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Fast telescopes v/s high resolution

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#51 TareqPhoto

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Posted 06 November 2018 - 11:38 AM

It's not really about any of those.

 

It is about enjoying what you are doing.

 

If the high-end stuff is what it takes to make one happy then I'm all for it.  If one can be happy with lesser equipment then that is at least as awesome.

That is not a point, i am sure all will be happy with any gear in hand from so cheap to so very expensive, it is just about when is that very expensive is a must or essential and when a cheap one is more than enough or overkill, otherwise if it is always the cheap affordable one are great and more than enough then it will be always high end a very big waste of money then, but i never read about that anywhere and i see more going from cheap to expensive and never the opposite, i can't afford very expensive for sure, but i just want to know is it worthy to do it if we got the opportunity to do with big deep budget somehow?



#52 TareqPhoto

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Posted 06 November 2018 - 11:41 AM

Also i wanted to start a new thread asking about very similar topic to this thread, as following:

 

Which is giving more noticeable results either quality or resolution:

 

1. High end scope and cheap ccamera

 

2. High end camera expensive and cheap scope

 

When i say cheap i don't mean $100-500, but something like $1000-2000 vs. high end $5k-20k.

 

Assuming that only used a very capable mount to hold above, so it is only camera or telescope factor.



#53 Jon Rista

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Posted 06 November 2018 - 12:54 PM

Yes, that is why i am still happy with my QHY mono camera and cheap filters and i am slowly upgrading to Astrodon but at 1.25" only, and about large aperture or scope i really don't know about that yet, in my mind for my small mount i was thinking about 8" or 10" RC maximum, but the trap of Facebook there are members told me to forget this small and go directly to 14" or 16" RC, i still don't know if 14" RC or 16" are ultimate scopes to be used over 8"-12" RC, and i already planned on a solid mount, so mount and scopes are likely covered or known, but still not sure about this camera pixel resolving story yet.

 

I saw a lot of nice images from ASI1600 and also my camera or similar ones, but i still read here and there about CCD vs. CMOS, so i don't know what i am missing there, and definitely when i see scopes and mounts and cameras i mentioned above combined then i know "Blindly" that the results are outmatched or in another level completely, so i was thinking are cheaper affordable one any good even with so nice amazing results or we only put those high end full setup as minimum and standard?

I would not consider a large RC like that until you really know for sure that you NEED it. Those are very big scopes, and would require high end mounts, and would definitely increase the demand on everything, including you and your skills. It doesn't really matter if you use big pixels on a big scope, or small pixels on a moderately sized scope like a 10" newt, in either case, you have to be able to track well enough to benefit from the increased resolution. You also need the skies and good seeing to really take advantage of it...IMO, if you don't have the seeing, a smaller, less costly system is a better option.

 

You should really KNOW, yourself, that you definitely need a big scope like that, before you invest the money in one.

 

This thread is fairly specific, in that the OP knows what he wants, knows he has very good skies, and knows he could take advantage of a higher end system with higher resolving power. Not everyone should be pursuing what the OP is pursuing. Keep in mind, while this is the CCD and CMOS forum, it is still a forum for more advanced imaging.

 

No offense is meant here, honestly, I want to give you sound advice...but I believe you are definitely more in the beginner to intermediate range, Tareq, and I think you should be careful about dumping money into high end gear until you yourself are familiar enough with all the nuances that you yourself know for sure what you need. I do not believe you are at that point yet...so I think you will benefit much more from using what you have, and pushing what you have farther and farther until you know for sure you have reached its limits. I think you can take the gear you have pretty far, you actually have quite a lot of gear...at the moment, I think the most beneficial thing for you is to hone your skills, your techniques, optimize  your current setups, work on improving the image quality you can currently get from your current equipment and processing skills. That only requires an investment of time, rather than money.



#54 555aaa

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Posted 06 November 2018 - 02:29 PM

To the OP (dhaval), I think I am in sort of the same boat - we get good seeing in the Fall - down in the 1 arc second or maybe even lower. When your seeing is that good, you can't make use of it unless your guiding is commensurately good, and it doesn't matter if it is a fast system with small pixels or a long focal length with big (or binned) pixels.  I don't think that the PHD RMS number is all that useful and I don't like how PHD does centroiding on off-axis star images for OAG or images with some dynamics. Basically what I see with my eye looks very consistent, but PHD thinks it's moving all over the place. I'm talking about a guide star FWHM around 1 arc second or so.  It seems to be optimized for people using short focal length guide scopes.  I also noticed that when I upgrade from a 10" SCT to a 16" SCT that my seeing "magically" improved. What actually happened was that more often, the big 'scope is taking advantage of those moments of good seeing where the 10" wasn't quite as capable in terms of actual angular resolution. So something I would definitely be careful about is going too long on exposures because a few moments of bad guiding or the passage of some cirrus or a bit of wind will totally blow out the star FWHM for your exposure, so you may be better off taking a small hit on read noise to get a higher percentage of subs that are at minimum FWHM. Also as your target moves down (up in air mass) it definitely gets worse in terms of resolution so you might be limited there in terms of how many subs you can get per night before the target is at too high an air mass (to low in the sky). I think you will want to experiment with this to get an idea of the optimal exposure in that way, not just for read noise - remember that FWHM is signal to noise; the more blurred the image is, the more the object pixels are contaminated with photons from the background sky.

 

I had a chance to do some measurements on some images from PanStarrs and I noticed that they were getting consistently 0.8 arc seconds FWHM.  If they were only getting 3.5 arc second FWHM seeing, they would be losing several magnitudes in terms of SNR limited sensitivity. They are way up on a mountain of course but it was very educational to see how much of their ability to image deep is because it is high angular resolution.

 

As I said above, there is a lot of interest in somewhat fast systems more because they are wide field and that is more visually pleasing; it gives the illusion of high resolution but it is not high resolution, especially in the corners. It's high resolution across the image presented to the user (because it may be a couple degrees of sky) but not angularly high resolution. Two degrees is 7200 arc seconds across. 


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#55 TareqPhoto

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Posted 06 November 2018 - 02:42 PM

I would not consider a large RC like that until you really know for sure that you NEED it. Those are very big scopes, and would require high end mounts, and would definitely increase the demand on everything, including you and your skills. It doesn't really matter if you use big pixels on a big scope, or small pixels on a moderately sized scope like a 10" newt, in either case, you have to be able to track well enough to benefit from the increased resolution. You also need the skies and good seeing to really take advantage of it...IMO, if you don't have the seeing, a smaller, less costly system is a better option.

 

You should really KNOW, yourself, that you definitely need a big scope like that, before you invest the money in one.

 

This thread is fairly specific, in that the OP knows what he wants, knows he has very good skies, and knows he could take advantage of a higher end system with higher resolving power. Not everyone should be pursuing what the OP is pursuing. Keep in mind, while this is the CCD and CMOS forum, it is still a forum for more advanced imaging.

 

No offense is meant here, honestly, I want to give you sound advice...but I believe you are definitely more in the beginner to intermediate range, Tareq, and I think you should be careful about dumping money into high end gear until you yourself are familiar enough with all the nuances that you yourself know for sure what you need. I do not believe you are at that point yet...so I think you will benefit much more from using what you have, and pushing what you have farther and farther until you know for sure you have reached its limits. I think you can take the gear you have pretty far, you actually have quite a lot of gear...at the moment, I think the most beneficial thing for you is to hone your skills, your techniques, optimize  your current setups, work on improving the image quality you can currently get from your current equipment and processing skills. That only requires an investment of time, rather than money.

All those are noted, but i think because i am new here people still don't understand my plan or my behave or how i get things.

 

I will not buy anything yet, not RC no Newtonian, but i will do buy a Takahashi scope and this is never be a wrong idea even if i am a beginner.

 

I asked about a combo, which is a camera and a telescope, and you brought or mentioned about large scope so that i mentioned RC.

 

I won't buy now, but if i want to be in the future at least i should know what i want to buy, believe me, even if i buy a PlaneWave now it will do a great job for me, it is only why you use a Newtonian or a refractor or an RC or even SCT, and when, i saw images of so so many targets from remote observatories and from Astrobin and from APOD and from Facebook and even here, so i definitely have big idea each target and each scope, but i still need to know more because it is not only about put this on that and fire, if my sky is bad then i am sure even a cheap camera and cheap telescope will never do much and definitely not high end, but when i ask for more or high end or upgrade then i am sure that the sky is allowing me.

 

I ask to collect knowledge, not necessary i have to use all gear, and what i have now i am still using and learning and i have patience, but i always ask what you call it as "Pre-questions", which i mean i ask about gear i may plan to get after 2-4 years from now, so maybe i buy or maybe not, it all depends on the answers, i can easily just stop asking and buy 5 scopes between cheap and very expensive and just get experience, and definitely i won't buy anything new until i know what i am doing or did already, but please, not everything is about "I must know how to use item A before i buy item B", we sometimes buy many items without knowledge and just taking time to use or learn, i bought 2 cameras before during photography and i really didn't spend years to learn and bought new high end cameras and i couldn't be happier at all, and i also got same answer as yours "Use what is in your hand now and don't rush", but i rushed as i bought what i like and i did ask A LOT about it so i was so confident that it will be right even before i buy it, and i was correct, RC is amazing no matter if i shoot from very dark sky or very bright, i saw images from all RC and in fact RC was the main first reason i got into astrophotography last year, my question isn't about if RC is good enough to be use or not, it is about which RC, there are different brands and different aperture sizes, i even say beginners got RC and used it happily, you are not offending me but you only think that if i buy so many gear i will only get disappointed, and i have ALL the time to try all gear i have now or i will get later, and i am still learning about processing skills either with my data or by others data.

 

Not hijacking the thread anymore, just asking simple question, is there a difference between using high end camera with cheap affordable scope and high end scope with cheap affordable camera? forget about big scopes now.



#56 Peter in Reno

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Posted 06 November 2018 - 02:51 PM

I think I prefer high end scope with cheap camera than low end scope with expensive camera. I mean sensor is a sensor and should not make a whole lot difference for different brands of camera. Usually more expensive cameras are "higher" quality like QSI, FLI and SBIG even though they can share same sensors as low quality cameras.

 

I think optics are far more important than cameras. I also think mounts are the most important.

 

Peter


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#57 TareqPhoto

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Posted 06 November 2018 - 02:57 PM

I think I prefer high end scope with cheap camera than low end scope with expensive camera. I mean sensor is a sensor and should not make a whole lot difference for different brands of camera. Usually more expensive cameras are "higher" quality like QSI, FLI and SBIG even though they can share same sensors as low quality cameras.

 

I think optics are far more important than cameras. I also think mounts are the most important.

 

Peter

Aha, great, this is very understandable, ofcourse the mount is most important that is why i assumed we use a high end or very capable mount already, so only left the camera and the scope, and your answer now giving me a better knowledge or idea about it, thank you very much, very helpful and straight forward clear answer.



#58 jhayes_tucson

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Posted 06 November 2018 - 03:40 PM

I think that both of you (John and Jon) tend to refer to what is best for "most common conditions."  That is perfectly fine - but should not be the mindset of someone who strives for max detail and resolution - such as the OP in his first post.  If you aim to optimize everything for typical conditions at a given site, you will be stuck with giant pixels on those rare nights when the seeing is exceptional.  And those are the nights that are most valuable - if you strive for high resolution.

 

And if you do have small pixels but the seeing is not exceptional - you can always bin or low pass after the fact - with little or no penalty as long as read noise is not a strong factor - and nowadays it usually isn't.  And even if the image is a bit noisier due to smaller pixels - you can image longer to achieve the same result - after binning.  Small pixels do not represent a fundamental limit to what can be achieved - whereas large pixels do.

 

With large pixels (and in many situations, 0.5" is quite large) you are just stuck with what you got - and the great seeing and amazing optics you paid for - are being wasted.

 

As always - if you don't actually care about max detail, or if you seeing is never great - 0.5" may be a decent minimum pixel size.  But the OP in his first post mentioned a desire for high res, and seeing around 1".  To me - both the mindset and the seeing point to 0.2" pixels.

 

Frank

 

Boy, I guess that things really do go full circle and I'm glad to see that you finally agree with me!  I've always argued that AP systems should be optimized for the best possible real-world conditions.  That's why my system can easily handle 1" conditions--if and when they occur.  The problem is that my site virtually never produces 1" conditions and when the conditions are good, they don't stay good for very long.  Putting a sensor with 3 micron pixels on any F/11 scope would put about 5 pixels across the Airy disk and that's about right for a space based system.  (As you know, the optical bandwidth limit for any circular system is 4.88 samples across the Airy disk.)  Seriously oversampling with long exposure imaging simply adds to the time needed to reach SNR targets and some of us (me included) already have to spend multiple lunar cycles to gather enough data for a single image.  Simply shooting minimally processed monochrome images that don't add up to much more than a few hours is a lot more forgiving than gathering 25-40 hours of high quality data needed to produce clean, fully processed mutli-color images.  I often have to toss out 75% of the data that I gather due to lousy conditions that might include dew, thin overcast, poor seeing wind, etc. and it makes no sense to make it any harder than it already is.  I found that going from a 9 micron pixel camera to a 6 micron camera on my system did nothing good.  The SNR was lower, the field was smaller, and it took way too long to get enough good data to actually complete an image.  As I've said, there is a trade off between SNR and sampling and it makes no sense to oversample since it does virtually nothing to improve image detail for long exposure imaging under any real-world conditions.  If anyone wants a higher sampling rate, drizzling works pretty well.  It won't do much to add any significant detail but it does a good job of producing clean star profiles.  In the grand scheme of things, it doesn't matter what camera you use; but, it's important to understand that gathering data is a lot more efficient when the pixel size is more closely optimized to the imaging system.

 

John


Edited by jhayes_tucson, 06 November 2018 - 04:24 PM.

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#59 WadeH237

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Posted 06 November 2018 - 04:07 PM

Also i wanted to start a new thread asking about very similar topic to this thread, as following:

 

Which is giving more noticeable results either quality or resolution:

 

1. High end scope and cheap ccamera

 

2. High end camera expensive and cheap scope

 

When i say cheap i don't mean $100-500, but something like $1000-2000 vs. high end $5k-20k.

 

Assuming that only used a very capable mount to hold above, so it is only camera or telescope factor.

Hey Tareq,

 

Please start that thread.  There is some good stuff to talk about here, but this is not the right thread for it.



#60 TareqPhoto

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Posted 06 November 2018 - 04:13 PM

Hey Tareq,

 

Please start that thread.  There is some good stuff to talk about here, but this is not the right thread for it.

No need, i already got the answer here from Peter, and now if the admin or mod wants to delete my posts it is ok, thanks for Peter and sorry to include it here in this OP thread.



#61 freestar8n

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Posted 06 November 2018 - 04:39 PM

Boy, I guess that things really do go full circle and I'm glad to see that you finally agree with me!  I've always argued that AP systems should be optimized for the best possible real-world conditions.  That's why my system can easily handle 1" conditions--if and when they occur.  The problem is that my site virtually never produces 1" conditions and when the conditions are good, they don't stay good for very long.  Putting a sensor with 3 micron pixels on any F/11 scope would put about 5 pixels across the Airy disk and that's about right for a space based system.  (As you know, the optical bandwidth limit for any circular system is 4.88 samples across the Airy disk.)  Seriously oversampling with long exposure imaging simply adds to the time needed to reach SNR targets and some of us (me included) already have to spend multiple lunar cycles to gather enough data for a single image.  Simply shooting minimally processed monochrome images that don't add up to much more than a few hours is a lot more forgiving than gathering 25-40 hours of high quality data needed to produce clean, fully processed mutli-color images.  I often have to toss out 75% of the data that I gather due to lousy conditions that might include dew, thin overcast, poor seeing wind, etc. and it makes no sense to make it any harder than it already is.  I found that going from a 9 micron pixel camera to a 6 micron camera on my system did nothing good.  The SNR was lower, the field was smaller, and it took way too long to get enough good data to actually complete an image.  As I've said, there is a trade off between SNR and sampling and it makes no sense to oversample since it does virtually nothing to improve image detail for long exposure imaging under any real-world conditions.  If anyone wants a higher sampling rate, drizzling works pretty well.  It won't do much to add any significant detail but it does a good job of producing clean star profiles.  In the grand scheme of things, it does't matter what camera you use; but, it's important to understand that gathering data is a lot more efficient when the pixel size is more closely optimized to the imaging system.

 

John

 

With my EdgeHD11 at f/10 and 1.1" seeing, the fwhm of the star itself is 1.1" by definition - and with 0.2" pixels I would sample that peak at 5 points to capture the shape of the curve.  When I then align and stack, the sampling is effectively reduced to about 3 points.  So in the final stack if I zoom in on those small stars, they will have a natural profile and no loss of detail - which means they also stand up better to processing and additional enhancement - though I don't tend to do that.

 

More importantly - the fwhm that I measure will depend on the size of the pixels - and will tend to be smaller - in arc-seconds - with smaller pixels.  This is all empirical and involves subtleties of how the sensor behaves and what happens when you align and stack.

 

There is no Nyquist going on here - it's just an image that is discretely sampled - and then you look at the samples - after first blurring them and stacking them.  So there are multiple factors involved that point to the benefits of small pixels - and much smaller than typical Nyquist arguments would suggest.

 

And at the very worst - if you do end up wanting to bin the result because it is a bit soft in those conditions - the only downside is a bit more noise - or equivalently a bit more imaging time needed for the same result with larger pixels.

 

Frank



#62 Ralph Paonessa

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Posted 06 November 2018 - 05:09 PM

Excellent and informative thread.

 

That said, with tiny pixels, it IS possible to get good resolution with smaller systems. It may not be the "best possible" resolution, but you can indeed still get very good resolution with much, much more affordable systems like the newt system I described above. You would be able to spend significantly less money. You would not achieve the same potential as a higher end system, but you should be able to get a majority of the way there.

In my case I started with -- and still prefer widefield imaging. I have f/5 106mm FSQ-106 and KAF-8300 CCD with 5.4 micron pixels giving 2.1"/px. Seeing usually 2", so I'm undersampled as I expect for widefield. In fact my next step is wider with Tak 0.72X reducer. I wish I could afford a larger sensor and filters.

 

But I also wonder what's my pathway to smaller objects as in distant galaxies and planetary nebulas.

 

Coming from a nature photography background, I still think in terms of smaller subject = longer focal length. As in EdgeHD 11".

 

But this thread has me wondering: What about a smaller sensor with smaller pixels? It seems one disadvantage is that I'd still be limited to the theoretical resolution of D = 106mm, so although the target would be better framed, the details wouldn't be so well-resolved, compared to 250mm?

 

Now I image at a fast f/5. Can I assume that at f/10 I will have to collect four times longer? There's always the Edge f/7 reducer.

 

I've never been quite clear on then finer points of matching sensor to scope. (But I do like a lot of pixels if I ever want to make a large print. Although images exist more and more on monitors and less and less on walls ...)

 

Thanks,

Ralph



#63 TareqPhoto

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Posted 06 November 2018 - 05:32 PM

Excellent and informative thread.

 

In my case I started with -- and still prefer widefield imaging. I have f/5 106mm FSQ-106 and KAF-8300 CCD with 5.4 micron pixels giving 2.1"/px. Seeing usually 2", so I'm undersampled as I expect for widefield. In fact my next step is wider with Tak 0.72X reducer. I wish I could afford a larger sensor and filters.

 

But I also wonder what's my pathway to smaller objects as in distant galaxies and planetary nebulas.

 

Coming from a nature photography background, I still think in terms of smaller subject = longer focal length. As in EdgeHD 11".

 

But this thread has me wondering: What about a smaller sensor with smaller pixels? It seems one disadvantage is that I'd still be limited to the theoretical resolution of D = 106mm, so although the target would be better framed, the details wouldn't be so well-resolved, compared to 250mm?

 

Now I image at a fast f/5. Can I assume that at f/10 I will have to collect four times longer? There's always the Edge f/7 reducer.

 

I've never been quite clear on then finer points of matching sensor to scope. (But I do like a lot of pixels if I ever want to make a large print. Although images exist more and more on monitors and less and less on walls ...)

 

Thanks,

Ralph

This scope is very much my next upcoming scope, and i have a small pixel and sensor mono CMOS camera, so i hope it will be fine with that, and i will include a reducer with this scope [0.73x] to be wider, so i will watch this thread and your posts more to learn.



#64 Jon Rista

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Posted 06 November 2018 - 06:44 PM

Excellent and informative thread.

 

In my case I started with -- and still prefer widefield imaging. I have f/5 106mm FSQ-106 and KAF-8300 CCD with 5.4 micron pixels giving 2.1"/px. Seeing usually 2", so I'm undersampled as I expect for widefield. In fact my next step is wider with Tak 0.72X reducer. I wish I could afford a larger sensor and filters.

 

But I also wonder what's my pathway to smaller objects as in distant galaxies and planetary nebulas.

 

Coming from a nature photography background, I still think in terms of smaller subject = longer focal length. As in EdgeHD 11".

 

But this thread has me wondering: What about a smaller sensor with smaller pixels? It seems one disadvantage is that I'd still be limited to the theoretical resolution of D = 106mm, so although the target would be better framed, the details wouldn't be so well-resolved, compared to 250mm?

 

Now I image at a fast f/5. Can I assume that at f/10 I will have to collect four times longer? There's always the Edge f/7 reducer.

 

I've never been quite clear on then finer points of matching sensor to scope. (But I do like a lot of pixels if I ever want to make a large print. Although images exist more and more on monitors and less and less on walls ...)

 

Thanks,

Ralph

I think a reduced EdgeHD 11" could pair well with the smaller pixels of CMOS cameras. I think at f/10, you would probably oversampled too much, but at f/7 you would be at 2000mm. With 3.8 micron pixels (i.e. Panasonic M, ASI1600/QHY163/Atik Horizon) you would have an image scale of 0.4"/px. With 2.4 micron pixels (i.e. IMX183, ASI183/QY183) you would be at 0.25"/px. With 1-1.5" seeing you would be sampled within either John's criteria (~3x) or Frank's criteria (~5x) with one of these systems. If  you had worse seeing, then either system would probably oversample too much, and you might need to expose much too long in order to get good SNR. 

 

SCT, CDK and RC scopes are not the only larger-aperture options, though. This is why I like newts these days. With newts, you can still get pretty large apertures of 200mm, 250mm, 300mm or even larger if you are willing to spend the money. That is in the same ballpark as a lot of popular SCTs, RCs and smaller CDKs. But they are f/4, or around there, and would pair much better with small pixel CMOS cameras, and be capable of delivering higher resolution and greater detail like larger systems. The very high Q.E. of the IMX183 is an important factor here...as it helps to balance things out. You might still not be able to get better SNR or better resolution than a 20-24" RC or CDK with 9-15 micron pixels. But you could get pretty darn close, and do it for only a few grand, rather than a few tens of grand. 


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#65 Ralph Paonessa

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Posted 06 November 2018 - 07:00 PM

Thanks.

If  you had worse seeing, then either system would probably oversample too much, and you might need to expose much too long in order to get good SNR. 

Could you explain why?



#66 Ralph Paonessa

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Posted 06 November 2018 - 07:03 PM

You might still not be able to get better SNR or better resolution than a 20-24" RC or CDK with 9-15 micron pixels.

Does this imply that these large aperture scopes should generally be paired with such large pixels? And why?



#67 Ralph Paonessa

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Posted 06 November 2018 - 07:09 PM

The very high Q.E. of the IMX183 is an important factor here...as it helps to balance things out.

Sorry for the confusion, but how is QE a factor here? Simply because I'm capturing more photons/second, so getting better SNR even though I'm using a slower scope?

 

And I agree, those newtonian astrographs look interesting. But does their longer physical length make them harder to mount/track than an SCT?

 

Thanks,

Ralph



#68 Jon Rista

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Posted 06 November 2018 - 07:15 PM

Sorry for the confusion, but how is QE a factor here? Simply because I'm capturing more photons/second, so getting better SNR even though I'm using a slower scope?

 

And I agree, those newtonian astrographs look interesting. But does their longer physical length make them harder to mount/track than an SCT?

 

Thanks,

Ralph

If we compare an 11" SCT with a KAF-16200 to a 10" newt with an IMX183. These two systems have very similar image scales and similar apertures, and are real systems you could get and use today. Both can deliver high resolution results that are comparable to each other.

 

The SCT system here has an advantage due to the larger aperture. The IMX183 has a very slightly larger image scale, but much higher quantum efficiency. The difference in Q.E. helps to counteract the aperture benefit of the SCT, effectively putting the two systems on equal footing.

 

The Newt system should still have higher throughput, thanks to the low read noise of the IMX183, it should be able to use shorter exposures, which reduces the cost of sub loss.

When it comes to tracking...image scale is going to be the biggest factor. An image scale of 0.45-0.5"/px very large, and that alone is going to put critical demands on tracking. The longer moment arm of the newt is a factor, but you really would not want to be under-mounted with either scope. Both are similar in weight, about 35-40lb with the scope alone, and several more pounds for additional equipment, dovetails, cameras, etc. So no low end mount is going to do, here. You would want something like a CGE, HDX, or a proper high end mount like an AP Mach 1, Paramount MyT, AP 1100, etc.

 

 


Does this imply that these large aperture scopes should generally be paired with such large pixels? And why?

 

When we start talking about 20-24" and larger scopes, the focal lengths tend to be pretty long regardless. Even with a CDK at f/6, a 20" scope has a 3000mm focal length. Outside of maybe solar system objects, small pixel CMOS cameras, even more moderately sized pixels, are going to be too small. Further, the field of view at 3000mm or longer with a Panasonic M or IMX183 sensor, even APS-C sized sensors, is going to be very small. So not only do bigger pixels pair better with big scopes like that, but you also want a big sensor. And read some of what John writes about his imaging experiences with the 16803 at just 14"...he usually gets a significant amount of data, often has to throw away a lot of it, and still usually needs to stack a lot of hours to get good SNR.

 


Thanks.
Could you explain why?

 

Continuing from above, and to answer this next question. Again...the image scale is the key factor here...doesn't much matter how big the pixels are, or even whether they have 60% or 75% or 84% Q.E...what matters is that each pixel only sees about half an arcsecond of sky, and you just need longer exposures and lots and lots of data to get good SNR at that kind of an image scale. At 0.2"/px, you would need four times the exposure (total exposure) as at 0.4"/px, and even more than that as at 0.5"/px, to get the same SNR. So bigger pixels are more effective here, if you want to be able to achieve a reasonable SNR in a reasonable amount of time. This is what John Hayes is saying. (Differences in Q.E. will ultimately pale in comparison to differences in aperture...if you had the means to get a 24" RC, and pair it with say a KAF-9000 with its 12 micron pixels, such a system would again have an image scale around 0.5"/px, and would be 4x as fast as say a 12" newt system that was also at 0.5"/px. Even with very high Q.E. you might gain about a 50% improvement over the KAF-9000, which is just not enough to overcome the benefit of the aperture. You could even build an even bigger system, with a 36" aperture, an even bigger sensor with even bigger pixels, and it would be even faster. Thing is you keep losing FoV here, and not everything is about getting the highest SNR out of the smallest FoV as fast as possible... :p)


Edited by Jon Rista, 06 November 2018 - 07:40 PM.


#69 jhayes_tucson

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Posted 06 November 2018 - 08:29 PM

With my EdgeHD11 at f/10 and 1.1" seeing, the fwhm of the star itself is 1.1" by definition - and with 0.2" pixels I would sample that peak at 5 points to capture the shape of the curve.  When I then align and stack, the sampling is effectively reduced to about 3 points.  So in the final stack if I zoom in on those small stars, they will have a natural profile and no loss of detail - which means they also stand up better to processing and additional enhancement - though I don't tend to do that.

 

More importantly - the fwhm that I measure will depend on the size of the pixels - and will tend to be smaller - in arc-seconds - with smaller pixels.  This is all empirical and involves subtleties of how the sensor behaves and what happens when you align and stack.

 

There is no Nyquist going on here - it's just an image that is discretely sampled - and then you look at the samples - after first blurring them and stacking them.  So there are multiple factors involved that point to the benefits of small pixels - and much smaller than typical Nyquist arguments would suggest.

 

And at the very worst - if you do end up wanting to bin the result because it is a bit soft in those conditions - the only downside is a bit more noise - or equivalently a bit more imaging time needed for the same result with larger pixels.

 

Frank

 

 

Just to be clear, the telescope itself is a low pass filter.  The highest spatial frequency passed by any telescope with a circular pupil is 1/(lamda*F/#) (in the image plane) and to reconstruct that information, you need at least two samples per cycle.  That means that sampling at any rate more than 4.88 points across the Airy disk contributes no additional information--no matter how small the pixels are.  Spatial frequencies above the cutoff frequency don't exist in the image plane.  This is the result of both optical diffraction and Nyquist information theory.  

 

John


Edited by jhayes_tucson, 07 November 2018 - 12:58 AM.


#70 rockstarbill

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Posted 06 November 2018 - 09:02 PM

I think there are different tools for different jobs, and different ideas of what quality is. We all have different, yet similar aspirations in this hobby. I like refractors, a lot. I like imaging with refractors, a lot. I like the stars they produce, more than other systems. That is just my preference though. At the same time, I also like to image things quickly. I live in the PNW and we dont get a ton of time most seasons to get data, so when I can get out, I like to take advantage of that. Speed is a factor in a lot of the decisions I make, but not all of them. I dont put speed of a system ahead of everything else, just for the sake of speed. I also have some aspirations in exploring NIR imaging, and going after things better suited to longer focal lengths. Sometimes we need to make a tradeoff for that, and I am personally just fine making it.

 

I prefer to have multiple optical systems and multiple cameras as options. Sure in some cases I will chase resolution as the top priority (and to that end - likely will end up with a QHY183M + CWF3 + OAG-S camera option). But I also like really huge wide fields, and enjoyed the 16200 sensor quite a bit. 



#71 freestar8n

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Posted 06 November 2018 - 09:04 PM

You are ignoring the blurring that happens during alignment and other factors like pixel crosstalk. Both act as an additional mtf that is happening on the scale of the pixels - and making the pixels smaller increases the bandwidth of the final result.

These are all very real effects that result in real benefits - in terms of smaller stars when the stars are well sampled.

Frank
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#72 jhayes_tucson

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Posted 06 November 2018 - 09:46 PM

Frank,

The image itself is bandwidth limited!  None of those effects will help to pull out additional information from a bandwidth limited image.  If you don't believe in diffraction theory then just say so up front and we'll move on.

 

John



#73 freestar8n

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Posted 06 November 2018 - 10:33 PM

The blurring on the pixel scale is an additional mtf much larger than diffraction. Diffraction is a fundamental limit but processing blurs things much more. And that blurring is reduced when the pixels are smaller.

This is all basic mtf filtration stuff. No matter how big or small the pixels are - when you align and interpolate you blur the image. If the pixels are 1um that additional blurring is less that if they are 10um. And that is only one of the ways pixels determine the final result. With a pixel level mtf.

Frank

#74 dhaval

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Posted 07 November 2018 - 12:43 AM

If we compare an 11" SCT with a KAF-16200 to a 10" newt with an IMX183. These two systems have very similar image scales and similar apertures, and are real systems you could get and use today. Both can deliver high resolution results that are comparable to each other.

 

The SCT system here has an advantage due to the larger aperture. The IMX183 has a very slightly larger image scale, but much higher quantum efficiency. The difference in Q.E. helps to counteract the aperture benefit of the SCT, effectively putting the two systems on equal footing.

 

The Newt system should still have higher throughput, thanks to the low read noise of the IMX183, it should be able to use shorter exposures, which reduces the cost of sub loss.

When it comes to tracking...image scale is going to be the biggest factor. An image scale of 0.45-0.5"/px very large, and that alone is going to put critical demands on tracking. The longer moment arm of the newt is a factor, but you really would not want to be under-mounted with either scope. Both are similar in weight, about 35-40lb with the scope alone, and several more pounds for additional equipment, dovetails, cameras, etc. So no low end mount is going to do, here. You would want something like a CGE, HDX, or a proper high end mount like an AP Mach 1, Paramount MyT, AP 1100, etc.

 

 

 

 

 

When we start talking about 20-24" and larger scopes, the focal lengths tend to be pretty long regardless. Even with a CDK at f/6, a 20" scope has a 3000mm focal length. Outside of maybe solar system objects, small pixel CMOS cameras, even more moderately sized pixels, are going to be too small. Further, the field of view at 3000mm or longer with a Panasonic M or IMX183 sensor, even APS-C sized sensors, is going to be very small. So not only do bigger pixels pair better with big scopes like that, but you also want a big sensor. And read some of what John writes about his imaging experiences with the 16803 at just 14"...he usually gets a significant amount of data, often has to throw away a lot of it, and still usually needs to stack a lot of hours to get good SNR.

 

 

 

 

Continuing from above, and to answer this next question. Again...the image scale is the key factor here...doesn't much matter how big the pixels are, or even whether they have 60% or 75% or 84% Q.E...what matters is that each pixel only sees about half an arcsecond of sky, and you just need longer exposures and lots and lots of data to get good SNR at that kind of an image scale. At 0.2"/px, you would need four times the exposure (total exposure) as at 0.4"/px, and even more than that as at 0.5"/px, to get the same SNR. So bigger pixels are more effective here, if you want to be able to achieve a reasonable SNR in a reasonable amount of time. This is what John Hayes is saying. (Differences in Q.E. will ultimately pale in comparison to differences in aperture...if you had the means to get a 24" RC, and pair it with say a KAF-9000 with its 12 micron pixels, such a system would again have an image scale around 0.5"/px, and would be 4x as fast as say a 12" newt system that was also at 0.5"/px. Even with very high Q.E. you might gain about a 50% improvement over the KAF-9000, which is just not enough to overcome the benefit of the aperture. You could even build an even bigger system, with a 36" aperture, an even bigger sensor with even bigger pixels, and it would be even faster. Thing is you keep losing FoV here, and not everything is about getting the highest SNR out of the smallest FoV as fast as possible... tongue2.gif)

Jon,

A lot in here, but you haven't spoken about well capacity - how does that play a role when comparing with some of these CMOS cameras, assuming similar scopes? I know a lot of people had apprehensions about the new KAF16200 chip, given that it was brought to market just around the 1600 Panasonic chip  and yet, by all accounts, I hear that the 16200 chip is just as good, if not better, than the 1600 chip (even when you have to take longer subs and such). 

 

BTW, thanks to all for the great depth of knowledge in this thread. Very much appreciated.

 

CS!



#75 TareqPhoto

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Posted 07 November 2018 - 12:44 AM

I think there are different tools for different jobs, and different ideas of what quality is. We all have different, yet similar aspirations in this hobby. I like refractors, a lot. I like imaging with refractors, a lot. I like the stars they produce, more than other systems. That is just my preference though. At the same time, I also like to image things quickly. I live in the PNW and we dont get a ton of time most seasons to get data, so when I can get out, I like to take advantage of that. Speed is a factor in a lot of the decisions I make, but not all of them. I dont put speed of a system ahead of everything else, just for the sake of speed. I also have some aspirations in exploring NIR imaging, and going after things better suited to longer focal lengths. Sometimes we need to make a tradeoff for that, and I am personally just fine making it.

 

I prefer to have multiple optical systems and multiple cameras as options. Sure in some cases I will chase resolution as the top priority (and to that end - likely will end up with a QHY183M + CWF3 + OAG-S camera option). But I also like really huge wide fields, and enjoyed the 16200 sensor quite a bit. 

This is my approach too, i want speed, i like to use refractors too but i don't mind another designs, and i also like to have options and not only using option over and over again for all targets all nights forever, and i was thinking between TAK FSQ106 or TOA130 now i think i will go with FSQ106 because i want so wide field which is with 106 and reducer more than 130 and reducers.




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