Frequently Asked Questions

How were the Guide Star Catalogs created?

• The basic steps involved include:
  • scanning and digitizing the Schmidt plate material,
  • image processing,
  • calibration for astrometry, photometry, and object classification,
  • Comparison to external catalogs

What is the difference between GSC-I and GSC-II?

• For a quick summary of the properties of these two catalogs see: http://www-gsss.stsci.edu/gsc/gsc2/CatalogProperites/GSCIIProperties.htm

  The GSC I catalog (version 1.0) was constructed to support pointing and target aquistion for the HST. The catalog contains approximately 19 million stars and other objects in the sixth to fifteenth magnitude range, primarily determined from an all-sky, single epoch collection of 6.4 degree by 6.4 degree Schmidt plates. The Schmidt plates for north of +6 degrees consists of a 1982-1984 epoch "Quick V" survey (IIaD, 20 minute exposure) obtained from the Palomar Observatory. The southern fields consist 0f 50-75 minute exposure IIIa-J Schmidt plates, from the UK SERC J/EJ survey (epoch 1975-1988) and a supplemental 4 minute IIa-D southern Galactic plane extension (epoch approximately 1988).

  For more information on the Schmidt plates used to construct the GSC I catalog see: http://www-gsss.stsci.edu/PlateMaterial/plateMaterial.htm

  GSC 1.1 is the catalog used for control and target acquistion for the HST. The improved catalog corrected a number of known problems in the GSC 1.0. The corrected defects included spurious entries (principally due to false detections on the diffraction spikes of bright objects), grossly incorrect entries for the brighter stars (V<7) that were produced from heavily over-exposed Schmidt images, and different entries for the same object having more than one name because of blend-resolution difficulties, as well as astrometric errors at the plate edges.

  GSC 1.2 An astrometric re-calibration of the GSC 1.1 reducing the systematic errors present in the GSC 1.1 positions. This catalog has not been been installed on the HST Guide Star Selection system, so it must not be used for HST observation planning.

  The GSC II will be an all-sky catalog based Schmidt plates at two epochs and three bandpasses, from the Palomar (POSS I and POSS II) and UK Schmidt telescopes (SERC/UK surveys). This catalog will contain postions, magnitudes, colors and proper motions for all objects to at least 18.5 in photographic F. The construction of the GSC II is in progress. A preliminary catalog, GSC 2.2 is now available.

  The GSC2.2 is an all-sky, magnitude-selected subset of this data that has been extracted to support telescope operations at the GEMINI and VLT telescopes. This Telescope Operations version contains positions, classifications, and magnitudes for 435,457,355 objects. The magnitude limits (18.5 in photographic F and 19.5 in photographic J) were implemented to ensure the photometric quality of the released data. Since bright objects are extremely overexposed on the Schmidt plates, positions from the Tycho-2 catalog were used.

  The final version (GSC 2.3), expected to be released in 2002, will also contain proper motions.

  For more information on the Schmidt plates used to construction the GSC I catalog see: http://www-gsss.stsci.edu/PlateMaterial/plateMaterial.htm

What material is the GSC II based on?

• See http://www-gsss.stsci.edu/PlateMaterial/plateMaterial.htm#GSCIIPlates

What reference frame is used for the GSC II catalog?

• The International Celestial Reference Frame (ICRF).

What magnitude system is used with GSC II?

• The magnitudes are in the natural system of the photographic plates. See http://www-gsss.stsci.edu/gsc/PlateMaterial/plateMaterial.htm for bandpass information.

How were the positions, magnitudes, and classification selected for
objects imaged on more than one plate ?

• Although the database from which the catalog was constructed contains multiple epochs and observations for a given source, the GSC 2.2.0 exported catalog positions, magnitudes, and classifications for each unique source were selected by the following criteria:

  Positions and magnitudes from plate observations were selected by choosing the observation which occurs closest to the plate center prioritized by bandpass F,J,V, and N respectively. Magnitude cutoffs were applied (F=18.5, J=19.5, V=19.5). If the observation occurred at different plate scanning resolutions, the highest resolution (15 um pixels) was selected over the lower resolution (25 um pixels) for each bandpass regardless of plate location. Classifications were assigned by voting observations into star (0) or nonstar(3) codes.

  For more details see: http://www-gsss.stsci.edu/gsc/gsc2/gsc22_release_notes.htm

What epoch was used for positions?

• The positions are at the epoch from the plate that they were taken from. We did not take an average or combine in any way the positions imaged on more than one plate. The positions were selected by choosing the observation which occurs closet to the plate center prioritized by bandpass F, J, V, and N repectively.

How were positions determined for bright stars?

• Since bright objects are extremely overexposed on the Schmidt plates, positions obtained from the Tycho-2 catalog were used (see http://www.astro.ku.dk/%7Eerik/Tycho-2/).

Can you provide the photometric transmission curves ?

• Yes, here ( http://www-gsss.stsci.edu/DSS/Transmission%20Curves/transmission_curves.HTML )

What is the GSPC?

• To support the photometric calibration of the Guide Star catalogs, CASB produced the Guide Star Photometric Catalog. GSPC I is an all-sky set of photoelectrically determined BV sequences in the magnitude range from 9 to 15, generally near the centers of the fields of the GSC-I plates. The GSPC II, a joint project between CASB and Osservatorio Astronomico di Torino, is generally an extension of GSPC I sequences to V=19 in B, V, and R passbands based on CCD photometry.

  For more detailed information see: http://www-gsss.stsci.edu/gspc/GSPChome.htm

What is the sky coverage of the GSPC II?

• http://www-gsss.stsci.edu/gspc/gspc2_coverage_map.htm

How do I obtain the GSPC data?

• http://www-gsss.stsci.edu/support/data_access.htm#table