alphajuno:
That's really nice. So how did you decide on 10 minute exposures? Is it better to stack more shorter exposures or fewer longer exposures?
Thanks:
There are a lot of factors including focal ratio, read noise, FOV size/quality of polar alignment, amount of aircraft traffic*FOV size, dark current and well depth.
The focal ratio speaks to how densly the light flux is concentrated on the sensor. If you have high magnifcation then you spread a given amount of light over a wider area so naturally the intensity/area falls off. It is the intensity of the light that determines the rate that you acquire signal. A doubling of F/# will spread the same amount of light flux over four times the area so that means you acquire signal at 1/4 the rate.
For dim objects the read noise can be signifcant and that can degrade S/N if you use exposures that are too short. Likewise if your dark signal noise is getting comparable to the read noise and that is comparable to the signal shot noise, then you can be degrading your s/n as well.
If the FOV is big and the polar alignment has a bit of error, then you will see worse field rotation than if the FOV is small. the longer you expose the more time for the rotation to occur so that's a factor.
Wide FOVs are more likely to snag aircraft traffic and in my backyard that is a consideration as well.
If you have Zero read noise and you sum 10 exposures of 1/10 the length of a single exposure you have identical s/n, but with real read noise it is degraded and the larger the read noise is compared to the signal you are collecting the more significantly it is degraded.
Well depth ultimately limits the exposure length because once it fills you have no contrast in the image. usually I try to avoid saturation of all but the brightest of stars and will only permit them to bloom if their blooming spikes don't "damage" the nebula or galaxy of interest in the image.
Sometimes that criterion can limit you to very short exposures so low read noise can be of critical importantance as will be shown in a chart linked below. M42 is a good example of such a target: the Trapezium stars will saturate and bloom very easily and that is in the middle of the object of interest. So you have to shoot short exposures and lots of them. But if you have high read noise you will not be able to resolve the faint nebulosity from the noise floor.
I have some charts that speak to these points here:
http://www.narrowbandimaging.com/using_multiple_subexposures_page.htm
and here:
http://www.narrowbandimaging.com/maximum_op_temp_vs_exposure_page.htm
A final factor that is often a deciding factor for me is the availability of a library dark. In my case that is why I selected 10 minutes, because I know my camera (FLI Proline3200) has low read noise (~5.25 e-)