George Jacoby and Taft Armandroff
The NOAO CCD Mosaic Imager at KPNO has been operating smoothly with its SITe thinned CCDs since July 1998. No dramatic changes have been made to the system. Several recent upgrades are noteworthy for prospective users.
New Filters: Vendors have been experiencing difficulty in making filters in the 6-inch size needed by Mosaic. Nevertheless, since our last Newsletter article (September 1999), we have received the additions listed below. Detailed transmission curves are available at the Mosaic web page (http://www.noao.edu/kpno/mosaic).
Name Central Wavelength Bandpass SDSS g´ 4813 1537 SDSS r´ 6287 1468 SDSS i´ 7732 1548 Washington C 3860 1034 Helium II 4690 51 Carbon III 4653 52 Continuum 475 4750 51
Reduced Noise: Minor hardware and software changes were made during January 2000 to reduce the pattern noise on the Mosaic CCDs. While the noise was only reduced by about one electron, it is much more random now; the low-level moiré is gone.
New DAT Tape Drive: We have added a new DDS-4 DAT tape drive at the 4-m installation on the computer Pecan. These tapes can hold up to 20 Gbytes (uncompressed), and compression can be used to increase the capacity by ~ 50%. This drive also can write DDS-3 format tapes (12 Gbyte capacity uncompressed), which makes a convenient "1 tape per night" format, at a cost of roughly $20 per tape. KPNO will sell DDS-3 tapes on the mountain, but at a higher price to recover overhead costs --- please bring your own.
New Display Software: The Mosaic display process (MSCDISPLAY) had been interpolating the 8K x 8K pixels to fit into a screen of about 1K x 1K. In doing so, some aliasing artifacts appeared, sometimes causing confusion to observers who could not be sure if they were seeing genuine noise patterns in the data. Frank Valdes has revised the display scheme so that no interpolation occurs; instead, the images are block averaged to fit into the available screen size. The only negative side effect of this change is that the displayed image may be slightly smaller than if interpolation were used.
Optical Quality at the 0.9-m: Chuck Claver has recently aligned the telescope optics especially for Mosaic. Previously, alignment had been done for the popular T2KA CCD with its smaller field corrector, and the images in one corner of the Mosaic array (in CCD #1) were always sub-par. Today, the images are much improved with far better uniformity across the entire field. Reports of sub-arcsec images over the entire 1° field are not uncommon, and reports of 0.7-0.8" are no longer rare.
Scattered Light: There have been a few reports of very low-level scattered light from bright stars just outside the Mosaic field. We have been systematically testing at the telescope to track down potential sources of the scattering and have begun a ray-trace study to diagnose the problems further.
New MSCRED Available: Frank Valdes has been evolving the CCD Mosaic reduction package, MSCRED, to a mature state. The latest version is available at ftp://iraf.noao.edu/iraf/extern/.
Astrometry Error: Regrettably, an error in the astrometric solutions got propagated into the Mosaic headers for data taken between October 1998 and August 1999 under certain circumstances. At the KPNO 4-m, those circumstances are:
Filter CCDs Affected U 6 B 1, 5 V 8 R 1 "UB" 5, 8
At the KPNO 0.9-m, CCDs #1 and #5 are affected for I-band data. Note that images taken with any filter besides those listed above will use the V-band solution, so CCD #8 at the 4-m would be affected.
The magnitude of the error is small (a few pixels) in the affected CCD. Coordinates in those areas of the field will be slightly incorrect, and misalignment of dithered images may occur. The headers can be repaired using the task MSCSETWCS, using astrometric database files found in mscdb (ftp://iraf.noao.edu/iraf/extern/mscdb.tar.Z ) in the directories mscdb noao/kpno/4meter/wcs and mscdb noao/kpno/36inch/wcs. Further details can be found on the Mosaic web page at http://www.noao.edu/kpno/mosaic/manual/recent.html .