Date: Thu, 28 Mar 2024 10:21:31 -0400 (EDT) Message-ID: <1532658046.995.1711635691533@ploutercf.stsci.edu> Subject: Exported From Confluence MIME-Version: 1.0 Content-Type: multipart/related; boundary="----=_Part_994_1640980602.1711635691530" ------=_Part_994_1640980602.1711635691530 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Content-Location: file:///C:/exported.html
The PS1 object identifiers (objID
in the database tables) a=
re computed from the object positions. We do not recommend trying to =
extract the positions directly from the objID
because subseque=
nt calibrations have significantly improved the positional accuracy of the =
RA and Dec columns. However, the objID
is the primary key in m=
ost PS1 tables, and it is useful to know how the objID
is rela=
ted to position. Partitioning the table by objID
approxi=
mately divides the table into declination strips.
The objID
inde=
x is derived from right ascension an=
d declination. While it is possible to calculate =
the RA and Dec from the objID<=
/span>
, it is not recommended to do this, because=
the objID
is based on =
the astrometric solutions from individual exposures and=
stacks as they are ingested <=
/span>during an early phase of the PS1 data processing. Consequently =
the positions derived from the objID
have not been calibrated =
against 2MASS or Gaia. It is recomme=
nded to use raMean
, decMean
from =
ObjectThin. Included below is the C =
code for the translation between R.A. and Dec., for users in=
span>terested in the relationship.
uint64_t CreatePSPSObjectID(double ra, double dec) { double zh =3D 0.0083333; double zid =3D (dec + 90.) / zh; // 0 - 180*60*2 =3D 21600 < 15 bits int izone =3D (int) floor(zid); double zresid =3D zid - ((float) izone); // 0.0 - 1.0 uint64 t part1, part2, part3; part1 =3D (uint64 t)( izone * 10000000000000LL); part2 =3D ((uint64 t)(ra * 1000000.)) * 10000 ; // 0 - 360*1e6 =3D 3.6e8 (&= lt; 29 bits) part3 =3D (int) (zresid * 10000.0) ; // 0 - 10000 (1 bit =3D=3D 30/10000 ar= csec) (< 14 bits) return part1 + part2 + part3; }
The figure below (adapted from a similar figure from Heather Flewelling)=
shows an example of computing the objID
from spe=
cific RA and Dec values.