  A number of synoptic sky surveys are underway or being planned. Typically
they are done with small telescopes and relatively short exposure times. A
search for transient or variable sources involves comparison with deeper
baseline images, ideally obtained through the same telescope and camera. With
that 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 to
generate 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 than
individual images. CRTS system does not use a filter in imaging, hence there is
no standard passband in which the optical magnitude is measured. We estimate
depth by comparing these wide-band unfiltered co-added images with images in
the $g$-band and find that the image depth ranges from 22.0--24.2 across the
sky, with a 200-image stack attaining an equivalent AB magnitude sensitivity of
22.8. We compared various state-of-the-art software packages for co-adding
astronomical images and have used SWarp for the stacking. We describe here the
details of the process adopted. This methodology may be useful in other
panoramic imaging applications, and to other surveys as well. The stacked
images are available through a server at Inter-University Centre for Astronomy
and Astrophysics (IUCAA).
