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Old 20-04-2012, 08:38 AM
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SkyViking (Rolf)
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Question How important is mirror quality for deep sky imaging?

Hi All, I was wondering how accurate does a primary mirror need to be for deep sky photography?
I'm thinking that for long exposures the accuracy of the optical surface just need to be good enough to achieve a resolution equal to that limited by the average seeing? (Well actually more like 1/3 of seeing, to get good sampling)
This would typically still be less stringent than 'diffraction limited' I suppose.

The reason I ask is because my primary mirror really needs a resurfacing, and local options here in NZ are limited especially when I also want a good protective overcoating. The cost of packing and shipping the mirror overseas, plus recoating, would most likely be higher than a new standard 10" mirror (for example one of the Bintel ones). And I would be without a mirror for a while too (oh the horror... )

My current mirror is just a no-name one that I bought 17 years ago as part of a kitset, with sonotube, cheap focuser etc. I have no idea of how good it is, but doubt it is anything special. But it does produce images that are fine for my purpose. As you may know I have taken images of various high resolution features over the years so the mirror definitely delivers. I'm just thinking that a cheap standard mirror would be just as good, or maybe even better? But I really have nothing to compare with as I have never owned another mirror.

So does anybody know if it really matters - is there perhaps a way to calculate the resolution expected from a mirror, given its properties such as strehl etc? In short, is a high end mirror really worth it for deep sky imaging?

Regards,
Rolf

Last edited by SkyViking; 20-04-2012 at 09:27 AM.
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Old 20-04-2012, 01:37 PM
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alocky (Andrew lockwood)
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In the days of film astrophotography that was certainly the wisdom: you don't need 1/8th wave figures to get good astrophotos. That will still apply where there is any sort of long exposure, but the advent of adaptive optics means that the optical performance will become more important. Planetary imaging with a high resolution video certaily shows up defects in the optics, the degradation due to slightly miscollimated optics certainly shows up, and I suspect so would a bad figure.
It all depends on the arc-second per pixel spec of your current imaging system, and your typical exposure length.
regards,
Andrew.
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Old 20-04-2012, 01:54 PM
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SkyViking (Rolf)
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Hi Andrew, yes planetary imaging is a whole different story for sure. I'm specifically thinking of deep sky imaging with exposures on the order of minutes or more, where the seeing effectively limits the resolution.

My native resolution is 0.86"/pixel and with that I seem to get FWHM's in the range of 2"-3" depending on seeing conditions, which seems quite reasonable to me. So perhaps my question really is how accurate does a mirror have to be in order to still achieve the same FWHM with that image scale?
When I image Jupiter using high FPS I can easily record sub-arsecond details, so that's why I assume that the accuracy of my (probably very average) mirror is clearly higher than necessary for deep sky imaging.

Last edited by SkyViking; 20-04-2012 at 03:39 PM.
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Old 20-04-2012, 02:08 PM
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Hi Rolf,
I had the same question as well and didn't really get an answer anywhere about practical differences.
I bought mine from Orion optics uk, and they have 4 grades from 1/4pv wavefront to 1/10pv. as for how much of a practical difference it makes with deep sky imaging, I'm very curious to know as well.
I asked them this question and they were more than confident that even their professional grade would yield far better results than standard GSO mirrors for DSO's. so maybe it does make a difference.
there is a good write up here http://www.orionoptics.co.uk/OPTICS/...topticspa.html
I will be testing my 10" soon with the same camera that I used with the GSO mirror. will be interesting to see the difference. I'd be very disappointed if there isn't much.
But generally it makes a larger difference with planetary imaging.

would love to hear what others would have to say.
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Old 20-04-2012, 02:12 PM
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Diffraction limited at 1/4 wave. Beyond that and the atmosphere won't let you see the difference unless you are on top of a mountain (and no mountain in Oz either!). Planetary high mag though, not so sure.


Greg.
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Old 20-04-2012, 04:04 PM
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Shiraz (Ray)
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good question Rolf. Best quality optics is always worth having, but your question is - is top quality necessary when the atmosphere dominates?.

FWIW, my current thinking:
1. highest possible quality is required for visual use - we are very good at seeing anomalies and can pick up the effects of relatively minor aberrations very efficiently (particularly if we are unfortunate enough to know what to look for). Assume that is why people still pay big bucks for tiny but perfect refractors for visual use.
2. system requirements dominate for planetary - collecting lots of photons is a large part of the game and a big, but less-than-perfect optic may outperform a smaller perfect one, since it allows higher frame rates to help with seeing problems. Other issues such as thermal management and mirror cell design for equatorial mounting are also vitally important. I have only once found conditions good enough to test the limits of my GSO 12 inch optics over a full year of planetary imaging - but it would still be nice to have top quality optics though, just in case that mythical perfect night comes along.
3. optical quality is much less important for deep space imaging. The atmophere limits the resolution much more than the optical system and you can correct for many defects such as lower contrast etc in post processing - which explains why RC systems are popular, even though they start out with low Strehl simply because they have such large secondaries. The main optical problem for DSO imaging is astigmatism, since out-of-round stars will remain somewhat out-of-round even after convolution with the atmospheric psf - but astigmatism is generally caused by mirror mounting problems rather than the mirror, so you should be able to correct this type of fault. Minor diffraction can also influence saturated star shapes, since you see the skirts of the saturated PSF and any minor sprays of light from intrusions into the light column will distort the star shapes, even though unsaturated stars look OK.

My guess is that you will find that a GSO mirror is great for DSO imaging. Interested to hear what others think. Regards ray.

Last edited by Shiraz; 20-04-2012 at 11:36 PM.
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Old 20-04-2012, 04:21 PM
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Finding this a fascinating discussion. I suppose theoretically you would need an occasional exceptional night of seeing/clarity to see the difference with a mirror better than 1/4 wave from what people are saying for DSO imaging.

I'd always wondered if my chinese mirror was a "goodie", but probably in reality it's 90% seeing limited as people say (and Brissie is never going to put Mauna Kea out of business)

Last edited by RobF; 20-04-2012 at 08:41 PM.
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Old 20-04-2012, 04:31 PM
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Quote:
In short, is a high end mirror really worth it for deep sky imaging?

Good question, but I don't know if there is an answer since it depends on your personal value judgement.
I use a GSO mirror and my images look ok to me, but I notice people with more expensive mirrors often times take much better images. Of course there are additional factors involved, including the skill of the operator!
I would say have a good look on the internets at all the images taken with GSO mirrors and set your expectation level accordingly.
If you want to go down the premium path, you could consider a Zambuto swap over. This is something I might consider one day too.
Just my 2 cents worth.
James
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Old 21-04-2012, 01:12 AM
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One way to answer this is what is the mirror accuracy of these GSO RCs?

I doubt its even 1/4 wave but it must be fairly close. There plenty of sensational images using them.

I imagine (I don't know for a fact) that a computer controlled polishing machine can most likely take a mirror close to 1/4 wave without too much trouble but to get it further requires more expensive, more precise machines and probably a fair bit of human labour and work to get that last little errors smoothed out.


Greg.
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Old 21-04-2012, 10:44 PM
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Hi again Rolf.

Looks like I was wrong in an earlier post to suggest that optical quality was not such a major issue for DSO imaging. I did a quick and dirty analysis using Aberrator to generate PSFs for a variety of scopes and then convolved these with a 2 arc sec Gaussian (in IRIS) that represents good seeing averaged over a fairly long integration time. The top row of the diagram represents the resulting combined PSFs (star shapes) for scopes from 80 to 300 mm, with brightness normalised and assuming perfect optics and with identical angular scale. The chosen scopes are a couple of refractors, three Newts and an RC. For scale, the 300mm Newt produces pretty close to a 2 arc sec combined PSF (half power).
The two PSF images below the 250 f5 entry are a 250 f5 with a mirror that only meets lambda/4 and the lower one includes some pinching and tube currrents as well.

the whole pattern is repeated lower down, but with the synthetic stars driven into saturation by a factor of 2x.

So:
1. for 2 arc sec seeing, you get no resolution increase above about 250mm aperture, but even the 80mm refractor does a pretty good job.
2. moderate spherical aberration results in larger stars from the 250mm Newt - the half power area is ~20% larger at lambda/4 than with perfect optics. Looks like optics with low SA are definitely worth having in this application as well, if you are imaging in above average seeing conditions.
3. tube currents and improper mirror mounting can really ruin the image

These results are a bit unexpected and I am now going to re-read the relevant parts of Suiter's book to make sure I have it somewhere near right with Aberrator - maybe someone else can try something similar as a check.
Regards ray
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Last edited by Shiraz; 23-04-2012 at 05:28 PM.
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Old 23-04-2012, 01:50 PM
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Higher end mirrored scopes often quote spot sizes in microns for centre and for a certain distance off centre.

That is another way to evaluate star sizes you will get.

Greg.
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Old 24-04-2012, 11:52 AM
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SkyViking (Rolf)
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Thanks to everyone for your valued input on this. I've decided to get a GSO mirror - they sell on Astronz for NZ$375 for a 10" f/5, I just can't really go wrong with that price and then I won't be without a mirror either (Imagine if Eta Carinae went bang while I was waiting for my mirror to be recoated... )

I've also just installed a new 70mm secondary from Bintel so that's taken care of too. I believe the Bintel mirrors are GSO as well?
It will be great to once again have nice shiny optics throughout, should have done this years ago...

I'll let you know how it goes with the new mirror once I get my hands on it.
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Old 24-04-2012, 12:18 PM
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and then I won't be without a mirror either (Imagine if Eta Carinae went bang while I was waiting for my mirror to be recoated... )
Sounds like a plan Rolf
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Old 24-04-2012, 02:59 PM
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You know for most of the time when your seeing is pretty average then I think the wisdom that has been told is spot on, but even here in Perth I can see where my mirror lacks in clarity.

On crystal clear, still winter nights my mirror isn't far off atmospherics. 0.89 arc sec per pixel.

My train of thought is the errors of the PV across the mirror don't "Hide" under the seeing and to some effect are additive, think of it this way if your Mirror was 100% flat, no peaks or vallys (extreme case) then the atmospherics would 100% be the cause of all errors, but if your adding in the light that comes though the aperature is already distorted then it hits the side of peak/vally it will not come back dead straight so its added to the error.

Why would you buy a RCOS telescope for its mirrors why would you get ION milled RCOS mirrors? when GSO sell the same?

Even looking at Mikes ETA and mine side by side, the Qhy9 has 5.4micron pixels Vs 9 micron pixels. The Arc second per pixel isn't too different. but there is a noticable change in resolution. I put it down to higher quality optics.
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Old 25-04-2012, 09:35 AM
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A couple of thoughts:

Missing from the discussion so far is the limitations imposed by the (CCD) detector.

To extract all the information at the focal plane you will need to oversample the image by at least a factor of 2x
(basic nyquist sampling theorem)

However, the following caution also needs to be included:
Do not assume that a CCDs sampling rate is defined by its pixel size.
A real world example of what I am alluding to can be found at the bottom of page 11 on the following PDF:
http://www.sta-inc.net/wp-content/up...-Astronomy.pdf
In this example, when a 2 micron star image was focussed on to an 9 micron array, 90% of the image was
registered by the target pixel, the surrounding pixels acquired the bulk of the remainder, rendering
it as a 27 x 27 micron cross. (13.5x the size of the original image) This is not due to the central pixel's well
depth being exceeded.

Implicitly therefore, for mirror quality to become a significant limitation in deep sky imaging, you need to
be over-sampling to a significant extent. The magnifications used by planetary imagers might serve as a data point here.

The bottom line is that if you put an ST10 at the prime focus of an 8" F5 GSO Newtonian equipped with a coma corrector,
the resulting spatial resolution in the image will be indistinguishable from what you would get with a 5" f8 Astrophysics
refractor, all else being equal (not withstanding the diffraction spikes & better image depth with the GSO).

best
~c
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Old 25-04-2012, 10:38 AM
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A hypothetical question to perhaps lend some context....

What is the first thing that comes in to your mind when you hear the words; Riccardi Honders...
I'll bet London to a brick that it would be something along the lines of; 'ultimate imaging tool... want.. might consider trading internal organ to acquire'

Now consider the effect of its 50% central obstruction and what level of spherical aberration
constitutes an equivalent offence to the PSF. fwiw) It is waaaaaaay more than 1/4wl of SA.

ergo.. for prime focus deep sky imaging, the only purpose that 1/10th wave optics serve is to gratify your ego.
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Old 26-04-2012, 02:15 AM
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Quote:
Originally Posted by clive milne View Post
The bottom line is that if you put an ST10 at the prime focus of an 8" F5 GSO Newtonian equipped with a coma corrector,
the resulting spatial resolution in the image will be indistinguishable from what you would get with a 5" f8 Astrophysics
refractor, all else being equal (not withstanding the diffraction spikes & better image depth with the GSO).

best
~c
Clive,
How about two 8" reflectors, one with a 1/4 PV wavefront spec, and other a 1/10 PV wavefront spec with the same secondary?
if mirror quality doesn't lend discernible differences for DSO's, does ccd quality be it build or sensor quality make more of a difference?
eg,similar spec'd qhy, sbig, FLI or other high end cameras? or is it sensor size similar to aperture with telescopes? bigger sensor, more detail?
I'm referring to a raw capture without any processing?
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Old 26-04-2012, 08:57 AM
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Hi Rolf, will be very interested to find out how the new mirror performs.

Clive, thanks very much for the info - never seen specs for lateral charge diffusion (assume that is what it is) published for any astro sensors, except maybe Kodak's claim to produce "low smear" chips.
This could possibly explain why planetary imagers find that going well beyond Nyquist sampling requirements yields additional detail - maybe we are using oversampling to deal with sensor diffusion blur. Heavy handed oversampling may also be worth pursuing for DSO resolution improvement as well. Regards ray

Last edited by Shiraz; 26-04-2012 at 10:16 AM.
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Old 26-04-2012, 08:53 PM
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Originally Posted by alistairsam View Post
Clive,
How about two 8" reflectors, one with a 1/4 PV wavefront spec, and other a 1/10 PV wavefront spec with the same secondary?
if mirror quality doesn't lend discernible differences for DSO's, does ccd quality be it build or sensor quality make more of a difference?
eg,similar spec'd qhy, sbig, FLI or other high end cameras? or is it sensor size similar to aperture with telescopes? bigger sensor, more detail?
I'm referring to a raw capture without any processing?
Hi Alistair,
To my understanding, the only real difference between the two OTA's you describe above is that the higher spec example will (in theory) be very, very slightly more tolerant of seeing conditions. I doubt however that you will ever be able to notice.... I would guess that it would be buried in the noise.

As far as variation between different cameras.. I have no feel for that, but intuitively, being that the manufacturer I quoted above is at the cutting edge of developing CCD's for professional astronomical research applications, I would be prepared to go out on a limb and suggest that their performance is close to 'best in class'.

~c
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Old 26-04-2012, 09:09 PM
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Originally Posted by Shiraz View Post

Clive, thanks very much for the info - never seen specs for lateral charge diffusion (assume that is what it is) published for any astro sensors, except maybe Kodak's claim to produce "low smear" chips.
This could possibly explain why planetary imagers find that going well beyond Nyquist sampling requirements yields additional detail - maybe we are using oversampling to deal with sensor diffusion blur. Heavy handed oversampling may also be worth pursuing for DSO resolution improvement as well. Regards ray
Hi Ray...

There may be two factors at play here... electrical charge diffusion and/or light spill through the side walls of the pixels. Without some manufacturer specific 'secret sauce' the signal spill may be inherent to all CCD's made from any given substrate... I don't pretend to know the answer to that.

Incidentally, if anyone wants to simulate the effect of adding the equivalent of 1/4 wl of spherical aberration, just add a 35% central obstruction to your OTA.

Interesting that RC's have a 35% central obstruction basically by definition, and they seem to work fine more often than not. The fact that they routinely deliver higher resolution images than Newtonians of similar aperture might be just a function of the over sampling achieved by the longer focal length.

Also, (i'll re-iterate the point) the fact that optical tube assemblies with 50% central obstructions are practicable as prime focus telescopes should serve as a data point for how much spherical aberration can be tolerated in this application... at a guess, this is equivalent to 1/2 wave, give or take.


~c

Last edited by clive milne; 26-04-2012 at 09:24 PM.
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