Subject: DIS red bias pattern

From: Mark Hammergren

Submitted: Thu, 13 Feb 97 13:10:11 -0800

Message number: 29 (previous: 28, next: 30 up: Index)

  I have inspected some DIS red biases that were obtained on Feb. 9.
These biases display the increased level and "bimodality" of the effective
read noise that has been mentioned previously.

  While there is no apparent dependence of bias level on two-dimensional
position on the chip, there is a distinct pattern in the bias dependent on 
pixel number. (That is, numbering the pixels in a raster from left to right, 
and bottom to top.)

  Both Fourier analysis and phase dispersion minimization show a strong
period at around 615 pixels. When binned in phase, the pattern resembles
a sawtooth, or perhaps a sine curve with one or two low overtones.

  Such a fluctuation in bias level could account for the bimodality
seen if the bias spends more time at the extremes of the imposed pattern.

  If the readout speed of DIS is around 40-thousand pixels per second, this
seems to imply a frequency of around 60 Hz for the interference causing the
new bias structure.

  All of these observations point to some sort of pickup noise as being
the source of the new bias structure in the DIS red chip. Was DIS's ground
changed during the telescope refurbishment? This should be examined in hopes
of removing the bias structure from future observations.

  I have not looked into the effectiveness of Fourier filtering at getting
rid of the bias pattern from existing data.

-- Mark Hammergren, UW
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