The CCD controller has been modified with the goal of achieving bias frames with a flatter shape and higher reproduceabilty. A comparison between two bias frames is shown in figure 9. One bias is made after the camera has been idle for an hour, and the other is made after a few minutes of constant read-out activity. The high similarity of the two frames indicate a very stable bias. It should be noted, though, that a few times during testing, a bias frame with a strong vertical gradient appeared, as seen with older controllers. The circumstances under which this happens has not been determined.
The CCD has hardware overscan regions of two columns at each end of the serial register. These areas somewhat too small for a high signal to noise measurement and are suspected to be affected by the illumination of the imaging area, so they are not recommended as bias level reference.
The recommended type of overscan can only be used for read-out windows extending to the edge of the CCD opposite of the amplifier being used. Overscan in the X-direction is set by the ``xover'' command, and in the Y-direction by ``yover''. The Y-overscan is of little practical use, though. To get the overscan region in the image, the window size must be specified to go beyond the border of the CCD by the width of the overscan. E.g. An image of the section [1500:2052,something] with a X-overscan width of 50 using the A-amplifier would be defined like this:
ampl A
xover 50
xbeg 1500
xsiz 602 (i.e. 2052-1500+50)
The default x-overscan size is 50 and zero in the y-direction. Post-processing by BIAS will always make the x-overscan appear at the right border of the image, whatever amplifier is specified.
The overscan level has been confirmed not to drift relatively to the bias level as a function of exposure time. There is a small drift with illumination, practically neglectable with an amplitude of 2 ADU for near-saturation illumination.
Another type of overscan can be defined by the ``over'' command. This type can be used for any window geometry, but has proved to be very unreliable. It should not be used under any circumstances, so do not change the default size of zero.
The bias level is approximately 400 ADU, but changes a little depending on the amplifier, gain and mpp mode selection.
Figure 9:
At the upper plot, the X-profiles of two bias frames are displayed.
In black is drawn a frame taken after an idle time of an hour, in grey
a frame taken after a period with continuous read-out.
From column 552 to 602 is the ``xover'' type overscan, and from column
602 to 652 the ``over'' type that is strongly offset in level.
In the plot below, the Y-profiles are shown, and in the two plots at the
bottom, the differences between the profiles are shown.
The differences between the images is evidently rather small, and both
overscan types track the bias level well. If the read-out window
does not extend over the edge of the CCD, the ``xover'' type will not be
available, and the ``over'' type will be unreliable.