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Optimized Newts vs APOs

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#1 ed_turco

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Posted 19 December 2012 - 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I waited 6 months; the offer is withdrawn; why should I put up with all this "mighty APO" BS?
 

#2 David Pavlich

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Posted 19 December 2012 - 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?

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#3 Astrojensen

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Posted 19 December 2012 - 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
 

#4 John Jarosz

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Posted 19 December 2012 - 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 :rofl5: :rofl5: )

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#5 Jon Isaacs

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Posted 19 December 2012 - 08:02 PM

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
 

#6 kfrederick

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Posted 19 December 2012 - 08:17 PM

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

#7 Jon Isaacs

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Posted 19 December 2012 - 08:30 PM

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
 

#8 ed_turco

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Posted 19 December 2012 - 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
 

#9 ed_turco

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Posted 19 December 2012 - 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
 

#10 ed_turco

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Posted 19 December 2012 - 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.
 

#11 Jon Isaacs

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Posted 19 December 2012 - 08:47 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


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
 

#12 tim53

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Posted 19 December 2012 - 08:49 PM

Long focus newtonians should be renamed "hyperapochromats".

Rant off.

-Tim.
 

#13 careysub

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Posted 19 December 2012 - 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.
 

#14 Jon Isaacs

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Posted 19 December 2012 - 09:30 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.


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
 

#15 Datapanic

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Posted 19 December 2012 - 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!"
 

#16 Datapanic

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Posted 19 December 2012 - 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.
 

#17 kfrederick

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Posted 19 December 2012 - 09:51 PM

funny
 

#18 magic612

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Posted 19 December 2012 - 09:52 PM

Long focus newtonians should be renamed "hyperapochromats".

Rant off.

-Tim.


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

#19 Meep_Esq

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Posted 19 December 2012 - 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
 

#20 Pinbout

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Posted 19 December 2012 - 10:44 PM

Daniel Mounsey did a comparison already

TEC140 vs. Ceravolo HD145
 

#21 StarStuff1

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Posted 19 December 2012 - 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) :tonofbricks:
 

#22 mikey cee

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Posted 19 December 2012 - 11:45 PM

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

#23 MKV

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Posted 20 December 2012 - 12:52 AM

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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#24 tim53

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Posted 20 December 2012 - 01:06 AM

Long focus newtonians should be renamed "hyperapochromats".

Rant off.

-Tim.


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


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

#25 MKV

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Posted 20 December 2012 - 03:13 AM

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

Attached Files


 






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