Mosaic temperature control: a warning. If you are using z' filter, read this!
In late 2003, it was discovered that the temperature control loop for the CCDs in the Mosaic camera is not closed. That is, the temperature of the focal plane is free to vary according to its instantaneous thermal load and coupling quality with the LN2 tank. A warning is sounded if this temperature moves more than ten degrees away from 165K, but otherwise, the temperature can vary by several degrees Celcius within a night and by as many as 10-12 degrees Celcius over the course of an observing block.
It is not clear how long this has been the case. Possibly (probably?) since the instrument was commissioned.
The consequence will be to produce QE variations at the red end of the spectrum. In particular data taken with the SDSS z' filter will be affected, as for any other filter where the red cutoff is set by the drop off of the CCD QE rather than the filter itself. The variation in QE of a silicon detector can show up to 1% variation per degree temperature change in the far red.
Created: 23 March 2004, by Tim Abbott