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The ISIS spectrograph is fitted with polarisation optics which allows the observer to rapidly convert the instrument into a spectropolarimeter capable of measuring both linear and circular polarisation. The polarisation assembly basically consists of two units: above the slit a halfwave and a quarterwave plate can be inserted, and immediately below the slit either a calcite plate or a polaroid are available as a polarisation analyser.
In some more detail the following components comprise the ISIS polarisation system, in the order the light passes them:
For linear spectropolarimetry, after setting up the spectrograph and acquiring the object, one only needs to move the halfwave plate into the beam at a specific angle, move the calcite slab into the beam and select the appropriate dekker mask. The calcite slab affects the spectrograph focus which is corrected by adding 9200 units to the standard collimator positions. The slit-view TV camera looks through the halfwave plate, which causes the TV image to be highly vignetted and also affects the TV focus. The optimal TV focus (at scale 5) is about 1500 units less than the standard setting without the halfwave plate. If your object is faint it will be necessary to remove the halfwave plates and the dekker in order to put the object on the slit.
The instrumental setup for circular spectropolarimetry is similar, but the quarterwave plate must be used in stead of the halfwave plate. For further details see Section 9.5 of this manual and the ISIS Spectropolarimetry Users' Manual.