By default, a positive value is used which results in no loss of information, assuming that the value's precision is double. Usually, this causes no problems.
However, to adhere strictly to the recommendations of the FITS standard, the width of the formatted value (including sign, decimal point and exponent) ought not to be more than 20 characters. If you are concerned about this, you should set FitsDigits to a negative value, such as -15. In this case, the absolute value (15) indicates the maximum number of significant digits to use, but the actual number used may be fewer than this to ensure that the FITS recommendations are satisfied. When using this approach, the resulting number of significant digits may depend on the value being formatted and on the presence of any sign, decimal point or exponent.
The value of this attribute is effective when FITS header cards are output, either using astFindFits or by the action of the FitsChan's sink function when it is finally deleted.
AST A Library for Handling World Coordinate Systems in Astronomy