The History of Astronomy

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CCDs are sensitive to cosmic rays

Bad columns

Cosmic rays

Glowing amplifier

Slide 20

CCDs are characterized before they are put on a telescope.

CCD Characterization

The parameters which are needed are:

The amplifier gain – how many electrons per count.

The linearity of the amplifier and electronics – there will always be some slight variation from perfection.

QE and CTE – how good is the CCD.

Any cosmetic or electronic blemishes (“trapping sites”, etc.) – every CCD is unique!

Slide 21

Gain and Readout Noise

Noise (ADU) as a function of the signal (ADU)

Slide 22

Observing with a CCD

1, 10, 100 and 1000 sec exposures of M100

S/N ratio improves with exposure time

Readout noise dominates in the shortest exposure

Photon noise in the sky dominates for the longest exposure

Slide 23

CCD Calibrations

NGC 2736, part of the Vela SN remnant.

Basic calibrations include BIAS, DARK and FLAT FIELD

Slide 24

CCD Calibrations - Bias

A BIAS frame is a zero-length exposure to show any underlying structure in the image from the CCD or electronics

The bias consists of two components

a non-varying electronic zero-point level

plus any structure present

CCD systems usually produce an overscan region to allow the zero-point for each exposure to be measured

The bias structure is a constant and may simply be subtracted from each image

Because of readout noise, average several (say, 10–20) bias frames to create a master bias frame

The image is scaled with only a few ADU from black to white

Little structure is evident

Statistical variation is only 0·4 ADU so this is a clean bias frame

Slide 25

To remove dark current, take a series of DARK frames

A dark frame is the same length as a normal exposure but with the shutter closed so no light falls on the CCD

Since CCDs also detect cosmic rays, take several darks and combine them with a median filter to remove cosmic rays from the combined dark frame. Combining several dark frames also minimizes statistical variations.

Subtract the combined dark frame from a normal image, provided they are of the same duration. (After the bias has been removed, of course.)

All images, including darks, contain the bias. A shortcut often used is to not separate out the bias but subtract the dark+bias.