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Can a CCD sensor obtain good images using very fast/short exposures?

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#1 DSO_Viewer

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Posted 08 March 2018 - 12:08 PM

My main interests are faint DSO's in particular galaxies and I want to start using very short exposures of 10 seconds or less using live stacking. I like the idea of fast exposures and stacking so I do not have to worry about guiding or using an equatorial mount. I am using a C11 SCT on an alt/az mount which limits my exposures and an Atik Infinity color CCD camera. I really like this camera but think it is best for an EQ mount in order to take advantage of longer exposures. I know that the readout noise is much higher in most CCD's than CMOS and was wondering if this limits the ability of the CCD camera to use much shorter exposures. I am seriously thinking about buying a CMOS camera from ZWO because of the much lower readout noise in order to do some type of lucking imaging/observing for my EAA. Any suggestions and help are appreciated.

 

Thank you!

 

Steve 



#2 theastroimager

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Posted 08 March 2018 - 01:48 PM

Ten seconds is a long time through a C11 that's not tracking, and you'll start seeing star trailing long before you are going to have to worry about readout noise.

 

So put it to the test...take some 10 second integrations, then some 5, then some 2...examine them until you determine how much star trailing you are willing to tolerate (which will vary with the declination of your target). Then measure your signal and background levels, and you should be able to come up with how much readout noise with your current camera (or with a CMOS camera) is going to affect your images. 

 

Keep in mind that with very short integrations you may to have to deal with non-linearity issues, depending on the design of the shutter your future camera has. 

 

Thanks,

 

John



#3 NiteGuy

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Posted 08 March 2018 - 02:13 PM

+1 for what John said. Even 1-second exposures at prime focus will trail on a C11 that has no tracking. However, if your Alt-Az tracking is working, you can probably experiment to see how long you can go before needing to add field rotation. I suspect 30 seconds or maybe even a couple of minutes depending on what part of the sky you're imaging. Also you might be able to get a wedge to convert your Alt-Az to equatorial and solve the whole problem.



#4 Umasscrew39

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Posted 08 March 2018 - 03:01 PM

Hi Steve

 

I used my C11 SCT on the Alt-AZ mount for a year and could stack up to 45s, sometimes 60s, with my UltraStar CCD camera which has the same Sony sensor as the Atik Infinity if I am not mistaken.  Most often, I used it for something like 5 or 6 stacks at 30s with no trouble at f/10 (prime), 5 and 2 if the mount is working properly.  Mine worked fine for months as long as I avoided the areas of field rotation.  Eventually, the mount simply was not functioning at its optimal performance so I finally de-forked it and placed it on an GEM.  So, there is no reason to limit yourself to 10s stacks unless the mount is acting up.  It should not have anything to do with the camera.

 

Bruce


Edited by Umasscrew39, 08 March 2018 - 05:28 PM.


#5 theastroimager

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Posted 08 March 2018 - 03:52 PM

Bruce,

 

Just want to make sure I understand...so you could take images between 45 and 60 seconds long with a C11 not tracking and you wouldn't get any star trailing?

 

Thanks,

 

John



#6 DSO_Viewer

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Posted 08 March 2018 - 03:57 PM

Ten seconds is a long time through a C11 that's not tracking, and you'll start seeing star trailing long before you are going to have to worry about readout noise.

 

So put it to the test...take some 10 second integrations, then some 5, then some 2...examine them until you determine how much star trailing you are willing to tolerate (which will vary with the declination of your target). Then measure your signal and background levels, and you should be able to come up with how much readout noise with your current camera (or with a CMOS camera) is going to affect your images. 

 

Keep in mind that with very short integrations you may to have to deal with non-linearity issues, depending on the design of the shutter your future camera has. 

 

Thanks,

 

John

Thank you for the help, John. My mount can track up to about 30 seconds max but the stars start to appear egg shape and I am trying for the roundest stars possible. This is why I want to go less than 10 seconds after I did my test around 8 seconds worked out best. After reading posts here my issue is with the higher readout noise with this camera and at  8 seconds the S/N is swamped by the noise. Stacking helps but only to a certain point due to the read noise. This is why I thought that a new CMOS camera with it's much lower readout noise will work out best for capturing very short exposures and stacking them. I should have mentioned that I live in a red/white zone.

 

Steve



#7 DSO_Viewer

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Posted 08 March 2018 - 04:00 PM

+1 for what John said. Even 1-second exposures at prime focus will trail on a C11 that has no tracking. However, if your Alt-Az tracking is working, you can probably experiment to see how long you can go before needing to add field rotation. I suspect 30 seconds or maybe even a couple of minutes depending on what part of the sky you're imaging. Also you might be able to get a wedge to convert your Alt-Az to equatorial and solve the whole problem.

Thank you for the help but I want to keep it simple if possible and use my C11 alt/az without a wedge because I must set up at every session. I can go 30 seconds tops but there is some trailing and my test showed that around 8 seconds works best.

 

Steve



#8 theastroimager

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Posted 08 March 2018 - 04:03 PM

Steve,

 

Well, again, it really is about the results you want. Once you find the edge of your comfort level with the shape of the stars, and you've determined that readout noise is the weak link then it sounds like you are on the right track. 

 

Somehow I came away with the impression that you were not tracking at all...thus my amazement.

 

Have fun with it!



#9 Umasscrew39

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Posted 08 March 2018 - 04:03 PM

Bruce,

 

Just want to make sure I understand...so you could take images between 45 and 60 seconds long with a C11 not tracking and you wouldn't get any star trailing?

 

Thanks,

 

John

John

 

No- I was tracking using the alt/az mount.  


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#10 DSO_Viewer

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Posted 08 March 2018 - 04:04 PM

Hi Steve

 

I used my C11 SCT on the Alt-AZ mount for a year and could stack up to 45s, sometimes 60s, with my UltraStar CCD camera which has the same Sony sensor as the Atik Infinity if I am not mistaken.  Most often, I used it for something like 5 or 6 stacks at 30s with no trouble at f/10 (prime), 5 and 2 if the mount is working properly.  Mine worked fine for months as long as I avoided the areas of field rotation.  Eventually, the mount simply was not functioning at it optimal performance so I finally de-forked it and placed it on an GEM.  So, there is no reason to limit yourself to 10s stacks unless the mount is acting up.  It should not have anything to do with the camera.

 

Bruce

Thank you, Bruce but it is field rotation that gets me most. Since I am in a very light polluted location red/white zone I have to observe as straight up as much as possible. I tried a light pollution filter but the exposure time needs to be increased.

 

Steve



#11 Umasscrew39

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Posted 08 March 2018 - 04:08 PM

 

Ten seconds is a long time through a C11 that's not tracking, and you'll start seeing star trailing long before you are going to have to worry about readout noise.

 

So put it to the test...take some 10 second integrations, then some 5, then some 2...examine them until you determine how much star trailing you are willing to tolerate (which will vary with the declination of your target). Then measure your signal and background levels, and you should be able to come up with how much readout noise with your current camera (or with a CMOS camera) is going to affect your images. 

 

Keep in mind that with very short integrations you may to have to deal with non-linearity issues, depending on the design of the shutter your future camera has. 

 

Thanks,

 

John

Thank you for the help, John. My mount can track up to about 30 seconds max but the stars start to appear egg shape and I am trying for the roundest stars possible. This is why I want to go less than 10 seconds after I did my test around 8 seconds worked out best. After reading posts here my issue is with the higher readout noise with this camera and at  8 seconds the S/N is swamped by the noise. Stacking helps but only to a certain point due to the read noise. This is why I thought that a new CMOS camera with it's much lower readout noise will work out best for capturing very short exposures and stacking them. I should have mentioned that I live in a red/white zone.

 

Steve

 

Steve

 

I suspect your mount is not tracking properly like mine had done.  Many use CCD cameras on an alt/az mount for up to 30s or more and no issues with stars looking egg shape unless the mount is not tracking properly or you are using a focal reducer in which you may have some star aberrations in the periphery.  

 

Bruce

 

Bruce



#12 DSO_Viewer

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Posted 08 March 2018 - 04:40 PM

Steve,

 

Well, again, it really is about the results you want. Once you find the edge of your comfort level with the shape of the stars, and you've determined that readout noise is the weak link then it sounds like you are on the right track. 

 

Somehow I came away with the impression that you were not tracking at all...thus my amazement.

 

Have fun with it!

No, I am tracking but it is limited due to my sky position.

 

Steve



#13 DSO_Viewer

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Posted 08 March 2018 - 04:42 PM

 

 

Ten seconds is a long time through a C11 that's not tracking, and you'll start seeing star trailing long before you are going to have to worry about readout noise.

 

So put it to the test...take some 10 second integrations, then some 5, then some 2...examine them until you determine how much star trailing you are willing to tolerate (which will vary with the declination of your target). Then measure your signal and background levels, and you should be able to come up with how much readout noise with your current camera (or with a CMOS camera) is going to affect your images. 

 

Keep in mind that with very short integrations you may to have to deal with non-linearity issues, depending on the design of the shutter your future camera has. 

 

Thanks,

 

John

Thank you for the help, John. My mount can track up to about 30 seconds max but the stars start to appear egg shape and I am trying for the roundest stars possible. This is why I want to go less than 10 seconds after I did my test around 8 seconds worked out best. After reading posts here my issue is with the higher readout noise with this camera and at  8 seconds the S/N is swamped by the noise. Stacking helps but only to a certain point due to the read noise. This is why I thought that a new CMOS camera with it's much lower readout noise will work out best for capturing very short exposures and stacking them. I should have mentioned that I live in a red/white zone.

 

Steve

 

Steve

 

I suspect your mount is not tracking properly like mine had done.  Many use CCD cameras on an alt/az mount for up to 30s or more and no issues with stars looking egg shape unless the mount is not tracking properly or you are using a focal reducer in which you may have some star aberrations in the periphery.  

 

Bruce

 

Bruce

 

Perhaps, Bruce, it is a time for a tune-up which I have never done as of yet. Thank you.

 

Steve



#14 Don Rudny

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Posted 08 March 2018 - 05:00 PM

Steve,

 

Like Bruce, I have a CPC1100HD with an Ultrastar C.  I can get 30 to 60s subs at f/5, but it can be frustrating at times dealing with field rotation and tracking inconsistency.  Attached is an example recently of a SN hunt at f/5 and 45s.  I finally decided to bite the bullet and buy an AP gem that arrives tomorrow.  

 

Don

 

C5F5C770-B3FF-4508-9A88-685A6A379DE7.jpeg

 


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#15 DSO_Viewer

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Posted 08 March 2018 - 07:08 PM

Steve,

 

Like Bruce, I have a CPC1100HD with an Ultrastar C.  I can get 30 to 60s subs at f/5, but it can be frustrating at times dealing with field rotation and tracking inconsistency.  Attached is an example recently of a SN hunt at f/5 and 45s.  I finally decided to bite the bullet and buy an AP gem that arrives tomorrow.  

 

Don

 

attachicon.gif C5F5C770-B3FF-4508-9A88-685A6A379DE7.jpeg

Very nice image Don and it is hard to beat a good AP gem. Thank you!

 

Steve


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#16 alphatripleplus

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Posted 08 March 2018 - 07:22 PM

Steve,

 

Quick answer when using a high-read noise camera: 

 

For a high surface brightness galaxy, shot noise will still likely dominate read noise even using short subs, so short subs may be fine. However, for a low surface brightness galaxy or a galaxy with faint details, read noise will be more significant relative to shot noise, so longer and fewer subs are going to be better. 

 

Here is an illustration from last weekend using a Lodestar X2 mono on M82, one of the  highest surface brightness galaxies, using a C6 @f/4.7. Do you see a difference using 10s vs 30s subs?

 

12 x 10s

 

M82.12_2018.3.4_21.06.08.png

 

 

4 x  30s

 

M82.#12_4x30s_ND_f4.7_2018.3.4_20.51.54.jpg

 

 

If your prey is going to be low surface brightness galaxies, say M101 instead of M82, and you are limited to 10sec exposures, you should get a low read noise camera. I used to have one (ASI224) and I could stack M101 happily with 5sec subs at f/5  and pull out  fainter details after a lot of subs.

 

However,  if your tracking can handle longer exposures, then you might want to still consider a higher read noise camera. Of course, there are other considerations like resolution and cost etc. By the way, Robin Glover has a good discussion here of the interplay between read noise, shot noise and sub exposure, and yes, it does apply to EAA.


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#17 Umasscrew39

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Posted 08 March 2018 - 07:43 PM

Steve,

 

Quick answer when using a high-read noise camera: 

 

For a high surface brightness galaxy, shot noise will still likely dominate read noise even using short subs, so short subs may be fine. However, for a low surface brightness galaxy or a galaxy with faint details, read noise will be more significant relative to shot noise, so longer and fewer subs are going to be better. 

 

Here is an illustration using a Lodestar X2 mono on M82, one of the  highest surface brightness galaxies, using a C6 @f/4.7. Do you see a difference using 10s vs 30s subs?

 

12 x 10s

 

attachicon.gif M82.12_2018.3.4_21.06.08.png

 

 

4 x  30s

 

attachicon.gif M82.#12_4x30s_ND_f4.7_2018.3.4_20.51.54.jpg

 

 

If your prey is going to be low surface brightness galaxies, say M101 instead of M82, and you are limited to 10sec exposures, you should get a low read noise camera. I used to have one (ASI224) and I could stack M101 happily with 5sec subs at f/5  and pull out  fainter details after a lot of subs.

 

However,  if your tracking can handle longer exposures, then you might want to still consider a higher read noise camera. Of course, there are other considerations like resolution and cost etc. By the way, Robin Glover has a good discussion here of the interplay between read noise, shot noise and sub exposure, and yes, it does apply to EAA.

Errol

 

Very interesting information.  What I am wondering though is if Steve still has a tracking problem.  I can't image he can only have 10s exposures on a C11 on an alt/az mount and it is due to the read noise of his Atik Infinity camera.  Mine was never that bad even when it was essentially losing its tracking performance. 

 

Bruce



#18 alphatripleplus

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Posted 08 March 2018 - 07:59 PM

Bruce,

 

I don't know anything about Steve's mount, but unfortunately with a focal length of say 1400mm, some alt-az mounts may struggle with 20s or 30s exposures. I myself experienced problems going unguided for much longer than 15s-20s and getting round stars with a nameless Celestron equatorial mount (I no longer own) operating at 1000mm. It can be a bit of a lottery with the mass market mounts; some are lucky, but not everyone.smile.gif



#19 Don Rudny

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Posted 08 March 2018 - 08:45 PM

Bruce,

 

I don't know anything about Steve's mount, but unfortunately with a focal length of say 1400mm, some alt-az mounts may struggle with 20s or 30s exposures. I myself experienced problems going unguided for much longer than 15s-20s and getting round stars with a nameless Celestron equatorial mount operating at 1000mm. It can be a bit of a lottery with the mass market mounts; some are lucky, but not everyone.smile.gif

Right, Errol.  Definitely a crapshoot.  Our CPC 1100 was great when we first got it.  Then something went wrong.  After three trips to the nameless manufacturer, they finally just sent us a new one.  It has been very good, but sometimes frustrating as I wrote earlier.  The advent of low read CMOS has changed EAA with alt-Az mounts, but for long focal length  narrower FOV, GEM is still the way to go.  Wide field with Hyperstar or short focal length scopes are great on alt-az mounts with any cam.  Having used my SV60eds on my Evolution Mount, you would think it’s an AP or SB.

 

Don


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#20 DSO_Viewer

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Posted 08 March 2018 - 10:03 PM

Steve,

 

Quick answer when using a high-read noise camera: 

 

For a high surface brightness galaxy, shot noise will still likely dominate read noise even using short subs, so short subs may be fine. However, for a low surface brightness galaxy or a galaxy with faint details, read noise will be more significant relative to shot noise, so longer and fewer subs are going to be better. 

 

Here is an illustration from last weekend using a Lodestar X2 mono on M82, one of the  highest surface brightness galaxies, using a C6 @f/4.7. Do you see a difference using 10s vs 30s subs?

 

12 x 10s

 

attachicon.gif M82.12_2018.3.4_21.06.08.png

 

 

4 x  30s

 

attachicon.gif M82.#12_4x30s_ND_f4.7_2018.3.4_20.51.54.jpg

 

 

If your prey is going to be low surface brightness galaxies, say M101 instead of M82, and you are limited to 10sec exposures, you should get a low read noise camera. I used to have one (ASI224) and I could stack M101 happily with 5sec subs at f/5  and pull out  fainter details after a lot of subs.

 

However,  if your tracking can handle longer exposures, then you might want to still consider a higher read noise camera. Of course, there are other considerations like resolution and cost etc. By the way, Robin Glover has a good discussion here of the interplay between read noise, shot noise and sub exposure, and yes, it does apply to EAA.

Great illustration Errol and thank you. I see that both M82 images are very good and look very similar. I will read the discussion from Robin Glover. Great having you guys helping me out.

 

Steve


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#21 alphatripleplus

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Posted 08 March 2018 - 10:41 PM

Steve,

 

I'll see if I have a comparison on a low surface brightness target like M101 with the Lodestar, but I do have a comparison on M51 which is intermediate in terms of surface brightness between M82 and M101 (but still pretty bright). Here  using my Lodestar X2 and a C8 @ f/4  are comparisons  of M51 with 5s, 10s and 20s subs (all the same total exposure) all on the same night:

 

36 x 5s;

 

M51.29.5s.subs_2017.5.15_22.21.04.jpg

 

18 x 10s;

 

M51.29.10s.subs_2017.5.15_22.28.43.jpg

 

9 x 20s;

 

M51.29.20s.subs_2017.5.15_22.34.46.jpg

 

Although the black level is a little bit different across the 3 exposures, the 20s subs do show more detail and less noise/banding when similarly stretched, compared to the 5s or 10s subs.


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#22 Wildetelescope

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Posted 09 March 2018 - 07:44 AM

Steve,

 

Quick answer when using a high-read noise camera: 

 

For a high surface brightness galaxy, shot noise will still likely dominate read noise even using short subs, so short subs may be fine. However, for a low surface brightness galaxy or a galaxy with faint details, read noise will be more significant relative to shot noise, so longer and fewer subs are going to be better. 

 

Here is an illustration from last weekend using a Lodestar X2 mono on M82, one of the  highest surface brightness galaxies, using a C6 @f/4.7. Do you see a difference using 10s vs 30s subs?

 

12 x 10s

 

attachicon.gif M82.12_2018.3.4_21.06.08.png

 

 

4 x  30s

 

attachicon.gif M82.#12_4x30s_ND_f4.7_2018.3.4_20.51.54.jpg

 

 

If your prey is going to be low surface brightness galaxies, say M101 instead of M82, and you are limited to 10sec exposures, you should get a low read noise camera. I used to have one (ASI224) and I could stack M101 happily with 5sec subs at f/5  and pull out  fainter details after a lot of subs.

 

However,  if your tracking can handle longer exposures, then you might want to still consider a higher read noise camera. Of course, there are other considerations like resolution and cost etc. By the way, Robin Glover has a good discussion here of the interplay between read noise, shot noise and sub exposure, and yes, it does apply to EAA.

Hi Errol     

        This is a helpful post for a newbie like me!  What should I be looking for in each image?

jmd 



#23 alphatripleplus

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Posted 09 March 2018 - 08:12 AM

Hi Errol     

        This is a helpful post for a newbie like me!  What should I be looking for in each image?

jmd 

 

 

What I was trying to show was that with a high surface brightness target like M82, there is not much, if any, noticeable difference in visible details or noisy background  between the short and long subs, i.e. it is hard to tell the pics apart. Even after zooming in and/or stretching, they look very similar.

 

With a much fainter/lower surface brightness target, those differences should be much more apparent with a higher read noise camera like the Lodestar X2 (about 7e) compared with, say, a low read noise CMOS sensor (with read noise in the 1-1.5e range).

 

Next night out, I will try and do a short vs long sub comparison on M101 as an example of a low surface brightness galaxy.



#24 Wildetelescope

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Posted 09 March 2018 - 12:29 PM

 

Hi Errol     

        This is a helpful post for a newbie like me!  What should I be looking for in each image?

jmd 

 

 

What I was trying to show was that with a high surface brightness target like M82, there is not much, if any, noticeable difference in visible details or noisy background  between the short and long subs, i.e. it is hard to tell the pics apart. Even after zooming in and/or stretching, they look very similar.

 

With a much fainter/lower surface brightness target, those differences should be much more apparent with a higher read noise camera like the Lodestar X2 (about 7e) compared with, say, a low read noise CMOS sensor (with read noise in the 1-1.5e range).

 

Next night out, I will try and do a short vs long sub comparison on M101 as an example of a low surface brightness galaxy.

 

Thanks Errol,

   I might do that myself tonight or tomorrow.   Eventually I plan to compare my Mallicam Universe (CCD) to my new ASI 294 and that sounds like a good test to look read noise for both cameras.

 

Cheers!

 

JMD



#25 DSO_Viewer

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Posted 09 March 2018 - 12:32 PM

Steve,

 

I'll see if I have a comparison on a low surface brightness target like M101 with the Lodestar, but I do have a comparison on M51 which is intermediate in terms of surface brightness between M82 and M101 (but still pretty bright). Here  using my Lodestar X2 and a C8 @ f/4  are comparisons  of M51 with 5s, 10s and 20s subs (all the same total exposure) all on the same night:

 

36 x 5s;

 

attachicon.gif M51.29.5s.subs_2017.5.15_22.21.04.jpg

 

18 x 10s;

 

attachicon.gif M51.29.10s.subs_2017.5.15_22.28.43.jpg

 

9 x 20s;

 

attachicon.gif M51.29.20s.subs_2017.5.15_22.34.46.jpg

 

Although the black level is a little bit different across the 3 exposures, the 20s subs do show more detail and less noise/banding when similarly stretched, compared to the 5s or 10s subs.

Thank you, Errol, this is an excellent example of how longer exposures will improve the image esp. when stretching when observing a lower surface brightness object with a CCD camera. A lot of the springtime galaxies have lower surface brightness and I would either have to work on my mount in order to obtain 30 to 60 seconds without trailing or buy a CMOS camera like the ASI290 in order to take advantage of it's lower readout noise. This way I will be able to use less exposure time and stack a lot of subs.

 

Steve




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