HST Phase 2 Target Coordinate Instructions

The HST reference frame is effectively defined by the positions of the Guide Stars that are selected for each pointing. Since launch, we have used the Guide Star Catalog (GSC-I) which was an all-sky catalog of stars down to 15th magnitude built from Schmidt Sky Survey plates. This catalog has been updated (GSC-II) with more recent epoch plates and calibrated to be on the International Celestial Reference System (ICRS). This has been adopted by the IAU as the new fundamental reference frame. This now simplifies the procedure for providing HST coordinates as it removes the necessity to tie the object coordinates back to the GSC-I and the plates used to construct it.

General Guidelines

• If your target has a position that is in a catalog using the ICRS you may use it directly.
  These include GSC-II, Hipparcos, Tycho, SDSS, 2MASS, FIRST etc.
• When using the GSC-II, please make sure you are using the most recent version (currently GSC2.3.2) to make sure that you take advantage of any corrections and updates.
• If your target has a known proper motion, PLEASE provide the epoch of the coordinate as well as the proper motion values. (For the GSC-II, this is listed in the output). Please note that the GSC-II coordinates for bright stars come from the Tycho2 catalog as these are more accurate than positions measured from the Schmidt plates. For the most accurate coordinates and proper motions of the bright stars we strongly recommend using the HIPPARCOS catalog.
• If your target is visible on the Digitized Sky Survey plate material, but is an extended object so that the observation position does not correspond to the GSC-II coordinates, you should download a FITS image of the field to measure your target coordinates. The latest version of the DSS now includes FITS standard WCS (world coordinate system) keywords which are on the GSC-II/ICRS reference frame.
  (please note that for backward compatibility the original non-FITS standard GSC-I keywords are still present in the FITS headers but should not be used from cycle 15 onwards).
  Please use the POSSII/UKSTU RED images if the web form does not default to that.
  
  CAUTION - It has been reported to us that and old version of SAOimage (1.18.9) provides incorrect coordinates. We have verified that the current releases of APT and SAOimage DS9 work correctly. It will be necessary to use an up to date application that supports the current FITS WCS standard.

• If your target is not visible on the DSS images then you need to obtain additional observations and transfer the ICRS coordinate frame to your data. Please contact your program coordinator if assistance is required.
• 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.
  CAUTION - please do NOT use coordinates from Cycle 14 or earlier images that are still in the GSC-I reference frame. WFPC2 and ACS images can be transformed to the ICRS if there are sufficient GSC-II/2MASS/SDSS objects in the frame.
  CAUTION - 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 have previously observed your target with HST, an alternative method of updating your coordinates is to apply a derived offset between GSC-I and GSC-II for the local Field of View using this tool. Enter the GSC-I based coordinates that you previously submitted with your earlier proposal. This is particularly important if your target has a known high proper motion. Please also provide the GSC-II mean epoch that is output by this tool.
• 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!
Check with your Program Coordinator for more details and/or exceptions