Does The Amount Of Images Matter, If I Want To Get A Good Image?

ZWO Seestar S50 Stacking Acquisition
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Szijártó Áron avatar
Szijártó Áron avatar
Hello and welcome! I have a question for anyone who uses Zwo Seastar or has experience in this. Does the number of images or exposure times affect the quality of a good image? I'm trying to achieve a very sharp, well-stacked image processed in software like Siril or PixInsight. I typically collect around 100 to 200 images, usually with exposure times of 20 to 30 seconds. However, the speed at which my device enhances the image can vary. Sometimes, it processes images very slowly, while other times, it can finish in under an hour, depending on the position of the deep sky object.

So, how does that work? Does anyone know the answer to this? I've just started this thing with astrophotography, so I do not know how many exposure tines or frames are enough to get a very sharp stacked image.
Ethan Sweet avatar
Ethan Sweet avatar
Many things affect the quality of your final image. 
​​​​​​More images in your stack increases the signal to noise ratio, increasing the quality of the image. However, if you have low quality subs in your stack it will do the opposite. Garbage in garbage out.

I don't own the Seestar, but since it isn't on an equatorial mount it has field rotation. This I can only guess would limit the sharpness of your images
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John Hayes avatar
John Hayes avatar
Ethan gave you a good answer but I'm going to add a couple of things.  First each image that you take must be well focused and well guided–as Ethan said.  Second, the SNR in the stacked result improves as the square root of the total exposure time.  If you take four times the exposure, the SNR increases by a factor of two.  Because light arrives in discrete packets called photons, the signal that you are tying to measure has inherent noise call "photon noise" and along with that, every sensor injects some additional noise into the process.  The goal is to make sure that the dominating noise comes from the object; not the camera.  I won't go into all of the sources of noise here but this is one reason that you take a LOT of images and then stack them.  If we simplify everything and assume that we have a perfect camera, then stacking say 16 images instead of 4 images will improve the SNR by a factor of 2.  So, in simple terms, the more images that you can stack, the better; although at some point you reach a point of demising returns.  For example if you stack 100 hours of exposure, going to 200 hours of total exposure will only produce a ~40% improvement.  For many fairly bright deep sky objects, achieving a total stacked exposure time of at least a couple of hours is sufficient to get a reasonably good SNR.

John
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Szijártó Áron avatar
Szijártó Áron avatar
John Hayes:
Ethan gave you a good answer but I'm going to add a couple of things.  First each image that you take must be well focused and well guided--as Ethan said.  Second, the SNR in the stacked result improves as the square root of the total exposure time.  If you take four times the exposure, the SNR increases by a factor of two.  Because light arrives in discrete packets called photons, the signal that you are tying to measure has inherent noise call "photon noise" and along with that, every sensor injects some additional noise into the process.  The goal is to make sure that the dominating noise comes from the object; not the camera.  I won't go into all of the sources of noise here but this is one reason that you take a LOT of images and then stack them.  If we simplify everything and assume that we have a perfect camera, then stacking say 16 images instead of 4 images will improve the SNR by a factor of 2.  So, in simple terms, the more images that you can stack, the better; although at some point you reach a point of demising returns.  For example if you stack 100 hours of exposure, going to 200 hours of total exposure will only produce a ~40% improvement.  For many fairly bright deep sky objects, achieving a total stacked exposure time of at least a couple of hours is sufficient to get a reasonably good SNR.

John

Ah, so it kind of matters. So it's better to get more exposure time to get a better image, but the improvement on the image is going to show off when we stack all sub-frames altogether, I guess. Right? So, in my case, it's better what I'm doing, to take 100 images or more, I think.
Jan Ossowski avatar
Jan Ossowski avatar
First of all remember that with seestar you are limited to more or less 10 sec subs because of field rotation. Exact number depends on your latitude, but 10s is a good rule of thumb. Secondly, yes, acquisition time does matter a lot, although it will not fix errors such as star trails. Basically the amount of noise will correspond with 1/sqrt(exposure time). So with 4x more images you will get 2x less noise. 
1 thing you should try is not using live stacking within seestar. You will get better results if you instead take all the subs and stack them in siril or DSS. Also don't forget about calibration frames, especially flats.
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John Hayes avatar
John Hayes avatar
Jan Ossowski:
First of all remember that with seestar you are limited to more or less 10 sec subs because of field rotation. Exact number depends on your latitude, but 10s is a good rule of thumb. Secondly, yes, acquisition time does matter a lot, although it will not fix errors such as star trails. Basically the amount of noise will correspond with 1/sqrt(exposure time). So with 4x more images you will get 2x less noise. 
1 thing you should try is not using live stacking within seestar. You will get better results if you instead take all the subs and stack them in siril or DSS. Also don't forget about calibration frames, especially flats.

Jan,
Be careful.  Noise always increases with exposure time.  With 4x more exposure time, you get 4x more signal and 2x MORE noise so the signal to noise ratio (SNR) goes up by a factor of two with four times the exposure.

John
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Szijártó Áron avatar
Szijártó Áron avatar
Jan Ossowski:
First of all, remember that with seestar, you are limited to more or less 10 sec subs because of field rotation. The exact number depends on your latitude, but 10s is a good rule of thumb. Secondly, yes, acquisition time does matter a lot, although it will not fix errors such as star trails., The amount of noise will correspond with 1/sqrt(exposure time). So with 4x more images, you will get 2x less noise. 
1 thing you should try is not using live stacking within Seestar. You will get better results if you instead take all the subs and stack them in Siril or DSS. Also don't forget about calibration frames, especially flats.


Yeah about that. If u could ever do calibration frames with Seastar, that would help a lot. But since when could you do this? It wouldn't work. Unless there's a trick that u can do with it, then I'm surprised.  Although I thought this could only be done with cameras that connected to very expensive and incredible telescopes like Redcat.
Jan Ossowski avatar
Jan Ossowski avatar
John Hayes:
Jan Ossowski:
First of all remember that with seestar you are limited to more or less 10 sec subs because of field rotation. Exact number depends on your latitude, but 10s is a good rule of thumb. Secondly, yes, acquisition time does matter a lot, although it will not fix errors such as star trails. Basically the amount of noise will correspond with 1/sqrt(exposure time). So with 4x more images you will get 2x less noise. 
1 thing you should try is not using live stacking within seestar. You will get better results if you instead take all the subs and stack them in siril or DSS. Also don't forget about calibration frames, especially flats.

Jan,
Be careful.  Noise always increases with exposure time.  With 4x more exposure time, you get 4x more signal and 2x MORE noise so the signal to noise ratio (SNR) goes up by a factor of two with four times the exposure.

John

You are absolutely right. I made a "though shortcut" (is this an expression? It is in Polish ) mea culpa. 
I of course meant SNR. 

Szijártó Áron:
Jan Ossowski:
First of all, remember that with seestar, you are limited to more or less 10 sec subs because of field rotation. The exact number depends on your latitude, but 10s is a good rule of thumb. Secondly, yes, acquisition time does matter a lot, although it will not fix errors such as star trails., The amount of noise will correspond with 1/sqrt(exposure time). So with 4x more images, you will get 2x less noise. 
1 thing you should try is not using live stacking within Seestar. You will get better results if you instead take all the subs and stack them in Siril or DSS. Also don't forget about calibration frames, especially flats.


Yeah about that. If u could ever do calibration frames with Seastar, that would help a lot. But since when could you do this? It wouldn't work. Unless there's a trick that u can do with it, then I'm surprised.  Although I thought this could only be done with cameras that connected to very expensive and incredible telescopes like Redcat.

I don't own a seestar, maybe there is something that would prevent you from taking them although I can't imagine why. Will it not take photos on demand? Does it only support taking photos when tracking? 
In any case there are a few good videos on YouTube which give some tips and tricks on how to make the seestar photos better. Like this one:
https://youtu.be/61V7LzKSleo
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