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  #21  
Old 28-03-2017, 10:15 PM
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Shiraz (Ray)
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Originally Posted by codemonkey View Post
I suspect we (or at least I) get better seeing than people assume is typical in Australia. I routinely get 1.8" FWHM L subs with my Esprit 120. Using Ray's handy spreadsheet I can reverse-engineer an approximation of the seeing at my site based off:

0.12m aperture
0.6" RMS guiding
1.8" FWHM stars in the resulting images
= ~ 1.28" seeing

Again using Ray's handy spreadsheet, upping my aperture to 0.2m would reduce the FWHM of stars in the resulting image to approx. 1.63"

I realise these are approximations, but don't know exactly how accurate Ray's formulas are. I do know the aperture of my scope, typical RMS guide errors and resulting star FWHM with confidence though.
the assumptions make it a bit ropey to backcast like that Lee, particularly in extricating seeing from the effects of guiding RMS. The actual seeing is likely to be in between what the spreadsheet gives with your measured RMS and with zero RMS, but there is no way of being any more precise, so it is what it is and that is still useful information.

you are fortunate to have seeing that good, but it is not unheard of in Australia - just not available in all locations http://articles.adsabs.harvard.edu//...00101.000.html
pretty good here tonight as well - between gusts it is well under 2 arcsec.

yeah, you need a bigger scope ..... and a bigger mount and .......
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  #22  
Old 29-03-2017, 06:44 AM
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Speaking of bigger mounts, I'm not sure if it is always plausible to assume that bigger aperture, even from the same manufacturer, will necessarily yield tighter stars given the same level of guiding is maintained with a heavier telescope. It is curious that for example Orion Optics states that for their fast Newtonians 12inch will yield the smallest spot size on and off axis, while their smallest 8" as well as their largest 16" Newtonians both have the largest spot sizes. Or is spot size unrelated to the FWHM we can expect in our images?
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  #23  
Old 29-03-2017, 07:23 AM
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At shorter focal lengths poor spot sizes will yield soft images while it becomes less important as focal length increases.
For instance, at 16" F/3.8 has a focal length around 1544mm. If used under the kinda average 2" seeing conditions that we've all grown to know and love, a star will cover 15 microns.

An 8" F/3.8 has a FL of 772mm and stars will cover 7 microns at 2" seeing.
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  #24  
Old 29-03-2017, 08:10 AM
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Thanks Colin. Would then a 12" f/3.8 from OO, with stars covering 11 microns yield better data than a 16" when seeing is about 2", since spot size for 12" is less than half of an 16"? Would this suggest that smaller aperture telescope is not always inferior to a larger one in terms of resolving power, even from the same manufacturer?
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  #25  
Old 29-03-2017, 08:46 AM
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Location location location!
If I was at the top of Atacama or in Chile I would be getting the 12" and whacking a FLI ML-814 (same chip as the QSI690) as it would be imaging at 0.68"/pixel. Considering the sky clarity and spot diagrams, it allows for drizzle integration for some serious resolution.

Out in the average Aussie burbs however, it doesn't matter a great deal
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  #26  
Old 30-03-2017, 04:39 PM
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Quote:
Originally Posted by Shiraz View Post
the assumptions make it a bit ropey to backcast like that Lee, particularly in extricating seeing from the effects of guiding RMS. The actual seeing is likely to be in between what the spreadsheet gives with your measured RMS and with zero RMS, but there is no way of being any more precise, so it is what it is and that is still useful information.

you are fortunate to have seeing that good, but it is not unheard of in Australia - just not available in all locations http://articles.adsabs.harvard.edu//...00101.000.html
pretty good here tonight as well - between gusts it is well under 2 arcsec.

yeah, you need a bigger scope ..... and a bigger mount and .......
Thanks Ray I should probably correct myself in that I routinely get 1.8" (sometimes better, but rarely) for part of the night, definitely not all night. As the target drops in the sky and I shoot through more atmosphere the FWHM goes up.

If I was to get a bigger scope I'd be looking at an 8" RC or Edge, both of which are smaller and lighter than the Esprit. But getting one would only be sensible if the resolution gain I'd get from the greater aperture wasn't offset by less great optics.

Quote:
Originally Posted by Slawomir View Post
Speaking of bigger mounts, I'm not sure if it is always plausible to assume that bigger aperture, even from the same manufacturer, will necessarily yield tighter stars given the same level of guiding is maintained with a heavier telescope. It is curious that for example Orion Optics states that for their fast Newtonians 12inch will yield the smallest spot size on and off axis, while their smallest 8" as well as their largest 16" Newtonians both have the largest spot sizes. Or is spot size unrelated to the FWHM we can expect in our images?
I don't know much of anything about spot size, or optics in general. Doesn't seem to be published for the Esprit either. I think I've seen Colin posting recently about the ideal spot size for a given camera, but I'm not sure of the details. Colin, can you (or someone else) elaborate on this?
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  #27  
Old 30-03-2017, 05:38 PM
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I have a quite limited knowledge on spot diagrams (can spot bad ones but struggle to read the difference between good ones), MTF curves and the like. Mostly I can spot bad ones and glean some very basic information, something I am slowly trying to teach myself

What I thought I would show is two images that I have taken. Same target, same lens, roughly the same integration time but different cameras.

LMC with Nikon D700 & Nikkor 85mm F/1.8G

Nikon D7200

The first was taken with the Nikon D700 which has 8.445 micron pixels and an anti-aliasing filter which means that I can never get better than a FWHM of ~8 pixels. It was taken wide open at F/1.8 so when I saw a little bit of blue/purple around some stars I really wasn't too surprised. The stars are showing some slight deformity at the edges of the FX frame. Also not too surprised at F/1.8.

The second is with a Nikon D7200 which I think has 3.91 micron pixels and no anti-aliasing filter. It was taken at F/4 so a far cry from wide open. It shows a LOT of purple (the outer star forming regions look purple and not blue) and you can see the stars deforming quite easily in the DX chip. There is however ~3500 galaxies in the PGC catalog in this image!

What these two show is that even an average lens can show almost zero aberrations if quite large pixels are used. Small pixels really show any optical imperfections, especially when small pixels are matched with a higher QE and lower read noise.

In some ways this also points back to what Greg has seen with shorter scopes with small pixels vs larger scopes with larger pixels. On average, most of the shorter focal length telescopes are cheaper and have smaller cheaper cameras and lesser quality correctors so even when you're imaging at the same scale, optical imperfections are more likely to be an issue.

I have recently been looking at short focal length optical nirvana, looking at telescopes like the Tak Epsilon series (F/2.8-3.3) or a Boren-Simon Power Newt 8" F/2.8 (8" F/4 with an ASA 0.73x Corrector/Reducer). These are on the cheaper side for their size and focal ratio and it shows when using cameras like the ASI1600.
Of course then there is the large price increase to the ASA 8" F/2.8 H which has spot diagrams that just make one drool which is effectively what you end up paying for.
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  #28  
Old 30-03-2017, 06:53 PM
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Very interesting discussion and I really enjoy reading what everyone has to say about the topic. Colin, I certainly prefer the look of the image taken with D7200 - and both indicate quite a few interesting regions for potential targets! As you can probably imagine, I really can't wait to finally test my new small refractor and see if it is capable to collect photons more elegantly than the best telescope I have owned until now - a TS 4" doublet.

As for spot diagrams for apochromatic refractors, below is an interesting excerpt from an article written by Pal Gyulai, optical designer from CFF Telescopes. I hope it is okay to quote entire paragraph.

In fact, if we construct a fast triplet APO lens of F/6 focal ratio and the optical design is optimized for the "best looking" spot diagrams, then after actually building the lens and testing it under the sky, the image quality will not be optimal. This is not surprising, as the classical method cannot be used for optical systems approaching (or at least getting reasonably near) the diffraction-limited quality. Today we expect better than diffraction-limited performance everywhere between the blue and red colour wavelengths, so, the classical methods (that can't explain why we see an Airy disk surrounded by diffraction rings in the eyepiece) cannot be used anymore. This methodology was perfect to design car headlights, room lighting and slow focal ratio achromat lenses and (judging by today’s standards) semi-APO lenses. But it is not usable to construct modern, fast APO systems.
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  #29  
Old 30-03-2017, 08:04 PM
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That is a really interesting read Suavi, something that I wouldn't have expected. My general assumption would be that when you have the best spot diagrams you would have the best performance, i.e. light it being focused at its best and most efficient within that system.

I've still got so much to learn!
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  #30  
Old 30-03-2017, 08:23 PM
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I reckon we all have a lot to learn, but that is arguably the best part of this hobby :-)

It makes sense to me that when we talk about diffraction-limited performance, we should consider light as a wave not particles. That is not to say that spot diagrams are not useful- they certainly have their place and are also relatively easy to read.
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  #31  
Old 31-03-2017, 01:28 PM
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Cheers Colin (and nice images both!). I've got a basic understanding of spot diagrams, can spot basic aberrations etc. My main question is what are you looking for in relation to pixel size? Is there a specific ratio that will yield optimal results?

Interesting quote, Suavi! Certainly for me at least, it's very difficult to compare scopes and make a decision that's better than a gamble.
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  #32  
Old 31-03-2017, 02:19 PM
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I don't think there is a ratio that you can use perse. You also need to consider the sensor size, the ASI1600 has the pixel density of a 65MP full frame sensor but you wouldn't want a full frame sensor with 3.8 micron pixels as there isn't many telescopes available (they're all VERY expensive) that can handle that pixel size off-axis. The shorter the focal length the less seeing conditions have an effect.

Shooting with a 50mm lens will ALWAYS be lens (and camera architecture) limited where as a 24" F10 will always be seeing limited unless using something like 24 micron pixels.
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  #33  
Old 31-03-2017, 03:47 PM
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Quote:
Originally Posted by codemonkey View Post
My main question is what are you looking for in relation to pixel size? Is there a specific ratio that will yield optimal results?

.
for anything but a very small scope, suggest that the pixel size in arcsec be 1/2-1/3 the seeing that you expect if you want the optimum DSO combo of sensitivity and resolution.

Quote:
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Shooting with a 50mm lens will ALWAYS be lens (and camera architecture) limited where as a 24" F10 will always be seeing limited unless using something like 24 micron pixels.
good point but maybe you could take it a little further - a 24" scope will always be seeing limited even if it has significant aberrations. eg, if a big scope has say 1/2 wave of SA, it's FWHM will blow out by say a factor of ~2x - to 0.5 arcsec. But that will still be way less than typical 1.5-2 arcsec seeing FWHM, so it will not matter (quite a bit worse could be tolerated - which is why scopes with horrendous central obstructions still work OK). Optical quality is a major factor with lenses and small scopes, but it is almost of secondary importance with big ones - unless your seeing is exceptional.
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  #34  
Old 31-03-2017, 04:51 PM
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Cheers guys. Yep, Ray, I've been under the impression that 1/2 - 1/3 the seeing in "/px is what you should aim for, but Suavi's earlier comments have me wondering, specifically in regard to the assumption that a bigger aperture is going to mean better resolution: what if it does the opposite because the optics suck?

As mentioned earlier, my "best common case" seeing is about 1.8" with a 120mm scope and 0.6" RMS guide error. We established earlier that puts my actual seeing (approx) between about 1.6" and 1.8".

So lets say I want to make the most of it, err on the side of resolution and oversampling so we'll use 1.6". Again erring on the side of oversampling, I'll go with 1/3 of the seeing in "/px - that tells me I want to be sampling at about 0.53" / px

Given the above, an RC8 combined with the ASI 1600 looks like a good fit. Increasing the aperture (assuming all else is precisely equal) should have reduced my FWHM to 1.6" instead of 1.8"... but how can I be sure that the RC8 is going to be a good choice? What if it's just plain "not sharp"? What metric can I use to compare my options before making such a purchase?

Funny thing this hobby... the longer I'm involved with it, the more I realise what I don't know.
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  #35  
Old 31-03-2017, 07:08 PM
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To put things in perspective I have attached some photos from Sky and Telescope in 1997 (20 years ago).
The first is a typical pixel planner for planetary or DSO imaging, and the second the difference in quality of large pixels on DSO images.

How things have changed with respect to cameras, as 9 micron is about the largest you can buy today and the average around 5 micron, with the newest cameras at 3 micron or less.

Cheers
Bill
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