The SkyFrame class supports the following System values and
associated celestial coordinate systems:
-
"FK4": The old FK4 (barycentric) equatorial coordinate system,
which should be qualified by an Equinox value. The underlying
model on which this is based is non-inertial and rotates slowly
with time, so for accurate work FK4 coordinate systems should
also be qualified by an Epoch value.
-
"FK4-NO-E" or "FK4_NO_E": The old FK4 (barycentric) equatorial
system but without the "E-terms of aberration" (e.g. some radio
catalogues). This coordinate system should also be qualified by
both an Equinox and an Epoch value.
-
"FK5" or "EQUATORIAL": The modern FK5 (barycentric) equatorial
coordinate system. This should be qualified by an Equinox value.
-
"GAPPT", "GEOCENTRIC" or "APPARENT": The geocentric apparent
equatorial coordinate system, which gives the apparent positions
of sources relative to the true plane of the Earth's equator and
the equinox (the coordinate origin) at a time specified by the
qualifying Epoch value. (Note that no Equinox is needed to
qualify this coordinate system because no model "mean equinox"
is involved.) These coordinates give the apparent right
ascension and declination of a source for a specified date of
observation, and therefore form an approximate basis for
pointing a telescope. Note, however, that they are applicable to
a fictitious observer at the Earth's centre, and therefore
ignore such effects as atmospheric refraction and the (normally
much smaller) aberration of light due to the rotational velocity
of the Earth's surface. Geocentric apparent coordinates are
derived from the standard FK5 (J2000.0) barycentric coordinates
by taking account of the gravitational deflection of light by
the Sun (usually small), the aberration of light caused by the
motion of the Earth's centre with respect to the barycentre
(larger), and the precession and nutation of the Earth's spin
axis (normally larger still).
-
"ECLIPTIC": Ecliptic coordinates (IAU 1980), referred to the
ecliptic and mean equinox specified by the qualifying Equinox
value.
-
"GALACTIC": Galactic coordinates (IAU 1958).
-
"SUPERGALACTIC": De Vaucouleurs Supergalactic coordinates.
-
"UNKNOWN": Any other general spherical coordinate system. No
Mapping can be created between a pair of SkyFrames if either of the
SkyFrames has System set to "Unknown".
Currently, the default System value is "FK5". However, this
default may change in future as new astrometric standards
evolve. The intention is to track the most modern appropriate
standard. For this reason, you should use the default only if
this is what you intend (and can tolerate any associated slight
change in future). If you intend to use the FK5 system
indefinitely, then you should specify it explicitly.