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Gary Bernstein, 07/21/2014 03:25 PM

U-Band Star Flats

21 July 2014 GaryB

Am I seeing CTI problems in the u-band star flats? Is this an issue for any low-background exposures, including our standards exposures?

I am constructing a star flat from the u-band exposures taken by Jonathan Hargis on night of 22 March 2014 (295292-295313).
Since flat lamps were not working that day, I constructed a u dome flat from 13 July 2014 calibrations biases & domes (335787-335805). Only departure from standard processing is that I do not adjust each CCD to a common mean in the flats. They are scaled by a single common normalization factor (SCALMEAN).

Here is the output of the star flat process. I omitted the central N4/S4 pair (since they have a "bright spot" in u band that will confuse the current issue) and the flaky S7. Orientation is that N chips are at the bottom, S7 at the right:

Note that most of the S chips have gradients along their y axes and the N chips have gradients the other direction. These gradients are about 0.1 mag across the device.
Just for reference, here is what the dome flat looks like (using the "local" scope in ds9 so each CCD has its own zscale). It does not show the gradients in this pattern.