Deep Co-Added Sky from Catalina Sky Survey Images

TL;DR

This paper presents the creation of deep, co-added sky images from Catalina Sky Survey data, significantly enhancing the depth and quality of baseline images for transient detection in a large sky area.

Contribution

It introduces a methodology for stacking nearly 0.8 million images over ten years, achieving deeper sky images without standard filters, and compares software options for optimal co-adding.

Findings

Deep images reach magnitude 22.0–24.2 across the sky.

Stacking 200 images yields an AB magnitude sensitivity of 22.8.

The methodology is applicable to other panoramic surveys.

Abstract

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).