HST Phase 2 Target Coordinate Instructions

The HST reference frame is defined by the HST Guide Star Catalog (GSC) that STScI has created. For observations that require accurate coordinates, such as those using the spectrographs which have small apertures, it is very important that you provide positions derived in the same reference frame as the original GSC plate material. Except in special cases, you do not need to provide this level of sub-arcsecond accuracy for imaging observations.

General Guidelines

• If you are observing a star, or nucleus of a galaxy that is bright enough to be in the GSC (down to ~ V=15), then you should retrieve and use the GSC coordinates.
  • Retrieve coordinates from the GSC
  • Instructions for using the GSC output
• If your target is visible on the original GSC plate material you should download a FITS image of the plate to measure your target coordinates.
  • Retrieve FITS image of the GSC plate material
  • Instructions for measuring coordinates
• If your target is a bright star (V<10) which is difficult to measure from a Schmidt plate then we suggest using the Hipparcos Catalog
  • Instructions for using Hipparcos coordinates
  • Note that this does NOT use the GSC reference system so care must be taken when using small instrument apertures.

  Please note that the northern hemisphere of the publically available Digitised Sky Survey CDROMs was NOT USED for the construction of the GSC because of the proper motion effects. These plates should NOT BE USED when accurate coordinates are required.

• If your target is not visible on the GSC plates then you need to obtain additional observations and transfer the GSC coordinate frame to your data. Please contact your program coordinator if assistance is required.
• If your target is in the LMC, SMC or M31, be aware that some fields in the regular survey plates are overexposed and that short-exposure supplemental plates were used to provide Guide Stars for these regions. If your target is in one of these fields, you may access the short exposure images using the DSS Plate Finder utility which lists ALL available images - please select the correct image from the list returned. Once downloaded you can import this image into the VTT to complete your phase 2 proposal. Please contact your Program Coordinator if in doubt.

  Field	Region Name	GS PLATE ID
  LMC	XX001	06B0
  SMC	XX002	06B1
  M31	XX005	0738

• In some cases you may be able to measure an objects position or offset by using an HST image in the archive. If you do, be sure that the coordinate system for that image is fully up to date. With WFPC2 images, you need to run the "uchcoord" task before using "metric" - see Sec. 28.4 of the HST Data Handbook. WARNING - please check the FGSLOCK keyword in headers to make sure it is not set to GYRO because such images can have highly imprecise coordinates.
• If you measure your target coordinates from the GSC plates and/or use the GSC coordinates, it is VERY IMPORTANT that you provide the 4 character PLATE ID (from the header) with your positions.
• After you have completed Phase 2, you will receive a Confirmation Chart from STScI showing the target coordinates (as entered in the proposal database) overlayed on a finding chart as a final check that the coordinates are correct.
  You are responsible for verifying that coordinates are correct!