lect12 - UCF Physics: AST 5765/4762: (Advanced)...

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UCF Physics: AST 5765/4762: (Advanced) Astronomical Data Analysis Fall 2008 Lecture Notes: 12. Combining Frames 1 Check in: 10:30 — 10:40, 10 min Questions before we start? 2 Flats: 10:50 — 10:55, 5 min High on board: ADU = g 1 ( g 2 ( E Q ( F * + F sky ) + I d ) + V off ) (1) ADU = g 1 g 2 E Q F * + g 1 g 2 E Q F sky + g 1 g 2 I d + g 1 V off (2) Each value still multiplied by g 1 g 2 E Q 1 million unknowns in image! Want to remove. Take image of uniform source, remove array effects, divide by median. Still don’t know g 1 , g 2 , or E Q , but know kg 1 g 2 E Q product at each pixel! Dividing by this corrects to kF * . Calibrate k by observing standard stars. 3 Median Combination: 10:55 — 11:05, 10 min Take many dithered images (leave 2 lines of space on board) Loop over pixel positions (Double loop over 3D image array! Yuk!) Find median of all images at each pixel Astronomical sources, cosmic rays, bad pixels rejected! (leave 2 lines of space on board) Subtract from raw image, divide by flat Sigma-rejection technique from HW5 even better, given enough images 1
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4 Normalized Median Combination: 11:05 — 11:10, 5 min If sky varies a lot, try normalized median: Take many dithered images Subtract dark frame (cold shutter, “closed” filter)
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This note was uploaded on 11/09/2009 for the course AST 4762 taught by Professor Harrington during the Fall '09 term at University of Central Florida.

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lect12 - UCF Physics: AST 5765/4762: (Advanced)...

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