KPNO Mosaic chip-chip cross-talk coefficients

David W. Hogg
New York University
2005 May 9

To read this table, do the following: Find the column corresponding to the "target" chip (the chip you want to read). Look for significant amplitudes. For each "cross-talk" chip that has a significant amplitude in the column, subtract the amplitude times the cross-talk-chip image from the target-chip image. (In principle, you need to do some column and row flipping, but in practice you don't because the non-zero amplitudes are for chip pairs with identical orientations.)

These coefficients were found with mosaic_crosstalk and mosaic_crosstalk_analyze in the astrometry.net codebase.

Briefly, the method is as follows:

1. Read in pairs of chips, one being called the "target" and one being called the "cross-talking".
2. Find the highest-variance 256x512 (yes, vertical rectangle is a good idea) sub-image of the cross-talking chip.
3. Find all the pixels in the sub-image that are simultaneously far from median for the cross-talking image and close to median in the target image. This reduces false apparent cross-talk from accidentally overlapping sources.
4. Fit the identified pixels in the target sub-image as a linear combination of the form
   a C + b + c x + d y where a,b,c,d are free parameters, C is the cross-talking image pixel values, a is your single-image estimate of the cross-talk, b is the "sky" level, and c and d allow for gradients in the sky level.
5. Repeat the above for all chip pairs on all on-source images taken in your run.
6. The median a values (median taken across images) are your cross-talk coefficients, given above.

The IDL code that performs these functions is

• mosaic_crosstalk_analyze.pro
• mosaic_crosstalk.pro
• mosaic_mrdfits.pro
• mosaic_data_section.pro