The astrometric solutions for NOAO Mosaic exposures taken between October 1998 and August 1999 contain errors for certain extensions in certain filters. The filters and extensions affected are given in the table below. KPNO 4 meter: U im6 UB im5, im8 B im1, im5 V im8 R im1 KPNO 36 inch: I im1, im5 CTIO 4 meter: All im2, im5, im6, im7 Filters other than U, UB, B, V, R, or I use the V solution; look at the WCSASTRM keyword to identify the solution used. There is currently only a V filter solution for CTIO data. The effects of the errors are generally small (on the order of a few pixels) and affect only parts of the field as indicated in the table above. Coordinates measured from the solution will be incorrect and dithered and stacked exposures will show misalignment of objects leading to variations in the PSF of stacked stars. To correct the headers get or update the NOAO Mosaic database directory. This directory is distributed in the file: ftp://iraf.noao.edu/iraf/extern/mscdb.tar.Z The installation instructions are found in the file mscdb.readme in the same directory. Within the directory tree, with root defined in IRAF as mscdb$ as per the installation instructions, the astrometry database files are in the subdirectories; mscdb$noao/kpno/4meter/wcs mscdb$noao/kpno/36inch/wcs mscdb$noao/ctio/4meter/wcs The files have names of the form cheb44.dat, where is the short filter identifier and the rest of the name indicates the functional form of the solution. The astrometry database files are applied to the mosaic multiextension files (MEF) to update the header keywords with the following command from the current MSCRED distribtuion: ms> mscsetwcs where is the appropriate astrometry database file for the list of from the same filter. This can must be done to data in multiextension format, though it can be at any stage of processing from raw data to flat fielded prior to MSCCMATCH or MSCIMAGE. Frank Valdes fvaldes@noao.edu