The optical scheme of the instrument is sketched in Fig. 1 (click here); it is designed to match the f/20 focal ratio of the TIRGO telescope.
Figure 1: Optical diagram of instrument.
The optical components of the instrument include (enumeration
follows the path of radiation): (1) field lens, (2)
secondary mirror and (3) primary mirror of the collimator (an inverted
cassegrain), (4) a plane mirror, (5) the grating, (6) the plane mirror,
(7) the paraboloidal mirror of the camera, and (8) the detector
Following the optical path from the telescope,
the beam encounters the window of the
dewar, the order sorting filter, a field lens, and the slit; the latter
resides at the focal plane of telescope.
The window and field lens are composed of calcium fluoride.
Filters and slits are respectively mounted on two wheels and can be quickly
changed during the observations. The field lens images the pupil on
the secondary mirror of an inverted cassegrain (with focal length of
1400 mm) that produces a parallel beam 70 mm in diameter. This
beam is reflected onto the grating by a flat mirror tilted by 
.
The grating, arranged in Littrow configuration, has 150
grooves/mm and a blaze wavelength of 
at first order; rotation
around the tilted axis allows the selection
of wavelengths and orders.
A modified Pfund camera (with focal length of 225 mm) following the grating,
collects the dispersed beams on the detector. The sky-projected pixel
size is 1.73 arcsec, and the total field covered along the slit direction
is 70 arcsec.
The back face of the grating is a flat mirror so that, when the grating is rotated by 180 degrees, the instrument functions as a camera, in the band defined by the filters, with a field of view of about 1.5 arcmin square. This facility can be useful for tests, maintenance, and for centering weak sources on the slit.
All the mirrors are gold coated to provide good efficiency over a
wide spectral range, and the optics are acromatic at least up to
. The optical components are cooled to about 80 K by means of
thermal contact with a cryogenic vessel filled with liquid nitrogen
at atmospheric pressure as described below. The mounting of optical
elements is designed to take into account the dimensional changes
between mirrors (in pyrex) and supports (in aluminium) generated by
the cooling and the differences in thermal expansion coefficents.
The resolving power is (for first order) about 600 in the center of J band, and 950 in the center of the K band, using a slit of two pixels (3.46 arcsec).