Learn WCS transforms from the Gaia catalog

[jeff-regier]

The Gaia dataset is an extremely accurate catalog of bright stars' locations. We can use them to learn the WCS transforms for each image rather than relying on the SDSS pipeline.

It's already stored on the NERSC file system.

/global/project/projectdirs/cosmo/work/gaia/chunks-sdss

In addition to getting more accurate WCS transforms, we might also hope to correct for a property of the CCD by calibrating with the Gaia catalog: photons from previous images get left behind on read out with a probability based on their position in the image. Light sources near the edges get shifted by a tenth of a pixel or so.