If we print out the FITS header cards describing the SkyFrameSkyFrame we wrote earlier (§16.5), we should obtain something like the following:
As you can see, this resembles the information that would be written to a basic ChannelChannel to describe the same SkyFrame (§15.8), except that it has been formatted into 80-character header cards according to FITS conventions.
There are also a number of other differences worth noting:
This last point is worth further comment and is necessary because the FITS standard only allows for certain keywords (such as COMMENT and HISTORY) to appear more than once. astWriteastWrite therefore appends an arbitrary sequence of two characters to each new keyword it generates in order to ensure that it does not duplicate any already present in the FitsChan.
The main risk from not following this convention is that some software might ignore (say) all but the last occurrence of a keyword before passing the FITS headers on. Such an event is unlikely, but would obviously destroy the information present, so astWrite enforces the uniqueness of the keywords it uses. The extra characters added are ignored when the information is read back.
As with a basic Channel, you can also suppress the comments produced in a FitsChan by setting the boolean (integer) CommentComment attribute to zero (§15.10). However, FITS headers are traditionally generously commented, so this is not recommended.