CCDs are sensitive to cosmic rays
CCDs are characterized before they are put on a telescope.
The parameters which are needed are:
The amplifier gain – how many electrons per count.
The linearity of the amplifier and electronics – there will always be some slight variation from perfection.
QE and CTE – how good is the CCD.
Any cosmetic or electronic blemishes (“trapping sites”, etc.) – every CCD is unique!
Gain and Readout Noise
Noise (ADU) as a function of the signal (ADU)
1, 10, 100 and 1000 sec exposures of M100
S/N ratio improves with exposure time
Readout noise dominates in the shortest exposure
Photon noise in the sky dominates for the longest exposure
NGC 2736, part of the Vela SN remnant.
Basic calibrations include BIAS, DARK and FLAT FIELD
A BIAS frame is a zero-length exposure to show any underlying structure in the image from the CCD or electronics
The bias consists of two components
a non-varying electronic zero-point level
plus any structure present
CCD systems usually produce an overscan region to allow the zero-point for each exposure to be measured
The bias structure is a constant and may simply be subtracted from each image
Because of readout noise, average several (say, 10–20) bias frames to create a master bias frame
The image is scaled with only a few ADU from black to white
Little structure is evident
Statistical variation is only 0·4 ADU so this is a clean bias frame
To remove dark current, take a series of DARK frames
A dark frame is the same length as a normal exposure but with the shutter closed so no light falls on the CCD
Since CCDs also detect cosmic rays, take several darks and combine them with a median filter to remove cosmic rays from the combined dark frame. Combining several dark frames also minimizes statistical variations.
Subtract the combined dark frame from a normal image, provided they are of the same duration. (After the bias has been removed, of course.)
All images, including darks, contain the bias. A shortcut often used is to not separate out the bias but subtract the dark+bias.