The CCD Mosaic Imager is being used almost every night for science; in Semester 1999A, Mosaic was used for 132 nights (48 on the Mayall 4-m telescope, 84 on the 0.9-m), with 121 nights scheduled for Semester 1999B (61 4-m nights, 60 0.9-m nights). In this article, we provide a few updates since our last Newsletter contribution (December 1998). These updates are in the areas of usage, filters, and electronic cross-talk. Please see the article in the CTIO section on the recent commissioning of Mosaic II, a twin of the KPNO Mosaic, at the Blanco 4-m Telescope. Also see the Massey and Slesnick article on photometry with Mosaic.
Two new Mosaic filters have been commissioned: Washington M and DDO 51. These allow luminosity discrimination between late-type dwarf and giant stars (Geisler 1984, PASP, 96, 723). The ongoing U filter construction saga that we reported in the last Newsletter article has been completed successfully. A liquid copper sulfate U filter, with transmission properties nearly identical to our standard 4-inch U filters, has been in use since January 1999. Please see the Mosaic Web page for a complete list of supported filters (http://www.noao.edu/kpno/mosaic).
There is some low-level electronic cross-talk between pairs of CCDs in the Mosaic that are attached to the same controller. The cross-talk is not bidirectional. For example, a bright star on CCD[2] produces a "ghost" on CCD[1], but the converse does not occur. The strength of the cross-talk ghosts appears to be directly proportional to the input signal on the "contaminating" chip, with scaling coefficients of order 1/1000. The coefficients have been measured for the science-grade Mosaic I camera, and can be used to remove cross-talk ghosts within the IRAF MSCRED package, applied as the first data processing step in CCDPROC. The residuals from this correction are less than or approximately equal to 1 ADU, even for saturated objects. Within the accuracy of the measurements, the coefficients have remained stable from January through June 1999. Cross-talk correction is now included in the default Mosaic data reduction procedures of IRAF V2.11.2's mscred.ccdproc task.
Taft Armandroff, George Jacoby, James Rhoads