Checking out the CCD

The CCD setup is simply a matter of typing SETUP_CCD <channelname> at the ICL interface, and taking the default for each prompt. CCD focus and tilt are set up once for each chip at each focal station, and the cryostat is then put back in precisely the same place using the micrometers attached. Tilt can only be checked and corrected on the sky.

The CCD should be checked by taking a bias frame (BIAS <channelname> or GLANCE <channelname> 0) and an exposure with the shutter opening (e.g. GLANCE <channelname> 1). Transfer the BIAS frame over to the SPARCstation and check the image header (e.g. with IRAF IMHEAD) to make sure all of the necessary information is there. In the event of problems:

• If the CCD fibre optic interface card (at the top of the second blue cabinet from the left hand end of the control desk) displays a light labelled G.O.A.T. then there is probably a fault with an optical fibre cable. A duty tech should be called.
• If there is an error light on the DICI card; or a FIFO FULL light on the CCD fibre optic interface card; then the DICI should be reset and the exposure repeated.
• If the data are clearly scrambled; or show a series of diagonal stripes then the CCD controller thinks the data frame is a different size to the DMS. The DMS should be reset, and the SETUP and WINDOW commands should be repeated at the ICL interface FOR EVERY CCD IN USE.
• If the previous procedure does not work then the CCD controller should be rebooted (with the REBOOT command rather than switching it off and on) and the procedure repeated.
• If the CCD shutter fails to open this will send a message to the ICL interface. This could be due to a fault on a shutter cable; to low nitrogen pressure to the telescope; or to a mechanical failure of the shutter. However before calling for technical assistance it is worth typing UNJAM at the relevant CCD controller to see whether this cures the fault.
• Diagonal stripes may be seen on BIAS frames. These are quite common and are due to electronic pick-up noise. These are usually at a sufficiently low level (ie. less than 1 ADU) as not to be a problem for most imaging applications, especially long exposures where they will be lost in the sky noise. However, if they are seen at higher levels, alert your SA or a Duty Tech.