The History of Astronomy

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Slide 1

CCDs in Astronomy

History of CCDs

How do CCDs work ?

Advantages of CCDs

Calibrations

Observations with a CCD

Charge Coupled Device

Slide 2

Photographic Imaging

Photographic Plates: 100mm thick emulsion spread over a glass base

Solidified gelatin w/silver halide salt grains

“Pixel size” (grain size): 6 mm

How it works: multiple photons strike silver halide grains – give off electrons and form silver atoms within grains

Developing converts exposed grains into opaque silver

Slide 3

Photomultiplier Tubes

“Single pixel” detector made of alkali metals

Photon strikes detector and knocks off one electron

That electron creates a cascade effect that ends in many electrons at the end of the detector

The incoming photon must have enough energy to knock loose the first electron or it is not “seen”

Slide 4

CCD History

First developed in the 1960s as memory storage devices

Sensitivity to light suggested imaging possibilities

In the 1970s, CCDs were used primarily as experimental devices

In the 1980s, their use became more widespread

By the 1990s they’d essentially completely taken over almost all imaging applications

video and still cameras, scanners etc.

Astronomy is a highly demanding application

low light

noise

cosmetics

CCD image of Uranus from 1975 (JPL & UofA)

8900 Å

61”, Mt. Bigelow

Methane at the south pole

Slide 5

How Does a CCD Work?

Rain = Photons

Water = Charge (photon strikes silicon semiconductor surface and knocks an electron loose by the photoelectric effect)

Buckets = pixels (electrons accumulate in “potential wells;” depth represents how much charge each pixel can hold)

The charge in each line of pixels is shifted to the readout register

The charge in each pixel is counted

Slide 6

How CCDs Work

9 pixel CCD, an output register, and an amplifier

Pixels divided into 3 regions to create potential wells

during an exposure, the higher potential of the central region (yellow) collects electrons

during readout, the potentials are changed to transfer charges to the next region.

Slide 7

Electrons transfered from pixel to pixel

Charges guided to the output register