Short Subs vs Long Subs ... Advantages, Disadvantages

Good questions! I did not many short sub exposures, and when, then I targeted usually a pretty bright object, like M42. My experience: I cannot get as many details showing up in short exposures compared to long ones, which I think is reasonable. Yes, you do not need excellent guiding, but the image itself does not give as much as one can get with longer exposures. There is also no dithering in place (usually), I can see this as a problem as well. And one shall not forget: processing time! Depending on the sensor of course the processing of 1000 images might take hours, probably days if you use for example the ASI6200. And my computer probably will melt long before a final image comes out.
Not saying that short exposures should not be done, but in general I am leaning to 5-10 minute exposures. From what I can tell they produce cleaner images and more detail. There are theoretical works out there that say more than 2 minutes is not needed with our common equipment. From experience I cannot follow this, yes, I understand the mathematical background, but nothing tells you more than trying it. 
Hence I encourage you to try, target a brighter object and take short exposures, and see what it reveals. Then target a faint object, and you will probably throw those images away since they do not see anything. I have seen some people here on Astrobin who did a marvellous job though with 1s exposures, I do not know how they did this, hopefully someone who did reads this and can shed some light on it.

Short exposure advantages.-No guiding-Less sensitive to having a good mount (vibrations), winds and seeing , to enable good stars-Low rejection rate of subs (clouds, airplanes)--more subs are good. Can also do a median rejection (or other rejection method) rather than a straight average when combining frames, faint satellite trails can be eliminated without throwing out affected frames. Short exposure disadvantages- There is some delay between subs typically depending on camera, cable, computer--can be 1 to 4 seconds. This is lost time for getting exposure time.- Requires more disk space and longer processing calibrating/registering/stacking. I do lots of short exposures and do not have issues with this. Disk space is cheap (compared to getting what you need for long exposures --great mount, guide camera/scope, dome), and you can just let your stacking runs  go on your cloudy nights, and your stack will be ready in the AM. Finally signal/noiseYour camera converts photons to electrons --the ADU  (analog-to-digital units, if your camera takes 16-bit images the maximum signal is 65,536 ADU). Your camera sensor doesn't know how you collect the photons. The final ADU signal you get it is exactly the same, no matter how you break it into subs.  Signal ADU only depends on the total time of exposure (and the brightness of your target). So, why take longer exposures at all? The only reason is read noise. Every time you take an image (a sub), the camera will add read noise. This depends on your gain--for many cameras higher gain gives less read noise. Obviously then, the fewer subs, the less total read noise. So how long should a sub exposure be to avoid the effect of read noise? There is a good way to decide. The key point is that if your background sky is bright, most of your noise (shot noise) comes from the sky. It overwhelms read noise. So if you sky is bright the read noise rapidly becomes insignificant compared to the sky shot noise. If your sky is very dark, then read noise is a more important contribution. There are some slightly different takes on the optimal sub exposure time. Here is one estimate for your optimal time for your sub, Tsub, considering your sky noise: Tsub = 10 (R^2/sky)  Sky=(ADUsky-ADUdark)/Ts - R is read noise in ADU. Measure the standard deviation in the background ADU from a single bias frame. Or use a literature value in electrons for R and divide by the gain to get R in ADU. Same ISO or Gain as your subs will be.-Sky is the sky signal in ADU for an arbitrary sub of length Ts. Doesn't matter how long, as long as your sky gives you a reasonable signal in ADU and is not saturated. You should subtract the ADU for a dark, though generally this is pretty small. Divide by the length of the sub exposure, Ts. Now you have all you need to calculate your optimal sub exposure Tsub. I am in a big city, nearly full moon bright, operating at F6.3--so  my optimal exposure is <10 s, more doesn't buy me anything. The darker the site, the longer your subs need to be to be optimal. But remember, in the end the same total exposure will give you the same number of photons, and averaging the subs will improve your S/N, no matter what your sub length was. And in the end, enough  of a  longer total exposure with shorter subs, could still match a shorter exposure with longer subs. Sorry, this is a bit long-winded.  If you want a simple answer, if your sky background is close to at least one-half your full ADU you are doing well with your sub. But for short exposures make sure you set your gain to try as much as possible your read noise. That gain or ISO varies a lot from camera to camera.

Since I do everything from the big city I only use short exposures. So far I have not had an issue imaging what I want to image, though I do need more exposure than a dark site. Which is only fair, something is wrong if you can't do more from a dark site. You can see what I have done, both on bright and dark images here @rveregin   The key is lots of total exposure--other things are secondary.

Clear skies
Rick

Higher gain gives you more signal and generally less read noise in many camera's including yours. The only issue with higher gain is that you reduce the dynamic range, so you risk blowing out and saturating bright stars and bright parts of the image. The theory says for S/N you want to maximize the gain and minimize read noise both. For your camera I see gain=350 minimizes read noise. https://astronomy-imaging-camera.com/product/asi294mc-pro-color Gain 390 might be a just a little higher read noise than gain 350, but of course gives you a bit more signal. Probably a wash between these settings. So you are in a good place, again as long as your bright parts are not blown out.

And yes, if you use shorter subs you should in general run at higher gain. And if you go longer from 5s to 5 minute or 50 minute subs, you would need to reduce the gain, otherwise you will totally blow out your highlights. 

Note, you will see some say that any lower dynamic range, which you get at higher gain, is unacceptable.  Lower dynamic range is not necessarily bad. Unless you are using the full dynamic range--your highlights are nearly to your max ADU, any extra unused dynamic range is just empty, wasted space that could have been used for your carefully gathered photons by pushing up the gain.

And yes, I found longer total integration time not only reduces S/N, it makes bringing up detail easier--you can do more deconvolution and sharpening. It makes colors easier to bring out and to be more brilliant. 
Rick

Thank you Rick and Dimitris for the excellent discussion and insights you  have provided!

Another area of discussion might be the length of time of the exposure on an unguided scope and any drift which would affect the resolution/detail acquired.

Would a decent scope and a 5 sec sub produce better detail than a guided image over a 2 to 5 min exposure? I am sure the accuracy of the guide and quality of stacking and mount drift come into play..

Thanks Jeffbax!

Very impressive image!
And 7000 1sec subs.

Thanks for the correction.
It makes sense, the method your using.

What was the gain set to?

Hi there, 

Couldn't agree more with Jeffbax. In my opinion, the primary goal of short exposures is to get a better resolution. Even with 2s subs, i obtained excellent details on some targets like ngc7331 or stephen's quintet ( see in my gallery ) that in my opinion i couldn't have obtained with long exposures and guiding on ground level.

The shorter the sub is, the more details you get ; but there is a tradeoff on sensitivity. However, with a well optimized setup, you can use short subs ( around 1s ) and still produce images with signal/noise ratio on par with long exposure images. You just need a good mono sensor witn low readout noise ( imx290 is still the best for now ) and a low F/D ratio.

Romain

Astrobin does not support exposition time under 1 second in its dso description.

Could we ask a modification of such a feature ?

David Russell:
what is the cross over point in terms of sub length where the extra detail for DSO starts to set in ?

I have been shooting DSOs very successfully at 30 seconds with a DSLR the last 12 months, but recently upgraded to a ZWO-2600MC and I increased the exposure time from 30 seconds to 60 seconds. I notice that many resulting stacked images, seem less sharp,and thought that might be because of the increase in  exposure length. I am investigating back focus as another potential source of the loss of detail. very interested in this thread.

Hello, well I think short exposures and sorting the frames really increase resolution under 1 second exposure time. In your case it might be a tracking or backfocus issue 😉

CS

JF

Rick Veregin:

David Russell:
what is the cross over point in terms of sub length where the extra detail for DSO starts to set in ?

I have been shooting DSOs very successfully at 30 seconds with a DSLR the last 12 months, but recently upgraded to a ZWO-2600MC and I increased the exposure time from 30 seconds to 60 seconds. I notice that many resulting stacked images, seem less sharp,and thought that might be because of the increase in  exposure length. I am investigating back focus as another potential source of the loss of detail. very interested in this thread.

Are you guiding or not? If you are not guiding, then the longer your exposures the more the image will get degraded, unless you have a really good tracking mount and really good polar alignment
Rick

Thanks Rick.

yes I am guiding using PHD2 and EKOS. lately my guiding RMS has been 0.8 to 1.2 roughly.

I have been fussy as hell with PA and using EKOS Polar align. last time out I got to 34 arc seconds, which I believe is considered very good or perhaps even excellent.

I think I understand what you are saying with how in longer sub exposures the errors get baked in. 

I am chasing clarity and detail, so investigating how sub exposure length effects those 2 things.

I am shooting from Bortle 1which helps. 

thanks Dave