CCD

A problem with CCD present with the old controller when mounted on the instrument at the telescope was some pick-up noise which showed up like a ``pepper-and-salt'' pattern of various pixels with a value well above and below the average level. With the new controller this noise has disappeared, but when the detector is mounted on the instrument at the telescope we now occasionally see some pick-up noise with a ``pepper'' pattern, i.e., a regular pattern of pixels with a value of $\sim$60 ADU ($\sim$20 e-) below the average level. From a frequency analysis the noise was found to have a frequency of 13.28kHz. Tests were done with ALFOSC off the telescope with the power for the CCD controller taken from either the telescope or a wall socket. Also the mains earth was disconnected for some tests. In addition, measurements were made with the dewar removed from the instrument and with the dewar mounted but deliberately electrically connected - note the normal situation is the dewar is electrically isolated from the instrument when mounted. For all these situations the pick-up noise was seen. However, also in some of the tests the pick-up noise is intermittent and it appears to be absent lately.

One difference with the new controller is that the data are written in 32 bits instead of 16 bits. This required some changes to the quality control reduction scripts. One direct advantage of the use of 32 bits is the increased dynamical range. With the old controller the CCD would saturate at the maximum value of $2^{16}$ ADU which corresponded to $\sim$50000 e-. The maximum level that can now be measured is limited by the level at which the CCD becomes significantly non-linear which is at $\sim$140000 e-, i.e. nearly a factor 3 higher.

Details tables were produced for the readout times with the new detector controller of the CCD for different read out speeds (113, 100, 400, 800 kpix/second), different window sizes and different binning (1x1 and 2x2). These values are provided on the detector web page and will be incorporated in the estimate of the execution time provided by the observing script generator.