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Choice of detectors

TAURUS-2 is available for use with either the CCD-IPCS photon counting detector or with a CCD. In deciding which detector to use, the normal tradeoff has to be made between the higher quantum efficiency of the CCD, particularly in the red, and the lower detector noise of the IPCS. Which detector gives the optimum signal to noise depends on a number of factors, discussed in more detail in Chapter gif

However, in the case of TAURUS there is an additional factor to consider. A TAURUS spectral scan normally consists of a large number (typically 60) of separate 2D images taken sequentially at different gap settings of the Fabry-Perot etalon, each giving a two-dimensional image in a different wavelength bin. In order to minimize the effects of changes in atmospheric conditions during a scan, the integration time at each step should be kept to a minimum so as to complete a scan through the wavelength range as quickly as possible. When using the CCD-IPCS as a detector, the fact that there is no overhead associated with reading out each frame means that it is possible to scan through an entire datacube very quickly (typically 5-10 seconds), whilst the low noise level means that it is possible to add together the results of a large number of successive scans in order to obtain the required signal-to-noise ratio. In contrast, when using TAURUS with a CCD detector, it is likely to take 1-2 hours to complete a single scan of a datacube. This means that observations taken with the CCD-IPCS are much less likely to be affected by changes in atmospheric transparency than observations taken with a CCD.

Tue Aug 15 16:42:46 BST 1995