If no Format value is set for a Frame axis, a default value is supplied instead. This is based on the value of the Digits, or Digits(axis), attribute and is chosen so that it displays the requested number of digits of precision.
When supplying a value for this attribute, beware that the "%" character may be interpreted directly as a format specification by some printf-like functions (such as astSet). You may need to double it (i.e. use "%%") to avoid this.
If the format string starts with a percentage sign (%), then the whole format string is assumed to conform to the syntax defined by the Frame class, and the axis values is formated as a decimal radians value.
AST A Library for Handling World Coordinate Systems in Astronomy