Canon dslr: noise v exposure length

[kennysarmy]

When doing star trail photography I note that when stacking exposures the max. Length of exposure is often quoted as 30 seconds.....this being the max. time most slr's will let you set before going to bulb mode....

However, I have a cheap eBay intervalometer that allows me to take photos of longer shutter speeds...

By taking say a batch of photos with a 60 second exposure I would have less photos to manipulate and process time would be less.....also I guess the startrails themselves might be better with fewer 'gaps'....

But....would the 'noise' of a 60 second exposure be significantly worse than say a 30 second exposure?

 

[hunnymonster]

Try it and see - however DeepSkyStacker's "How to create better images" (link)does say this:

How many frames?
The more, the better but above some threshold it is less efficient.
The signal to noise ration in increasing with the square root of the number of combined frames regardless of the exposure time of each frame.
This is true with all the combining methods (average, median, kappa-sigma clipping, auto-adaptive weighted average, ...) except entropy weighted average since this one in using the entropy to weight each pixel and thus is increasing the noise that is a big entropy contributor..

This means that if your base SNR is 1, when you combine 10 images the SNR increases by 3.16 (square root of 10). For 30 images it is 5.47, for 50 images it is 7.07, for 100 images it is 10, for 300 images it is 17.32.

From which I read more shorter exposures is better than fewer longer ones.

 

[DuncanDisorderly]

I'll add that the 30s technique allows you to work around the ambient light problem.
I've managed a star trail demonstration in 80% cloud coverage at full moon in the middle of a town.
If you can see stars with the naked eye, then you can make star trails.

However, both techniques have different advantages and problems.
Try them both and see which works best for you.