Specifying Sky Positions

A sky coordinate value can consist of a set of up to three ``fields''. Each field consists of a numeric value (which can have a fractional part) followed by an optional character specifying the units of the field. The following characters are used to specify particular units:

h

- hours

d

- degrees

m

- minutes or arc-minutes (depending on whether the leading field is an hours or degrees field).

s

- seconds or arc-seconds (depending on whether the leading field is an hours or degrees field).

r

- radians.

If no such characters are supplied, defaults are assumed for the field units (eg if the string ``11 12 56'' is supplied as a Right Ascension value it is treated the same as ``11h 12m 56s''). Note, when making command line assignments to parameters from within ICL, it is necessary to enclose the string to be assigned to the parameter in double quote marks.

Fields may be omitted, in which case they are assumed to have the value zero. If a degrees or hours field is to be omitted, then the leading field in the supplied string should always include a character to specify its units. For instance, the string ``15 23'' is treated as the RA value ``15h 23m''. If, in fact, the leading field is a minutes field then ``15m 23'' could be specified.

In addition, an input string may contain a single field in an ``encoded'' form. ``Encoded'' fields are identified by the fact that the field contains 5 or more digits to the left of the decimal point (including leading zeros if necessary). These fields are decoded into hours or degrees as follows: Any fractional part is taken as the fractional part of the seconds field, the tens and units digits are taken as the integer part of the seconds field, the hundreds and thousands digits are taken as the minutes fields, the remaining digits are taken as the degrees or hours field. Thus -12345.4 would be interpreted as (- 1 hour 23 mins 45.4 seconds) or (- 1 degree 23 mins 45.4 seconds).

The supplied values must be in their ``normal'' ranges (i.e. 0h 0m 0s to 23h 59m 59.99s for RA values, -90d to +90d for DEC values, etc). Values outside these ranges cause an error to be reported. The exception to this is if the string is prefixed with a ``*'' character, in which case any numeric value may be supplied.

Any coordinate value can be specified in degrees, hours or radians. If a value is given in unusual units, the corresponding normal units are used. For instance, if a declination value of ``1.105r'' was given (specifying 1.105 radians) the equivalent value in degrees is used (i.e. 63.3118 degrees). A right ascension of ``45d'' would cause a value of ``3h'' to be used, etc.