Imaging limits with Light Pollution

[BlueAstra]

I remember reading somewhere that your CCD exposure times for DSOs are ultimately limited by your local light pollution levels. Is this right, and if so what method would you use to determine the maximum exposure time for your local light pollution level? Is it absolute, or can you recover the situation with software? Is it different for LRGB and NB?

[dph1nm]

The real killer with light pollution is not the colour shift but that it adds noise to your image (more photons from the sky, and the noise is sqrt(number of photons)).  This means that to get the same signal-to-noise ratio as at a dark site (with the same camera/telescope) you have to exposure for longer. There is no other way round the problem (ok, so lp filters might help a little bit).

So in principle there is no limit - you just keep adding data and getting deeper. However, if you work out the increase in exposure time needed to match a truly dark sky it can be pretty horrific (e.g. 100x and more), so my advice is not to think about it!

NigelM

[BlueAstra]

Thanks for the response. I was just wondering if LP imposed some practical limit on exposure time. I could in principle expose subs for 1m, 10m or 30m on a target, but is there a point where LP 'saturates' the image, limiting the exposure time? And if so what do you measure to determine this exposure time limit? If it doesn't, do you just go for the longest practical exposure time ignoring LP and stack as many subs as possible to reduce the noise?

[RogerTheDodger]

I have moderate LP on the far west edge of Birmingham, I just extend or shorten the exposure time depending on the target and how the histogram for the subs look. Some nights are worse than others.

[dph1nm]

Yes, LP will saturate the image eventually. Lower ISO will put off the evil hour. Look on the bright side though - with bad light pollution, read noise won't be an issue, so you can get away with short subs without loss of efficiency!

NigelM

[BlueAstra]

So, should I choose the longest exposure time that makes sure the histogram (including LP) is not saturated?

[russellhq]

Samir has some good information on this subject that's well worth reading. You can find it from the following link:

http://www.samirkharusi.net/sub-exposures.htm

And this one:

http://www.starrywonders.com/snr.htmll

[glappkaeft]

So, should I choose the longest exposure time that makes sure the histogram (including LP) is not saturated?

If you have stars in the image the histogram will pretty much always be partially saturated. There are several ways to calculate an optimal exposure, some of them are described here: http://starizona.com/acb/ccd/advtheoryexp.aspx