SkyFrames use AST_FORMAT for formatting coordinate values in the same way as other Frames (). However, they offer a different set of formatting options more appropriate to celestial coordinates.
The Digits attribute of a SkyFrame behaves in essentially the same way as for a basic Frame (), so the precision with which celestial coordinates are displayed can also be adjusted in this way. However, the range of format specifiers that can be given for the Format(axis) attribute, and the default format resulting from any particular Digits value, is different.
The syntax of SkyFrame format specifiers is detailed under the description of the Format(axis) attribute in . Briefly, however, it allows celestial coordinates to be expressed either as angles or times and to include one or more of the fields:
- degrees or hours
- arc-minutes or minutes
- arc-seconds or seconds
with a specified number of decimal places for the final field. A range of field separators is also available, as the following examples show:
Format Specifier Example Formatted Value d 219 d.3 219.123 dm 219:05 dm.2 219:05.44 dms 219:05:42 hms.1 15:44:13.8 bdms.2 219 05 42.81 lhms.3 15h44m13.88s +zlhms +06h10m44s ms.1 13145:42.8 lmst.3 876m22.854s s.2 788742.81
Note the following key points:
The formatting performed by a SkyFrame is also influenced by the AsTime(axis) attribute, which has a boolean (integer) value for each SkyFrame axis. It determines whether the default format specifier for an axis will present values as angles (e.g. in degrees) if it is zero, or as times (e.g. in hours) if it is non-zero.
The default AsTime value depends on the celestial coordinate system which the SkyFrame represents which, in turn, depends on its System attribute value. For example, equatorial longitude values (right ascension) are normally expressed in hours, whereas ecliptic longitudes are normally expressed in degrees, so their default AsTime values will reflect this difference.
The value of the AsTime attribute may be set explicitly to over-ride
these defaults if required, with the formatting precision being
determined by the Digits/Digits(axis) value. Alternatively, the
Format(axis) attribute may be set explicitly to specify both the
format and precision required. Setting an explicit Format value always
over-rides the effects of both the Digits and AsTime attributes (unless
the Format value does not specify the required number of decimal places,
in which case Digits is used to determine the default number of decimal
AST A Library for Handling World Coordinate Systems in Astronomy