For FOS-1, observations can use several methods of sky subtraction, summarised in Table . The most common way of operating FOS-1 is to use mode D -- that is, a central 25 arcsecond aperture, giving both orders, without any field lens. For most applications, there is sufficient sky as well as the target spectrum in the 25 arcsecond aperture for sky subtraction, especially for point like sources. The format of a FOS-1 mode D spectrum is shown in Figure .
Mode A uses the spectrograph on-axis and allows a 12 arcsec separation between the star and sky aperture; no field-lens is required. By completely separating the star and sky order pairs, Mode B allows a limited 2-dimensional capability of 25 arcsec slit length. Since the star-sky separation is 85 arcsec, a higher throughput is obtained by using a clear field-lens behind the slit. For slit lengths greater than 25 arcsec (Mode C), it is sensible to observe first order only, and the second spectral order is filtered out using a GG495 field-lens behind the slit. A long slit can of course be used with the two spectral orders, but sky subtraction is then very difficult.
Table: Operating modes for FOS-1
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Figure: Format of a FOS-1 mode D spectrum