1. CCD Detectors CCD=“charge coupled device” Readout method:
Silicon semiconductor chip
array of up to crossed electrodes -> potential wells
current models: up to 2048x2048 wells trap photoelectrons
Readout method:
Electronics adjust electrode potentials cyclically to shift charge from one electrode to the next: “bucket brigade”.
Bottom row read out, then all others shifted down simultaneously.
ADC
Memory
Amplifier:
Cooled by LN2
Amplifier:
takes charge from corner pixel
thermal noise with fast readout
Analogue-to-digital converter:
Converts voltage to digital units (ADU)
a.k.a. data numbers (DN)
Memory:
Stores image

2. The pros and cons of CCDs
Advantages:
Quantum efficiency (QE) ~ 80 % (400 nm - 1 m)
Linearity to (better than) << 0.1 %
Dynamic range: Pixel well depth ~ 106 e–, RMS readout noise ~ 4 to 10 e–
Fixed format pixel grid
Can extend blue response (thinned back-illuminated chip or coronene coating)
Disadvantages:
Readout noise 4 to 10 e– RMS
Slow readout ≥ 10 to 100 s
Cosmic-ray hits limit exposure times
Saturation via wells filling up and limited ADC range
Charge “bleeding” down columns, then across rows
Blemishes (charge traps, hot pixels)
Gaps between pixels

3. CCD calibration Two main steps: Raw image bias subtraction
flat-field division
“Flat-field frame”:
Measures pixel-to-pixel
sensitivity variations under
uniform illumination.
“Bias frame” = zero-exposure image
Measures constant signal added by
readout electronics

4. Measuring bias and readout noise
Calibrate by taking mean or median of many zero-exposure images and/or
“Overscan” the CCD by reading out additional rows of data for which no physical pixels exist.
Cosmic ray hits must be removed, e.g. by taking median of many frames.
“Readout noise” =  (Bxy):
Voltage drift may cause <Bxy> to vary in time:
Need to scale bias frame to match overscan.

5. Flat-field division Direct imaging: Spectroscopy:
twilight sky or inside of telescope dome
OR median of many dark sky frames of different fields (median eliminates stars)
Spectroscopy:
spectrum of internal comtinuum source (tungsten lamp)
Pixel-to-pixel sensitivity variations => Fxy is never uniform, even with uniform illumination.
Take 10 to 30 flats with high exposure levels
subtract bias form each
scale to common mean value (if lamp/sky brightness drifts)
take average or median (to reject cosmic-ray hits)
fit a polynomial to flat field and divide so that <Fxy> ~ 1. This preserves data numbers/photons while correcting pixel-to-pixel variations.

6. Measuring the gain of a CCD -- 1
The gain of a CCD is the number of photo- electrons per ADU (DN).
Let X be a random variable representing number of ADU recorded in a pixel.
There are 3 types of noise that contribute to variance 2(X)
Readout noise 2
Poisson noise :number of photons detected = X / G
“Scale noise” with variance f2X2 -- usually dealt with by flat-fielding
At low count-rates, readout noise (=constant) dominates.
At high count-rates, Poisson noise ~X1/2 dominates.

7. Measuring the gain of a CCD -- 2
Take several flats with different levels of illumination.
Divide into sub-areas and measure <X>,2(X) locally
Make log-log scatterplot of  (X) vs. <X>:
Determine values of (0,G) that give best-fitting curve of form:
G=1
G=3.2
G=10
slope=1/2
4.2
In this illustrative case we find
readout noise = 4.2 DN and
gain G = 3.2 electrons per DN