Subtract the bias field from the dark field and multiply the result by
.
Then, subtract the modified dark and bias fields from the image.
Finally, divide the image by the flat field and then multiply it by the median of the flat field.
Bias = basically, what the ccd 'sees' when no picture is taken. It's a short (< 1 sec) exposure.
Dark = A long exposure shot (at least as long as the longest exposure image desired) taken with the shutter closed (or equiv). This field is designed to find and background noise from either thermally created electrons or 'hot pixels'. With the newer ccd cameras, the dark field's contribution is negligable - the dark field was ignored in all the texas data.
Flat = Another short (5-6 sec) image taken of a blank sky region. The flat field corrects for pixel to pixel gain variations and for shadowing and other flaws in the optics.
As with all ccd images, numerous copies of the above field are taken and then averaged together to eliminate cosmic rays and other flaws in individual images.
Subtract the bias field from the dark field and multiply the result by
Then, subtract the modified dark and bias fields from the image.
Finally, divide the image by the flat field and then multiply it by the median of the flat field.