The key to implementing improvements to the astrometry is the use of headerlets, self-contained FITS extensions containing a WCS transformation which can be attached to a FITS file and applied to the primary WCS. An observation can have multiple headerlets, each of which may have astrometry derived by differing methods. As HST data is processed/reprocessed, all available headerlets will be present as FITS extensions in the archived image with the best solution applied to the primary WCS. More details on how the WCS information is stored in headerlets may be found on the page Astrometry in Drizzled Products.

\section{WCS Naming Conventions}

Successfully aligning an observation to Gaia using the \textit{a posteriori} processing will result in an update of the 'active' WCS of the image with the new solution and the new headerlet extension. This headerlet not only includes the WCS keywords which define the transformation from pixels to Gaia-aligned positions on the sky, but it also contains information about how this solution was derived along with the errors to be expected based on the fit.

The various WCS solutions are identified by the WCSNAME keyword found in each FITS headerlet and use the following naming convention:

wcsName = OriginalSolution - CorrectionType

where OriginalSolution may be either

\begin{itemize}
\item OPUS : initial ground system wcs, no distortion correction
\item IDC\_xxxxxxxx : initial distortion corrected wcs (where xxxxxxxx = geometric distortion model used, e.g. the rootname of the IDCTAB reference file)
\end{itemize}

and CorrectionType may have several forms

\begin{itemize}
\item GSC240 : 'a priori' WCS where guide star coordinates are corrected from the original reference frame (e.g. GSC1.1 or GSC2.3) to the Gaia DR1-based GSC2.4.0
\item HSC30 : 'a priori' WCS corrected from the original reference frame to the Hubble Source Catalog (HSC v3.0) frame, which is based on Gaia DR1
\item FIT-IMG-RefCat : 'a posteriori' WCS matched to a reference catalog, where 'IMG' implies each FLT is separately aligned to the reference catalog
\item FIT-REL-RefCat : 'a posteriori' WCS matched to a reference catalog, where 'REL' implies that FLTs within the same filter within the same visit are aligned before a global catalog alignment
\item FIT-SVM-RefCat : 'a posteriori' WCS matched to a reference catalog, where 'SVM' implies that FLTs in multiple filters within the same visit are aligned before a global catalog alignment
\end{itemize}

More details on interpreting the WCS names may be found on the Astrometry in Drizzled Products page. A list of possible 'active' WCSNAME values populated in the image headers is provided in Table~\ref{tab:table_2}.

\begin{table}[h]
\centering
\begin{tabular}{lll}
WCSNAME & WCSTYPE & Comment \\
\hline
OPUS & `distorted not aligned' & No distortion correction has been applied; analysis of these FLT/FLC files may only be performed if corrected by the instrument-specific pixel area map \\
IDC\_0461802ej & `undistorted not aligned' & Distortion-corrected using the IDCTAB reference file '0461802ej\_idc.fits', but not aligned to an external catalog \\
IDC\_0461802ej-GSC240 & `undistorted a priori solution based on GSC240' & Alignment based on Guide Star Catalog v2.4.0 (GSC240). Absolute errors \textasciitilde 0.1'' \\
IDC\_0461802ej-HSC30 & `undistorted a priori solution based on HSC30' & Alignment based on Hubble Source Catalog v3.0. HSC30 errors are typically smaller than GSC240. If both corrections are available, HSC takes precedence. \\
IDC\_0461802ej-FIT\_REL\_catalog & `undistorted a posteriori solution relatively aligned to catalog' & Exposures aligned to one another, and then aligned as a set to the reference catalog \\
IDC\_0461802ej-FIT\_REL\_NONE & `undistorted a posteriori solution relatively aligned to NONE' & Exposures relatively aligned to one another, but the quality of the fit to an absolute reference catalog is unverified and should be checked by the user \\
IDC\_0461802ej-FIT\_IMG\_catalog & `undistorted a posteriori solution aligned image-by-image to catalog' & Exposures individually aligned to the reference catalog (not as a set) \\
IDC\_0461802ej-FIT\_IMG\_NONE & `undistorted a posteriori solution aligned image-by-image to NONE' & Exposures individually aligned to a reference catalog, but the quality of the fit is unverified and should be checked by the user \\
IDC\_0461802ej-FIT\_SVM\_catalog & `undistorted a posteriori solution relatively aligned filter-by-filter to catalog' & **NEW** Exposures aligned to a reference catalog and include improved relative alignment across filters in a visit \\
\end{tabular}
\caption{Sample active WCSNAME keyword values and the corresponding WCSTYPE description}
\label{tab:table_2}
\end{table}

\section{Caveats}