The first step in calibration is to prepare a bias frame. A bias frame is basically an image taken with the shutter disabled. The image will consist only of read-out noise and noise caused by interference of the computer.
What a bias frame does is set the zero point of the CCD output and the pixel scales to the same value. This makes the final image more accurate since the zero points are equal and no
nonlinear pixel values exist.
A bias frame will also take into account thermal current that collects while the frame is being
downloaded to your computer.
Note: If one uses darks that are the same duration as the light frames, bias calibration is
unnecessary because it is included in the dark data. Bias frames are only required If one
intends to scale their darks.
Taking Bias Frames
Not all CCDs can take pure bias frames. Some CCDs just are not designed to take an exposure
without using the shutter. In those cases you can simply skip the bias frame and go straight to
the dark frame (which will include some of the same data as the bias frames) or you can make a
psuedo-bias by taking an exposure with the shutter at the shortest possible exposure length and
your system completely blocked of any light.
If your camera can take a bias frame, it is likely already setup in your software. A bias frame is a
0 second exposure, at the same temperature.
A dark frame measures the thermal readout of your CCD, a.k.a. its
temperature. A dark frame is an exposure where the shutter is
opened but no light is allowed to hit it so it only measures the
energy from the CCD itself (dark current). This is normally done by
placing a dust cap on the telescope and then covering it with a
blanket, cloth, or something opaque to light. Darks also compensate
for hot pixels, which are defects in the CCD chip that makes pixels
look like they are permanently "on" or "lit". Darks are very easy to
take and are the most important calibration step so there is no reason not to take darks.
The Handbook of Astronomical Image Processing recommends the "Image-Times-Five" rule. Page 142
The more dark frames you take, the more accurate the frame and the lower the noise
For a sample of 100 electrons, the uncertainty is 10%, for 10,000 it is 1%. , so at lease 10 Darks Frames
A good rule of thumb is to make sure the total exposure time of all your dark frames equals five times that of the image you are calibrating.
So if you are taking a 2 minute image of expose time.
you can do 5 x 2 minute darks or 10 x 1-minute darks.
Example: 120 sec x 10 frames = 1200 sec or 20 mins ,
The Dark would be 20mins x 5 = 100mins of dark!!!
Total : 120mins
If the numbers of frames is 10 mins x 100mins = 1,000 mins = 16 hours
5 x 100mins = 500 mins = 8 hours
A night observation = 20 mins L + 20 Mins R + 20mins G + 20mins B = 80mins + 100min Dark
Total 180Min or 3 hours
I am using only one filter time exposure to calculate the dark frames , in this case L20 mins (20 x 5=100), and the total of the 4 filters
Inspect each frame to make sure that cosmic ray events do not contaminate them. They will appear as a bright spot
on your dark frame. It depends on altitude, physical size of the CCD chip, and exposure time but in general expect
about 1 cosmic ray event every few minutes of exposure time. When you have a bunch of darks, average them together to create a master dark. (You can skip the cosmic ray inspection by median combining at least 3 frames instead of averaging them; but then your final dark will have slightly more noise.)
Tip: It is possible to create a library of dark frames for different temperatures and operating environments.
However, these dark frames will never be as good as ones you take during the observing session.
Experiment with your own system and see how a library of darks affects your final error and then decide
whether the savings in hassle is worth it for your current project.