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ISIS can be used as an orthodox focal reducer, with a convenient plate scale of 0.3 arcsec per 22 micron pixel. The optical quality of the instrument is excellent on-axis, although towards the edges of the field there is some chromatic distortion (this does not matter in a spectrometer). The mirror which replaces the grating in this mode is silver-coated, and so the throughput of the red channel is good (5 reflections at 98% each). The mirror should be used at the angle appropriate for the channel in use (55500 for the red arm or 53250 for the blue arm).
The important use of the instrument in this mode is as a specialised imaging polarimeter, as the existing modulator as used for spectropolarimetry, can be employed. The analyser is a Savart plate which is placed in the multi-slit slide, with a field of 80 arcsec diameter. As in the case of spectropolarimetry, a dekker mask is necessary whose duty fraction is fixed by the angular throw of the below-slit analyser, and by cross-talk (scattered light) between the separated images in each plane of polarisation. The instrument has been successfully used with a dekker consisting of a series of slots 1.2 mm wide with an occulting bar 3.5 mm wide in between. This accommodates wavelengths as far as R -- the throw of the analyser decreases with increasing wavelength. This dekker arrangement, gives slots on the sky 5 arcsec wide, limited in the other direction by the 80 arcsec diameter field of the Savart plate. separated by 15 arcsec. This pattern is repeated in the narrow direction six times before the field is vignetted by the modulator (the half-wave plate). This dekker is rather conservative and a larger duty fraction could probably be used.
The principal limitation in practice is that filters have to be changed manually, as the small below-slit filter slides in ISIS have to be used. Likewise swapping from red to blue channels involves manual operations.