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Detector

LDSS-2 should work with any standard La Palma detector system. For the purposes of this document, it is assumed that the Tektronix 1024 CCD which was used during commissioning will be used. Table 1 gives the parameters for this CCD.

It is wise to carry out some checks to ensure that the CCD is working as expected. These can be carried out using the DMS to inspect test images and calculate simple statistics. The DMS is currently run from a window on the Sparcstation, and there are also menu options accessed with the mouse attached to the DMS display. Also data in any of the DMS buffers can be transferred to the Sparcstation (see Section 3.3). This allows image inspection using more sophisticated software such as IMEXAMINE and other IRAF routines.

The ICL computer should be used to set up the correct windowing so that the required part of the CCD is read out. Some CCDs also have a fast readout mode which reduces the readout time at the expense of increased noise. Normally you will need the full CCD area and the slow readout mode to minimize noise, but there are times during certain setup operations when it is useful to have a small window and/or fast readout so that the total elapsed time to readout the CCD is reduced.

The following aspects of the CCD detector need to be checked.

• Noise levels: Take a bias frame (zero exposure) and do statistics on the image using the DMS menu functions. The noise level recorded should be close to the expected readout noise (see Table 1).

• Uniformity and cosmetics: A bias frame should not show significant structure. Ramps over one dimension of the chip probably indicate the presence of dark current. If you see this, first check that the CCD is at the correct operational temperature. You can also check for anomalous rows or columns and individual pixels. These can also be a symptom of incorrect operating temperature since traps are temperature dependent.

• Charge transfer efficiency: The most practical way of checking this at this stage is to examine cosmic ray events. By taking cuts through them in the X and Y directions (use DMS functions X-CUT, Y-CUT, X-PROF, Y-PROF), you can see if there is any evidence for trailing or asymmetry in the charge distribution. If so, this may indicate problems with charge transfer efficiency but first check several cosmic ray events since some events are intrinsically asymmetric! A better check would be to examine unblended lines in a wavelength calibration exposure (Section 5.5.2).

Throughout the night, be sure to check the temperature of the CCD from time to time and make sure that the dewar has been filled at the start of the night.