Catalogs and Surveys Group
Cycle 14 Phase 2 Target Coordinate Instructions
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
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.
- If you
are observing a star, or nucleus of a galaxy that is bright enough
in the GSC-I (down to ~ V=15),
then you should
and use the GSC coordinates.
If you use
the GSC coordinates, it is VERY IMPORTANT that you provide the 4 character
PLATE ID (from the output) with your positions.
your target is visible on the original GSC-I plate material you should
download a FITS image of
the plate to measure your target coordinates.
you measure your target coordinates from the GSC plates it
is VERY IMPORTANT that you provide the 4 character PLATE ID (from the
header) with your positions.
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 a bright star (V<10) which is difficult to measure
from a Schmidt plate then we suggest
using the Hipparcos Catalog.
Note that this does NOT use the
system so care must be taken
when using small instrument apertures.
- If your
target is not visible on the GSC plates then you need to obtain
and transfer the GSC coordinate
frame to your data.
your program coordinator if
assistance is required.
- If your
target is in the LMC, SMC or M31, be aware that some fields
plates are overexposed and that
plates were used to provide Guide
Stars for these regions. If your
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
Coordinator if in doubt.
some cases you may be able to measure
an objects position or offset
using an HST
image in the archive. If you
do, be sure that the coordinate system
that image is fully up to date. With WFPC2 images, you need to
the "uchcoord" task
using "metric" -
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.