NOAO Abstracts for SPIE Meeting in March
Design of a Grating Mechanism for a Near Infrared Spectrograph
Authors:
Woon-Yin Wong, Randy W. Cuberly and John R. Andrew
Abstract:
The design of a grating mechanism for the Gemini Near-Infrared
Spectrograph (GNIRS) is a difficult optomechanical design problem.
The mechanism must operate in a 65K cryogenic environment. It needs
to provide repeatable motions and stable positioning in two axes,
for both grating selection and grating tilt. These requirements
are particularly stringent because the excellent image quality
anticipated from the Gemini telescopes leads to the use of
long-focal-length cameras, and the low backgrounds expected require
stability over long integrations. The substantial beam size - nearly
100 mm - and the need for multiple gratings imply a mechanism of
considerable size. Although the GNIRS is quite large, with a diameter
of 1.2 m and length of 2 m, internal space and weight are limited
by the large number of optics and mechanisms within the cryogenic
dewar - 9 major opto-mechanical assemblies, 4 large folding mirrors
and 9 motor drives. The requirement for high repeatability is met
by a design using a detent and ratchet mechanism on each axis. The
grating turret rotates via an external motor drive to select one
of five optics: four gratings and a flat mirror. Tilt of the
selected grating over a range of +/- 7.5 degrees in 0.25 degree
steps is provided by a cam follower mechanism, also driven by an
external motor drive. Novel features of the mechanism include the
coaxial two-axis drive, ratchet and detent mechanism, two-axis cam,
and external motor drives. The grating turret dimensions are 400
mm by 500 mm by 500 mm and its weight is about 72 kg.
[NOAO at the SPIE Meeting in
March]
National Optical Astronomy Observatories, 950 North Cherry Avenue,
P.O. Box 26732, Tucson, Arizona 85726 Phone: (520) 318-8000
Posted: 03Mar1998