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Seeking Advice on Small Pixel Camera Purchase

astrophotography ccd CMOS dso equipment imaging refractor
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#1 BenKolt

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Posted 04 June 2020 - 04:50 PM

Greetings!

 

I must confess that I've been overwhelmed with all the previous discussions related to this topic.  I'd like to please ask my specific questions fresh as they relate to what I am wishing to do.

 

I wish to use my refractor for smaller scale imaging on tight FOV framing of DSO's, specifically galaxies.  This is a TS 130mm APO refractor with 910mm FL.  This is for monochrome LRGB or RGB imaging, primarily in a suburban light polluted environment.

 

Presently, using my FLI ML16200 and QSI 683 CCD's, I achieve image scales of 1.36 and 1.22 arcsec/px, respectively.  It is my wish to achieve sub-seeing resolution on par with what I get with these cameras on my Celestron 11" scope, FL 2800mm.  With that scope, these two cameras achieve 0.44 and 0.40 arcsec/px, respectively.  I want to reproduce this capability with my refractor.  (As an aside in case you are wondering, sub-seeing resolution allows me to better explore the possibilities of deconvolution and other processing steps.)

 

What camera would you recommend I consider for getting smaller resolution with the TS scope?   Note that I am only interested in monochrome, cooled sensors.

 

I have no personal experience with CMOS cameras, although their pixel sizes seem to be a good match for what I wish to do.  I've looked more closely at the ASI line of cameras because ASI has a filter wheel that can use my existing set of 31mm unmounted filters.  The ASI183MM Pro and 1600MM Pro, for example, would achieve resolutions of 0.54 and 0.86 arcsec/px.  The 183 would get me close to what I want.  (Note that I am not interested at this time in paying for a more expensive camera such as the ASI8200MM Pro, and that one would also necessitate I purchase a new set of filters.)

 

Would the ASI 183 be a recommended camera for this purpose?

 

Some of the issues that concern me, however, are what effects will I encounter with this camera in comparison with my CCD experience.  To what extent does 12-bit instead of 16-bit acquisition cause an issue?  The dynamic ranges of both the 183 and 1600 look impressive, but does one truly get to use that range when in 12-bit?  How bad of an issue is amp glow in the 183; to what extent does it affect calibration and processing?

 

Another question I have is the effect of Dawe's limit using this scope and camera combination.  The TS 130/910 has a Dawe's limit of 0.89 arcsec, whereas my resolution would be 0.54 arcsec/px.  I know this would present a problem were I interested in discerning binary stars, however to what extent is this an issue with DSO imaging?  Does Dawe's limit present a physical limit to the effectiveness of camera resolution when imaging a galaxy as well as star separation?  I note that with my larger pixel CCD's with the C11, the camera resolution is matched right at that scope's Dawe's limit.  But, again, with the 183 on the TS 130/910, the resolution is about half of the scope's Dawe's limit.

 

Finally, I realize I've only looked so far into these particular ASI CMOS cameras.  Are there other small pixel CMOS or perhaps small pixel CCD's that I ought to consider?  If there are suitable CCD's (with suitable FW's that can take my 31mm filters), that might be an even better choice since I presently have no experience with CMOS cameras.

 

Thank you for your help with this decision, and I look forward to your responses.

 

Best Regards,

Ben

 

 

 

 



#2 andysea

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Posted 04 June 2020 - 06:14 PM

As far as I know the Dawes limit is a hard and fast number and you can’t get around that. I may be wrong but in our area your imaging is going to be seeing limited even at 1.2”~1.4”/px. I would be very interested in seeing what you are able to coax out of your data using deconvolution and over sampling.

#3 freestar8n

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Posted 04 June 2020 - 07:14 PM

Dawes limit isn't ideal for these discussions since it involve separating stars under ideal conditions so there is a gap between them.

 

Instead I think it's clearer to think just in terms of fwhm in arc-seconds.

 

A 130mm scope with no secondary will have a diffraction limited fwhm of about 0.9" in green light.  If your inherent seeing is around 2" then this additional diffraction blurring will have a fairly small bloating effect - maybe to 2.2".  Either way - what matters is the actual size of stars in your images.  They could end up 3" fwhm or 1.5" fwhm.  You won't know until you guide and image with the final system - but you can see what high end imagers in your area achieve in terms of small fwhm.

 

With 910mm focal length, 3.75um pixels will be 0.85" wide.  That would not be big enough to see the Airy pattern well, but it will do a reasonable job of revealing the shape of the stars, with about 3 pixels across the fwhm.  For high res work and max detail, smaller pixels would probably yield more detail at that focal length - but it may not be worth the trouble if it means a smaller sensor or much more cost.

 

Once you know the ota, the next question would be what objects you want to image and how big a sensor you need.  Once you know that you can consider pixel size - but as long as read noise is low there is little downside to very small pixels.  There are multiple trade offs going on and smaller pixels will likely provide more detail - but the benefit will begin to fade out once they are small enough to have about 3 pixels across the fwhm you can achieve.

 

Frank


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#4 andysea

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Posted 04 June 2020 - 08:49 PM

I can tell you that the best fwhm that I got in our area is ~1.8” with the toa130 or with the 10” dsi rc.
A lot of times the fwhm is ~3”. The seeing Around here is usually pretty bad.
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#5 dayglow

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Posted 05 June 2020 - 02:54 AM

Here is an estimation of achievable resolution vs pixel size over a range of atmospheric conditions.  This takes into account diffraction limit of optics, atmospheric seeing, mount / tracking accuracy and parameterizes pixel size.  These results are based on a 2.5x sampling factor, meaning that resolution improves through higher sampling up to 2.5 times the minimum star size and beyond that rate no further improvement in resolution is obtained.

 

I can provide equations later if you like.

 

refractor 130mm

 

-- David F.



#6 ChrisWhite

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Posted 05 June 2020 - 07:12 AM

Ben there really arent too many choices for the tiny pixel game.  I've used the 183 and the 1600 extensively, and hands down the 183 would be my choice.  There are so many things I dont like about the 1600 and the only thing that I dont like about the 183 is the severe ampglow... but that calibrates out perfectly with good calibration practices. 

 

The 183 has a much nicer noise profile and the calibrated files are a pleasure to work with.  Very CCD like IMO, which I think you would appreciate.

 

EDIT-  since you are also interested in CCD suggestions with small pixels. The QSI 6120 would fit the bill.   It produces the cleanest data I have EVER seen.  I'm just blown away by how clean the data is.  (A very subjective term I know).  There are zero quirks with this camera and it just performs.  You can take CMOS short exposures, as the RN is so low (1.9 to 2.1e-) but with slower scopes does brilliantly with 10 and even 20 min exposures (My edge at f7 for example).  Pixel size is 3.1um so it's right between the 183 and 1600.  If you want to see some data from this camera, I am happy to share.  It's pricey, but can be found on the used market for a good deal.  


Edited by ChrisWhite, 05 June 2020 - 07:17 AM.

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#7 BenKolt

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Posted 05 June 2020 - 09:04 AM

Dawes limit isn't ideal for these discussions since it involve separating stars under ideal conditions so there is a gap between them.

 

Instead I think it's clearer to think just in terms of fwhm in arc-seconds.

 

A 130mm scope with no secondary will have a diffraction limited fwhm of about 0.9" in green light.  If your inherent seeing is around 2" then this additional diffraction blurring will have a fairly small bloating effect - maybe to 2.2".  Either way - what matters is the actual size of stars in your images.  They could end up 3" fwhm or 1.5" fwhm.  You won't know until you guide and image with the final system - but you can see what high end imagers in your area achieve in terms of small fwhm.

 

With 910mm focal length, 3.75um pixels will be 0.85" wide.  That would not be big enough to see the Airy pattern well, but it will do a reasonable job of revealing the shape of the stars, with about 3 pixels across the fwhm.  For high res work and max detail, smaller pixels would probably yield more detail at that focal length - but it may not be worth the trouble if it means a smaller sensor or much more cost.

 

Once you know the ota, the next question would be what objects you want to image and how big a sensor you need.  Once you know that you can consider pixel size - but as long as read noise is low there is little downside to very small pixels.  There are multiple trade offs going on and smaller pixels will likely provide more detail - but the benefit will begin to fade out once they are small enough to have about 3 pixels across the fwhm you can achieve.

 

Frank

Thanks for the inputs, Frank.  Your point about FWHM from seeing vs. pixel size is well taken.  I didn't get into this point with my original post, but it has been on my mind.  I'll ponder this some more.

 

I can tell you that the best fwhm that I got in our area is ~1.8” with the toa130 or with the 10” dsi rc.
A lot of times the fwhm is ~3”. The seeing Around here is usually pretty bad.

I know what you mean, Andy!  Interestingly, I've observed from my location in Puget Sound, particularly during the summer months, that the seeing can at times be quite good, as low as 1.5" or maybe even lower, during the early morning hours past midnight.  This is not the norm during most of the year, but I've hit that sweet spot at times.
 

Ben



#8 BenKolt

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Posted 05 June 2020 - 09:05 AM

Here is an estimation of achievable resolution vs pixel size over a range of atmospheric conditions.  This takes into account diffraction limit of optics, atmospheric seeing, mount / tracking accuracy and parameterizes pixel size.  These results are based on a 2.5x sampling factor, meaning that resolution improves through higher sampling up to 2.5 times the minimum star size and beyond that rate no further improvement in resolution is obtained.

 

I can provide equations later if you like.

 

 

 

-- David F.

David, please post or send me the equations used to make this chart.  I'm curious about what goes into that equation.  Thanks!

 

Ben



#9 BenKolt

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Posted 05 June 2020 - 09:09 AM

Ben there really arent too many choices for the tiny pixel game.  I've used the 183 and the 1600 extensively, and hands down the 183 would be my choice.  There are so many things I dont like about the 1600 and the only thing that I dont like about the 183 is the severe ampglow... but that calibrates out perfectly with good calibration practices. 

 

The 183 has a much nicer noise profile and the calibrated files are a pleasure to work with.  Very CCD like IMO, which I think you would appreciate.

 

EDIT-  since you are also interested in CCD suggestions with small pixels. The QSI 6120 would fit the bill.   It produces the cleanest data I have EVER seen.  I'm just blown away by how clean the data is.  (A very subjective term I know).  There are zero quirks with this camera and it just performs.  You can take CMOS short exposures, as the RN is so low (1.9 to 2.1e-) but with slower scopes does brilliantly with 10 and even 20 min exposures (My edge at f7 for example).  Pixel size is 3.1um so it's right between the 183 and 1600.  If you want to see some data from this camera, I am happy to share.  It's pricey, but can be found on the used market for a good deal.  

Thank you, Chris!  I notice that you don't list the 183 in the footer of your messages.  Do you still own one now?  Your comment about such long exposures is interesting.  Would that be for NB?

 

Ben



#10 ChrisWhite

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Posted 05 June 2020 - 09:53 AM

Thank you, Chris!  I notice that you don't list the 183 in the footer of your messages.  Do you still own one now?  Your comment about such long exposures is interesting.  Would that be for NB?

 

Ben

Ben,

 

correct.  I do not have the 183 anymore.  With the 6120 I really didnt need both cameras.  Same chip size, small pixels, low RN... The 183 is a great camera in my opinion, but the 6120 is quite a bit better.  Is it 4x better (4x more expensive new)?  Probably not... but would I go back?  No.

 

I sold it and picked up a 683 so I could do longer exposures on my fast widefield scope and have a wider FOV. 

 

Regarding the long exposures I'm doing 600s RGB and 1200s Narrowband on my Edge reduced to f7.  I'm quite oversampled here, but I can always downsample in post processing and the RN is so low I dont have trouble swamping. 

 

I did remember one gripe I have with the camera.  I'm not stoked about the quality of images with the 6120 when binned 2x2.  1x1 is amazing, but 2x2 is not.  So I never bin, and I always downsample.  Not a problem IMO.


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#11 BenKolt

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Posted 05 June 2020 - 12:20 PM

Ben,

 

correct.  I do not have the 183 anymore.  With the 6120 I really didnt need both cameras.  Same chip size, small pixels, low RN... The 183 is a great camera in my opinion, but the 6120 is quite a bit better.  Is it 4x better (4x more expensive new)?  Probably not... but would I go back?  No.

 

I sold it and picked up a 683 so I could do longer exposures on my fast widefield scope and have a wider FOV. 

 

Regarding the long exposures I'm doing 600s RGB and 1200s Narrowband on my Edge reduced to f7.  I'm quite oversampled here, but I can always downsample in post processing and the RN is so low I dont have trouble swamping. 

 

I did remember one gripe I have with the camera.  I'm not stoked about the quality of images with the 6120 when binned 2x2.  1x1 is amazing, but 2x2 is not.  So I never bin, and I always downsample.  Not a problem IMO.

Chris:

 

Interesting!  I looked at the QSI (Atik) specs for your 6120 and noticed some differences with the 183.

 

First, what is your RN with this camera?  The specs say 1.6 e- RMS at the top of the page, but then 3.5 - 4 e- RMS further down.

 

I note that the 6120 has 3.1 um pixels, the 183 has 2.4 um, so the 6120 size is right in between the sizes of the 183 and 1600.

 

Do you run into any issues with the 6120's well depth of 9,000 e-?  Depending upon what the actual RN noise, this seems to limit the bit range compared with some of the other cameras I'm looking at.  But is this not really a problem?

 

Thanks!

 

Best Regards,

Ben



#12 dayglow

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Posted 05 June 2020 - 02:43 PM

Ben,

 

Here are the equations used to make the graph.

estimating image system resolution

 

I have a paper explaining the analysis in some detail which will be available on DropBox for a while

 

https://www.dropbox....short.pptx?dl=0



#13 ChrisWhite

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Posted 05 June 2020 - 03:48 PM

Chris:

 

Interesting!  I looked at the QSI (Atik) specs for your 6120 and noticed some differences with the 183.

 

First, what is your RN with this camera?  The specs say 1.6 e- RMS at the top of the page, but then 3.5 - 4 e- RMS further down.

 

I note that the 6120 has 3.1 um pixels, the 183 has 2.4 um, so the 6120 size is right in between the sizes of the 183 and 1600.

 

Do you run into any issues with the 6120's well depth of 9,000 e-?  Depending upon what the actual RN noise, this seems to limit the bit range compared with some of the other cameras I'm looking at.  But is this not really a problem?

 

Thanks!

 

Best Regards,

Ben

Ben,

 

The 6120 RN spec at 1.6e- is a little optimistic.  I think the lowest I have seen for any copy of this camera is one that rockstarbill owned, and that was around 1.8e- if I remember correctly.  been a while, so you could ask him.  Curiously, the deeper you cool with this camera the lower the RN.  I generally cool to about -20C, and I get 2.3e- of RN.  In winter I cool more and get a little bit less.  Here is a thread I started when I first got the camera:  https://www.cloudyni...ial-impression/

 

The 3 to 4e- spec is probably if you are using high-speed download.  Download with this camera is NOT fast with high image quality.  Around 17 seconds, so lucky imaging is out.  But, since you are looking to use it for galaxy season with a slower scope you would not be taking very short exposures.  The data is so good, I don't mind the loss in download time.  Here is an early image I took with it.  5 hours narrowband with 300s subs.  ZERO calibration frames.  No fancy processing...  https://www.astrobin.../?real=&nc=user

 

The 6120 well depth is a limitation for exposure length, but no-more-so than the 183 and it's actually much better than the asi 1600 when it comes to clipping stars.  I used it regularly with my Newt at f4.6 and took 180s RGB and 300s NB. 

 

Personally, I loathe the ASI 1600.  I would not recommend that camera to anyone.  There are so many things I dislike about it but wont get into it here.  I've written about this many times and usually get backlash from the community so I'll just leave it at that.  I DO like the ASI 183 however.  It has some limitations with higher gain and long exposures, but does seem to calibrate well.  The noise profile is very nice to work with.  You might ask Jon Rista about it, he has extensive experience with this camera, and has pushed the gain and exposure length limits I believe. 


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#14 BenKolt

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Posted 05 June 2020 - 04:57 PM

Ben,

 

Here are the equations used to make the graph.

 

 

I have a paper explaining the analysis in some detail which will be available on DropBox for a while

 

https://www.dropbox....short.pptx?dl=0

Thanks, David!



#15 BenKolt

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Posted 05 June 2020 - 04:58 PM

Ben,

 

The 6120 RN spec at 1.6e- is a little optimistic.  I think the lowest I have seen for any copy of this camera is one that rockstarbill owned, and that was around 1.8e- if I remember correctly.  been a while, so you could ask him.  Curiously, the deeper you cool with this camera the lower the RN.  I generally cool to about -20C, and I get 2.3e- of RN.  In winter I cool more and get a little bit less.  Here is a thread I started when I first got the camera:  https://www.cloudyni...ial-impression/

 

The 3 to 4e- spec is probably if you are using high-speed download.  Download with this camera is NOT fast with high image quality.  Around 17 seconds, so lucky imaging is out.  But, since you are looking to use it for galaxy season with a slower scope you would not be taking very short exposures.  The data is so good, I don't mind the loss in download time.  Here is an early image I took with it.  5 hours narrowband with 300s subs.  ZERO calibration frames.  No fancy processing...  https://www.astrobin.../?real=&nc=user

 

The 6120 well depth is a limitation for exposure length, but no-more-so than the 183 and it's actually much better than the asi 1600 when it comes to clipping stars.  I used it regularly with my Newt at f4.6 and took 180s RGB and 300s NB. 

 

Personally, I loathe the ASI 1600.  I would not recommend that camera to anyone.  There are so many things I dislike about it but wont get into it here.  I've written about this many times and usually get backlash from the community so I'll just leave it at that.  I DO like the ASI 183 however.  It has some limitations with higher gain and long exposures, but does seem to calibrate well.  The noise profile is very nice to work with.  You might ask Jon Rista about it, he has extensive experience with this camera, and has pushed the gain and exposure length limits I believe. 

Thanks for the input, Chris.  Yes, I'm aware that Jon Rista has been using the 183 with some of his images.  I will check with him as well for advice.

 

Ben



#16 Jon Rista

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Posted 05 June 2020 - 08:46 PM

If you are looking for a low cost CMOS camera, I truly do think the IMX183 is a tough sensor to beat. It does have the starburst glow, but in practice I've never found that to be a problem. The data is very clean. The dark signal is very well behaved. The noise profile is wonderful. The small pixels are very flexible (you can bin/downsample 2x or 3x to get pixels that better match your actual skies on a range of focal lengths.) The sensor requires extremely intense stars to start creating any kind of microlens artifacts, and even then, the pattern it produces is significantly more pleasant than the ASI1600 (I've only seen it on Sirius with very long exposures, and it was a very mild effect.) 

 

I think it is hard to go wrong if you aren't going to be imaging large objects at moderate focal lengths (around 1000mm, give or take 100mm), where you'll hit the sweet spot for image scale/sampling for ~2" skies. 

 

Other than the IMX183....the only other sensor that really excites me these days is the IMX455. I think that is the true game-changer CMOS sensor we have all been waiting for. Big frame. Small pixels. Very flexible settings, highly configurable. Could be matched to just about any scope, and with ROI (not absolutely certain it has this, I think it does) you could even re-shape the effective image sensor area to fit any image circle. It produces very clean data (significantly cleaner than the Kepler sCMOS sensors, IMO...and I think it is because the very high Kepler dark current makes it very tough to get clean backgrounds), and its dark signal seems to be well behaved. It is kind of the dream CMOS sensor. Could probably be used for any application, from DSO to wide field, to SSO (sol, luna, and the planets), etc. Also comes in an OSC and Mono version, so you could pair them up as well if you wanted to get color data fast, then also get NB and/or L simultaneously, etc. etc. 


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#17 BenKolt

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Posted 06 June 2020 - 01:23 PM

If you are looking for a low cost CMOS camera, I truly do think the IMX183 is a tough sensor to beat. It does have the starburst glow, but in practice I've never found that to be a problem. The data is very clean. The dark signal is very well behaved. The noise profile is wonderful. The small pixels are very flexible (you can bin/downsample 2x or 3x to get pixels that better match your actual skies on a range of focal lengths.) The sensor requires extremely intense stars to start creating any kind of microlens artifacts, and even then, the pattern it produces is significantly more pleasant than the ASI1600 (I've only seen it on Sirius with very long exposures, and it was a very mild effect.) 

 

I think it is hard to go wrong if you aren't going to be imaging large objects at moderate focal lengths (around 1000mm, give or take 100mm), where you'll hit the sweet spot for image scale/sampling for ~2" skies. 

 

Other than the IMX183....the only other sensor that really excites me these days is the IMX455. I think that is the true game-changer CMOS sensor we have all been waiting for. Big frame. Small pixels. Very flexible settings, highly configurable. Could be matched to just about any scope, and with ROI (not absolutely certain it has this, I think it does) you could even re-shape the effective image sensor area to fit any image circle. It produces very clean data (significantly cleaner than the Kepler sCMOS sensors, IMO...and I think it is because the very high Kepler dark current makes it very tough to get clean backgrounds), and its dark signal seems to be well behaved. It is kind of the dream CMOS sensor. Could probably be used for any application, from DSO to wide field, to SSO (sol, luna, and the planets), etc. Also comes in an OSC and Mono version, so you could pair them up as well if you wanted to get color data fast, then also get NB and/or L simultaneously, etc. etc. 

Thank you, Jon, for your response here and PM's.

 

Ben




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