As can be seen in Fig. 2 (click here), where we show some parts of the mechanical structure of the instrument, the core of the instrument is the liquid nitrogen reservoir, which has a toroidal shape with rectangular cross section and a capacity of 3 liters. It provides support and cooling for two optical benches, which are located on opposite sides of the vessel. The central hole of the toroid allows the beam to pass from one optical bench to the other.
Figure 2: Mechanical arrangement of instrument.
The optical parts have the same enumeration as in Fig. 1 (click here).
(A) liquid nitrogen vessel,
(B) support of mirrors in the back side of instruments,
(C) support of the detector,
(D) support of primary mirror of collimator,
(E) filter wheel,
(F) slit wheel (each slit support a field lens),
(G) grating mounting and wheel for positioning in wavelength
The grating motion is assured by an external stepper motor via a ferrofluidic feedthrough, and the position is controlled by an encoder connected to the motor axis outside the dewar. Two springs acting on the worm gear guarantee good stability of the grating position. Two internal stepper motors, modified to operate at cryogenic temperatures (Gennari et al. 1993), drive the filter and slit wheels.
Mechanics and optics are enclosed in a radiation shield. The internal cold structure is supported by nine low-thermal-conductivity rods, which are fixed between the internal liquid nitrogen reservoir and the external vacuum shield, and are rigidly linked to the focal plane adaptor of the telescope. Externally, the instrument has the form of a cylinder with a base of about 40 cm in diameter and length of about 60 cm.
A small amount of active charcoal is present to maintain the
value of the pressure required (less than 10-4 mb) for a
sufficiently long time (more than 20 days).
The charcoal is cooled by an independent cryogenic system; in the rear
optical bench there is a smaller nitrogen vessel (
0.5 l)
thermally insulated from the surrounding environment.
The regeneration of the charcoal must be carried
out once a month in order to maintain a sufficiently high absorption rate.
This operation consists of heating the charcoal to 300 K, while
the pressure inside the dewar is maintained below 10-1 mb by means
of a rotary vacuum pump.
Because the charcoal is cooled by an
independent cryogenic system, the heating of the optics and the
main part of the mechanical structure
is not necessary and the operation can be completed in about four hours.
To cool and warm the entire instrument reasonably quickly, the dewar is filled with gaseous nitrogen at a pressure of about 200 mb during the cooling and heating phases: in this way the thermal transients prove to be shorter than seven hours. The rate of evaporation of the nitrogen from the main reservoir allows about 16 hours of operation in working conditions, more than a winter night of observation.