1. Charge Coupled Device Advantages

2. CCD Advantages
Wide dynamic range (can measure both very faint targets and very bright ones)
Low noise
High QE
Accuracy (both linearity and stability)
Sensitivity over a wide spectral region (to 1 mm)
Dimensionally stable
Regular grid of pixels
Digital
CCD mosaic from Kitt Peak
Four 2048x2048 pixels

3. Quantum Efficiency
QE is a measure of how efficient a device is in turning input energy (in this case light) into a measurable signal.
Best film
Amateur CCD
Professional CCD
Greater efficiency means that more data can be gathered in a shorter time, or that in the same time you can measure a fainter signal.

4. Comparing Detector QEs
1 The CCD detector
However, CCDs possess advantages which clearly distinguish them from photographic plates, and from all other detectors in general.
2) The above figure compares the quantum efficiency of a CCD with the one of other types of detectors. The eye- the first astronomical detector- possesses at visible wavelenght a quantum efficienty of around one percent. In other words, we can only detect one single photon among one hundred who hit our eye. In contrast, more than 50 ( 80 at certain wavlenghts) of photons falling on the surface of a CCD are detected.
3) Additionnaly, the domain of spectral response of our eye is much more narrower compared to the one of the CCD. This limiation affects also other detectors as photocatods, photographic plates, ...
It is noticable that when a CCD is illuminated from above, it is rather insensitive to ultraviolet light and X rays, the electrodes surrounding each individal pixel beeing opaque to these types of photons. We can improve by about 20 the quantum efficiency of CCDs in the ultraviolet by covering the upper surface by a thin layer of phosphore which main task is to convert ultrviolet photons to photons who have a higher wavelenght. A much more performant method consists in making the CCD very thin and illuminating it from the bottom. The incident photons can then enter in contact with the photosensible area of the CCD, without beeing absorbed by electrodes. However, at this face of the CCD (bottom) appears a potential well who tends to trap the photoelectrons at the bottom surface.
Ingeneers have found two solutions to deal with this problem. The first method ( called “back side charging”) consists in flashing the CCD by means of an ultraviolet light before using it. In this way, an excess of photoelectrons is produced which destroys the potential well at the bottom surface of the CCD.

5. Linearity
CCDs have a linear response to light, i.e. the measured signal is directly proportional to the amount of light which was received. This is not true for film.
A linear response means that if the exposure is doubled, then the measurable signal will double. Also, twice the signal means the source is twice as bright.
CCD linear response
Film non-linear response

6. Designing a CCD Most commercial CCDs are “front-side illuminated”
A 3-d circuit on a base of silicon (the light sensitive layer)
Light has to go through the circuitry, which causes losses
Astronomical CCDs are “back-side illuminated” with QEs of 90% or greater
silicon is be thinned to a few tens of microns
need to support the silicon.
some charge diffusion in the silicon
Anti-reflection coating applied to the CCD surface reduces loses
Front-side illuminated CCDs have low blue QE
devices can be coated with “Lumigen” – an organic substance similar to the “glow” in highlighter markers (Lumigen converts blue/UV photons to 520 nm, where the CCD has higher QE)

7. Readout Noise - A CCD has an analog output
How accurately can we measure the number of photons detected by a CCD pixel?
A CCD has an analog output
Photons are converted to a charge and then to a voltage for measurement.
An amplifier on the CCD boosts the signal to a useful level
Is it possible to measure exactly how many photons fell on each pixel?
No. The noise inherent in the conversion and amplification process introduces some noise
The lowest noise CCDs now used in astronomy have a readout noise (RMS error ~2e–)

8. Bias low-level structure in the bias
The CCD amplifier also introduces a “bias level” to the output voltage
typically a few hundred electrons
The bias level is measured from the “overscan” region and subtracted off
“Bias structure” may also be present in a 2D image
The electronics as well as the physical make-up of a CCD can also imprint a faint background structure on the images.