It is well known that the oblique reflection off the tertiary mirror of the 3.5m induces a photometric modulation in observations of polarized sources. What is possibly less well known is that the internal oblique refections in DIS and GRIM can induce similar photometric modulation even in observations of unpolarized sources. This happens because the tertiary induces polarization, the instruments also induce polarization, and, under normal circumstances, the instruments rotate with respect to the tertiary. The peak-to-peak amplitude of the photometric modulation of an unpolarized source is 2 * P_t * P_i, where P_t is the induced polarization from the tertiary and P_i is the induced polarization from the instrument. If each mirror induces a polarization P_m and the instrument has n mirrors in the same orientation, then P_t = P_m and P_i = n * P_m to first order in P_m. GRIM in f/5 mode has one mirror; GRIM in f/10 and f/20 modes have two mirrors; DIS has three mirrors (I assume that reflection off a blazed grating is similar to reflection off a mirror and I ignore any polarization from transmission through the dichroic). P_m can be written in terms of the optical constants of the reflecting material (see Born and Wolf), but unfortunately the constants given for Al in the standard references are for clean, bulk Al and not for oxidized Al coatings. Ken Nordsieck tells me that he measured a value of 10% in the visible for one astronomical mirror using his spectropolarimeter. Adopting this value, we find peak-to-peak modulations of: 2% GRIM f/5 4% GRIM f/10 and f/20 6% DIS The RMS modulations are roughly 0.7%, 1.4%, and 2.1%. Whether this is of concern to you depends on how accurately you wish to perform photometry. The modulation for unpolarized sources can be eliminated by fixing the instrument with respect to the tertiary (i.e., fixing the rotation of the instrument with respect to the horizon). You can perform similar calculations for polarized sources, which suffer both a modulation as the tertiary rotates on the sky and an offset because of instrumental polarization. The `direct' instruments (DSC and SPICAM) have no instrumental polarization (P_i = 0) and will not suffer a modulation for unpolarized sources. They will, however, suffer a modulation for polarized sources because of the tertiary. Alan Watson APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 133 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO