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Improvements in The Grating Drive of the Main...(1Mar94)
Spectrometer
(from NSO, NOAO Newsletter No. 37, 1 March 1994)

There are two gratings mounted back to back in the main spectrometer: a
16.5 x 12.5 inch replica grating used to cover the spectrum 300 nm to
2.5 um and the 18.5 x 14.5 inch original grating which extends the
spectrum to 12.5 um.  The spectrum is scanned with a screw drive
pulling on a steel tape wrapped around the spool containing the
gratings.

Photographs of the hyperfine structure of Hg 546.1 nm demonstrate a
resolving power of about one million.  However, when used in the
scanning mode, as we normally use the spectrometer, it is much less -
the linearity and smoothness of the drive are slightly worse than we
had hoped and need - errors of 1 x 10**-4 to 5 x 10**-4 nm.  To reduce the
error, we have redesigned the tape-drive and are installing a new
high-precision lapped screw and nut in February 1994.

Keith Pierce

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