Determining the PLATE-ID of HST Observations

All HST data have at least one guide-star ID in the Standard Header Packet (SHP). Data from the SHP will be recorded in the shh (WFPC2) or the spt (STIS) file that accompanies all HST observations. The guide stars that were used in any pointing will be recorded in the SHP as character strings associated with specific keywords in the header.

The primary or dominant guide star is associated with the keyword DGESTAR and the secondary or subdominant guide star is associated with the keyword SGESTAR. The guide-star ID is a concatenation of the region number from the Guide Star Catalogue, the star number, and the FGS used in the pointing, e.g. "0123406789F2", as a quoted character string. The actual guide star in this example is 01234 06789 and FGS2 was used.

To find the PLATE-ID that was used for the coordinates, use the WWW Guide- Star Catalogue interface:

http://www-gsss.stsci.edu/support/rgscid_form.html

The form will prompt you for the guide-star ID (in two fields)-you should use the primary guide star only. This web page will provide the coordinates, estimated magnitude, associated errors, flags internal to the catalogue's structure, and the PLATE-ID. Note that for most guide stars there will be multiple entries (corresponding to multiple PLATE-IDs), as the plates taken for the Guide-Star Catalogue overlap. The first line from your output will be for the primary plate-the preferred plate for the guide star in question, and, if two guide stars were used, the PLATE-ID you should specify in your Phase II program. If the observation used a single guide star, it is possible that a secondary plate was used. Please consult the help desk (help@stsci.edu) or an Instrument Scientist if this is the case.