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Improved Photometry for the DASCH Pipeline

The Digital Access to a Sky Century@Harvard (DASCH) project is digitizing the ~500,000 glass plate images obtained (full sky) by the Harvard College Observatory from 1885-1992. Astrometry and photometry for each resolved object are derived with photometric rms values of ~0.15mag for the initial photometry analysis pipeline. Here we describe new developments for DASCH photometry, applied to the Kepler field, that has yielded further improvements, including better identification of image blends and plate defects by measuring image profiles and astrometric deviations. A local calibration procedure using nearby stars in a similar magnitude range as the program star (similar to what has been done for visual photometry from the plates) yields additional improvement for a net photometric rms ~0.1mag. We also describe statistical measures of light curves that are now used in the DASCH pipeline processing to autonomously identify new variables. The DASCH photometry methods described here are used in the pipeline processing for the Data Releases of DASCH data (Grindlay et al. 2012; http://dasch.rc.fas.harvard.edu), as well as for the long-term variables discovered by DASCH in the Kepler field (Tang et al, in preparation).