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Hi Leon
I have the Pentax *istDS and I have used it in a limited manner for astrophotography.
When I use the Pentax Remote software and expose for the (maximum) of 30 seconds using the interval function in the software, then I leave the auto-dark frame to the camera as it chugs along exposing the light frame followed by the obligatory 30 second dark. That is, I do not take any separate dark frames.
However, if I were to use the bulb mode and expose for say, 60 secs or 2 to 3 minutes, then I would take an equivalent dark frame with the lens cap on and manually subtract it from the light frame.
The important things are:
The dark frame should be exposed for the same duration as the light frame.
The dark frame should be acquired at the same temperature as the light frame.
Every 6 deg C drop in temperature halves the dark current.
Hope that helps.
Cheers
Dennis
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