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Scopes resolving power

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#1 Edmond S.

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Posted 18 April 2021 - 12:49 PM

Just a quick question. Theoretically, if I have a 10" RC f8, the resolving power with the camera would be, say 0.4". Will it be any difference ( in resolving detail, splitting stars... etc ) compare that with 10" Newtonian f4, but using drizzle x2 integration during processing  ? 

 

Of course the RC will require a FF, and Newtonian will require a coma corrector, and I assume ideal situation for optical performance.

 

 


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#2 ravenhawk82

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Posted 18 April 2021 - 01:43 PM

Assuming equal quality optics, a 10" primary will have the same resolving power regardless of what optical system it's part of.

However, when you drizzle images you are effectively generating data by interpolating your images and letting your software fill in the blanks. It's a very good estimate of what would be there, but it's not the same as exposing at that resolution natively. In the two scenarios you described I'd expect slightly more detail from the RC. How noticeable the extra detail is though would be up for some debate considering that atmospheric seeing on most nights is going to be worse that .4".

For visual use this is a different story. The 10" mirror in either scope would be able to resolve the same fine details, you'd just need eyepieces with half the focal length on the F4 newt for an equivalent magnification.


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#3 dan_1984

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Posted 20 April 2021 - 11:28 AM

Just a quick question. Theoretically, if I have a 10" RC f8, the resolving power with the camera would be, say 0.4". Will it be any difference ( in resolving detail, splitting stars... etc ) compare that with 10" Newtonian f4, but using drizzle x2 integration during processing ?

Of course the RC will require a FF, and Newtonian will require a coma corrector, and I assume ideal situation for optical performance.


Besides lucky imaging, the scope resolution will be limited by seeing. Above 8 inches in aperture, the seeing limits the resolution.... I really don't get why many buy big scopes in backyards
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#4 carolinaskies

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Posted 20 April 2021 - 01:19 PM

Besides lucky imaging, the scope resolution will be limited by seeing. Above 8 inches in aperture, the seeing limits the resolution.... I really don't get why many buy big scopes in backyards

Because seeing doesn't limit resolution, only magnification.  Take a 60mm scope and look at Lunar detail at low power.  Take a 100mm, 200mm, 400mm and look at lunar detail at the same low power... you'll see more detail with the big aperture at low power.   

Seeing limits magnification but there are some ways to work around it too.  These include lucky imaging for planetary work, and adaptive optics for longer exposures.   


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#5 Jon Isaacs

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Posted 20 April 2021 - 02:45 PM

Besides lucky imaging, the scope resolution will be limited by seeing. Above 8 inches in aperture, the seeing limits the resolution.... I really don't get why many buy big scopes in backyards

 

That probably depends on the particular backyard... I've split doubles under 0.5" from my backyard. Something about a very mild climate, South of the jet steams a few kilometers from a massive body of water called the Pacific Ocean.. 

 

Starsplitter Back March 2015 CN.jpg

 

It's also worth remembering a Dawes limit split is a very difficult split. The Dawes limit for a 120 mm is just about 1.0".  In 1.0" seeing, a 120 mm probably won't make the split, a 10 inch will split it nicely..

 

Jon



#6 WadeH237

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Posted 20 April 2021 - 05:53 PM

Because seeing doesn't limit resolution, only magnification.

Huh?

 

Seeing absolutely affects the details that you can resolve in an image, either photographically or visually.



#7 carolinaskies

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Posted 20 April 2021 - 06:30 PM

Huh?

 

Seeing absolutely affects the details that you can resolve in an image, either photographically or visually.

If aperture were a limit, use a large flat mirror and a small flat mirror and tell me your view of the sky is better in the smaller mirror on a poor night.  The general 'thought' is that larger aperture has more chance to be affected by the inconsistent light wavefront.  That the smaller aperture is accepting a smaller slice therefore it's less affected by the wavefront.  The problem with this idea is that it's at a far fringe of what any one of us would say was a reasonable night for any but extreme low power viewing thru binoculars.  A 90mm F/8 isn't going to be any better on that night than a 200mm F/10 because they can't get to a low enough power.  But a 7x50mm pair of binoculars might be low enough power that our eyes at least somewhat compensate, like image stabilizers.  

But I'm not here to worry about those poor nights, my life isn't so involved with astronomy that I have to seek out the worst nights to observe.  



 


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#8 Nippon

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Posted 20 April 2021 - 06:52 PM

Agree when the seeing is so bad that Castor looks like a blob in a 4"apo I would think reaching it's resolution potential is being reduced.



#9 Jared

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Posted 20 April 2021 - 06:56 PM

Just a quick question. Theoretically, if I have a 10" RC f8, the resolving power with the camera would be, say 0.4". Will it be any difference ( in resolving detail, splitting stars... etc ) compare that with 10" Newtonian f4, but using drizzle x2 integration during processing  ? 

 

Of course the RC will require a FF, and Newtonian will require a coma corrector, and I assume ideal situation for optical performance.

It's actually not as simple a question as you might think to answer.

 

First, fast optics often have compromises in terms of their resolution even on axis.  To get good off-axis performance (using a field flattener, a reducer, etc.), the designer will be balancing loss of on-axis performance with gains in off-axis performance, so the Paracorr that you are using to get a nice flat, coma free field out of the 10" Newt will, at least a little bit, damage its resolution in the center of the field of view. The amount of damage might be quite small, though, to the point of being immaterial. For example, I would say that my 12" f/3.7 telescope has essentially the same resolution on-axis as my 12.5" f/8 scope. It's hard to know for certain since I can't run them on the same night, but if there are differences they are quite small.

 

You asked for a bit more than that, though. You asked whether you would get the same resolution with the same camera using drizzle. Probably not, but the devil as always is in the details. If your 10" f/8 scope is sampling at 0.4" per pixel, then you could probably switch to an f/4 telescope at 0.8" per pixel and notice little if any difference. That assumes your seeing conditions are the typical 2" to 3" that a lot of us seem to run into. The consensus seems to be that if you have 2" skies, you will get benefits with smaller pixels down to about 0.7" per pixel or so. After that, it's not just diminishing returns, but undetectable ones. If your skies on a given night are significantly better than 2"--it can happen!--then you might get significantly more out of the f/8 scope of the same aperture.

 

The next thing to keep in mind is that optical quality matters, and slower scopes are generally easier to make at a given level of quality than faster scopes. It may not be a huge difference between the two scopes you mentioned, though, since Newtonians only need one asphere and a flat while the RC will need two aspheres, and the primary will be quite a bit faster than the overall telescope, maybe faster than the parabolic primary in the Newtonian. For these two particular telescopes, either might be the better optically.

 

A lot of people seem to think that aperture doesn't really matter once you get past about 5" or so (at least in terms of deep sky imaging since it involves long exposures). Seeing determines your resolution. The truth is a bit more nuanced than that. All elements of your seeing train have an effect on your final resolution. Diffraction, optical quality, collimation, equilibration, tracking, focus, sampling rate, and seeing all play a role. As your aperture increases, seeing starts to dominate, but it isn't to the point that the other elements don't even matter. Even for long exposures, you'll still get more resolution out of a 14" than a 12", more out of a 12" than a 10", etc. It's just that the differences get smaller and smaller. Those buying larger telescopes are primarily hoping to get better signal to noise ratio in a given integration time, but there are still some improvements in resolution as you get larger all the way through the common sizes of scopes used by amateurs. Diminishing returns, though, in terms of resolution. I have looked back at the resolution I have achieved (as measured in FWHM values on an integrated luminance master) across the various scopes I have used over the years. My 12" scopes yielded a touch more resolution than my 10" scope. My 10" yielded quite a bit more than my 5" scope. My 5" yielded a LOT more than my 80mm. This is after accounting for how good my guiding is/was. As it happens, though, the highest resolution I ever got out of a deep sky image was using the 10" scope. I had one night of particularly good seeing--will under 2"--that resulted in a super sharp image. In general, though, I don't think I would buy a scope larger than 8" or so under my average skies with the hopes of improving resolution. And I have generally achieved the same resolution across a given field of view with fast and slow scopes as long as my sampling was sufficiently high with the faster scope. 

 

Finally, a quick comment on Drizzle... It's useful when you are under sampled, but it isn't magic. If you are significantly under sampled a 2x Drizzle can help significantly, but not to the point of being equivalent to 2x the sampling rate. And if you aren't significantly under sampled, then Drizzle may not help at all. In the cases you outlined (0.4"/pixel or 0.8"/pixel), I would be surprised if Drizzle helped at all. And in terms of signal to noise ratio, there is no question that 0.8" /pixel will be much better. Frankly, if I were at 0.4"/pixel I would be tempted to bin 2x2 on all but the very best nights.

 

- Jared


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#10 pweiler

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Posted 20 April 2021 - 07:09 PM

This may be posted on CN elsewhere. See table at the bottom of the article to see the limits of various aperture scopes under 3 different sky quality.

https://www.allabout...lly-matter.html
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#11 teashea

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Posted 20 April 2021 - 07:10 PM

Because seeing doesn't limit resolution, only magnification.  Take a 60mm scope and look at Lunar detail at low power.  Take a 100mm, 200mm, 400mm and look at lunar detail at the same low power... you'll see more detail with the big aperture at low power.   

Seeing limits magnification but there are some ways to work around it too.  These include lucky imaging for planetary work, and adaptive optics for longer exposures.   

Does anyone here have adaptive optics?



#12 teashea

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Posted 20 April 2021 - 07:19 PM

This may be posted on CN elsewhere. See table at the bottom of the article to see the limits of various aperture scopes under 3 different sky quality.

https://www.allabout...lly-matter.html

This is so well done and so intelligent and so meaningful.



#13 WadeH237

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Posted 20 April 2021 - 10:12 PM

If aperture were a limit, use a large flat mirror and a small flat mirror and tell me your view of the sky is better in the smaller mirror on a poor night.  The general 'thought' is that larger aperture has more chance to be affected by the inconsistent light wavefront.  That the smaller aperture is accepting a smaller slice therefore it's less affected by the wavefront.  The problem with this idea is that it's at a far fringe of what any one of us would say was a reasonable night for any but extreme low power viewing thru binoculars.  A 90mm F/8 isn't going to be any better on that night than a 200mm F/10 because they can't get to a low enough power.  But a 7x50mm pair of binoculars might be low enough power that our eyes at least somewhat compensate, like image stabilizers.  

But I'm not here to worry about those poor nights, my life isn't so involved with astronomy that I have to seek out the worst nights to observe.  


 

I have no idea what you are talking about.

 

There was a claim made in post number 3 of this thread that seeing limits a scope's resolution.  That statement, even in the context it was made, is absolutely true.  I have no idea why you are taking exception to it and offering non-sequiturs as explanation.


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#14 Jared

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Posted 20 April 2021 - 11:27 PM

Does anyone here have adaptive optics?

Yes. The amateur version of adaptive optics are slightly better than traditional guiding when the seeing is bad. They are no better than guiding when the seeing is good. The fundamental problem is that they aren't nearly fast enough for most of the effects of seeing, and they are correcting for centroid motion at a point that is out of the frame rather than right at the subject--and typically the seeing outside the frame does not exactly match the seeing in the middle of the frame. In addition, unless you are lucky with the location of a good guide star and/or have an extremely fast telescope, it is rare that you can get a correction rate that is significantly higher than traditional guiding just because the SNR on the guide star isn't high enough for accurate centroid calculations.

 

The professional version of adaptive optics involves deformable mirrors. In addition, they measure the seeing on the actual subject, not on a nearby portion of sky slightly out of the frame. Even so, adaptive optics have an extremely limited field of view--great for planetary nebulae and smaller galaxies, but not so helpful with even objects the size of most globular clusters.

 

Lucky imaging can definitely increase your resolution to something that more closely approximates the actual resolution of your telescope. However, because the shutter speeds are necessarily very short--fractions of a second--this form of imaging is really only helpful for objects with high surface brightnesses. When zero read noise cameras become more price competitive (currently EMCCD cameras are well into the tens of thousands of dollars), we will all be able to use tons and tons of extremely short exposures to reduce the effects of seeing. It still won't eliminate the effects since not all of the aberrations of seeing are simple translation, but that certainly dominates. The computer resources required for, say, calibrating, registering, and integrating 100,000 separate 125 MB frames are somewhat daunting, but who knows what storage will cost in ten years?

 

Until then, we are still stuck with relatively long exposures for deep sky objects (typically 30s or longer) where the effects of seeing are at their worst.


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#15 Jared

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Posted 20 April 2021 - 11:34 PM

Because seeing doesn't limit resolution, only magnification.  Take a 60mm scope and look at Lunar detail at low power.  Take a 100mm, 200mm, 400mm and look at lunar detail at the same low power... you'll see more detail with the big aperture at low power.   

Seeing limits magnification but there are some ways to work around it too.  These include lucky imaging for planetary work, and adaptive optics for longer exposures.   

In the context of deep sky imaging, resolution is typically measured by the Full Width/Half Maximum diameters of stars measured in arc seconds. Given that context, there is no question that seeing has a profound impact on resolution. In the case of planetary imaging, sub exposures are in the low tenths of seconds or even hundredths of seconds. That doesn't really apply to deep sky objects at this point in time. And the adaptive optics available to amateurs have, at best, a very small impact on resolution. I have used adaptive optics for years on SBIG cameras. Fantastic feature if you have a poor quality mount. Marginal feature if you have a high quality mount. The biggest benefit is when the seeing is at its worst, and even then it isn't a dramatic improvement on traditional guiding. Seeing still has a profound impact on resolution when using amateur adaptive optics. 




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