AGASC Catalog Description and History¶
Getting started¶
The most common usage of the agasc package is getting the AGASC catalog data
for stars near a location on the sky or by AGASC ID.
The three main functions are:
get_agasc_cone(): Get stars within a radius of RA, Decget_star(): Get information for one star by AGASC IDget_stars(): Get information for a list of stars by AGASC ID
AGASC catalog order¶
Catalogs with version 1.7 or before are ordered by increasing Declination. Cone search is done by selecting a band of Declination and then searching for stars within the required spatial cone.
Starting with version 1.8, catalogs are ordered by HEALpix index. HEALpix is a library that provides a fast way to uniquely tile a sphere with tiles of uniform area, providing a map between any point on the sphere and the index number. The HEALpix index is a pixel number between 0 and 12*NSIDE**2-1, where NSIDE is the HEALpix resolution parameter. For AGASC 1.8, NSIDE=64.
HEALpix-ordered catalogs include a healpix_index table that provides the row range
for each HEALpix index. Cone search is done by finding the HEALpix pixels that overlap
the required spatial cone and extracting the stars from the catalog that are in those
pixels. This strategy is faster than the Declination-ordered catalog search and requires
less memory.
Note that for testing, some version 1.7 catalogs are available in HEALpix order. These
are denoted with a _healpix suffix in the filename.