This attribute is used to control the orientation of the offset coordinate system defined by attributes SkyRef and SkyRefIs. If used, it should be set to hold the longitude and latitude of a point within the coordinate system specified by the System attribute. The offset coordinate system represented by the SkyFrame will then be rotated in order to put the position supplied for SkyRefP on the zero longitude meridian. This rotation is about an axis from the centre of the celestial sphere to the point specified by the SkyRef attribute. The default value for SkyRefP is usually the north pole (that is, a latitude of $+$90 degrees in the coordinate system specified by the System attribute). The exception to this is if the SkyRef attribute is itself set to either the north or south pole. In these cases the default for SkyRefP is the origin (that is, a (0,0) in the coordinate system specified by the System attribute).

If an integer axis index is included in the attribute name (e.g. " SkyRefP(1)" ) then the attribute value should be supplied as a single floating point axis value, in radians, when setting a value for the attribute, and will be returned in the same form when getting the value of the attribute. In this case the integer axis index should be " 1" or " 2" (the values to use for longitude and latitude axes are given by the LonAxis and LatAxis attributes).

If no axis index is included in the attribute name (e.g. " SkyRefP" ) then the attribute value should be supplied as a character string containing two formatted axis values (an axis 1 value followed by a comma, followed by an axis 2 value). The same form will be used when getting the value of the attribute.

SkyFrame

• If the position given by the SkyRef attribute defines the origin of the offset coordinate system (that is, if the SkyRefIs attribute is set to " origin" ), then there will in general be two orientations which will put the supplied SkyRefP position on the zero longitude meridian. The orientation which is actually used is the one which gives the SkyRefP position a positive latitude in the offset coordinate system (the other possible orientation would give the SkyRefP position a negative latitude).

• An error will be reported if an attempt is made to use a SkyRefP value which is co-incident with SkyRef or with the point diametrically opposite to SkyRef on the celestial sphere. The reporting of this error is deferred until the SkyRef and SkyRefP attribute values are used within a calculation.

• If the System attribute of the SkyFrame is changed, any position given for SkyRefP is transformed into the new System.