Subject: DIS red chip bias

From: holtz@nmsu.edu

Submitted: Fri, 7 Feb 1997 12:06:10 -0700 (MST)

Message number: 27 (previous: 26, next: 28 up: Index)

We have noticed a large "effective" read-noise on the DIS red chip on 
two separate nights within the past two weeks. Upon inspection of image
histograms of bias frames, we see that there appears to be a fluctuation
of bias level which spreads out the pixel histogram. On one night, the
histogram is distinctly bimodal, with each peak having a width of the
previously determined readout noise of ~15 electrons, but with two peaks
separated by ~20 DN, leading to an effective readout noise of ~28 electrons.
The two peaks do NOT come from two distinct regions on the chip, odd vs.
even rows/cols, or in any discernable pattern. On the second night, the
histogram is broad and flatter topped, with a comparable effective rn of
~28 electrons, with a hint of bimodality, though not as clear.

Has anyone noticed similar behavior recently? Can recent DIS users check
their data for a similar effect? If this is a new problem, it needs to
be addressed ASAP, because an effective readout noise of 28 electrons is
really quite large!

Jon Holtzman
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