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Rob,
Let's say that you shot 10 each
600 second exposures of the red, the green, and the blue with an ST-10E camera
and typical SBIG filters ... (I think the ratio of exposure times to get
"normal" color is something like R=1.0 G=.9 B=1.7 or 1.8 ... this assumes that
you are not going to do any stretching of the data to achieve neutral color). So
... if you were to combine the equal exposure times your image would come out
really yellow ... not enough strength in the blue.
To get normal color out of those
equal exposure times, you are going to have to stretch the blue values ...
multiply what you have by 1.7 to get the correct strength. Uh - oh ... when you
do that, your NOISE just went up 1.7 times !
If the S/N of the red and green
are approximately 1.0, then to get the same S/N out of the blue when it is
stretched, it needs to have 1.7X the S/N compared to the R&G. Now when the
blue is stretched and the S/N decreases, it comes back to 1.0 just like the red
and green. So we need to shoot a bunch more of the 600 second blue exposures to
increase our S/N ... we know that later we will be stretching the blue data to
get neutral color.
The huge advantage with this is
having only one master dark frame ... oh that is soooo nice!
Hope this makes sense
...
Tony
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