Horizontal charge transfer is paused while charge packages at the output register are transfered vertically to an output amplifier and then read one by one
The cycle repeats until all the charges are read
Readout times can be as long as a few minutes
Charge transfer in a CCD
Changing the voltages φ1 and φ2 moves the location of the potential well to the right, and the electrons follow along
Wide dynamic range (can measure both very faint targets and very bright ones)
Accuracy (both linearity and stability)
Sensitivity over a wide spectral region (to 1 mm)
Regular grid of pixels
CCD mosaic from Kitt Peak
Four 2048x2048 pixels
QE is a measure of how efficient a device is in turning input energy (in this case light) into a measurable signal.
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.
Comparing Detector QEs
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
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)
A CCD has an analog output