Long exposure astrophotos using dslr camera

Long exposure is a lot of fun, whether it be making star trails or ND110 work. One thing with stitching star trails, if it's cloudy like the example above you tend to get breaks in the cloud from the interruptions which I don't personally like (although I suppose blend mode might be able to solve this? don't know).

Combining stuff is just a way to improve signal Vs noise and the result is basically an improvement. I used it on this moon shot with a simple sigma 150-500 and it worked for me but YMMV =)

I've never tried this, and am wondering about the concept of "dark frames." How do you make those? Just take a shot with the lens cap on?