Longest exposure question.

[DazC]

What is the longest single exposure time I can get away with for a constellation image without having elongated stars?

Thanks.

D.C

[ollypenrice]

Sorry but this is impossoble to answer withut more information.

What mount?

Alt Az or equatorial?

Motorized?

Autogided?

Your signature says 250 Dob but for constellations you need a focal length of about 50mm or less, so camera lens territory. Even a very basic equatorial tracking mount wth a 50mm camera lens can do several minutes without guiding at 50mm focal length.

For a better answer we need to know more about the equipment you intend to use. Fire away!

Olly

[DazC]

Sorry Olly, my mistake. 

I'm using a Nikon D80 with a 50mm lens. No mount or motors.

D.C

[rubecula]

The general concensus is to follow the 500 Rule.  That is 500 divided by your lens FL. So in your case 500/50 = 10 seconds. Some also use the 600 rule. Trial shots at each setting will show you what works best for your set-up.

[SonnyE]

I ask my friend who quickly calculates the suggested ISO, Time, Aperture, in his head.

Or I use my "Hit and Miss" trial and error method. Then the "Delete the Incriminating Evidence" before anybody see's the errors. I find "Delete" to be one of the best features of the digital age.