Table 3 summarises the characteristics of the types of CCD chips in current use for all instruments at the WHT. Availability of CCDs changes frequently, and updates to this information will be published regularly in Gemini. At the time of writing the CCDs available are: EEV3 and EEV6 which are EEV P88300s; TEK1 and TEK2 which are Tektronix 1024 square CCD; and GEC5 which is a GEC P8603. In the very near future EEV8, an EEV P88500 will become available. See Figure 6 for the quantum efficiency curves for the TEK2 chip and a typical EEV chip.
Table 3: CCD types available at the WHT (as of May 1993)
There is a choice of five readout speeds for the CCDs, known as ``Slow'', ``Standard'', ``Quick'', ``Turbo'', and ``Nonastro''. It is possible to select all modes from the ICL interface with the commands SLOW,STANDARD, QUICK, TURBO, and NONASTRO respectively. The advantages of faster speeds are a faster readout (less overhead), and a higher dynamic range. The disadvantage is a higher readout noise. Note that the bias (overscan) level also changes with readout speed. For each CCD there is a distinct value of the ``gain''; the conversion factor between the output of the analogue to digital converter and the number of detected photoelectrons per pixel, for each readout speed. The values of ``gain'', usually expressed in e /adu, will be known to local staff. Table 4 lists the changes in CCD characteristics with changing readout speed for the TEK2 CCD. See the CCD Quality Control Manual (green binder) in the WHT Control Room for details of the various chips, e.g. linearity, gain, read-noise, read-out speed, bias levels. The CCD Quality Control information is also available on the World Wide Web (WWW) at this URL: ftp://ftp.ast.cam.ac.uk/pub/lpinfo/CCDs/. See the preceding WWW page (URL: http://www.ast.cam.ac.uk/ lpinfo/) for more general information on the ING telescopes and instruments, and the La Palma site.
Table 4: Gain and noise as a function of readout time for the TEK2 CCD (as of 28/7/94)
For normal broadband observing the observer may require either QUICK or TURBO speeds; for most applications QUICK is recommended (note that at TURBO speed the TEK2 chip becomes nonlinear at 50000 ADU). For narrow band imaging where readout noise is more important and integrations are longer, STANDARD speed is recommended.
All CCDs produce an image which is larger in both dimensions than the active area of the CCD, this area is given in column 4 of Table 3. The additional image area consists of underscan and overscan and dark reference regions. The difference between these types of reference regions, and the location of the regions for CCD types in use on the WHT, are discussed by P.R. Jorden in ING La Palma Technical Note No. 93. The bias level in each data frame, which is an electronic offset with a low frequency time dependence, is estimated most reliably from the serial underscan region, in regions of the image frame which are given in Table 5.
On-chip binning and windowing are available for any CCD, and the parameters can be controlled using the ICL commands BIN and WINDOW. On-chip binning is useful not only in reducing the amount of data read out from the CCD, and thus the dead time between exposures, but also in reducing the effective readout noise in the data. Because the readout noise occurs only once for each summed pixel, the effective readout noise is reduced by a factor of the square root of the number of pixels summed. On the other hand, because the output of the analogue-to-digital converter limits the amount of charge that can be read from the CCD in any one pixel, on-chip binning reduces the maximum dynamic range that the CCD can provide by this same factor.
Table 5: Recommended bias regions for WHT CCDs
Figure 6: Quantum efficiency of WHT Imaging CCDs