NAME:    
	write_asc_xmm_posfromxy
  PURPOSE: 
	makes a gaia-readable ascii catalogue for overplotting source list 
	onto image. Positions stored as RA & dec but extrapolated
	from x & y fields in db.
  CALLING SEQUENCE: 
	write_asc_xmm,xmmdb,outfile[,colour=colour]
  INPUTS: 
	xmmdb: name of xmm database containing positions
  OPTIONAL INPUT PARAMETERS:
	colour: colour to over plto with (default is green)
  OUTPUTS: 
    	outfile: an ascii file containing the table to be loaded in gaia
  EXAMPLE: 
	write_asc_dbd,'dbase/xmm','xmmssc/xmm.tab',colour='green'
  DEPENDENCIES: 
	xmmdb must contain ra and dec fiedls
  RESTRICTIONS: 
        UNTESTED
  CALLED PROCEDURES AND FUNCTIONS: 
	astrolib functions
  MODIFICATION HISTORY: 
     	24-Nov-2000  Original by Suzanne AIgrain

(See /home/xmmssc/soft/idl/pro/gaia/write_asc_xmm_posfromxy.pro)