ARCBAR

       Draw an arc bar on an image showing the astronomical plate scale

       ARCBAR, hdr, arclen,[  COLOR= , /DATA, LABEL= , /NORMAL, POSITION =,
                              /SECONDS, SIZE=, THICK= ]

       hdr - image FITS header with astrometry, string array
       arclen - numeric scalar giving length of bar in arcminutes (default)
               or arcseconds (if /SECONDS is set)

       COLOR - integer scalar specifying the color to draw the arcbar (using
               PLOTS), default = !P.COLOR
       /DATA - if set and non-zero, then the POSITION keyword is given in data
              units
       LABEL - string giving user defined label for bar.  Default label is size
               of bar in arcminutes
       /NORMAL - if this keyword is set and non-zero, then POSITION is given in
               normalized units
       POSITION - 2 element vector giving the (X,Y) position in device units
               (or normalized units if /NORMAL is set, or data units if /DATA
               is set) at which to place the  scale bar.   If not supplied,
               then the user will be prompted to place the cursor at the
               desired position
       /SECONDS - if set, then arlen is specified in arcseconds rather than
               arcminutes
       SIZE  - scalar specifying character size of label, default = 1.0
       THICK -  Character thickness of the label, default = !P.THICK

       Place a 3' arc minute scale bar, at position 300,200 of the current
       image window, (which is associated with a FITS header, HDR)
       IDL> arcbar, HDR, 3, pos = [300,200]

       When using using a device with scalable pixels (e.g. postscript)
       the data coordinate system must be established before calling ARCBAR.
       If data coordinates are not set, then ARCBAR assumes that the displayed
       image size is given by the NAXIS1 keyword in the FITS header.

       AD2XY, EXTAST, GSSSADXY, SXPAR()

       written by L. Taylor (STX) from ARCBOX (Boothman)
       modified for Version 2 IDL,                     B. Pfarr, STX, 4/91
       New ASTROMETRY structures               W.Landsman,  HSTX, Jan 94
       Recognize a GSSS header                 W. Landsman June 94
       Added /NORMAL keyword                   W. Landsman Feb. 96
       Use NAXIS1 for postscript if data coords not set,  W. Landsman Aug 96
       Fixed typo for postscript W. Landsman   Oct. 96
       Account for zeropoint offset in postscript  W. Landsman   Apr 97
       Converted to IDL V5.0   W. Landsman   September 1997
       Added /DATA, /SECONDS keywords   W. Landsman    July 1998
       Use device-independent label offset  W. Landsman   August 2001

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