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3.2 CCD Characteristics

Presently, the dedicated CCD at the coudé spectrograph is a Ford 3K x 1K CCD. There is also a TI thinned 800 x 800 CCD available on advance request. This latter chip has slightly better resolution than the Ford CCD, but significantly smaller spectral coverage (25%). The operating characteristics of these devices are given in Table 16 and a graph of the quantum efficiency vs. wavelength is shown in Figure 4. The CCD Characteristics Manual (a copy is kept at each telescope) has pictures of each CCD which show the bad columns and other cosmetic details.


 

Table 16: CCD characteristics


  

Figure 4: CCD Quantum Efficiencies.


TI CCDs suffer from a small degree of charge transfer inefficiency at low light levels (25 ADUs1. This can be a problem when looking at weak emission lines without background continuum. A low level of ``preflash" is sometimes used to bring the background up to about 20 ADUs to alleviate this problem. It is not generally a problem on normal stellar spectra where the continuum level brings the signal up to a sufficiently high level. Please discuss this technique with your instrument assistant. Aligning the spectrum parallel to rows on TI CCDs will also alleviate the charge transfer problem along the dispersion.

The CCD can currently be operated in two different observing modes: with on-chip pixel summing, or without. In the on-chip summing mode pixel size can be any integral number of pixels in one or both dimensions. The readout noise per pixel is still approximately the same as that of one pixel even though the charges from several pixels per resolution element are summed together. The amount of pixel summing is set in the DETPARS task (§6.2). While the readout noise per resolution element can be reduced in this manner, one must consider how the data will be reduced to arrive at the optimal pixel size. For example, the DOSLIT task works with the spatial profile of the spectrum to find cosmic rays and will not be effective if the spectrum is too narrow. It is important to realize that the CCD response will become non-linear above certain ADU levels. For the TI CCD's this is ~7,000 ADUs, and is essentially A/D saturation (32,767 ADUs) for the Ford CCD using the default gain. The TI upper limits can be exceeded when on-chip summing is in use, but caution is advised since the degree of summing varies.

The CCD detector will not be damaged by normal room light. You can safely enter the spectrograph room to cover or uncover the spectrograph optics with the lights on. Do not enter the room with a light during an integration as the CCDs suffer from some degree of persistence after exposure to high light levels.

1Analog to Digital Units


next previous contents
Next: 3.3 Dewar Design and Maintenance
Previous: 3.1 Introduction