U-Band Star Flats » History » Version 4

Gary Bernstein, 07/21/2014 03:39 PM

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h1. U-Band Star Flats
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*21 July 2014 GaryB*
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_Am I seeing CTI problems in the u-band star flats?  Is this an issue for any low-background exposures, including our standards exposures?_
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I am constructing a star flat from the u-band exposures taken by Jonathan Hargis on night of 22 March 2014 (295292-295313).  These exposures have ~6 ADU of background (55s, dark time, u band).
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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).
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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:
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_This has me worried that there is a slope in the y direction (stars look fainter farther from the readout) because this could be a sign of CTI losses in the stars.  Indeed trails are visible for the brighter stars.  But I am surprised this is a 0.1 mag effect._
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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.
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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.
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I bypassed the photometry solution to make a simple comparison between instrumental mags in two exposures, 295296 & 295298, which have roughly 1 CCD's worth of N-S shift between the exposures, so that the stars on the N3 CCD of the first are on S3 in the second.  The difference in mags indeed shows a clear slope as we move E-W, in the direction expected for CTI.  Next I measured the slope of the N-vs-S mag difference vs the y pixel coordinate, dividing the stars into bins of magnitude.  This was repeated for 5 pairs of CCDs astride the "Mason-Dixon line" dividing N from S.  
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*There does not appear to be any dependence of the slope on the brightness of the stars.*  This seems inconsistent with charge traps, which typically affect faint stars more.  It behaves as a roughly a loss of ~10% of photons after 4k serial transfers, or CTI=2e-5, independent of the size of the star's charge packets, but dependent on the sky background, since this problem does not occur in the other bands' star flats with higher bg.
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I will next look at the u-band star flat data taken 29 Aug 2013 to see if it does the same thing.