Here is an example of using a dark frame scaled down to adjust for a shorter light exposure.
The image consists of 7x8 minute frames at -20șC with ST10xme. The frames were average combined and blooms automatically removed.
Note that all dark frames (8-minutes and 20-minutes) and bias frames were taken more than half a year ago in my refrigerator. Bias master consists of 40 frames sigma-reject combined.
Here is the image reduced using a master dark frame consisting of 20x8min frames sigma-reject combined at -20șC:
Same image fragment, but zoomed-in 300%:
Now, here is the same image reduced using a master dark frame consisting of 20x20minute exposures @ -20șC, also sigma-reject combined. Notice some additional cold pixels:
Same image zoomed-in 300%:
And here is the same image (with the dark master scaled from 20 minutes to 8 minutes), but with cold/hot pixel removed:
And, finally, just for fun, here is the same image reduced using a master dark frame of 15 minutes @ -15șC. Again, 20 frames were combined using sigma-reject to create this master dark. Notice the increased population of hot/cold pixels.
Interestingly, the 15min@-15șC master dark needed to be scaled by a factor of 0.27 to match light frame of 8min@-20șC:
Copyright © 2005 Paul Kanevsky