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Polarisation optics

There are four optical components immediately above the dekker slide and immediately below the slit assembly which are used exclusively for polarimetric observations. The principles behind spectropolarimetric observing, and the ISIS/FOS-II system, are described in detail in The ISIS Spectropolarimetry Users' manual, by J. Tinbergen and R.G.M. Rutten. Briefly the polarisation optics consist of:-

• A quarterwave plate, at present borrowed from the People's Photometer, effective over the wavelength range 3000-11000 Å, which can be inserted into the beam, set to any position angle, or rotated continuously at a speed of several Hz. The quarterwave plate converts circular into linear polarisation, so that the Savart plate (linear beamsplitting polariser) can detect its presence. Rotating the quarterwave plate rotates the linear polarisation striking the Savart plate.
• A halfwave plate, 40mm diameter, which can similarly be set to any angle or rotated continuously. Rotating the halfwave plate through n degrees results is a rotation of 2n degrees of the polarisation vector of the light. The halfwave plate is usually mounted below the quarterwave plate, which gives the largest field of view and best slit viewing for linear polarisation studies. It is possible to interchange these plates, although this requires that ISIS be taken off the telescope.
• A calcite block or Savart plate, located in the FCP (Field lens, Calcite, Polaroid) tray immediately below the slit. This is effective over the wavelength range 3300-11000 Å, and gives two beams separated by an amount which depends upon wavelength, but is in the range 2.1 - 2.6 mm over the effective wavelength range. The two beams are 100% polarised, orthogonally, and their relative intensity depends upon the polarisation vector of the incoming beam. Use of the Savart plate requires the spectrograph to be refocussed by 9600 microns in the Blue arm, and 9300 microns in the red in the sense that both collimator positions must be increased. Full details of the Savart plate are given in The ISIS spectropolarimetry users' manual.
• A polaroid filter, located in the FCP tray. This is used when full spatial coverage is required, and it is therefore impossible to use the dekkers which are used with the Savart plate.