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diff --git a/noao/astutil/asttools/astlvac.x b/noao/astutil/asttools/astlvac.x
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+# AST_LVAC -- Convert air wavelength to vacuum wavelength.
+#
+#     Convert LAMBDA(air) to LAMBDA(vacuum) using the formulae
+#
+#     (n-1)*1e8 = 8342.13 + 2406030/(130-s**2) + 15997/(38.9-s**2)
+#
+#                 where  s = 1/lambda(air)   (for lambda in MICRONS)
+#
+#     lambda(vac) = n * lambda(air)
+#
+#     NOTE: lambda is given in ANGSTROMS for this program.
+#
+
+procedure ast_lvac (lair, lvac, npts)
+
+double	lair[npts]		#I Air wavelength (Angstroms)
+double	lvac[npts]		#O Vacuum wavelength (Angstroms)
+int	npts			#I Number of points
+
+int	i
+double	s2, n
+
+begin
+	do i = 1, npts {
+	    s2 = 1D8 * (1. / lair[i]) ** 2
+	    n = 1 + ((8342.13 + 2406030 / (130-s2) + 15997 / (38.9-s2)) / 1D8)
+	    lvac[i] = n * lair[i]
+	}
+end