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  #61  
Old 22-11-2013, 02:43 PM
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Originally Posted by alpal View Post
You can clearly see that the spot size for a Newt. is much greater than an RC.
(The SCT is shocking.)
I wish I could find a higher resolution image of this graph of optical results.
Its a classic example of reviewers/companies abusing peoples lack of knowledge about spot diagrams or optical designs to further their own causes.

If you would like to see what a large Newt can do with a 3" Phillip Keller Corrector on a Newt ( seeing and all other issues aside ) have a look at these spot diagrams of a 400mm F3.5 with 3" corrector to the edge of a 45mm diameter circle.

The square box is 20 micron in diameter . As these spots are just larger than the airy disc at that F ratio I think it is valid to look at spot diagrams, but seeing will of course bloat the discs larger .
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  #62  
Old 22-11-2013, 03:37 PM
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Agree. Top of the head, I think that a 2 arc sec atmosphere blur circle will be about as big as the full 20 micron squares in the diagram - this system will be seeing limited across the field and as good as it is possible to get for seeing down to below 1 arcsec. That is, if I have interpreted the spot diagram correctly.
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  #63  
Old 22-11-2013, 04:31 PM
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Originally Posted by gregbradley View Post

I have wondered about this diffraction limited theory and limited by the atmosphere. It does not match my experience unless most scopes do not come close to being limited by the seeing.

Use an Astrophysics APO refractor or a top Tak or other tp brand APO and you will see much greater sharpness than say a lesser figured but still close to 1/4 wave ED80 or similar.

Longer focal lengths hit the wall of seeing much earlier than shorter focal length scopes down to FSQ type scopes not really seeing affected at all.

Rick from Planewave was telling me that they can get a mirror and measure it on a table and get 1/4 wave. But when the mirror is installed in a mirror cell that 1/4 wave can disappear and so a lot of attention was placed on the mirror cell. So these mirrors that are tested are no doubt tested on a bench before installation. If they were tested after installation they may reveal much lower ratings.

Nevertheless optics like Orion Optics UK mirrors which offer a 1/1th wave upgrade would on the surface seem a waste of money as 1/4 wave is considered diffraction limited. But as you can plainly see from Mike's and John's images the 1/10th wave mirror definitely adds to sharpness. Same with Rolf's Newt and David Fitz's mirror. They are really performing.

So perhaps getting the highest rated mirror/lens is the way to go to allow for the anomalies introduced by mounting the lens/mirror.

Marj Christensen posted once she has seen Roland rub a lens with his finger to get that last tiny bit of imperfection out. We must be talking some almost unmeasurable imperfection that would respond to that.
But you can definitely see it.

Perhaps its because APOs generally are short to modest focal length scopes and so aren't always hitting the limit of the seeing so much as the longer focal length scopes of about 1500mm plus.

This seeing limited argument does not bar getting the highest optics you can possibly get in my opinion.

Greg.
With your indulgence Allan

the angular size of the resolution spot from a scope will decrease with increasing aperture, but the seeing spot remains the same size. Small scopes have spot sizes roughly similar in size to the seeing spot, so the scope quality does have a major impact - the scope imperfections are not swamped by the seeing and a better scope is obvious. This also means that small short scopes are not affected so much by seeing because the spot size (or resolution) is mainly determined by the scope and not the seeing in many conditions. Sampling also comes into it when imaging, but that is another issue.

Above about 6-8 inches in Australian conditions, the seeing dominates and the scope quality becomes less important for DSO imaging - many of the best big scopes have huge secondary obstructions that really spread out their diffraction spots, but they still work perfectly because the resulting spot, while large compared to that of unobstructed optics, is still smaller than the seeing. Some people buy high spec mirrors and then put in an MPCC to fix the coma, which probably turns their 1/10 wave Newtonian into worse than a 1/2 wave system - others use focal reducers or field flatteners that do who-knows-what to the spot diagrams of their high end scopes - but none of this matters because the resulting scope is still better than the seeing. By all means get high quality optics - you will not go wrong that way. But for DSO imaging with apertures above about 6-8 inches, it probably does not matter much. What matters more is the mechanical quality of a good scope (eg the mirror cell/focuser/thermal) and that can have a profound effect on performance - nothing is worse than a scope that drifts out of focus and alignment with temperature and attitude - on a second rate mount.

The best illustration I know of showing how fundamental seeing can be is the before and after images of the 1987 SN taken by the AAT. Top quality big scopes do not work better than smaller scopes in poor to average seeing - they just work better in very good seeing.

http://www.astronnewsroom.com/2012/0...after-march-1/

Last edited by Shiraz; 22-11-2013 at 04:59 PM.
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  #64  
Old 22-11-2013, 04:56 PM
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Originally Posted by Shiraz View Post
But for DSO imaging with apertures above about 6-8 inches, it probably does not matter much.....
I beg to differ.

Hubble pre and post servicing is a good example.

Throwing some light into the first or second diffraction rings I find to be rather moot.

"Oh dear my MTF is less than perfect APO refractor of the same aperture!"

All I can say is: lots of luck with your 12-14-16 inch refractor

Fact of the matter is: a high quality optic will give you sublime focus, and with some exotic designs this will extend across an entire field (eg KAF 16803).

This is not the same is scattering incoming starlight all over the aperture and/or simply not being able to focus...which is what poor optics do.
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  #65  
Old 22-11-2013, 05:05 PM
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Hubble is a bad example - no atmosphere.

I agree with you - as I said, central obstruction is not a problem in bigger scopes.

as long as the optics is good enough to outperform the atmosphere, the result will be as good as you can get. As Mark's post showed, a Newtonian with a corrector can do as well as anything.

Last edited by Shiraz; 22-11-2013 at 05:34 PM.
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  #66  
Old 22-11-2013, 05:31 PM
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Originally Posted by Shiraz View Post
Hubble is a bad example - no atmosphere.

as long as the optics outperforms the atmosphere, the result will be as good as you can get.
I guess you missed the point..... an atmosphere would only confuse the issue.

The RC design, polish, baffling secondary size etc. in the Hubble remained identical with pre and post servicing.

Problem was it had a 1/2 wave of spherical error....and images looked like crap. Even in a vacuum.

Co-star fixed the error. Images became world famous.

Now add seeing to the dud mirror....this is your (bad) baseline. Capish?

Last edited by Peter Ward; 22-11-2013 at 05:35 PM. Reason: clarification
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  #67  
Old 22-11-2013, 05:55 PM
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To be fair the error on Hubble mirror was 1/2 wave RMS - much greater than you will find on a lower end typical consumer scope .

There has been a thread on this before - but planetary imagers need focal lengths of 5 to 15 meters to literally oversample the airy pattern with pixels to exploit every bit of quality in their optics and detail possible in the image. Its all to do with pixel scale versus airy disc size vs. seeing . I think there would be few deep sky imaging rigs exploiting optical quality fully , but I do feel that the better the optics , the less havoc tilt and defocus of the wavefront due to seeing , will have on the FWHM. There is no doubt that the guys with known good optics that are well mounted and guided are coming up with the premiun images.
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  #68  
Old 22-11-2013, 06:03 PM
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Originally Posted by Satchmo View Post
To be fair the error on Hubble mirror was 1/2 wave RMS - much greater than you will find on a lower end typical consumer scope .
Hummm.... I did have a gander through a dark blue tubed OTA...made in southern California at the time (they've moved further south since then)..... some years ago.... Couldn't see any bands on Jupiter.

"Jupiter has bands!!!???" protested the owner.

Reminded me of "Young Frankenstein" ...Marty Feldman saying " I have a hump!!???"
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  #69  
Old 22-11-2013, 07:11 PM
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Originally Posted by Bassnut View Post
Thats just plain wrong!. ....
Took this with an AP RHA with a 16803 sensor an average night...OK with an AO-X as well....

No decon. A single 20 minute H-alpha exposure. Flat/Dark calibrated only (hence the noise/gamma rays)

Here is the link

(sorry to keep it web friendly, 66% res)

Fish bums and star tightness come to mind at the edges...
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  #70  
Old 22-11-2013, 07:13 PM
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Originally Posted by Bassnut View Post
.

So, ignoring skyglow, for the average punter, seeing is NOT the limiting factor, it can be suitably overcome with megadata and decon.

IMO other gear/setup factors count far more before seeing becomes an issue.
That is undoubtedly true.

But ..when a system is finally working well enough that it outperforms the seeing, you have reached an absolute limit. It doesn't matter how much better the scope is than that minimum requirement - once it has enough resolution to be seeing limited, physics steps in and that's the best quality data you will get from it or from any scope in the conditions, regardless of pedigree.

So, getting back to the original post, having spot sizes that are 1/8 those of an RC is not likely to make the slightest difference to image quality, since a well made RC will most likely be big enough and of sufficient resolution to be seeing limited over a reasonable field - and that will be as good as it gets.

Last edited by Shiraz; 22-11-2013 at 07:29 PM.
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  #71  
Old 22-11-2013, 07:27 PM
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Originally Posted by Peter Ward View Post
Took this with an AP RHA with a 16803 sensor an average night...OK with an AO-X as well....

No decon. A single 20 minute H-alpha exposure. Flat/Dark calibrated only (hence the noise/gamma rays)

Here is the link

(sorry to keep it web friendly, 66% res)

Fish bums and star tightness come to mind at the edges...

Stop making me so jealous.
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  #72  
Old 22-11-2013, 07:30 PM
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Originally Posted by Shiraz View Post
That is undoubtedly true.

But ..when a system is finally working well enough that it outperforms the seeing, you have reached an absolute limit. It doesn't matter how much better the scope is than that minimum requirement - once it has enough resolution to be seeing limited, physics steps in and that's the best quality data you will get from it or from any scope in the conditions, regardless of pedigree.

So, getting back to the original post, having spot sizes that are 1/8 those of an RC is not likely to make the slightest difference to image quality, since a reasonable RC will most likely be big enough and of sufficient quality and image scale to be seeing limited - and that will be as good as it gets.

Good post,
the best seeing I've had so far in Melbourne is a FWHM of 2.6 arc seconds.
I am using a Newt. + corrector & it doesn't seem worth upgrading
if that is the best seeing I can get.
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  #73  
Old 22-11-2013, 07:47 PM
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Originally Posted by alpal View Post
the best seeing I've had so far in Melbourne is a FWHM of 2.6 arc seconds.
....
During nights of "fuzz-ball" seeing, non-deformable (read affordable ) AO optics really don't help much.....but during "slow seeing" tip-tilt AO systems can and do make significant improvements to FWHM's.
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  #74  
Old 22-11-2013, 08:26 PM
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Quote:
Originally Posted by Peter Ward View Post
Took this with an AP RHA with a 16803 sensor an average night...OK with an AO-X as well....

No decon. A single 20 minute H-alpha exposure. Flat/Dark calibrated only (hence the noise/gamma rays)

Here is the link

(sorry to keep it web friendly, 66% res)

Fish bums and star tightness come to mind at the edges...
So what does that prove??, seeing doesnt matter that much??. You took this single non-deconed sub in Sydney skies right?. So, seeing can be beat with good gear/set up and AO. Thank you, so seeing isnt the limiting factor, obviously
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  #75  
Old 22-11-2013, 09:23 PM
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So what does that prove??, seeing doesnt matter that much??. You took this single non-deconed sub in Sydney skies right?. So, seeing can be beat with good gear/set up and AO. Thank you, so seeing isnt the limiting factor, obviously
Well...

I look at it this way....if an optical system is, to coin an old chestnut, is barely "diffraction limited" then *even the slightest* seeing distortion, by definition makes the images it produces, poorer than that limit.

If on the other hand, the optical system is better than "diffraction limited"
(yes... this is possible) it can suck up, for example, a 1/6th wave of atmospheric distortion before *total aberrations* take it into the 1/4 wave "diffraction limit"

Hope that makes sense
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  #76  
Old 22-11-2013, 11:42 PM
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I agree with Peter. He has expressed better than me but he certainly is pointing out the phenomena I have observed which was my point.

His RHA image says it all. You can't get those tiny stars with other gear. The other scope I have seen tiny stars like that is the infamous ASA 12. When it works its a tight machine.

There is apart from seeing and distortions the factor of scatter.

If your argument were true then there would be no point in getting those Ion Milled RCOS's that essentially had perfect figure. Yet have a look at those Rob Gendler images taken on a 14.5 inch RCOS with Ion Milled optics in WA on his astropic site.

They have very tight stars, show detail in commonly imaged southern objects you just don't see on images from this site except Martin's from Sierra Nevada.

So there is obviously another factor or factors that come into play than simply seeing limited, 1/4 wave limited etc. Scatter is one, sharp focus as Peter points out is another. Having had many scopes some definitely snap to focus better than others. Some have sharp focus all across the field and others super sharp in the centre and not so much in the corners etc. Addded to that would be deformations in the performance of the mirror/lens with different temperatures (Paul Haese and Bird can tell you about the importance of cooled optics) as well as deformations caused by stress from a lack of proper mirror support in the scope (as Rick Hedrick from Planewave was telling me, your 1/10th wave mirror once mounted may be way less than you think). So mechanical issues as above become very important factors in getting that sublime optical system which is quite a rare instrument. I guess that is why the high end stuff often seems overengineered like Tak scopes, AP scopes, etc.

Greg.
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  #77  
Old 23-11-2013, 07:33 AM
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Originally Posted by gregbradley View Post
If your argument were true then there would be no point in getting those Ion Milled RCOS's that essentially had perfect figure. Yet have a look at those Rob Gendler images taken on a 14.5 inch RCOS with Ion Milled optics in WA on his astropic site..
I've seen wavefront maps of these large RC surfaces before and after ion milling . The ion milling is more about honing down assymetries and non - rotationally symetrical defects in large surfaces to get them better than 1/4 wave and rotationally symmetric . Such localised polishing by hand would be very time consuming .A 32" F3 Hyperpola for an RC has a wavefront departure of over 120 waves on the wavfront from the base sphere. It is very hard to polish out this amount of glass without non rotationally symetric defects coming in .
A test of the combined wavefront with the highly aspheric secondary ( which also will have non- rotationally symmetric errors gives the operator of an ion milling machine a chance to smooth the wavfront better than 1/4 wave on these large surfaces , by milling down assymetries.

Robert Gendlers photos look good , not because the optics are 1/20 wave but becasue the combine wavefront error of the optical train could definately be better than 1/4 wave in terms of random - non symmetric errors.
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  #78  
Old 23-11-2013, 08:05 AM
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If on the other hand, the optical system is better than "diffraction limited"
(yes... this is possible) it can suck up, for example, a 1/6th wave of atmospheric distortion before *total aberrations* take it into the 1/4 wave "diffraction limit"

Hope that makes sense
Peter ,

I doubt that there is a single deep sky imager on this forum who is sampling the airy pattern with enough pixels or has near diffraction limited seeing with a mid size instrument to even put that theory to the test .

I don't think it is sufficient to point to experienced amateurs with high quality gear and say the differences are down to their having 1/10 wave rather than 1/4 wave optics , or that this extra quality is `money in the bank ' when it comes to all the different quality scopes trying to image a star that is say a real 1 arc second fuzzball in the sky .

Its worth noting that generally large observatory mirrors are specified by how much light they are expected to focus into X sub arc second spot, not by Strehl ratio.

The situation is made that more complex that smaller scale errors like 1/6 wave may be acroos 6 or 8" apertures but much greater over largere ones taking into account the tilt components between the cells .

My gut feeling is that fast optics will bloat more easily via the defocus component of the seeing error, but fast systems are the most likely to be well undersampled compared to the potential resolution of the telescope

The type of seeing is all important to . I'm sure there are many scientific papers around on the topic which are probably beyond our grasp .

There is some Hartmenn testing software around by SBig ( probably free if you ask nicely ) - it would be really interesting to see someone do some systematic measures at the beginning of their imaging session to see how often the real errors in their optical train are visible and compare to the FWHM they get with the actual images. I suspect most of the time you would be getting snapshots of the seeing .

Any takers? The question of `how good is good enough ' when it comes to amateur gear is one really worth looking into.
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  #79  
Old 23-11-2013, 08:25 AM
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I've seen wavefront maps of these large RC surfaces before and after ion milling . The ion milling is more about honing down assymetries and non - rotationally symetrical defects in large surfaces to get them better than 1/4 wave and rotationally symmetric . Such localised polishing by hand would be very time consuming .A 32" F3 Hyperpola for an RC has a wavefront departure of over 120 waves on the wavfront from the base sphere. It is very hard to polish out this amount of glass without non rotationally symetric defects coming in .
A test of the combined wavefront with the highly aspheric secondary ( which also will have non- rotationally symmetric errors gives the operator of an ion milling machine a chance to smooth the wavfront better than 1/4 wave on these large surfaces , by milling down assymetries.

Robert Gendlers photos look good , not because the optics are 1/20 wave but becasue the combine wavefront error of the optical train could definately be better than 1/4 wave in terms of random - non symmetric errors.


I thought that ion milling obtained 1/100th wave performance?

http://www.rcopticalsystems.com/tele...n_milling.html

Quote:
"Post figuring is a common procedure in modern glass optics.
The Keck telescope mirror segments, for example,
were polished to the l/20 (optical) level and then post figured to the l/100 level by ion milling"
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  #80  
Old 23-11-2013, 08:36 AM
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Originally Posted by gregbradley View Post
....

His RHA image says it all. You can't get those tiny stars with other gear. The other scope I have seen tiny stars like that is the infamous ASA 12. When it works its a tight machine..............

Greg.
Prior to getting the Honders, I had cursory look at other contenders like the ASA, Orion etc. that use Wynne correctors on their Newtonians. Yet it looked to me that the design still had residual field curvature, astigmatism, coma and chromatism.

The AP Riccardi-Honders, however has effectively perfect colour correction from 400 to 1000 nanometers, is fully corrected for spherical, coma, astigmatism, field curvature and distortion, as well as longitudinal and lateral chromatic aberration.

So I guess I'm one of those deluded nutters who waited years and parted with some serious bucks...
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