HAP Single Visit Mosaics Now Available

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On December 17, 2020, MAST began production of new ACS and WFC3 products in the HST data calibration pipeline: Hubble Legacy Archive (HLA)-style mosaics comprising the data from a single HST visit which are aligned to a common astrometric reference frame. These are the first of two types of Hubble Advanced Products (HAP) that will be produced in the HST data pipeline and made available the MAST Discovery Portal. These mosaics will be available as they are produced in the pipeline; it will take some time (roughly one to two months) before all the archival data have been processed to create mosaic products.

Three levels of products are available as part of this release: 

• Exposure level products contain data from a single HST exposure.
• Filter level products are produced from all exposures in a visit with a common filter.
• Total level products combine all exposures from a visit and are intended as a detection image for producing catalogs. 

The HAP Single Visit Mosaics (SVMs) differ from the standard HST drizzled data products, which are aligned filter-by-filter to Gaia.  SVM data products, on the other hand, are all drizzled onto the same north-up pixel grid and may have improved relative alignment across filters within a given visit, enabling easy comparison of the images through multiple filters or for images to be combined to create color mosaics. When possible, sources in the images have been aligned directly to the Gaia source catalog to improve the WCS of the images. SVM data products with both relative alignment (by filter) and absolute alignment to Gaia will contain the string 'FIT_SVM_GAIA' in the 'WCSNAME' keyword in the science extension of the image header. More discussion on HAP alignment, may be found on the webpage 'Improvements in HST Astrometry'.

We will also soon begin production of source catalogs for the filter and total level products in the ecsv format that can be read in Python as an astropy table. The columns for the point source and segmentation source catalogs are different, but in general the x and y pixel values, the right ascensions, declinations, fluxes with errors, and magnitudes with errors of each detected source are given. The catalog for the total level product gives a summary of the filter level catalogs listing the measured magnitudes for each source through the different filters used for the visit. Catalogs may be rejected (i.e., not archived) if it is determined that they are dominated by cosmic rays.

We have also released tools for users to create custom mosaics. As it is not always possible for the pipeline software to choose the best combinations of exposures to include in a mosaic, users may wish to refine the list of exposures used or use custom drizzle parameters for their own science use cases.  The tools are included as part of the drizzlepac package maintained by STScI.