The modulation principle described in the previous section boils down to a sequence of 4 exposures, differing only in the position angle of the halfwave plate, each exposure yielding two spectra on the detector. Since in the mode of observation described in this section the modulation is of a (very) slow kind we shall refer to it as the staring mode. A more complicated observing scheme, in more truly modulating mode, is described in Section 5 and is for special cases only.
Large CCDs should be in operation in both arms of ISIS. 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, correct telescope and spectrograph focus and select the appropriate Dekker mask. At present, only one Dekker mask is available for spectropolarimetry; see Fig. 11. Special masks can be produced; see Appendix B).
For point sources a Dekker with three apertures, one for the source and two for the sky, could be used. In this setup the chip may be windowed to reduce the time taken by readout and data transfer. For extended sources a comb dekker with 4''.5 apertures is available (Dekkers 4 and 5), covering a 2.6' field in 9 apertures. The currently available comb Dekker has a duty ratio of 1/4 and hence observation of the full slit requires 4 complete polarization measurements with the telescope at offset positions along the slit to allow different parts of the slit to be observed.
Note that the calcite slab affects the spectrograph focus; this is corrected by adding 9200 units to the standard collimator positions.
The slit-view TV camera looks through the halfwave plate, which 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.
A standard sequence of exposures is:
Exposures 1 and 2 yield Stokes Q, and exposures 3 and 4 Stokes U.
Q and U, hence also the polarization angle, are defined relative to
some instrumental coordinate system. The orientation of this
instrumental reference system relative to the N-S meridian must be
defined and recorded. It is convenient to keep the orientation
of the instrument fixed relative to the sky (set Cassegrain rotator
tracking), but equally it is important to keep the relative
orientations of the telescope and ISIS constant (this will mean there
is only one global system to calibrate for polarization zero-point).
Our compromise recommendation is to track in angle during the entire
observation, but to aim at having the slit vertical halfway through
the observation. Fig. 5 can help you to plan this; in future it may be
an on-line facility. Be sure to record the position angle of the
halfwave plate (for each exposure) and of the Cassegrain rotator, and
to understand the exact definitions.
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Figure 5: Planning aid to set slit vertical at mid-exposure; courtesy
Robert Laing. The curve labels are declination values.
Experiments with the dichroic beamsplitter in position showed reflected light from the rear of this component. Such light is displaced along the slit, partly into the spectrum of the other polarization; this spoils the polarimetry, so for the time being we must, reluctantly, advise against use of the dichroic (i.e. against simultaneous use of the ISIS red and blue arms; the blue folds - prism or mirror - are OK, you can alternate between red and blue in one run).