a priori WCS
A WCS derived using pre-existing information about how the observation was taken. The primary example of an a priori WCS would be the GSC240 WCS solutions from the astrometry database which were derived by correcting the coordinates that were used for guiding the observation with updated coordinates for those guide stars derived from the GAIA catalog.
a posteriori WCS
A WCS derived using information measured in the observations themselves and fit to some external reference. A primary example would be the HSC30 WCS solutions from the astrometry database. These were computed using the Hubble Source Catalog (HSC) positions of sources from the observation, then cross-matching and fitting them to sources from the GAIA catalog. Another example would be the -FIT WCS solutions computed during standard pipeline calibration processing. These solutions get derived by identifying sources from each exposure, then cross-matching those sources with GAIA catalog sources and applying the fit to the WCS.
single visit
This term refers to the set of observations taken by a single instrument using the same guide stars potentially spanning multiple orbits. All observations taken by all detectors of that instrument which were requested by the PI in the original proposal are included as this ‘single visit’ set.
single-visit mosaic (SVM)
A set of drizzled products for all the observations taken in a single visit. All observations in this set of products should be aligned to each other with the exposures for each detector/filter combination being combined into it’s own product. All products have the same WCS definition and therefore the same pixel grid to enable direct pixel-by-pixel comparison of the data across all the detectors/filters.
singleton
A single exposure taken as part of a visit that is not part of any pre-defined association based on the proposal.
association
A group of exposures taken together in the same visit, typically defined as a single defined observation in the proposal. For example, a proposal may specify use of a DITHER-LINE pattern and request that it be used to take 15second observations with the F555W filter. All those exposures would be ‘associated’ by the proposal and result in defining an association table that specifies the names of each of the 15second exposures and the name of the image created by combining all those exposures.
association table
A FITS table listing the filenames of all the input exposures that should be combined together, along with the name or names of the products to be created by combining the input images.
FLT/FLC image
This term refers to the pipeline calibrated version of the input exposures. These files have either _flt.fits(FLT) or _flc.fits(FLC) in their filename, thus the term FLT/FLC. The FLC files are the CTE-corrected versions of the FLT files, while both copies have identical WCS solutions.
Active WCS
Primary WCS

The WCS solution in the same image header as the science array which defines the transformation from pixel coordinates to world coordinates. The set of keywords that make up this WCS are:

* CRVAL1, CRVAL2
* CRPIX1, CRPIX2
* CD1_1, CD1_2, CD2_1, CD2_2
* CTYPE1, CTYPE2
* A_\*_\*, B_\*_\*

Alternate WCS
This term refers to any set of WCS keywords which have a single alphabetic character (A-Z) appended to the end, such as CRVAL1A, as defined by FITS WCS Paper I.