doit   1
	if ($1 >0) {
		device postencap dispersion.ps
		ctype black}
	if ($1 <0) {
		device x11 
		ctype white}

	expand 1.5
	lweight 3
	data dispersion.dat
	location  3500 31000 15000 31000 

	read {airmass 1  zenith 2 sinz 3 dispersion 4 dispacross 5 dispalong 6}

	limits 0 60 -0.5 3.5
	box
	connect zenith dispacross
	connect zenith dispalong
	
	xlabel zenith angle (deg)
	ylabel dispersion (arcsec)

	relocate 12 1	
	label along slit

	relocate 40 0
	label across slit

	lweight 1
	ltype 1
	do i=-.4,3.4,0.2{
	    relocate 0 $i
	    draw 60 $i
	}
	do i=0, 60, 5{
	    relocate  $i -20
	    draw $i 20
	}
	lweight 3
	ltype 0		


	device x11