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FOS offers two (and possibly three) orders on a cross dispersed format. First order covers the region 5170Å to 10830Å, second order covers 3400Å to 5670Å and third order approximately 3400Å to 3780Å. The short wavelength cutoff is governed by the ultraviolet edge of the glass components in the FOS optics. In modes A B and D both orders can be taken simultaneously. In mode C (long slit) clearly it gets rather confusing, since the second order signal lies on top of the sky from the first order. If there are several objects you must be careful that the spectra do not overlap. On the other hand the software will do its best to subtract any adjacent signal, so there may be occasions when long slit, two order mode is acceptable, although it is not to be recommended.
The sensitivity of FOS in the second order is now extremely good, since the installation of the blue coated GEC CCD in May 1988. Second order resolution is twice as good as first order, and so narrow features show up more easily than in first order.
Some third order response is now evident, but little work has been done on calibrating it.
The full chip modes B and C require a field lens in the below slit colour filter slide to avoid vignetting. To kill second order insert a GG495 field lens. To keep both orders insert the clear field lens. A copper sulphate filter is also available in the below slit neutral density slide. This preserves second order but destroys first order longwards of about 6000Å. It is useful for multiobject work in second order.