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THE MASTER DARK
If you take a picture of the noise in your camera (by leaving the cap on the lens) this is
known as a Dark Frame. The Dark Frame must be the same length exposure, and ASA setting,
and the same TEMPERATURE as your Lights. Because of the temperature factor you'll normally
have to capture Dark Frames during the same night that you capture your Lights.
The stacking software subtracts the Dark Frame from each of your subs. The effect is to
reduce noise in your subs.
If you create a single dark frame you actually create a photo of the noise in the pixels
of your camera as they were at the moment when you took the photo. If you take another
dark frame then due to the random nature of noise the second dark frame will look slightly
different from the first.
Ahah! What if we take lots of dark frames and then average them?? Heh, we get a picture
that has an average noise value for each pixel! How useful is that! This is the fabled
Master Dark. It is an Average Dark.
The Master Dark works by subtracting the "average" noise value from each pixel
in each sub. We are therefore more likely to be subtracting a value that is close to the
real noise value present in the pixels of each sub.
If you could create a Magic Dark Frame that removed the TRUE noise value (and not just the
average) from each sub you'd be on a winner because you would remove all noise from every
sub. You could amplify (brighten) your target image to whatever value pleased you or to
infinity, whichever came first. Until someone actually develops the Magic Dark Frame then
the average beauty of the Master Dark will have to do.
I like to capture enough data to do a Master Dark every session. It's usually a SUM of 100
dark VIDEO frames. I then use this as my Master Dark frame and scale it by 0.01 before
subtracting it from each Light frame. Astrovideo does the scaling and subtraction
automagically. It works a treat for me. Much better noise reduction than using just a
single frame dark.
Note that I'm using video (2.54 second subs) so you'll probably NOT want to capture 100
DSLR darks before you begin capturing lights. My guess is that to prevent boredom you will
only want to take 4 or 5 DSLR darks (assuming 5 mins for each dark). Remember the darks
have to be the same duration as your Lights. You may want to experiment with this because
the advantage of a very good Master Dark is that it is scalable. Just say you wanted to
take Lights that were half as long as normal then you could scale your Master Dark
appropriately to match the shortened Lights.
I once created a 1000 frame Master Dark for video and that was high quality but the
problem with doing that is that the dark data took up most of the tape, not leaving much
room for the actual Light data. Ho hum.
Anyhow, try creating a Master Dark with a reasonable number of darks and you'll be glad
you did! Chalk and cheese are words that spring to mind.
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