Reduction Steps for the Y4K Camera on the Yale/SMARTS 1-m telescope

CCD reductions

The 4kx4k CCD version of the Y4K Camera went into operation at the Yale 1-m in July 2005, and we have been privledged to be working with some of the first images obtained with it. Data for our program is obtained in queue mode by the 1-m observers, and provided on the Yale SMARTS 1-m ftp site the next day. In the course of our reductions, we have learned a lot about this instrument.

We have also developed IRAF reduction scripts for the basic CCD processing, and automatic photometry of objects. We are making these all available in a tarball. Things like directory names have been hard-wired for the photometry; if you know a little bit about scripts they should be readily adaptable by others. Enjoy.

Some facts:

Basic Reduction Steps

The CCD is read out through 4 amplifiers, each with its own bias level, and each with a slightly different gain. So, in principle the following steps should work fine: At that point, one expect to see a seemless image with sky level, and on which one can do photometry even on stars on the boundaries. This was certainly the case for the old Tektronix chips read out by 4-amps deployed elsewhere on Tololo.

However, the bias structure of the Y4K Camera is quite unstable during the night. No matter how I fit the overscans, the combined bias frame did not match the actual bias structure in program frames to better than 3-4 ADUs. This always left small "jumps" at quadrant boundaries.

The implications of this are greater than one might guess: if using a 5-pixel radius aperture on a seam, the DIFFERENCE in contribution of the sky would be roughly 500e-, or 5% for a star with 10,000 e-! So, our reduction procedure simply masks out a pixel-region along the seams, preventing photometry in this region.

The problem is well illustrated by comparing the average bias levels in each of the four quadrants for ten consecutive zero second exposures. See the example here. For each quadrant the variations are 10-20 ADUs. (For a typical CCD one might expect variations of the order of 1 ADU.) Furthermore, the structure of these biases vary considerably, as can be seen in the plots of the overscans. By fitting a high-order spline to the overscans, one almost takes this out, but not quite. `

A further complication is that the image headers are lacking any CCD section/trim/overscan information. Thus, ``standard" IRAF routines, such as Steve Heathcote's powerful "quadproc", which are strongly header-driven, will not work with these data without a lot of fiddling. So, we wrote our own.

Our procedures are as follow. The scripts must be definited in your file, located where ever you keep your IRAF home directory (i.e, with your Note that the following are hard-wired to work only on UNBINNED (1x1) images.

Note that there are several associated files:

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