How many subs is "enough"?

[Simms]

So it looks like I `might` get a clear night tonight - now I need to decide whether to add more subs to my current M81+M82 project (see thread in Image - DSO forum) or look for something else to image. The plus side tonight is I will be using some rudimentary guiding with PHD, a finderscope and Philips webcam - so I can probably opt for some Lower ISO/longer exposure subs. However, I was wondering is there a point when one has enough subs and considering the light pollution in my area its unlikely I can get anymore detail out of an object? Or doesnt it hurt just to keep adding to a project and even a few pixel of extra detail should be achievable.

I currently have around 50 subs of 3 mins at ISO1000.

[syd_malicious]

This is a rather subjective answer.

In theory you can't have enough subs AS LONG AS they are decent ones, have been calibrated, and are of the correct length. In particular the last point is important (see here for more help); essentially they need to be as long as possible without increasing the background signal. The background signal can be compromised by things like light pollution, light skies etc.

Practically though I have been like you and tried to maximiaze the targets I can shoot in the limited time we have in this country. What I have tended to do is take as many as possible in one session (I am now on mono so take 4 as LRGB per cycle) and then process them. If the shot is "good enough" (again subjective) then I've done enough; if I feel the result would benefit from more (lacking faint detail, poor S/N ratio) then I simply do more subs. there is a point where more subs would help but there will be a law of diminishing returns and that is for you to decide.

Unfortunately in this country the weather really hurts spending loads of time on one subject.

Having said all that 50 subs of 3mins seems more than reasonable but why not have a go at processing?

Paul

[Psychobilly]

I had been doubling time on each target every season... 4 hours at around f4.7 with t a moded Camera gives me a reasonable result... but i used to aim for 8 hours for a keeper...

You can get a feel for how much more data you need in the post processing .. with sufficient data the image just seems to pop out...

Peter...