What are mosaics? How do they differ from Level 2 combined images?
HLA mosaics, also referred to as Level 3 data, are images that combine data from multiple HST visits to cover a contiguous area of the sky. Level 2 combined images are restricted to data taken within a single HST visit, which means that the data can only cover a limited region of space (HST cannot change pointing by more than 2 arcmin within a single visit) and are obtained in consecutive HST orbits. In principle, mosaics can cover a very large area of the sky, such as the areas observed as part of the GEMS and COSMOS programs (0.25 and 2 square degrees, respectively). As mosaics can encompass data taken at different roll angles, with different guide stars, and at different times, alignment and handling of time-dependent effects (sensitivity, geometric distortion) pose greater challenges than for Level 2 combined images.
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