Specifies a source for the observation data, including
WCS information. The keyword has the multiple role of
specifying the source of WCS information, supplying
approximate WCS information directly, and supplying the
observation data (time, observatory, temperature and
pressure, and colour) which ASTROM needs if it is to
attempt one of its higher-order fits. This information is
only needed if it is missing from the NDF or any FITS
extension it incorporates, and if you wish to attempt the
slight increase in accuracy of the more elaborate fits.
Do not specify a keyword more than once with distinct values.
The value of the --obsdata option can specify
approximate astrometry through a comma-separated list of
key=value pairs. For example, to supply a
value for the centre of a CCD, overriding any astrometry
in the file, you might write
This value indicates the centre of the CCD even when this is not the centre of distortion, which would be the case when, for example, the CCD represents an off-centre section of a Schmidt plate.--obsdata=ra=14:24:00,dec=-12.34
The program will take its astrometry from only one of the
possible sources (see source below) -- so that
it will not, for example, take a position from the
obsdata option and a plate scale from the
file's own astrometry. By default, a position given here
overrides any other WCS information; thus to supply a
position as a backup, in case there is no
astrometry elsewhere, you could write
The available astrometry keywords are in Table 3.--obsdata=source=AST:FITS:USER,ra=14:24:00,dec=-12.34
| Keyword | Description |
|---|---|
| source | This is a
colon-separated list of sources of WCS
information. The values may be `AST', indicating
that the information should come from the AST WCS
component of the NDF, or `FITS', indicating that
it should come from any FITS extension there, or
`USER', indicating that the following
obsdata keywords should be used. The
default is source=USER:AST:FITS, so
that any WCS information given in the
obsdata keyword has precedence. The
keywords are not case-sensitive. If the program
gets through this list without finding any WCS
information, it exits with an error. |
| ra | Right ascension of the centre of the pixel grid, in colon-separated HMS or decimal degrees. Required. |
| dec | Declination of the centre of the pixel grid, in colon-separated DMS or decimal degrees. Required. |
| angle | Position angle of the pixel grid. This is the rotation, in degrees anti-clockwise, of the declination or latitude axis with respect to the 2-axis of the data array. Default: 0. |
| scale | The plate scale, in units of arcsec/pixel. Default: 1. |
| invert | If true (that is, specified as
invert=1 or just
invert), the axes are inverted;
otherwise (invert=0 or
noinvert) the axes are unflipped.
Default: 0. |
Keywords for the
--obsdataoption.
The position angle is the same as the AIPS
CROTA2 convention. Specifically (and by
definition), these definitions map to
the CDn_n conventions of
[fitswcs2] through
where 
is scale/3600 (ie, the scale
in units of degrees/pixel), 
is the position
angle pa, and 
is 
when
invert is false, and 
when
invert is true.
Additionally, there are keywords which specify observation
data: time, station coordinates, meteorological data and
colour. These are described in Table 4.
For fuller details on these, see the ASTROM
documentation, SUN/5. Note that the source keyword
discussed above
does not affect the priority of these keywords: if they
are specified, they override any values obtained from the
NDF or its FITS extensions.
| Keyword | Description |
|---|---|
| time | An observation time, given as a Julian
epoch (format r), or a local sidereal
time (format i:i) or UT
(i:i:i:i:r specifying four-digit
year, month, day, hours and minutes). |
| obs | An observation station, given either as one
of the SLALIB observatory codes; or else in the format
i:r:i:r[:r], giving longitude,
latitude and optional height.
Longitudes are east longitudes -- west
longitudes may be given either as minus degrees or
longitudes greater than 180. |
| met | Temperature and pressure at the telescope,
in degrees Kelvin and millibars. The defaults are
278K and a pressure computed from the
observatory height.
Format r[:r]. |
| col | The effective colour of the observations, as a wavelength in nanometres. The default is 500nm. |
Observation data keywords. In these specifications,
irepresents an integer,ra real, optional entries are in[...], and the separator:may be either a colon or whitespace.
Type: string; defaults: see above.