MDM Trouble Report for 2016 May 04

Telescope: 2.4m
Observer(s): Ryan Chornock (Ohio University)
Instrument: OSMOS/R

Problem(s) Encountered:

The upper-left quadrant (as oriented when opened in ds9) of R4K has developed a rather extreme increase in noise, as though the readnoise jumped by an order of magnitude.
To illustrate this, I subtracted the overscan of a bias frame with proc4k.py and plotted a random row in the upper half (row 610 of a rroi4x1k image).  In the right side (corresponding to the upper-right quadrant), the mean is 0.14 with an RMS of 1.9 counts, very much what we expect.  On the left side, the mean is -0.29 with an RMS of 13 counts.
See here for the dramatic increase in noise in the left half compared to the right:  http://www.phy.ohiou.edu/~chornock/science/bad-bias.tiff
Or get the FITS file here: http://www.phy.ohiou.edu/~chornock/science/n1.0002b.fits

This electronic noise appears to be additive, not multiplicative, and it is sufficiently close to random that averaging N biases lowers the RMS by sqrt(N) without revealing any clear pattern.  However, the amplitude is sufficiently large to make that quadrant of the detector useless for faint object spectroscopy.

I looked back at my biases over the past year and found that this was not present last fall. In March, that quadrant was noisier than the others, but at a lower level than it currently is.  So clearly there is a bad trend.

I was doing long-slit observations of single objects, so I put objects in the bottom half to avoid this bad quadrant.  However, the lower-left quadrant has its own issues.  There is obvious pattern noise after subtracting the overscan which is not stable from frame-to-frame, but which you just have to live with.


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Submitted on 2016 May 5 [5:47:13]