FUNCTION safe_headfits, file, ext, maxtry

; Description: This function tests that a FITS image file is available on disk before
;              reading in the required header corresponding to the specified image 
;              extension. If the file is not available, then the module waits 1 second
;              before trying again. The module will attempt to read the image header a
;              total of "maxtry" times before giving up and returning the string 
;              "ERROR".
;
; Input Parameters:
;
;   file - STRING - The full directory path and file name of the FITS image file to be
;                   read in.
;   ext - INTEGER/LONG - The FITS file image extension for which the header is to be
;                        read in. If "ext" is set to a negative number, then the
;                        function will read in the primary FITS header (extension
;                        number "0").
;   maxtry - INTEGER/LONG - Maximum number of attempts at reading the image header
;                           before giving up and returning the string "ERROR" (Default
;                           value = 1).
;
; Return Value:
;
;   The required header is returned as a STRING VECTOR.
;
; Author: Dan Bramich (dan.bramich@hotmail.co.uk)
;
; History:
;
;   19/04/2008 - Module created (dmb)

;Check that "file" is a variable of STRING type

;If "ext" is not a non-negative number of the correct type, then reset its value to "0"

;If "maxtry" is not a positive number of the correct type, then reset its value to "1" 

;Wait until the input file is available

;Return the required header