Correcting the Astrometry of DASCH scanned plates

We describe the process implemented in the DASCH pipeline which applies a reliable astrometric correction to each scanned plate. Our first blind astrometric fit resolves the pointing, scale and orientation of the plate in the sky using astrometry. net code. Then we iteratively improve this solution with WCSTools imwcs. Finally, we apply a 6th order polynomial fit with SCAMP to correct the image for distortions. During a test on 140 plates, this process has allowed us to successfully correct 79% of the plates. With further refinements of the process we now reach a 95% success rate after reprocessing all our scanned plates (~11 000 in Nov. 2010). We could extract a lightcurve for 2.85 times more objects than with the previous Pipeline, down to magnitude 17. The resulting median RMS error is 0.13" for objects with mag. 8 to 17.