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 .

 

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.