FanCam
Near Infrared Linear Polarimetry

We have a small sample of standard star data showing that FanCam with the Wollaston prism analyzers does detect linear polarization. Some preliminary results were presented at a SPIE conference in May 2006. We know very little about how well it works, because we have not yet identified the best standard procedures for acquiring and reducing the data. The basic technique is precision differential photometry of point sources in near infrared images. Much experimentation is required to compare the results of different methods of image reduction and flux measurement (e.g. flat fielding, background subtraction, aperture photometry, PSF fitting), and this cannot be done without a large collection of raw data.

Be prepared to go back and re-reduce all data from scratch for a long time until we are sure we have a stable system with a database of ongoing standard star observations that is continually updated. Observing programs should always include standard stars. NOTE: This is monumentally dull and tedious work, but unless you do it first, you have no basis for claiming that you observed or measured anything else at all with FanCam in polarization mode.

• Observe unpolarized standard stars to determine the instrumental polarization in each passband. How large is it? How precisely can it be determined? Does it depend on wavelength? Does it vary over the field of view (because of the detector or the optical system or both)? Does it depend on source brightness or color? What is causing it? How stable is it from night to night?
• Observe polarized standard stars to calibrate the systematic position angle offset and the analyzer efficency in each passband, and to verify the accuracy and stability of the system.