THE 182 CM TELESCOPE

THE 182 CM TELESCOPE AFOSC: Observing modes - SPECTROPOLARIMETRY
Spectropolarimetry with AFOSC is obtained putting the SP25 mask in the slit wheel, the polarimeter (POL) in the filter wheel and the selected grism in the grism wheel. Echelle grisms are not available in this configuration. The SP25 mask is formed by four slitlets 2.5 arcsec wide and 20 arcsec long. Four spectra will be recorded in the CCD images, corresponding to the four polarimetric channels.
If the observer is interested in polarization variations across the lines he has to take two different sets of spectra corresponding to two positions of the rotator adapter namely at 0 and 90 degrees. In this way a switch between corresponding polarimeter channels, 0 to 90 and 45 to 135, is performed, to avoid systematic errors due to differences between the channel efficiencies. See technical report n. 18 for more informations on how to take and reduce the spectropolarimetric data. Flat field The flat field procedure is similar to that for the spectroscopy observing mode, apart from two main differences: Light incoming both from sky and dome flats is polarized. For this reason two exposures at two different positions of the rotating adapter, 90 degrees separated, are necessary. Because of the splitting of the incoming beam into four channels, exposure times listed in the spectroscopy mode flat field table, should be multiplied by four. Wavelength calibration Thr wavelength calibration procedure is exactly the same as for the spectroscopy observing mode. A calibration for each of the four spectra in the image is necessary because of the presence of lateral chromatism. An example of a typical calibrated image in the figure below can be seen.
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