Optimal dark frame exposure time for CCD?

[JamieInCLT]

Hi - It is cloudy and rainy at home, so I am building my calibration library.

How do I determine the optimal dark frame exposure time?  I understand that PI does some form of "dark scaling" - but I would hate to take a slew of 30m darks when a bigger slew of 10m ones would do. 

Camera is a cooled mono atik-16200 if that helps in the conversation. 

Thanks,

-jamie

[aworonow]

Dark noise is fairly linear, as I am informed. The common way to scale the dark frames usually involves making a master based on the longest exposures you will use, and scaling down. But that's not a law. Remember, you need a master bias frame to correctly scale the master dark.
Alex