The History of Astronomy

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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–)

How accurately can we measure the number of photons detected by a CCD pixel?

Slide 15

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.

low-level

structure in

the bias

Slide 16

How efficiently can charge be moved across the pixels and the readout register? Will every electron be moved or will some be lost?

Charge Transfer Efficiency (CTE)

The earliest CCDs had a CTE of only ~98%

Today CTE is typically better than 99·995% in commercial devices (“4 nines”)

Much higher in scientific devices - 99·9999% (5-6 “nines”).

Poor CTE means that not all of the photons which arrived on the CCD will be counted, and the further from the readout register the worse the effect.

Slide 17

What if a pixel’s potential well “fills up” with electrons? The physical size of the pixel determines how much charge it can hold. Larger pixels can hold more charge.

Saturation

Pixels are saturated when their potential wells are full. Electrons “bleed” along columns into nearby pixels

As a CCD pixel nears saturation, the response of the pixel becomes non-linear

The data number read out from a saturated pixel cannot exceed the largest number allowed by the “analog to digital converter” that converts the voltage to a digital signal (typically 16 bit, or 65,536)

Bleeding from bright stars

Slide 18

Thermal Noise or Dark Current

The finite temperature of the CCD leads to the production of thermally induced electrons in the silicon

Dark current increases linearly with time

A function of the temperature of the CCD

CCDs cooled to around 170°K (-100°C) to reduce thermal noise

Dark current can be removed with careful calibration

Slide 19

A few more problems with CCDs:

The amplifier in some CCDs glows!

Defects in the silicon wafer can cause cosmetic problems