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  #41  
Old 28-04-2012, 07:35 PM
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Quote:
Originally Posted by bmitchell82 View Post

I might as well just get the bog stock standard run of the mill mass produced optics at 1/4 of the cost!

Could somebody put it in terms that are quantifiable and not in high end optician language?
The issue of whether optical quality is justified for deep sky imaging is intimately bound up by the choice of camera and pixel size vs airy disc size. If you are using your eyeball, high quality optics will always win out as your eyes are capable of seeing the moments of clarity in even average seeing Planetary imagers who try to over sample the airy disc significantly with pixels will be fairly sensitive to mirror quality.

My attitude is that the lines of optical quality are probably more 'blurry' for prime focus imaging at faster f #'s but it helps to have good optics, but good optics will not help if you can't focus or guide properly.
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  #42  
Old 28-04-2012, 07:40 PM
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Quote:
Originally Posted by DavidU View Post
Here is one of Rohr's optical tests. This is a superb mirror in all respects , now check out the resolution it is capable of in the 3.3u artificial stars test.In there is also the star resolution test of a GSO mirror and a few others. It clearly shows fine resolution is better with an extremely good mirror.
http://translate.googleusercontent.c...M0Kw#post30967
I'm pretty sure the advantage of good optics isn't under question, my understanding of the original post was whether you could actually translate that superior resolution into a better image in real world conditions. Peter Ward has demonstrated what is possible with good optics, and I suspect more importantly, good seeing. It would be nice to know how close his image approaches the theoretical perfomance of the system, or would a c14 have done just as well that night? Would a 'cheapy' 20" have done even better (notwithstanding the challenges of mounting larger instruments)?
cheers,
Andrew.
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  #43  
Old 28-04-2012, 07:54 PM
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Quote:
Originally Posted by Satchmo View Post
The issue of whether optical quality is justified for deep sky imaging is intimately bound up by the choice of camera and pixel size vs airy disc size. If you are using your eyeball, high quality optics will always win out as your eyes are capable of seeing the moments of clarity in even average seeing Planetary imagers who try to over sample the airy disc significantly with pixels will be fairly sensitive to mirror quality.

My attitude is that the lines of optical quality are probably more 'blurry' for prime focus imaging at faster f #'s but it helps to have good optics, but good optics will not help if you can't focus or guide properly.
I have no issues of guiding for how ever long I want and regularly see less than .5 px deviation at 5.4 micron pixel size. Focus well... here you can see i know how to focus too. Hence why the original post had me very interested as soon I will be upgrading my old Synta 10"

Quote:
Originally Posted by alocky View Post
I'm pretty sure the advantage of good optics isn't under question, my understanding of the original post was whether you could actually translate that superior resolution into a better image in real world conditions. Peter Ward has demonstrated what is possible with good optics, and I suspect more importantly, good seeing. It would be nice to know how close his image approaches the theoretical perfomance of the system, or would a c14 have done just as well that night? Would a 'cheapy' 20" have done even better (notwithstanding the challenges of mounting larger instruments)?
cheers,
Andrew.
Mid this year I am going to the Hallowed lands of unbelievable seeing! If i knew just how to make this measurement I would tell you.
Mark do you know how to preform this test?
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  #44  
Old 29-04-2012, 10:12 AM
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Quote:
Originally Posted by Peter Ward View Post
Beg to differ with Clive's analysis.

With imaging it comes down to field correction, spot size and seeing. A well made Riccardi-Honders delivers 5 micron stars across around a 70mm flat field. Most other systems have spot sizes 2x larger on axis, and
degrade to *way more* off axis due to field curvature, astigmatism etc.

Other factors such as thermal and mechanical stability also come into play, and literally shift focus as an exposure is being taken. Exacting focusers and thermally stable materials usually come at a cost. My experience so far has been, you get what you pay for.

Peter,
I think are getting the wrong end of the stick here.
I used the Riccardi Honders as an example because it is without question as good as it gets for deep sky imaging, but let's be realistic about the factors that make it so. Even a perfectly constructed OTA (of any configuration) that has a 50% central obstruction is in reality no better than 1/2 wave once you include the effects of diffraction.

Ergo, optical quality really isn't the weakest link in most prime focus deep sky imaging chains. This is what the OP was asking. Field aberrations, mechanical construction, mount stability, seeing conditions, tracking error, operator skill, etc) are different issues and are really what separate the sheep from the goats.

fwiw) Roland does not suggest that a Riccardi-Honders delivers 5 micron images (which it cannot) What he says is that a 12" f3.8 produces an airy disk with a diameter of 5 microns, and that his OTA is diffraction limited over a 60mm field. It is left to the educated reader to fill in a couple of blanks: ie)
1) Only 50% of the encircled energy will actually fall within the airy disk, most of the rest being pushed into the first diffraction ring with a diameter of 10 - 20 microns depending on the wavelength... there is your limit (diffraction)
And 2) the point I was trying to establish.... That spec is entirely good enough for prime focus deep sky imaging at the highest level. The results speak for themselves.

best,
~c

Last edited by clive milne; 29-04-2012 at 10:41 AM.
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  #45  
Old 29-04-2012, 10:39 AM
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Quote:
Originally Posted by Peter Ward View Post
Proof?? the M104 image I took some years ago with great optics in superb seeing. The link is here

That is truly an excellent image Peter... I think the point that you have just established is that it is possible to take a world class image with an OTA that has the equivalent of 1/4 wave spherical aberration.

(An RC with a 32% central obstruction has exactly that)

Click image for larger version

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ID:	114179
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  #46  
Old 29-04-2012, 11:12 AM
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Quote:
Originally Posted by bmitchell82 View Post
Okay now im lost!

On one hand people are saying that commercially made optics are just fine as all this PV measurements are just a load of horse whooharr

Could somebody put it in terms that are quantifiable and not in high end optician language?

My feeling is that good quality optics are worth the extra cost.
If you are using the scope as a visual instrument, then it's an absolute no brainer.
For imaging the merit function is a little different, but let me put it to you like this.... Just as an example, if you wish to get something around the 12" mark using a 16803 chip, the mount is going to cost you $10-20K, the camera a similar amount (you are up around the $20-40K mark already) The cost differential between OTA's is such that it isn't that much extra to go for something decent in the overall scheme of things.

The mechanical stability of the higher end products is probably worth more than anything else.

But you know... let's dispense with the idea that stratospheric strehl ratios quoted for instruments with large
central obstructions is anything other than an exercise in marketing hype.

Last edited by clive milne; 29-04-2012 at 11:25 AM.
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  #47  
Old 29-04-2012, 12:08 PM
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Quote:
My feeling is that good quality optics are worth the extra cost.
As I said in my first post, this is a personal value judgement we each have to make. I guess that why this such an interesting discussion to follow.
In my case I put the mount & camera ahead of the optics. Hence my current combination is PME + STL + GSO 300mm f/4. Feel free to have a look at a fairly 'honest' example or here for the full list of this combination.
In my assessment the things holding me back from better images would be (in rough order) the weather, light pollution, seeing, my skills, focus and collimation. Better optics would be nice, but it's down the list. For now I'll just pretend its as good as the test report posted by Dave.
James
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  #48  
Old 29-04-2012, 01:05 PM
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Nice work James... Looks like you have got that 12" working well.

Your report on the OTA mirrors my experience with GSO. The optics in the smaller sizes (12" and below) are actually pretty reasonable, and are certainly good enough for imaging. It amazes me that they haven't lifted their game with the mechanical components though. I also wish that Andrew's would spare us the marketing B.S. I don't imagine that too many people would take it seriously.

btw) Absolutely nothing wrong with this image:
http://deepspaceplace.com/images/ic4628.jpg

Well done.
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  #49  
Old 29-04-2012, 02:39 PM
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Quote:
=Moon;846803 For now I'll just pretend its as good as the test report posted by Dave.
I wonder who made that mirror. Its certainly not a GSO being 2007.
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  #50  
Old 29-04-2012, 04:42 PM
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Quote:
Originally Posted by clive milne View Post
That is truly an excellent image Peter... I think the point that you have just established is that it is possible to take a world class image with an OTA that has the equivalent of 1/4 wave spherical aberration.

(An RC with a 32% central obstruction has exactly that)

Attachment 114179
Well not quite, just because the airy disk energy is, well, not in the disk and has moved to the first ( or more ) diffraction ring doesn't mean the spot sizes will be bloated.

A modulation transfer function only tells part of the story....as does a ray trace which I think has more relevance to deep sky imaging.....

Differences in deep sky images from telescope with a figure error (eg 1/4 spherical) and one with none is obvious....with the latter putting all the energy into a small spot ( albeit larger than the "perfect" airy disk) rather than smearing it over tens of microns due the figure error.

One need look no further than Hubble images pre and post servicing missions to better understand my point....the secondary obstruction remained the same
but the figure and images were sooo much better

Last edited by Peter Ward; 29-04-2012 at 08:00 PM. Reason: Typo
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  #51  
Old 29-04-2012, 05:31 PM
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It is a hand made European mirror, possibly Italian or French.
Quote:
Originally Posted by Satchmo View Post
I wonder who made that mirror. Its certainly not a GSO being 2007.
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  #52  
Old 29-04-2012, 06:38 PM
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Quote:
Originally Posted by Moon View Post
Better optics would be nice, but it's down the list. For now I'll just pretend its as good as the test report posted by Dave.
James
James,

no need to pretend or guess. That is why Roddier invented his test. Capturing extrafocal images of a star is rather trivial nowadays, and I'm quite good at running the test.
Next best thing to a proper interferogram, that's for sure. No guessing involved one bit.

(attached analysis of Stefan's 16" D-K in average seeing)
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  #53  
Old 29-04-2012, 06:46 PM
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Thanks for reminding me about that Bratislav. I actually downloaded the software & joined the yahoo group but never got round to it.
I must do it next time I have the camera off.
James

.
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  #54  
Old 29-04-2012, 11:10 PM
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Quote:
Originally Posted by clive milne View Post
...Even a perfectly constructed OTA (of any configuration) that has a 50% central obstruction is in reality no better than 1/2 wave once you include the effects of diffraction.

.........
...............Roland does not suggest that a Riccardi-Honders delivers 5 micron images (which it cannot) What he says is that a 12" f3.8 produces an airy disk with a diameter of 5 microns, and that his OTA is diffraction limited over a 60mm field.
I understand the modulation transfer function to only be a measure of frequency preservation....hence and not sure what you mean by saying a 50% obstructed telescope is like having a telescope with 1/2 wave of spherical error... hence.... I still don't agree with your analysis

That much spherical error looks bad visually and easily shows up in images. (I've had some clunkers in my time )

You are indeed correct about Roland's spec claims... but I did some extra research prior to getting the scope, and the Honders has spot diagrams well within the diffraction limit across a 60mm field (attached)

I'd be interested to see a ray trace of a 1/2 wave spherical error....but so far I've not manage to track any down.
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  #55  
Old 30-04-2012, 08:23 AM
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Many valid points have been raised, and surely a Riccardi-Honders is great in practice with its large fully corrected field, but isn't that design mostly of benefit in wide field applications?

I found some numbers to shed a bit more light on my original question:
Median FWHM of Paranal site: 0.75"
Median FWHM of Kitt Peak site: 1.10"
Airy disk size for 8000mm (VLT) aperture: 0.03"
Airy disk size for 250mm aperture: 1.03" = 6.2microns @ f/5

So the conclusion from that must be that atmospheric seeing is by far the limiting factor for a site like VLT, effectively reducing the theoretical resolution by a factor of 25.
For my location, which isn't on a remote mountain top with premium seeing conditions, I'd expect the average achievable FWHM to then be around maybe 2.00". I'm just guessing here, but it must surely be worse than the professional sites. So that means in my case the atmospheric seeing reduces my theoretical resolution by a factor of ~2. The question then is wether a GSO 250mm mirror is good enough to deliver star sizes no larger than the average seeing of 2.00"? and based on my experience I think it would be good enough, since with my old no-name mirror I can see much greater detail that this visually when looking at Jupiter for example.

So simply in the context of 'which f/5 mirror to use for long exposure deep sky inaging', I think a GSO would more than suffice based on what I can gather here.

In any case this is a very interesting discussion overall, keep it coming!

Last edited by SkyViking; 30-04-2012 at 09:22 AM.
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  #56  
Old 30-04-2012, 09:13 AM
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Quote:
Originally Posted by Peter Ward View Post
I'd be interested to see a ray trace of a 1/2 wave spherical error....but so far I've not manage to track any down.
Peter, I think a 50% obstruction produces modification of the Airy pattern similar to a Strehl ratio equivalent to 0.5 wave or so spherical aberration in a perfect system. When you get down near the diffraction limit geometric ray trace diagrams are of intellectual interest only because of course the real image is subject to diffraction effects and the Strehl ratio tells us a lot more . I'll look into it for you.

Off the top of my head the geometric ray spot bundle at best focus for a 1/8 wave of pure spherical aberration optic ( wavefront ) is about half the theoretical airy disc diameter , and a 1/4 wave wavefront system has all it rays within 3 Airy Disc diameters.

Casual onlookers must keep in mind that in general discussions about wavefont quality vs utility for visual or imaging we are always talking about simple pure spherical aberration at those tolerances ( to keep things simple ) , and not other kinds of errors, which may or may not have similar effects on the Strehl ratio. The famous 1/4 wave `Rayleigh Criteria' which many mistakenly call `diffraction limited' referred only to pure spherical aberration, and even then was really a generally tolerable lower limit. Lord Rayleigh added that the effect on visibility of planetary detail was `already decidedly prejudicial ' at 1/4 wave.

I personally define the meaning of Diffraction Limited to mean that the only disturbance to the Airy Pattern attributable to the optics being the diffraction effects itself, and that won't happen until spherical aberration is reduced to 1/8 to 1/10 wave in the system . This caveat was first expressed by Danjon and Couder in the 1930's. Unfortunately few optics display SA in its pure form , but have generally more complex errors. In all the commercial optics I've looked at I find figure of revolution problems to be the greatest issue, and pure astigmatism itself is rare in favour of more complex irregular shapes.
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  #57  
Old 30-04-2012, 10:02 AM
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Originally Posted by Peter Ward View Post
I understand the modulation transfer function to only be a measure of frequency preservation....hence and not sure what you mean by saying a 50% obstructed telescope is like having a telescope with 1/2 wave of spherical error... hence.... I still don't agree with your analysis

That much spherical error looks bad visually and easily shows up in images. (I've had some clunkers in my time )

You are indeed correct about Roland's spec claims... but I did some extra research prior to getting the scope, and the Honders has spot diagrams well within the diffraction limit across a 60mm field (attached)

I'd be interested to see a ray trace of a 1/2 wave spherical error....but so far I've not manage to track any down.

There is no need to simulate 1/2 wave of spherical, as all modern raytrace programs will be able to easily show the results of 50% of obstruction.

I don't have access to Riccardi's variant used in AP, and can't be bothered to modify my existing designs based on Honders' original ideas, but have another similarly capable design handy (Companar at 12" f/3.3, 50% obstruction). See attached spot diagrams for spot diagrams from zero to 2 degrees off axis (full 70mm circle) and corresponding diffraction affected PSF plots on axis and at 2 degrees off axis (apologies for low quality gifs, don't have access to all my tools at the moment). For PSF diagrams, square dimension is ~18 microns; those diffraction rings are bright enough to swell the calculated 4 micron RMS into 15 micron blur for anything but faintest stars.

I've exchanged many emails with Claas Honders in late 90's and finally managed to convince him to get design into the open, but unfortunately his untimely death put stop to that. Busack now claims to be the earliest inventor of such designs, but without Claas to tell us when did HE come with such ideas, we will never know.
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Last edited by bratislav; 30-04-2012 at 10:42 AM.
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  #58  
Old 01-05-2012, 01:46 AM
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There is no need to simulate 1/2 wave of spherical, as all modern raytrace programs will be able to easily show the results of 50% of obstruction....
Guys...just so we are clear about the point I'm making....

An otherwise perfect optic with a 50% central obstruction does not degrade a telescope's resulting images in the same manner as a 1/2 wave spherical error....or turned edge or...or astigmatism etc. etc.... i.e the sort of errors you may find in an optic built to a price, rather than spec.

(download Aberrator...it's free...& play with the values )

One is a frequency domain filter (eg central obstruction), the other is a focus error.....which really will bugger up images, al la Hubble before it was fixed.
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  #59  
Old 01-05-2012, 06:31 AM
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Hi Peter, the central obstruction is indeed a frequency filter but it does seem to affect a telescope's ability to resolve contrast.

There is a fairly comprehensive analysis here: http://www.telescope-optics.net/tele...bstruction.htm

There is also Thierrys Legault's page here: http://legault.perso.sfr.fr/obstruction.html
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