up, wait ~ 20 min, and rest assured that you are getting the best possible views you can with that size scope. Whereas for the newt, you always need to be concerned about things like collimation and thermal issues. If you can manage those well, and can do it for a larger aperture newt, that is going to give you some of the best possible views in my mind. For ease of use and peace of mind in smaller apertures (< 6") though, I think that the APO is very strong. Futhermore, they need not be that expensive either if you consider things outside of Tak, AP, etc. to be APOs. I picked up a used Vixen ED100sf with a cg5 clone with tracking for < $600. I'm sure people may have different views depending on viewing habits or scope use, but that is my two cents. Best Regards.


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wh48gs
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5596498 - 12/30/12 04:29 PM

Quote:

We've all be deluged with all sorts of optical theory, when all I was asking for was a gentle comparison by owners




Well, your opening post does state that your notion of what the difference between optimized Newtonian and an apo agrees with what Suiter says in his book. That directly involves the "theory", doesn't it? The problem is that Suiter does not consider the effect of coatings nor, for that matter, specifics of the effect of larger-scale surface roughness, quite common with mirrors, while his assessment of the effect of dirt/chemical deposits on a mirror surface seem to be overly optimistic against some real-life data.

In order to really optimize Newtonian, one needs to know what is it that can impair its performance - all of it - and, at least approximately, by how much.

Vla


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MKV
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Re: Optimized Newts vs APOs new [Re: wh48gs]
      #5596522 - 12/30/12 04:41 PM

Quote:

Putting it all together, combining, say, 0.95 Strehl degradation factor due to larger-scale roughness with 0.98 degradation factor due to 3-year old coating, and 0.95 degradation factor due to a year-old chemical/dirt deposits, gives 0.88-0.89 Strehl degradation factor for a single mirror surface, and down to 0.78 for two surfaces (note that, unlike coatings/dirt, larger-scale roughness does not widely scatter light, but leaves it relatively close to the Airy disc, which is the energy negatively affecting planetary contrast).



Vla, the OP proposed a windowed Newtonian. If the tube is sealed for all practical purposes, and the optics cleaned and collimated prior to a comparative test, speaking of dirt accumilation on the mirrors is moot.

If the Newt were to be made specifically for this comparative study, as proposed, then speaking of 3-year old coating is also moot. There are Questars 3.5 still in use that were made in 1964 (there is a NASA report on this), and their mirrors are still in excellent condition, no doubt because the tube is sealed.

We could just as easily look for image degrading factors in objective lenses as well...for the sake of fairness. We can assume less then perfect baffling, or dirt on the lens, fingerprints, what have you, etc.

Reagrds,
Mladen


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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5596549 - 12/30/12 04:53 PM

Quote:

And perhaps herein lies the problem Mladen ... have you ever looked at Jupiter through a fine APO? If so, and if the view looked the same to you as that in a similar aperture Newtonian, then I agree ... an APO would pretty much be a waste of money for you.



Dave O, no need to change the goalposts. Just tell me have you ever looked through an optimized 6-inch f/10 windowed Newotnian with a 12% obstruction? Neither have I. So neither one of us really knows what difference would be, and what price you and I would be willing to attach to it.

Mladen


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Re: Optimized Newts vs APOs new [Re: MKV]
      #5596555 - 12/30/12 04:54 PM

I think optimizing newtonians is a waste of time, effort, money, for a small percentage gain.

What you need is a high quality main mirror like my 10" Zambuto f/5.6. All I did was replace my 30 year old Coulter mirror and SHAZAM!!! Super Quality viewing just like the best photos at ALPO-Japan latest. $2000 total investment in this dob beats ANY APO mere mortals can afford.

Bolted on 2x4 struts and some air-up wheels and it's portable even by old men.


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bratislav
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Re: Optimized Newts vs APOs new [Re: wh48gs]
      #5596576 - 12/30/12 05:05 PM Attachment (8 downloads)

Quote:


In other words, just due to these mainly inherent differences between the two telescope designs, a doublet comes out with some 16% contrast gain, which can be compared to the effect of 30% linear central obstruction contrast-wise. Actual obstruction in the reflector further increases this gain.





Thanks for adding some science to lots of hearsay in this thread.

What I think is even more damaging to reflectors is thermal behavior. Our main heat storing element (primary mirror) is at exactly the wrong end of tube - bottom, where it sees very little of sky (so it can't radiate). It is also coated on one side with coating that is very opaque at IR, preventing cooling even further. Even with fans, massive primary will lag if the ambient temperature keeps dropping. Which is unfortunately exactly what happens most evenings. Seen those videos from Bryan Greer? TWO DEGREES of difference is all it takes to completely wipe out all fine planetary detail. Just one degree will drive you nuts as telescope only sporadically gives away hints of what is really capable of. To make things even worse, all reflectors require a multiple path of the wavefront through the same thermal soup (in more extreme cases several times). Add to that the fact that refractor's entrance pupil is naturally far away from the observer (and typically much higher than reflector, above the worst ground boundary layers), and it all adds up.

Like in car race where one car has little bit better brakes, little bit better aerodynamics, little bit better balance, tiny bit better handling, a wee bit more downforce where it counts ... Once the light goes green, that car surges ahead and never looks back, increasing the margin lap after lap after lap.

And before anyone suggests more 'optimizing', have a good look at my Maksutov. FIVE fans, cooling the primary, scrubbing the boundary layer, ventilating the inside, you name it. Does it work ? Yes it does. Sometimes.

So why don't I use an APO then? Well, I did consider making an 6" APO. An f/10 oil spaced doublet with FPL glass will be as perfect 6" as you will ever see. But it is only 6 inches, and that in my books isn't even entry level planetary scope.
So I stick with my beloved Newtonians. They are not perfect, but they will do me just fine.


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MKV
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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5596577 - 12/30/12 05:05 PM

[quote}
Quote:

You continue to refer to those horrible images that Mladen put up as if they have something to do with this discussion.



They ARE a part of this discussion ... they come straight from this thread ...



Guys, you're missing the point: those are un-doctored or "raw' images someone captured and didn't use his "artistic" talent with Photoshop to make them look "pertty". It doesn't matter, the example was comparative. In other words, I could have sued any quality picture. the point was to show a relative loss of detail and/or contrast with two systems.

regards,
Mladen

Edited by MKV (12/30/12 06:35 PM)


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Daniel Mounsey
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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5596581 - 12/30/12 05:08 PM

Norme,

Thank you for sharing your experiences as they are quite interesting. I plan to respond to your constructive post shortly to get your thoughts. In the meantime, I've posted a link here for others to see what I meant by "optimized" Newtonian and what some of those features might be. I have conducted field comparisons of many telescopes from 4" 5" 6" 7" 8" 10" refractors and all types of reflectors, particularly 12.5" 14.5" and 20" Starmasters, 12.5" Portaballs, several Kennedy Optics dobs from 24" to 28" and 32" as well as many Maks. Of all the telescopes I've used whether it be apochromatic or reflective, this one is the most sensational of the lot.

The cores of globular clusters look like pin pricks and Saturn is a sight to behold from Charlton Flats. The views around 2am are so stable even Antares barley makes a pulse and drops to about .5 arc seconds on a decent night. The reason this telescope performs is not only because the optics are exquisite, but primarily because of the thermal management you have with a solid tube like this one and the aesthetics of the curved spider. It's quite an unusual view compared to a conventional reflector we are all used to looking through.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3879584/page...


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MKV
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Re: Optimized Newts vs APOs new [Re: robboski2004]
      #5596632 - 12/30/12 05:47 PM Attachment (11 downloads)

Quote:

The lens design , Roland Christen published in the Sky and Telescope , was later corrected , due to being over-corrected
for spherical and not optimised for colour.

These glasses are capable of producing, what in 1981, was considered apo correction. i.e green , red , blue within the airy disc



Hi Ian, well, below is a testimony of Christen's correction (April 1982). It's now better corrected for spherical aberration, but (a) it has introduced coma, (b) it has lost its symmetry the 1981 article boasted about, and (c) it is not corrected for red, green and blue, not even on-axis, so it doesn't qualify as an APO, but it was offered as one!

It was intended to cover an impressive 52 mm FOV (because that's where it would have outperformed any Newotnian without additional optics) but, as you can see, it didn't do such a good job. Incidentally, Sky & Telescope did not publish spot diagrams, just the configuration parameters. I think the reason is clear.

All other subsequent configurations are proprietary. BY 1985 Christen was offering (hopefully more optimized) 6-inch f/8 APO OTA on an equatorial mount for $2,595 (figure out what that's worth in 2012 dollars!).

Regards,
Mladen


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wh48gs
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5596636 - 12/30/12 05:49 PM

Mladen,

Quote:

Vla, the OP proposed a windowed Newtonian. If the tube is sealed for all practical purposes, and the optics cleaned and collimated prior to a comparative test, speaking of dirt accumilation on the mirrors is moot.




If we talk about optimized Newtonian, we are starting with the standard configuration. The window is commonly thought of as the mean of avoiding spider vane effect, but real-life data suggest that it is probably even more important for added protection of mirror surfaces. However, even windowed Newtonian is still not as well protected from airborne contaminants as an apo, especially if using prism diagonal. In addition, transmitting coatings are not only more than fourfold smaller error source than reflecting coating for given thickness variation, they are also generally more durable to decay. Considering the effect that likely limited exposure to airborne contaminants and dirt have on observatory mirrors, it is only beneficial to take this into account.

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If the Newt were to be made specifically for this comparative study, as proposed, then speaking of 3-year old coating is also moot. There are Questars 3.5 still in use that were made in 1964 (there is a NASA report on this), and their mirrors are still in excellent condition, no doubt because the tube is sealed.




I'd like to hear some specifics on that "excellent" attribute. Again, as the observatory measurement imply, the effect to exposure is significantly smaller with respect to reflectivity, than scatter. As for the one-time purpose of a telescope, I'm pretty sure that everyone is interested in optimizing performance in a prolonged period of time.

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We could just as easily look for image degrading factors in objective lenses as well...for the sake of fairness. We can assume less then perfect baffling, or dirt on the lens, fingerprints, what have you, etc.




I know my post was long, and you may have missed it, but I did exactly that. The three factors considered were larger-scale surface roughness and surface dirt/contamination (the two most significant degradation factors in the Newtonian, the last counted only for one mirror, since the front lens surface is also exposed (in all fairness, it is considerably easier to keep clean than reflecting coating, and also deteriorating at a considerably slower pace), as well as roughness due to coating unevenness (the least significant with properly applied, relatively fresh coating, but it shouldn't be assumed).

Vla


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bratislav
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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5596651 - 12/30/12 06:05 PM

Yes, a well made 13" Newtonian will better smaller APOs any day. Actually, make that SOME days. That full thickness primary will take a long time to radiate heat out. 10 kilos of Pyrex store an awful amount of heat. Try it during rapidly falling temperatures (after a sunset).

You see, it is easy to remember ONE best of everything. I still vividly remember a night when my Mak was so painfully sharp on Saturn that I kept piling the magnification until I ran out of eyepieces (that was at 650x, in a little 180mm Mak!!!). It was the night I tried to capture elusive Encke in smallest instrument to date. I failed (I am now convinced it takes at least 10 inches and perfect seeing to see it as I've never seen it in anything smaller than 12").

But most OTHER nights Mak is an exercise in frustration. As is my SCT, my Newtonians, Schiefspieglers and all other reflective scopes I know and use. APO is far from perfect, and it won't make great seeing from a mediocre night. But it WILL show you what atmosphere allows much more often than any other type. Optimized or not.

Bottom line - yes, a Newtonian (Maksutov/Schiefspiegler/Yolo/whatever) CAN match visually an equal aperture APO on planets.

Once or twice per year where I live.
If I'm lucky.


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wh48gs
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Reged: 03/02/07