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Matthew Burger - 2022 May 18

MVM color image of skycell-p2261x11y16 for a large region surrounding M51. The ACS/WFC F435W layer is shown in blue, the ACS/WFC F555W layer in green, and the WFC3/IR F110W layer in red.

On April 26, 2022, the HST data calibration and archive pipelines began producing a new Hubble Advanced Product (HAP) to be distributed through MAST. These are cross-visit, cross-proposal mosaics called Multi-Visit Mosaics (MVM), which combine public observations of fields observed multiple times by ACS and WFC3 into a set of products drizzled onto a common, pre-defined pixel grid. These new products complement the already existing HAP Single Visit Mosaics (SVM).

The sky has been divided into cells using a method similar to the PanSTARRS1 tessellation patterns. Cells are defined at two levels: Projection cells are 4.2º x 4.2º on a side and spaced 4º apart on the sky. Each projection cell is divided into a 21x21 grid of sky cells 0.2º on a side with 0.04" pixels. WFC3/IR mosaics are also produced at 0.12"/pixel scale. All sky cells within a projection cell share the same tangent WCS (i.e., the same CRVAL1, CRVAL2, and pixel size). This makes it easy to stitch together adjoining skycells when an object spans multiple skycells. Skycells from different projection cells cannot be stitched together, but the large overlap between projection cells should make the need to do this rare.

Products are created for each skycell, detector, and filter combination, designated a layer. All drizzled products in a layer share a common pixel grid which allows users to easily stack images through different filters. It does however make the individual fits files rather large. The footprint of each skycell is the smallest rectangular region that contains all the exposures in a skycell. STScI provides user tools in Python and a MAST API (both discussed below) to extract subsets of regions or patch together adjoining skycells. More details on the products including naming convensions and file formats can be found here.

The starting point for creating these products are the single-visit mosaics currently in the archive. When these underlying products are reprocessed (generally triggered by reprocessing of the standard pipeline products), the affected MVM skycells will also be reprocessed keeping the products up-to-date with the latest reference files and calibration software.

Products are created in the pipeline for ACS/WFC and both the UVIS and IR detectors on WFC3 using all exposures meeting the following criteria:

  • For ACS/WFC and WFC3/UVIS, the exposure files must be corrected for charge transfer effects (CTE) on the detectors; i.e., _flc.fits files must exist for the SVM products. Some subarrays with these detectors do not get CTE corrected and are not combined with CTE corrected exposures.
  • Solar system (moving target) observations are not used as these cannot be correctly aligned to the Gaia catalog or other exposures.
  • Grism observations are not included.
  • All observations must be publicly available.
  • Some exposures taken on gyros are not included, although an assessment of image quality is done internally in the software and some observations on gyros are deemed suitable for inclusion.

Currently all exposures of a field that meet these criteria are included in the products. For some fields with observations spanning a long time range this may not be appropriate. Therefore STScI provides tools in Python as part of the drizzlepac package for users to create custom mosaics using only the desired exposures. Documentation for MVM processing can be found at the drizzlepac documentation pages. MAST provides an API for requesting custom cutouts of MVMs including support for objects spanning multiple skycells. For example, this request will retrieve NGC 1569 which falls across two skycells. Click here for documentation about the HAPcut.MAST API.

MVM color image of hst_skycell-p0165x08y19 for a small region (2’x1.3’) in the Carina Nebula. The ACS/WFC F658N layer is shown in blue, the WFC3/IR F126N layer in green, and the WFC3/IR F167N filter in red.