White Balance and Overexposure

[Ingo]

How should I set the white balance for a night sky? I know during normal photography I use a gray card, but I don't think that'd work for the night sky. Should I just leave it on auto?

Also, whenever I use my 24-70mm at say 15 seconds exposure, it always comes out extremely overexposed at F2.8, is it because of the light pollution? Staring straight up, it's pretty dark which is where I position my camera, but towards the horizons, there's a lot of light pollution. I still get overexposure at 24mm and 15 seconds exposure.

[iceman]

What ISO do you have it on?

I use "auto" white balance - make sure you capture in RAW, which means you can change the white balance/colour temperature at any time during post-processing. Therefore it won't matter what you have white balance set to when you capture.

[Ingo]

I usually have it on ISO1600 to get the most light into the camera, my 300F4 is good on ISO1600 for DSO's, but the 24-70 F2.8 comes out overexposed.

[Dennis]

You need a dark sky, away from city or town lights, to be able to expose a wide angle lens wide open for 15 secs. Living in the suburbs of Brisbane, I find that 3 to 4 secs is sufficient to easily reveal light pollution, at 800ASA.

Pointing at the zenith (up) lessens the effects of light pollution and I can expose for 5 to 8 secs before it become objectionable.

Cheers

Dennis