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Old 28-05-2008, 04:47 PM
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Terry B
Country living & viewing

Terry B is offline
 
Join Date: Mar 2006
Location: Armidale
Posts: 2,789
Bias and offset are the same thing. It is just the amount added to each pixel count so that there are no negative counts. Essentially a zero integration of a dark exposure. You do a few of them and average them. If you then look at the pixel count for the image, it will be some +ve number with hopefully very little variation across the image.If the variation between pixels across the sensor is minimal you can just fill an image with the mean of the image creating a perfectly flat image and save that as your offset.
Flats need to have an exposure that roughly 1/2 fills the maximum well depth for your sensor. If the exposure is short enough then you don't needto worry about darks etc for the flats.
I have taken to using a flash aimed at a wall that the scope points at with a T shirt over the front of the scope. I then use a 1 sec exposure and fire the flash manually. This gives a lovely flat field and the exposure time is not important.
You need to subtract the bias frame from your flats or they don't work well.
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