Subject: DIS A/D


Submitted: Thu, 7 May 1998 17:51:19 -0600 (MDT)

Message number: 52 (previous: 51, next: 53 up: Index)

This is to report on the noise problem noted by Charles Hoopes in his 
May 5 run with DIS. Charles noticed exceptionally high noise in both
the red and blue chips of DIS. A histogram of intensities in individual
bias frames shows a bi-modal distribution of intensities; two histograms
with an empty region inbetween. In looking at this more
carefully today, it appeared that the offset between the two histograms
is 256, which suggests a problem in the A/D conversion, where a bit
is dropped or added in the conversion. I am not sure why this problem
would occur in both the red and blue chips (don't they have two 
different convertors?). Because the difference of 256 is so large,
it has in fact been possible to correct bias frames and data by subtracting
256 from the pixels that are clearly too high. I should also note that
regions of high intensity in the data (where the A/D conversion 
goes over to the next bit) appeared to be normal again. These pixels do not
require a correction, and needed to be identified and fixed by visual
inspection, after the automatic subtraction of 256 from high pixels.

While we can probably salvage the one set of spectra we managed to get
that night, this problem raises several questions:

(1) Why was this problem apparently no longer present the following night?

(2) Does it make sense that it showed up in both B and R chips at the same

(3) Could the A/D convertor(s) be flaky, also causing the apparent increase
in "readnoise" in the red chip, which seems to come and go. Note that 
an error in a different bit could easily cause some pixels to be high 
or low by any multiple of 2. For example, adding 32 to a bunch of pixels
would go a long way to explaining a large increase in readnoise.
The bi-modal noise histograms noted for the red chip by several users
show a peak separation that may well be caused by one of these smaller

I would like to get some feedback from DIS experts on this, and suggest that
DIS users carefully check their data for possible A/D convertor errors.
Would it be difficult to replace the A/D convertors?

Rene Walterbos

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