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High Spectral Resolution Achieved in Double-Pass...(1Mar95)
with the McMath-Pierce Grating Spectrometer
(from NSO, NOAO Newsletter No. 41, 1 March 1995)

The 120 groove/mm Harrison IR grating (size: 470 X� 368 mm) has a
theoretical resolving power (/ = Nm) of 110,000 at 4.6 um. In practice,
considerably lower resolving power is realized due to illumination under-fill
and finite slit widths. The McMath-Pierce 13.5-m spectrometer, which uses
this grating, is designed to operate either single-pass or double-pass. At
4.6 um we find that resolution is substantially improved in the double-pass
mode.  Improvement is dramatic for atmospheric ozone lines but is also
noticeable for solar CO.  Going from single- to double-pass is accompanied
by a throughput loss of about 2.5.

                           [Figures not included]

         Region at 4.6891 um showing atmospheric ozone lines and
         solar CO at 4.6871 um and 4.6885 um; (a) observed with the
         1-m FTS (note false lines because of instrumental sync
         function ringing), (b) double-pass with 200 um input and
         output slits, and (c) single-pass, same slits.  Air masses are
         slightly different among the three observations, accounting
         for some variation in line depth.

William Livingston

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