ISIS ({Section 
)
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 aberration (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 important use of the instrument in this mode is as a specialised imaging polarimeter, as both the existing modulator and analyzer, as used for spectropolarimetry, can be employed. 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. This accomodates wavebands as long as R -- the throw of the analyser decreases with increasing wavelength. The current arrangement gives seven slots on the sky of 6 x 100 arcsec unvignetted, separated by 11 arcsec.
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.