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Fast telescopes v/s high resolution

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#1 dhaval

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Posted 01 November 2018 - 12:49 PM

This may just end up being a ramble, so bear with me. I also am not technical enough to know a lot of things so please pardon things that I mention in this post that are not according to how people understand them. I am trying to be as laymanish as I can be. 

 

I have been following some telescope designs lately and I see a lot of telescope makers who design astrophotography telescopes are pushing towards designing and manufacturing fast telescopes (low focal ratio). The aperture hasn't increased (meaning, if they were manufacturing 10in, 12in and other dimension scopes before at a higher FR, the aperture seems to have stayed constant, thereby reducing the focal length to attain the necessary lower FR). Why the sudden push towards faster optics? I, for one, am disappointed in that trend to be honest. I image from Bortle 1 skies using really nice mounts and I want to pursue resolution - I am not worried to be imaging at F8 and above (with a larger aperture scope, I would really love imaging at 3M and above FL if I can). My only options currently are PW and/or Celestron EdgeHD/Meade ACF scopes - although even Celestron seems to be moving towards manufacturing RASA scopes that are fast. I asked a question on AGOptical FB page as to why they were no longer making "slower" scopes and the answer was vague - that's what our users would like to see (or something to that effect). BTW, this post is not against AGO, I am just making a point.

 

One thing that I should also mention is that cameras are also getting smaller pixels - would that mean that resolution is not necessarily being sacrificed? 

 

Like I mentioned before, I am not a big fan of large FOVs, I'd much rather go deep and get resolution. Am I in the minority for thinking like that?

 

What gives, I wonder?

 

CS! 

 



#2 Gipht

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Posted 01 November 2018 - 01:05 PM

With my 10"  f/4 reflector telescope and the ASI183MM-C Pro my image scale is 0.5"/pixel.   Most of the time, my seeing will not allow for any more resolution then that.   These smaller pixel CMOS cameras offer an alternate way to reach high resolution.


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#3 Jeff Struve

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Posted 01 November 2018 - 01:06 PM

Does it have to do with physical size?

 

My friends 8" F12

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#4 OleCuss

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Posted 01 November 2018 - 01:11 PM

Generally speaking, a longer focal length means more weight, more difficulty tracking, more sensitivity to atmospheric turbulence.  The convenience of fast optics is significant - and for those of us with inferior mounts it is huge.

 

The potential resolution of your optics is dependent on the aperture.  Longer focal length changes the image scale rather than the potential resolution of the optical component of your system..

 

Smaller pixels do not necessarily bring better resolution.  If you over-sample by using pixels which are too small for your focal length then you are increasing the noise (and decreasing SNR) and are likely not increasing the resolution at all.  If you are using a cooled low-noise modern CMOS camera then there is very little penalty from over-sampling but if you have a noisy CCD you will have a substantial decrease in SNR from over-sampling.


Edited by OleCuss, 01 November 2018 - 01:14 PM.

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#5 Der_Pit

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Posted 01 November 2018 - 01:26 PM

As Gipht writes - you should/have to match your focal length to the detector size, and base it on the resolution you want to achieve.  There of course you do have two parties - DSO and planetary.  DSO is limited by seeing, so going substantially below 0.5"/px doesn't pay off in most cases.  For those, the shortening seems logical.   Millisecond exposures are of course a different thing - there you want to (over)sample the resolution of the optics.  Then scales of 0.15"/px or less makes sense.  However, a scope with such 'natural' focal length might get impractical.  A high quality barlow is probably the better solution there...

 

So yes, I think there's more people that benefit from shorter FL scopes, and  I do believe AGO if they claim user requests.....


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#6 WadeH237

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Posted 01 November 2018 - 01:39 PM

I suspect that the answer that you got from AGO was genuine.  They build what people will buy.

 

All of the development in cameras right now is going into smaller and smaller pixels.  The image scale at longer focal lengths can get pretty crazy.  For example, if I use my ASI1600 with my C14 at native focal length, my image scale is at 0.2 arc seconds per pixel.  Nobody has seeing good enough to support that image scale.  And they are making cameras with pixels even smaller now.

 

The way that I think about resolution is that my skies on a decent night are around 2 arc seconds.  Using the rule of thumb of using an imaging scale that is 1/3rd the seeing, it means that I'm typically looking for an image scale of around 0.6 or 0.7 arc seconds per pixel.  The next factor is the resolving power of the telescope.  If I've done the math correctly, the Dawes limit suggests that a telescope of 193mm aperture can resolve down to 0.6 arc seconds.  That means that I need an aperture of about 8" to get to about the best resolution I can with my skies.

 

So right now, I have an EdgeHD 8 set up with an F/7 reducer.  It gives me about 1450mm of focal length and an image scale of 0.55 arc seconds per pixel.  So I am pretty close to the sweet spot for resolution.  Going to longer focal length isn't really going to help me unless I can find better skies.

 

So given that I've got the resolution about the best I can, what would help me even more?  The answer is a faster telescope.  If I want to optimize my imaging time (living in an area with nearly 300 overcast nights a year), I would benefit from reaching target signal to noise ratio as quickly as possible.

 

I've got plenty of mount capacity to handle a whole lot more telescope.  Let's say that I want to step up to a 14" scope.  To get a 14" scope without pushing the focal length beyond about 1500mm, I'm looking for an F/4 instrument.  Such a thing would be ideal for me and would dramatically reduce the time for me to reach a target signal-to-noise ratio.  Note, though, that this is even faster than what's currently being offered in this size scope.

 

As such, I'm happy that they are building faster instruments.


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#7 CygnusBob

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Posted 01 November 2018 - 01:46 PM

While a fast low F ratio telescope has a lot of practical advantages, a significant disadvantage is that it is more difficult to achieve diffraction limited imaging.  Collimation tolerances become tighter, focus depth of field decreases.  Fabrication of the optics is more difficult.  However, it may be worth it depending on what you are doing.

 

Bob


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#8 dhaval

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Posted 01 November 2018 - 01:52 PM

Thanks to all for responding so far.

 

I get (at least I hope I do) the intricasies of focal length, focal ratio, aperture, pixel size and seeing and its relative impact on resolution, image scale, etc. I also realize that most people are seeing limited, mount limited, etc. and if that is what gives, then it would make sense for telescope manufacturers to keep producing faster scopes, still leaving a gap (at least in my mind) for those that want and can do high resolution imaging.

 

CS!

 

PS: Jeff, that post is funny! I guess there is no practical way to imaging with that scope on that mount! 


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#9 Jeff Struve

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Posted 01 November 2018 - 02:02 PM

Thanks to all for responding so far.

 

I get (at least I hope I do) the intricasies of focal length, focal ratio, aperture, pixel size and seeing and its relative impact on resolution, image scale, etc. I also realize that most people are seeing limited, mount limited, etc. and if that is what gives, then it would make sense for telescope manufacturers to keep producing faster scopes, still leaving a gap (at least in my mind) for those that want and can do high resolution imaging.

 

CS!

 

PS: Jeff, that post is funny! I guess there is no practical way to imaging with that scope on that mount! 

It is incredible visually... and the AP mount handles it fine... have not tried AP with it, but maybe!



#10 Terry R

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Posted 01 November 2018 - 07:38 PM

 

One thing that I should also mention is that cameras are also getting smaller pixels - would that mean that resolution is not necessarily being sacrificed? 

 

 

 

CS! 

 Smaller pixels do not mean higher resolution.  That more image scale.  Resolving comes with an increase in aperature.   


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#11 Jon Rista

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Posted 01 November 2018 - 10:48 PM

This may just end up being a ramble, so bear with me. I also am not technical enough to know a lot of things so please pardon things that I mention in this post that are not according to how people understand them. I am trying to be as laymanish as I can be. 

 

I have been following some telescope designs lately and I see a lot of telescope makers who design astrophotography telescopes are pushing towards designing and manufacturing fast telescopes (low focal ratio). The aperture hasn't increased (meaning, if they were manufacturing 10in, 12in and other dimension scopes before at a higher FR, the aperture seems to have stayed constant, thereby reducing the focal length to attain the necessary lower FR). Why the sudden push towards faster optics? I, for one, am disappointed in that trend to be honest. I image from Bortle 1 skies using really nice mounts and I want to pursue resolution - I am not worried to be imaging at F8 and above (with a larger aperture scope, I would really love imaging at 3M and above FL if I can). My only options currently are PW and/or Celestron EdgeHD/Meade ACF scopes - although even Celestron seems to be moving towards manufacturing RASA scopes that are fast. I asked a question on AGOptical FB page as to why they were no longer making "slower" scopes and the answer was vague - that's what our users would like to see (or something to that effect). BTW, this post is not against AGO, I am just making a point.

 

One thing that I should also mention is that cameras are also getting smaller pixels - would that mean that resolution is not necessarily being sacrificed? 

 

Like I mentioned before, I am not a big fan of large FOVs, I'd much rather go deep and get resolution. Am I in the minority for thinking like that?

 

What gives, I wonder?

 

CS! 

The big change these days is smaller pixels that still have relatively high efficiency and, for the pixel size, surprisingly large FWC. 

 

While small pixels don't usually have the kind of FWCs that huge pixel sensors of the past had (which could range from 40k to 100k electrons), they still have 15k-25k, and as CMOS technologies continue to improve, those FWCs on a per-area basis are actually still climbing (while read noise continues to drop, which improves DR.) 

 

If apertures are, as you noted, being maintained, then you can achieve the same resolution with smaller pixels. The diffraction limited resolution would be the same, so as long as the image scales are the same...a smaller, faster system will perform in the same ballpark as much larger, slower systems paired with huge pixels. 

 

The trend overall is to lower cost. 

 

Push come to shove, the immense dynamic range of a KAF-16803 with a 11-14" SCT will still outperform an 11-12" f/4 Newt with an IMX183, Panasonic M, and similar small pixel sensors. It is tough to beat 100ke- FWC and 13.2 stops of dynamic range no matter how you slice it (although, the new Kepler 4040 about does it, albeit without the cost savings of the smaller/faster systems). But ALL the costs increase if you consider a large newt with a huge camera. You need a big mount to handle the weight of the scope and camera, you need a big focuser to handle the weight of the camera. All of these things are not particularly portable, so you need somewhere to put it...an observatory. The cost is MANY times more than a fast newt with an IMX183.

 

But...an IMX183 with an f/4 newt will perform pretty close to a KAF-16803 on an f/10 SCT, with very similar image scales. As long as diffraction is appreciably smaller than seeing, you can have your high resolution details with either system. (And, in fact, the newt system would actually produce a larger FoV...and STILL deliver the same resolution...)


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#12 dhaval

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Posted 02 November 2018 - 09:51 AM

The big change these days is smaller pixels that still have relatively high efficiency and, for the pixel size, surprisingly large FWC. 

 

While small pixels don't usually have the kind of FWCs that huge pixel sensors of the past had (which could range from 40k to 100k electrons), they still have 15k-25k, and as CMOS technologies continue to improve, those FWCs on a per-area basis are actually still climbing (while read noise continues to drop, which improves DR.) 

 

If apertures are, as you noted, being maintained, then you can achieve the same resolution with smaller pixels. The diffraction limited resolution would be the same, so as long as the image scales are the same...a smaller, faster system will perform in the same ballpark as much larger, slower systems paired with huge pixels. 

 

The trend overall is to lower cost. 

 

Push come to shove, the immense dynamic range of a KAF-16803 with a 11-14" SCT will still outperform an 11-12" f/4 Newt with an IMX183, Panasonic M, and similar small pixel sensors. It is tough to beat 100ke- FWC and 13.2 stops of dynamic range no matter how you slice it (although, the new Kepler 4040 about does it, albeit without the cost savings of the smaller/faster systems). But ALL the costs increase if you consider a large newt with a huge camera. You need a big mount to handle the weight of the scope and camera, you need a big focuser to handle the weight of the camera. All of these things are not particularly portable, so you need somewhere to put it...an observatory. The cost is MANY times more than a fast newt with an IMX183.

 

But...an IMX183 with an f/4 newt will perform pretty close to a KAF-16803 on an f/10 SCT, with very similar image scales. As long as diffraction is appreciably smaller than seeing, you can have your high resolution details with either system. (And, in fact, the newt system would actually produce a larger FoV...and STILL deliver the same resolution...)

Jon,

Thanks for the excellent write up. Much appreciated. I guess one of the things that I did want to find out was, are we, getting our money's worth by using "fast" systems with small pixel CMOS cameras. 

 

Going by experience, I have received some data from a friend who is in the process of automating their 27in scope (believe it is F6, so not necessarily the slowest that's out there, but not that "fast" as well, with around 4100mm FL). They are using a 16803 chip camera and the data that I received, was around 5 hours (LRGB), was simply stunning. I could not believe the amount of detail that I was viewing in that image. 

 

That got me thinking around why is it that telescope manufacturers are pushing for fast systems? The real question was, are we sacrificing quality in a bid to lower costs? But I suppose there is no clear answer to this - it seems that manufacturers want to drive the cost down (which is great), but it also sounds like there is not a whole lot of degradation from a quality standpoint (based on your reading, which is also great). I am hoping that is true and remains so! 

 

CS!



#13 Jon Rista

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Posted 02 November 2018 - 10:21 AM

Jon,

Thanks for the excellent write up. Much appreciated. I guess one of the things that I did want to find out was, are we, getting our money's worth by using "fast" systems with small pixel CMOS cameras. 

 

Going by experience, I have received some data from a friend who is in the process of automating their 27in scope (believe it is F6, so not necessarily the slowest that's out there, but not that "fast" as well, with around 4100mm FL). They are using a 16803 chip camera and the data that I received, was around 5 hours (LRGB), was simply stunning. I could not believe the amount of detail that I was viewing in that image. 

 

That got me thinking around why is it that telescope manufacturers are pushing for fast systems? The real question was, are we sacrificing quality in a bid to lower costs? But I suppose there is no clear answer to this - it seems that manufacturers want to drive the cost down (which is great), but it also sounds like there is not a whole lot of degradation from a quality standpoint (based on your reading, which is also great). I am hoping that is true and remains so! 

 

CS!

I suspect that a 27" f/6 (PlaneWave CDK?) would certainly be of higher optical quality than something like an ONTC Newt from Teleskop Service. The TS mirrors seem to be fairly good, better than the general consumer-grade GSO mirrors, but I doubt they are of the same caliber as an ion-milled mirror. That said, you CAN get some very good newtonian astrographs with very high end mirrors. I've read about some with 1/10th wave (and even better, but you have to phone in the orders so I don't know how much better you can go). The scopes are again pricey, but if you want the quality in the optics, and also want the scope to be fast, it is possible to get both.

 

Camera-wise, a KAF-16803 has the edge when it comes to dynamic range and has a 16-bit ADC. It is difficult to clip  stars with it, even with very long exposures (at around 4000mm FL it is possible to get 20 minute LRGB subs with minor clipping!) Data quality wise, I am not sure that the out-of-camera performance is really all that great. I've seen some very clean 16803 data, as well as some pretty darn dirty 16803 data. Column defects on such a large chip are not uncommon, and it also seems it can have amp glow in one corner. 

 

The IMX183 has some strong amp glows, strong enough that it does introduce a small amount of additional shot noise in those areas of the frame. However it seems to calibrate out fine. From a calibrated sub standpoint, my personal experience is the KAF-16803 and IMX183 data is on par. The 16803 usually has some remnant column defects and sometimes a bit of remnant glow, the IMX183 sometimes has some slight random horizontal banding. In both cases, cosmetic correction will correct most of the remnant FPN, and stacking averages out the remaining random noise. 

 

The bigger differences between current generation CMOS cameras and long-established CCD cameras are going to be things like reliability, robustness, longevity. CMOS cameras have only been around for a couple of years. CCD cameras have been around for decades, and some have even resided in remote observatories, unattended to, and operated for a decade or more in some cases. There are certainly issues and failures with CCD cameras, but overall they have a pretty good track record of being reliable for long term remote operation. That cannot be said about CMOS cameras yet, so if you NEED high reliability and robustness, and NEED very high grade optical quality, then those are certainly reasons to consider the much more expensive, larger systems. However, if you want good IQ at a lower price, then I think it can certainly be attained with some of the current generation CMOS cameras...and at significantly lower cost. (Lower cost not just because of the camera, but because you can get smaller scopes, which overall lead to lower weight, which is less demanding on focuser and mount, so the whole entire system can shrink.)



#14 jhayes_tucson

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Posted 02 November 2018 - 11:11 AM

Jon,

Thanks for the excellent write up. Much appreciated. I guess one of the things that I did want to find out was, are we, getting our money's worth by using "fast" systems with small pixel CMOS cameras. 

 

Going by experience, I have received some data from a friend who is in the process of automating their 27in scope (believe it is F6, so not necessarily the slowest that's out there, but not that "fast" as well, with around 4100mm FL). They are using a 16803 chip camera and the data that I received, was around 5 hours (LRGB), was simply stunning. I could not believe the amount of detail that I was viewing in that image. 

 

That got me thinking around why is it that telescope manufacturers are pushing for fast systems? The real question was, are we sacrificing quality in a bid to lower costs? But I suppose there is no clear answer to this - it seems that manufacturers want to drive the cost down (which is great), but it also sounds like there is not a whole lot of degradation from a quality standpoint (based on your reading, which is also great). I am hoping that is true and remains so! 

 

CS!

 

Dhaval,

Jon has provides some excellent responses here, but I want to add that as the scope gets bigger, the atmosphere becomes the limiting factor with respect to resolution.  However, consider this:  The resolution of my C14 at F/11 is limited by the atmosphere to about 1" under the best conditions.  If I were to move to a 20" PlaneWave at F/6.7 using the same KAI-16803 sensor, the image scale would be nearly the same and the resolution would still be around 1"; but, the signal would be around 2.5x higher (due to the focal ratio advantage roughly balanced against the obscuration ratio.)  That would allow either significantly shorter exposures or allow going significantly deeper with the same exposures.  That's the advantage of having a faster scope.  Even though the atmosphere limits the resolution, I would get nearly the same image scale with significantly more signal.  Under the right conditions, bigger and faster are always better...and that's what ultimately led to large telescopes in the world of professional astronomy.

 

John


Edited by jhayes_tucson, 02 November 2018 - 07:16 PM.

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#15 dhaval

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Posted 02 November 2018 - 12:56 PM

Thanks Jon and John. I very much appreciate the insightful responses!

 

We are putting up a C11 EdgeHD with QHY814A camera. The system will be severly oversampled even for our Bortle 1 skies with really good seeing - we usually get seeing between 1"-1.5" in the winter months, but with 0.27"/px @ 1x1 binning, I wonder how the system will perform? I that does not work, we may have to bin 2x2 or change the camera (I do have the QHY16200 as backup). 

 

CS!



#16 Jon Rista

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Posted 02 November 2018 - 01:29 PM

Thanks Jon and John. I very much appreciate the insightful responses!

 

We are putting up a C11 EdgeHD with QHY814A camera. The system will be severly oversampled even for our Bortle 1 skies with really good seeing - we usually get seeing between 1"-1.5" in the winter months, but with 0.27"/px @ 1x1 binning, I wonder how the system will perform? I that does not work, we may have to bin 2x2 or change the camera (I do have the QHY16200 as backup). 

 

CS!

With seeing of 1", an image scale of 0.27" would sample a bit over 3.5x. If you want to maximize resolution potential, then that would be pretty ideal, IMO. With 1.5" seeing, you would be sampled at around 5.5x, which is probably a bit much. On average I guess you would be sampling around 4x or so. However, from a background signal standpoint, the difference between 1" and 1.5" seeing actually wouldn't have any impact...it would be f/10 either way. So it isn't like sampling STARS at 5.5x vs. 3.5x will actually change your object signal. You'll get the same object signal either way. 

 

I think that, combined with effective deconvolution, that kind of sampling could be pretty darned ideal for high resolution imaging. Well sampled stars at 3-4x will lose a minimal amount of detail during registration, and should deconvolve very well. So you might not need to bin.

 

If you feel you do need more signal, then it might be better to drop a reducer on the scope, rather than bin. That would be about a 40% change in sampling, rather than a 2x change in sampling, which might lead to a better balance of resolution vs. SNR.


Edited by Jon Rista, 02 November 2018 - 01:30 PM.

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#17 Noah4x4

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Posted 02 November 2018 - 01:29 PM

I'm lost amongst all this technical stuff. But what about considering 'fast scopes PLUS high resolution' rather than "fast scopes versus high resolution"? Perhaps more affordable CMOS cameras now enable both?  

 

My Atik Horizon OSC on Hyperstar with its 16 megapixel output to an Intel NUC with Iris Plus 640 Graphics then over Thunderbolt display cable to a 28" 4K UHD monitor is (IMHO) pretty awesome. I obviously can't argue that it is better than a RASA set up (and probably isn't). But Hyperstar is about a third of the incremental cost of acquiring a RASA if you already own a decent OTA. When paired with a high resolution CMOS camera (here I include ZWO ASI1600) the results are (IMHO) quite compelling. There are now plenty of Atik Horizon (ZWO ASI1600)/Hyperstar images in the galleries for people to consider. This post is more about practical implications for a novice (like me).

 

The FOV is enormous due to removal of my secondary mirror and that has an inevitable corresponding effect on object size (e.g. smaller than OTA rear mounted). But that is compensated by the zoom capabilities of having available a 3840 x 2160 pixels display and a camera outputting 4644 x 3506. I can zoom in like Buck Rogers at warp speed retaining beautiful detail. Hyperstar at f/1.9 also means a 30 second image delivers the same degree of detail as a 14 minutes at f/10. So am I enjoying the best of both worlds, fast scope + high resolution?

 

To be fair, my focus is more on EAA rather than AP and I am not inclined to sit out all night on a single image trying to become astrophotographer of the year.  Hyperstar allows me to "cheat" (no polar alignment, no wedge, no guiding). But with a suitable camera, computer and display the 4k UHD experience is great for those like me that have limited photographic experience and are hence perhaps unable to conventionally tease out extra detail given our limited AP skills. I can be viewing beautifully detailed DSO objects inside a few minutes. The Atik camera including software instruction manual is also a mere 28 pages.  My Nikon DSLR has a baffling 200+ pages. Res Ipsa Loquitor.


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#18 jhayes_tucson

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Posted 02 November 2018 - 07:24 PM

Thanks Jon and John. I very much appreciate the insightful responses!

 

We are putting up a C11 EdgeHD with QHY814A camera. The system will be severly oversampled even for our Bortle 1 skies with really good seeing - we usually get seeing between 1"-1.5" in the winter months, but with 0.27"/px @ 1x1 binning, I wonder how the system will perform? I that does not work, we may have to bin 2x2 or change the camera (I do have the QHY16200 as backup). 

 

CS!

 

Dhaval,

Let me know how it works, but I suspect that you are going to be a bit underwhelmed by the signal level.  Oversampling at F/11 leads to a very low signal along with a lot of photon noise.  The good news is that if you can bin it 2x2, it will work noticeably better.  

 

I've had both the 16803 and the 16200 on my scope and the 16803 works a LOT better.  I have identical frames with identical cooling and exposures from both cameras that I keep meaning to post to demonstrate the difference.  I'll have to get around to it one of these days.  It's a good demo of the effects of sampling differences with the only variable being the pixel size.

 

John


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#19 Jon Rista

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Posted 02 November 2018 - 08:30 PM

Dhaval,

Let me know how it works, but I suspect that you are going to be a bit underwhelmed by the signal level.  Oversampling at F/11 leads to a very low signal along with a lot of photon noise.  The good news is that if you can bin it 2x2, it will work noticeably better.  

 

I've had both the 16803 and the 16200 on my scope and the 16803 works a LOT better.  I have identical frames with identical cooling and exposures from both cameras that I keep meaning to post to demonstrate the difference.  I'll have to get around to it one of these days.  It's a good demo of the effects of sampling differences with the only variable being the pixel size.

 

John

Don't forget that the ICX814 has very high Q.E. and relatively low read noise. I've seen numerous images at scales between 0.2 and 0.3 arcsec/px with the ASI1600. It has ultra low read noise but more average Q.E. and DR and the results were pretty good. 


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#20 dhaval

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Posted 02 November 2018 - 08:33 PM

Dhaval,

Let me know how it works, but I suspect that you are going to be a bit underwhelmed by the signal level.  Oversampling at F/11 leads to a very low signal along with a lot of photon noise.  The good news is that if you can bin it 2x2, it will work noticeably better.  

 

I've had both the 16803 and the 16200 on my scope and the 16803 works a LOT better.  I have identical frames with identical cooling and exposures from both cameras that I keep meaning to post to demonstrate the difference.  I'll have to get around to it one of these days.  It's a good demo of the effects of sampling differences with the only variable being the pixel size.

 

John

If that is the case we will have three options - first one will be to bin 2x2. I am guessing we should be ok with that. The second option may be to use the ZWO 1600 CMOS on the C11. I know how that performs - typically, I have been very impressed by images from that set up. Some of the very best that I have seen. Of course some of that is imager dependent, but still. The third option will be to use the QHY16200 on the scope. That should be similar to binning 2x2, just a larger FOV given the size of the sensor. Which might not be that bad as well.

 

The good thing about IC814 is that it is a Sony sensor with a high level of QE and very low noise - in fact, from what I have heard, it is so clean that people don't take darks with that. It is a CCD, so no worries about binning as well.

 

We shall see...

 

CS!



#21 dhaval

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Posted 02 November 2018 - 08:35 PM

Don't forget that the ICX814 has very high Q.E. and relatively low read noise. I've seen numerous images at scales between 0.2 and 0.3 arcsec/px with the ASI1600. It has ultra low read noise but more average Q.E. and DR and the results were pretty good. 

We must have been typing at the same time Jon.

 

Yes, what I have heard about the IC814 chip is consistent with what you are saying, so I am hopeful.

 

CS!



#22 jhayes_tucson

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Posted 02 November 2018 - 11:35 PM

Don't forget that the ICX814 has very high Q.E. and relatively low read noise. I've seen numerous images at scales between 0.2 and 0.3 arcsec/px with the ASI1600. It has ultra low read noise but more average Q.E. and DR and the results were pretty good. 

 

Yeah, I'm familiar with the high QE and low read noise for that camera (which are quite impressive.)  The low read noise makes it possible to use shorter exposures but to get the unbinned photon noise back to the level of a camera with larger pixels, you still have to use a lot more total exposure time to achieve the same SNR.  So unless you are doing super short exposures (aka lucky imaging,) the gain in sampling doesn't do anything to produce more image detail at the expense of spending a lot more time gathering the data.  Giving up on "detail" and simply binning is the fix to that problem.

 

John



#23 dhaval

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Posted 03 November 2018 - 12:21 AM

Yeah, I'm familiar with the high QE and low read noise for that camera (which are quite impressive.)  The low read noise makes it possible to use shorter exposures but to get the unbinned photon noise back to the level of a camera with larger pixels, you still have to use a lot more total exposure time to achieve the same SNR.  So unless you are doing super short exposures (aka lucky imaging,) the gain in sampling doesn't do anything to produce more image detail at the expense of spending a lot more time gathering the data.  Giving up on "detail" and simply binning is the fix to that problem.

 

John

John,

Typically, with CCDs, we do at least 20 minute subs for LRGB and around 30 minutes for NB. I hope the AP900 with appropriate guiding is up to the task for such exposure times. If it is, the total integration time will be around 20-25 hours for each subject. I am hoping that should suffice. 

 

CS!



#24 jhayes_tucson

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Posted 03 November 2018 - 12:43 AM

John,

Typically, with CCDs, we do at least 20 minute subs for LRGB and around 30 minutes for NB. I hope the AP900 with appropriate guiding is up to the task for such exposure times. If it is, the total integration time will be around 20-25 hours for each subject. I am hoping that should suffice. 

 

CS!

 

Dhaval,

That should work.  I typically total anywhere from 15 - 40 hours shooting 20 minutes subs with my ML16803.  They are completely different cameras so I look forward to seeing how well it works for you and (maybe) comparing some results along the way.

 

John


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#25 freestar8n

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Posted 03 November 2018 - 03:09 AM

This may just end up being a ramble, so bear with me. I also am not technical enough to know a lot of things so please pardon things that I mention in this post that are not according to how people understand them. I am trying to be as laymanish as I can be. 

Hi dhval-

 

Your first question was about why there is a trend toward faster scopes - and to me it's puzzling because there are still many people who think f/ratio is a dated concept with no relevance, while others think it is crazy to try to image with a scope slower than f/8.  In the case of sct's for imaging, I think the market tends to feel that f/10 is on the slow side and you need exceptional seeing and guiding to get benefit from that image scale.  But if you have Bortle 1 skies and seeing in the low 1" - and if you are striving for max detail - then you are in a position to benefit from long focal length and small pixels - as long as the objects you want to image will fit on your sensor.

 

There are many indirect benefits of small pixels that allow them to continue to gain resolution as they get smaller - and much smaller than typical "matching" criteria.  The fwhm corresponds to the diameter of the central peak part of a star profile - and for best alignment and stacking - and good star appearance - it is good to sample it very well.  So if your seeing might be in the low 1" - and if you value detail - then 0.2" per pixel is not overkill and would likely be much better than 0.4".

 

At the same time, if you did image an object hoping for great detail - and the result was somewhat soft - you can always bin or smooth it in post-processing with little loss compared to using a larger pixel camera in the first place.  But if you only have a large pixel camera - there is no way to recover detail lost by using those big pixels.

 

If you have very dark skies then you will be in a position to be concerned about read noise - and the cameras you mention have a wide range or read noise.  In that case a small pixel cmos camera with low read noise may do much better - both in terms of resolution and snr - than a larger pixel ccd camera.

 

I image with EdgeHD11 at a range of 0.2" pixels at f/10 with ASI290 uncooled lucky-ish imaging with 2s exposures - up to about 1.4" per pixel with hyperstar - and images are on my astrobin and MetaGuide web pages.  My main recommendation is that if you strive for max detail, go for small pixels, long focal length, and low read noise.  But be prepared to expose longer in order to go deep.  You can always expose longer to go deeper with small pixels - but with large pixels you can't do anything to extract finer detail - except maybe drizzling but I don't find that works well.

 

A large ccd with large pixels in combination with a smaller cmos camera with small pixels may work well - as long as you can easily switch them for different objects and different imaging goals.

 

Frank


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