"FITS header card within a FitsChan, the first card having an index of 1. The choice of current card affects the behaviour of functions that access the contents of the FitsChan, such as astDelFits, astFindFits and astPutFits.
A value assigned to Card will position the FitsChan at any desired point, so that a particular card within it can be accessed. Alternatively, the value of Card may be enquired in order to determine the current position of a FitsChan.
The default value of Card is 1. This means that clearing this attribute (using
" the FitsChan, so that the first card is accessed
next. If Card is set to a value which exceeds the total number of cards in
the FitsChan (as given by its Ncard attribute), it is regarded as pointing
" . In this case, the value returned in response to an
enquiry is always one more than the number of cards in the FitsChan.