The History of Astronomy

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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

Slide 8

What moves the charge?

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

Slide 9

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

Slide 10

QE is a measure of how efficient a device is in turning input energy (in this case light) into a measurable signal.

Quantum Efficiency

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.

Slide 11

Comparing Detector QEs

Slide 12

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

Slide 13

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)

Slide 14

Readout Noise -

A CCD has an analog output