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9.1 Bias

There are 55 bias exposures, taken on 6 different nights, as listed in Table [*].
Table: The Bias Exposures
set # night #frames dfsc #
1 2 10 1514-1526 (not: 1516 & 1518)
2 5 10 1909-1918
3 6 5 1921-1925
4 8 10 2192-2201
5 10 10 2499-2508
6 12 10 2558-2567
Notes: The names of the raw images from the DFOSC are in the form dfscnnnn.

The frames within each set were combined. In this process, values in pixels affected by cosmic-ray-events were excluded. The bias level is around 124.2 ADU, but it is not the same in the 6 sets, it varies within $\pm 0.5$ ADU. There is a positive gradient in the y-direction, but the slope is not the same for the 6 sets, it ranges from 0 to 0.5 ADU over the length of the chip (1024 pixels). There is no simple relation between the change of level and gradient, and time. These two phenomena are illustrated in Figure [*].

  \begin{figure}% latex2html id marker 20704\mbox{
rest of the images, one gets the same physical part of the CCD).}\end{figure}

As can also be seen in Figure [*] (left), there are some stripes which run along y (i.e., across x). They are the same in the 6 frames, and are part of the bias image.

All the 6 frames were combined into one (weighting by the number of frames they had been made from). No further processing was done. We would have liked to smooth the image to remove the pixel-to-pixel noise, but since we believe the stripes to be part of the bias image, we could not do that. The mean bias level is 124.2 ADU/pixel. The standard deviation on the final bias image in areas between the vertical stripes is 0.31 ADU/pixel.

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Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997

Bo Milvang-Jensen (