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Wavelength range and resolution

Table 3.4 summarises the wavelength range and resolution for the IDS gratings using a coated GEC CCD and either the 235 mm or the 500 mm cameras. With the exception of the 1800 and 2400 lines mm gratings, the wavelength coverage does not significantly vary with the central wavelength.

The tables give for each configuration the wavelength range, the dispersion in Å pixel, and the slit width projecting to one pixel at the detector. For proper sampling, the wavelength resolution cannot be less than about 2 pixels. To match the slit to the resolution ot the spectrograph, a slit width should be used which projects to about 2 pixels at the detector. If a wider slit is used to increase the total light collected by the instrument, this will degrade the wavelength resolution. This might be a good strategy in bad seeing conditions or for extended objects.

One point worth noting is that if a grating were to be mounted with the wrong orientation, (i.e. the blaze directed towards the other camera) it would result in a dramatic decrease of throughput. The dispersion will be affected too, and, in fact, a resolution improvement of about 20 can be obtained at high anamorphic magnification, but only at the cost of a drastic loss of efficiency (see J.V. Wall 1984, Gemini 12).