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azure1961p
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Reged: 01/17/09

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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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Reged: 12/21/11

Loc: Sri Lanka
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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Mike I. Jones
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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5595784 - 12/30/12 10:00 AM



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ed_turco
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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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David PavlichAdministrator
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Reged: 05/18/05

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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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Reged: 03/02/07

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
professor emeritus


Reged: 09/12/04

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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Loc: Lincoln, RI
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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Reged: 02/01/08

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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Jeff Morgan
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Reged: 09/28/03

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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
member


Reged: 11/07/10

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
Carpal Tunnel
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Reged: 03/02/07

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
Carpal Tunnel
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Reged: 01/20/11

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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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tomharri
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Reged: 09/19/08

Loc: USA
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
sage


Reged: 09/07/06

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
Carpal Tunnel
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Reged: 01/20/11

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
Vendor (Woodland Hills)
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Reged: 06/12/02

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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