Introduction to Image Reduction

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Motivation & background Instrumental Signature Removal Combining multiple images Photometric calibration Important parameters: • quantum efficiency How a CCD works Amplifier • full well and saturation/bleeding • charge transfer inefficiency One pixel • pixel size and (mostly) other variations: flatfield response • readnoise and gain (next page)
Motivation & background Instrumental Signature Removal Combining multiple images Photometric calibration Noise and S/N If electrons accumulate at a rate r, after a time t a pixel will contain rt electrons and the Poisson noise will be √ rt. The readout electronics add a Gaussian noise σ so that the signal-to-noise ratio is: S N = rt √ σ + rt For long (“sky noise limited”) exposures S N ∝ √ t so it’s hard to go deep! (Note tradeoff with area.) Be aware short exposures suffer a relatively large read noise penalty. The amplifier applies a “gain” factor G to obtain a digital number (DN) or analog to digital unit (ADU): where G is in e − /ADU. ADU = rt G
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Introduction to Image Reduction

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