All,
I had an action item from a past phonecon to dig out Andrew's thesis and
report on his method for correcting for differential refraction in
difference imaging. Here's a quick summary of the prescription:
1. Register and warp the red template image so that it overlays the blue
template image (there is a scale difference between red and blue in the
Macho camera), subtract sky from both, and form a ratio (color) image.
2. Prior to forming a difference image at some airmass, remap each blue
template image pixel to the position it would refract to based on the
airmass and the template color from (1). In calculating the
differential refraction, the assumption is made that the objects forming
the image are stars radiating as a blackbody at some temperature.
3. Carry through with the usual image subtraction procedure.
The effectiveness of this procedure is not well documented in the
thesis, but there is a comparison of a difference image done with and
without the correction which certainly shows a big improvement in the
residuals around the stars. It is at high airmass (2.4) of course, but
the Macho pixel size was 0.62", so the effect is sub-pixel even there.
I do recall Andrew showing me light curves with and without the
refraction correction, but none of this made it into the thesis. We
could ask him if he still has that info around.
Cheers,
Tim
Robert Lupton wrote:
February 16, 2006 1:00 pm : ImageDetWG 'Phone Con
866 330 1200, passcode 300-2363#
Agenda:
Action items
Andy Becker reporting on Drake's paper
on difference imaging in the presence of refraction
Zeljko: requirements on focal plane rigidity
Today's Exciting Topic:
Galaxy Photometry and other Parameters. What should we do?
Isophotal
Kron
Petrosian
Models
How sophisticated?
Texture etc.
Shapelets?
Shape measures for lensing
How should we handle multiple exposures?
Any other business
R
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