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A number of synoptic sky surveys are underway or being planned. Typicallythey are done with small telescopes and relatively short exposure times. Asearch for transient or variable sources involves comparison with deeperbaseline images, ideally obtained through the same telescope and camera. Withthat in mind we have stacked images from the 0.68~m Schmidt telescope on Mt.Bigelow taken over ten years as part of the Catalina Sky Survey. In order togenerate deep reference images for the Catalina Real-time Transient Survey,close to 0.8 million images over 8000 fields and covering over 27000~sq.~deg.have gone into the deep stack that goes up to 3 magnitudes deeper thanindividual images. CRTS system does not use a filter in imaging, hence there isno standard passband in which the optical magnitude is measured. We estimatedepth by comparing these wide-band unfiltered co-added images with images inthe $g$-band and find that the image depth ranges from 22.0--24.2 across thesky, with a 200-image stack attaining an equivalent AB magnitude sensitivity of22.8. We compared various state-of-the-art software packages for co-addingastronomical images and have used SWarp for the stacking. We describe here thedetails of the process adopted. This methodology may be useful in otherpanoramic imaging applications, and to other surveys as well. The stackedimages are available through a server at Inter-University Centre for Astronomyand Astrophysics (IUCAA).

Deep Co-Added Sky from Catalina Sky Survey Images