A rediculously simple question... *grin*

Hi all,

A simple question, in general lets say I'm imaging a globular cluster, is it better to shoot a bunch of .5 sec exposures or fewer of say, 10 sec? Does this also follow with nebula? I'm using an Atik 1HS cams.

Thanks!!
Paul

Hello:
If you're stacking images, the shortest exposure time you should use is one that gives an image where the sky noise (after you flat field) dominates over the read noise by some large number (5, 10, or whatever is your favorite). Whereas counts from the sky obey Poisson statistics and thus its noise can be beaten down by sqrt(n), dark noise does not and taken many more images to beat down to a similar level. So you should takes images long enough so that you're sky noise dominated. When this happens depends on your camera, filter, f/ratio, and probably a host of other things.

Cheers
Mike Connelley

What Michael wrote about S/N is correct and is most applicable to nubulae.
Most people like to shoot globular clusters RGB unbinned with an exposure time as long as possible without saturating the stars.

dan kowall

Paul,
I agree with Dan and Mike. If I have a limited time or a time budget, I will generally favor longer/fewer than shorter/more. I think this will generally apply to all but lunar and planet imaging.

However, the 8-bit Atik will offer some challenges balancing the sky noise, read noise and pixel saturation. It may mean that there is a narrower "optimum" exposure time range than with that of a 16-bit camera.

I think globulars would present an interesting test case. I agree with what the others say when it comes to nebula imaging.

But it might be true that imaging a globular is more like imaging a planet: lots of detail to resolve along with fairly high surface brightness. I'd like to see a comparison between the 'traditional' ways of shooting globs (say a dozen 30-second images) vs a stack of 1000 images of 0.36 seconds each....both methods with the same total exposure time of 6 minutes. A control shot of a single 6-minute exposure would be nice too.

Or is my premise of imaging a glob being similar to a planet completely faulty? Thanks,

Tim Iafolla