Catalogs and Surveys Group
HST Phase 2 Target Coordinate Instructions
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.
- 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
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.
- 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.
- 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