The ISIS triple spectrograph, at the Cassegrain focus of the 4.2 m. William Herschel Telescope of the Observatorio del Roque de Los Muchachos, came into operation as a common-user instrument in the autumn of 1989. Although the basic spectrograph was finished, a number of additional features were still being developed. One of these features is the option to convert the spectrograph into a spectropolarimeter, capable of measuring linear and circular polarization. The polarimetric capability became available for testing in the spring of 1990, and as a common-user option (for linear polarization) by the end of that year. We have gained experience doing spectropolarimetry with ISIS during commissioning tests and during scheduled observing. The tests show that the system is performing well, and common users find that spectropolarimetry with ISIS is hardly more complicated than standard spectroscopy.
This is the first version of the polarimetry manual for common users of the WHT/ISIS/FOS/CCD system and an introduction to the different classes of polarimetric observations that are possible with that system. Both it and the observational resources on La Palma will continue to evolve. For details of the current state of affairs consult any of the persons listed in Appendix J.
Section 2 of this manual contains a short introduction to polarimetric techniques and a schematic description of the ISIS polarimetry unit. Section 3 describes standard (linear) spectropolarimetric measurement and is intended as a common-user guide, sufficient for most observers. Section 4 deals specifically with circular polarimetry. Section 5 is intended for specialists whose astronomical application requires some non-standard mode of operation. Appendices provide information on the optical components, results from the commissioning tests and lists of standard stars, references, contact persons, etc.
Array-detector polarimetry is a relatively new technique and we request feedback from the observers to improve the system. Please communicate your experience to any of those in the list of Appendix J.
We thank Dave Clarke, Dave Axon and Jim Hough for their contributions to this manual, during the commissioning and/or proof-reading stages.