The History of Astronomy

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Most research CCDs have very low dark current, so dark frames may not be necessary.

CCD Calibrations - Dark

Slide 26

Pixel-to-pixel variations are removed with a “flat field” image

A flat field is an image of a featureless, uniform source (such as the twilight sky or a dome projector screen)

A flat field shows the minor pixel variations, as well as all the defects in the optical train (e.g. vignetting and dust spots)

After bias and dark subtraction, divide the image by the “normalized” (image mean reduced to 1.0) flat field

Dividing by the flat field image corrects for variations in sensitivity on the detector and throughput of the telescope and instrument

CCD Calibrations – flat field

Center to edge variations and donuts are both are about 1%

Slide 27

Flat Field Calibration - Divide (a) by (b) to get (c)

Slide 28

Cosmic Rays

CCDs are good cosmic ray detectors

Cosmic rays are always found on long exposures

To correct for cosmic rays, take at least three object exposures, and combine them with a median filter

Slide 29

Other artifacts

poor guiding

donuts

Slide 30

Data Histograms

A histogram is a plot of the number of times a particular data number occurs vs. data number

Slide 31

Sources

Kodak KAF-1302E(LE) CCD - Image courtesy of Eastman Kodak Company. KODAK is a trademark. http://www.kodak.com/US/en/digital/ccd/kaf1302ELE.shtml

Other images © Steven Lee http://www.aao.gov.au/local/www/sl/

Bob O'Connell's Fall 2003 Lecture Notes

U. Florida notes on electronic camerasThe_Electronic_Camera_in_Astronomy.ppt

Notes from the Max Planck Institute: http://www.mpia-hd.mpg.de/70CM/lecture/prak_astro.ppt