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Old 16-08-2014, 08:24 AM
glend (Glen)
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Mirror Quality for Imaging?

Here is a question for a rainy weekend. I while ago I saw a comment in a thread here that suggested that mirror quality in a newtonian was not of concern when it is used for imaging, as opposed to those used exclusively for visual observation. Can anyone elaborate on that statement? Is this true or false? Do mirror upgrades have any benefit for imaging Newts? I'll assume that they also get some visual use as well.

Last edited by glend; 16-08-2014 at 08:40 AM.
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Old 16-08-2014, 01:19 PM
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Originally Posted by glend View Post
Here is a question for a rainy weekend. I while ago I saw a comment in a thread here that suggested that mirror quality in a newtonian was not of concern when it is used for imaging, as opposed to those used exclusively for visual observation. Can anyone elaborate on that statement? Is this true or false? Do mirror upgrades have any benefit for imaging Newts? I'll assume that they also get some visual use as well.

To have diffraction limited results = 1/4 wavelength accuracy at the CCD chip-
you need a Newtonian with at least a 1/16 wave rms primary & secondary.

The light comes in & hits the 1/16th wave primary.
It's reflected wave equals 1/8th accuracy.

It then hits the secondary as 1/8th accuracy & reflects to the
CCD chip or eyepiece as 1/4 wave accuracy.

This is just enough to give full diffraction limited results.


Note: some of the now ( no longer produced ) RCOS scopes were ion milled to 1/100th wave accuracy.

To answer your question - good optics do matter.
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Old 16-08-2014, 01:40 PM
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On the same cloudy weekend...

The atmosphere will limit you to maybe 1.5 arcsec FWHM in excellent seeing on long exposure imaging (if you are lucky). There is not much point in getting optics that will do a whole lot better, since you will not see the difference. Average commercial 1/4 wave reflecting optics of 8 inches and above will do way better than 1.5 arcsec FWHM if properly mounted, so they are more than good enough - you will not get any better results with 1/12 optics than you would with 1/4 wave optics. Even if you have top notch optics, you will compromise that when you put in the necessary coma corrector. A good example is the MPCC coma corrector, used effectively by a lot of Newtonian imagers. It is likely that it turns diffraction limited optics into 2/3 wave optics, but even that is quite good enough in most seeing. If you have truly exceptional seeing, a better CC may just possibly be warranted, but 1/4 main optics will still be more than good enough.

Situation is different if you are observing visually or at high framerates and magnification - that is where better optics may pay off, since you can get fleeting bursts of good seeing that allow viewing or imaging near the diffraction limit. But even then you may only see some advantage if the mirror is cooled close to ambient to keep boundary layer effects at bay.

Then there is the difference in mirror mounting systems. If you buy an expensive scope, you should expect astigmatism, pinching etc to be sorted out, but a cheap OTA may have both in abundance - not because the optics are no good, but because the mounting systems are not up to scratch. Poor alignment stability may also make it more difficult to set up a coma corrector in a Newtonian, so scope mechanical quality definitely does matter.

Last edited by Shiraz; 16-08-2014 at 05:16 PM.
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Old 16-08-2014, 04:13 PM
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I agree that you won't be able to see much if anything between say a GSO mirror and a Royce mirror with a ccd but it's the mounting system that makes it so good.
What does make a big difference is the corrector used. I found the the Mk 3 MPCC very frustrating with my scope(Royce 10" f4). I changed to a type 2 Paracorr and it was instant goodness. It does slow the scope down to f4.6 but that a very good trade off for how well it performs.

https://farm3.staticflickr.com/2911/...3c71a20e_b.jpg
Corrector test by Raymond Collecutt, on Flickr
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Click for full-size image (Corrector testsmall.jpg)
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Old 16-08-2014, 08:49 PM
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Originally Posted by Rac View Post
I agree that you won't be able to see much if anything between say a GSO mirror and a Royce mirror with a ccd but it's the mounting system that makes it so good.
What does make a big difference is the corrector used. I found the the Mk 3 MPCC very frustrating with my scope(Royce 10" f4). I changed to a type 2 Paracorr and it was instant goodness. It does slow the scope down to f4.6 but that a very good trade off for how well it performs.

That's interesting.

I wonder what my Baader RCC coma corrector is doing then?

see here:
http://www.teleskop-express.de/shop/...-distance.html

ASA has more expensive ones here:

http://www.astrosysteme.at/eng/correctors_newton.html

I wondered if they would really be better or noticeable?

If I have good seeing in Melbourne I get say 2 arc seconds at the center
of the FOV & about 2.8 at the edge of the FOV with a KAF8300 chip.
I wonder if that's all I'll ever get?
If I went to high altitude to grab 1 arc second seeing then I suppose I could really test it.

cheers
Allan
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Old 16-08-2014, 09:11 PM
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Originally Posted by Shiraz View Post
On the same cloudy weekend...

The atmosphere will limit you to maybe 1.5 arcsec FWHM in excellent seeing on long exposure imaging (if you are lucky). There is not much point in getting optics that will do a whole lot better, since you will not see the difference. Average commercial 1/4 wave reflecting optics of 8 inches and above will do way better than 1.5 arcsec FWHM if properly mounted, so they are more than good enough - you will not get any better results with 1/12 optics than you would with 1/4 wave optics. Even if you have top notch optics, you will compromise that when you put in the necessary coma corrector. A good example is the MPCC coma corrector, used effectively by a lot of Newtonian imagers. It is likely that it turns diffraction limited optics into 2/3 wave optics, but even that is quite good enough in most seeing. If you have truly exceptional seeing, a better CC may just possibly be warranted, but 1/4 main optics will still be more than good enough.

Situation is different if you are observing visually or at high framerates and magnification - that is where better optics may pay off, since you can get fleeting bursts of good seeing that allow viewing or imaging near the diffraction limit. But even then you may only see some advantage if the mirror is cooled close to ambient to keep boundary layer effects at bay.

Then there is the difference in mirror mounting systems. If you buy an expensive scope, you should expect astigmatism, pinching etc to be sorted out, but a cheap OTA may have both in abundance - not because the optics are no good, but because the mounting systems are not up to scratch. Poor alignment stability may also make it more difficult to set up a coma corrector in a Newtonian, so scope mechanical quality definitely does matter.

So why were some of the now ( no longer produced ) RCOS scopes ion milled to 1/100th wave accuracy?
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Old 16-08-2014, 09:20 PM
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So why were some of the now ( no longer produced ) RCOS scopes ion milled to 1/100th wave accuracy?
no idea Allan. Maybe that RC design had very tight tolerances on one or other mirror profile - or maybe it was seen to be a good marketing point? For a Newtonian imager though, 1/4 wave is way better than the seeing will allow.
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Old 16-08-2014, 09:30 PM
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no idea Allan. Maybe that RC design had very tight tolerances on one or other mirror profile - or maybe it was seen to be a good marketing point? For a Newtonian imager though, 1/4 wave is way better than the seeing will allow.
I don't know really.

I would tend to go for the best optics I could afford.

Some of those RCOS ion milled 14.5" telescopes made such good pics:

http://gallery.rcopticalsystems.com/gallery/m83.html

http://gallery.rcopticalsystems.com/gallery/ngc253.html
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Old 16-08-2014, 09:54 PM
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I'm a great believer in the idea that "good enough is". If you cannot get any noticeable improvement by going to better optics, why do so - spend the money where it makes a difference, such as on a quality tracking mount or access to a remote high altitude site.
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Old 16-08-2014, 10:17 PM
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I'm a great believer in the idea that "good enough is". If you cannot get any noticeable improvement by going to better optics, why do so - spend the money where it makes a difference, such as on a quality tracking mount or access to a remote high altitude site.

Well certainly an RCOS ion milled 14.5" telescope would be wasted
in the suburbs of Melbourne.
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Old 17-08-2014, 02:13 PM
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Originally Posted by glend View Post
Do mirror upgrades have any benefit for imaging Newts? .
At typical focal ratios, the focal plane is sufficiently under sampled by any ccd that you are likely to encounter that there is no noticeable improvement to be gained by a mirror more accurate than 1/3 wave.
GSO mirrors are typically much better than that so are as good as you need. The issues surrounding cheap tube assemblies are related to deficiencies in the mechanical design (not the optics)
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Old 17-08-2014, 05:28 PM
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At typical focal ratios, the focal plane is sufficiently under sampled by any ccd that you are likely to encounter that there is no noticeable improvement to be gained by a mirror more accurate than 1/3 wave.
GSO mirrors are typically much better than that so are as good as you need. The issues surrounding cheap tube assemblies are related to deficiencies in the mechanical design (not the optics)
You know, thats a well put understated definitive conclusion thats should solve all endless arguments about optics. Ive compared subs Ive taken with my Meade 12" LX200R and an RCOS 10" in urban skies. With the LX200R in perfect focus and colomation and perhaps a bit of decon with decent data, I cant tell the difference between them apart from the extra depth from the 12".

The huge difference is in the tube mechanical stability, I focus the RCOS every 2 weeks or so and the secondary focuser/thermal management (fan/heater control) is insanely accurate and reliable. I havent colomated the RCOS in years.

If your willing to put the effort (im not) into countering these mechanical deficiencies, then you can save a lot of money for the same result. The old rule applies, less money, more effort for the same result, its doable.
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Old 17-08-2014, 05:31 PM
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There is still a nagging question:

So why were some of the ( now no longer produced ) RCOS scopes ion milled to 1/100th wave accuracy?
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Old 17-08-2014, 05:42 PM
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There is still a nagging question:

So why were some of the ( now no longer produced ) RCOS scopes ion milled to 1/100th wave accuracy?
Really, really dark and stable skies
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Old 17-08-2014, 06:34 PM
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For all those that could afford to send one into space and have their own personal mini-Hubble.
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Old 17-08-2014, 07:20 PM
glend (Glen)
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There is still a nagging question:

So why were some of the ( now no longer produced ) RCOS scopes ion milled to 1/100th wave accuracy?
Perhaps that's why they are no longer produced, assuming they were priced accordingly.
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Old 17-08-2014, 07:27 PM
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There is still a nagging question:

So why were some of the ( now no longer produced ) RCOS scopes ion milled to 1/100th wave accuracy?
For the same reason people buy ultra high end audio cable. They believe that buying the highest cost components is the optimum solution. (A false premise)
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Old 18-08-2014, 06:37 AM
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For the same reason people buy ultra high end audio cable. They believe that buying the highest cost components is the optimum solution. (A false premise)
So - you think that it 's all bragging rights then?

I think if I searched the internet I'd find comparisons that would disprove your theory.
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Old 18-08-2014, 06:38 AM
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You know, thats a well put understated definitive conclusion thats should solve all endless arguments about optics. Ive compared subs Ive taken with my Meade 12" LX200R and an RCOS 10" in urban skies. With the LX200R in perfect focus and colomation and perhaps a bit of decon with decent data, I cant tell the difference between them apart from the extra depth from the 12".

The huge difference is in the tube mechanical stability, I focus the RCOS every 2 weeks or so and the secondary focuser/thermal management (fan/heater control) is insanely accurate and reliable. I havent colomated the RCOS in years.

If your willing to put the effort (im not) into countering these mechanical deficiencies, then you can save a lot of money for the same result. The old rule applies, less money, more effort for the same result, its doable.

Do you have some results that you could post please?

That would be only stretched examples - no other processing & at full scale.
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Old 18-08-2014, 06:57 AM
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If the CCD is the limiting factor why is there a market (apparently) for $30k RCs like Officina Stellare when there are pretty good ones available for under $8k? Other than the snob factor or the need to have a red one? Probably the same thought process that influences luxury car purchases, because they can. People are free to spend their money as they please.
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