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FLATS AND DARKS
DARKS
Another way to reduce noise significantly is to create a dark frame. A dark frame
is a picture that you take when the lens of the camera has the cap on. No light gets in
but the noise is still there. You in effect take a picture of the noise. You then use
computer software to subtract the noise layer from your photos.
This works because not all aspects of noise are random. For example, some pixels in the
camera's CMOS/CCD chip are always noisier than others. Also there is a certain predictable
"average" level of noise for each pixel (sort of like how temperature is an
average measure of the random movement of molecules).
Armed with your dark frame you then use the computer software to subtract it from your
photos. You'll be amazed at how well this works. Fuzzy noisy photos clear up like magic.
FLATS
To simplify it a Flat is a picture that you take of a moderately, and evenly, lit
featureless wall or twilight sky. The purpose of it is to show up the optical defects in
your equipment. For example, dust specs, vignetting and even differences in pixel
sensitivity are all visible in a Flat.
You then use the Flat in the software to even out these irregularities and improve the
overall image. Using a Flat is, IMHO, less important than getting your darks working.
Darks are I believe critical.
I haven't yet graduated to creating images of a quality where I feel it necessary to
invest in taking flats - maybe one day soon...:o) I hope.
The big thing to remember with flats is that they are only useful if the flat is taken
with the EXACT same optical setup as your lights. If you rotate the lens, or the camera in
the telescope, or if you change other things in the optical path you'll have to take
another flat to match.
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