<title>Scatter and reflectivity measurements on large telescope mirror coatings</title>

The reflectivity and scatter of several large telescope mirrors have been measured immediately after coating and after various use times. Mirrors evaluated inside those of the AEOS 3.67-m Telescope and the SOR 3.5-m Telescope. Reflectivity and scatter measurements were made on the actual mirrors and witness samples using a (mu) Scan<SUP>TM</SUP> reflectometer/scatterometer, a Minolta 2002 hand-held spectro-reflectometer, and laboratory spectro-photometers and scatterometers. The reflectivity measurements made on coating witness samples were compared to measurements in round-robin tests by nationally recognized optical measurement laboratories. From the results of the round- robin measurements, correction factors were determined for the hand-held instruments and used to establish actual reflectivities of the large mirrors as a function of location on the mirror and time after coating deposition. Measurements of the reflectivity and scatter of the SOR 3.5- m primary mirror taken immediately after coating, nine months after coating, and 39 months after coating, and measurements of the reflectivity and scatter of the AEOS 3.67-m telescope primary mirror immediately after coating are presented and discussed. Correlations with coating data taken on other large mirrors are also presented. Depending on coating selection, initial coating quality, operational conditions, and cleaning procedures, coating lifetimes may vary from less than one to more than five years.