The core of the instrument is the liquid nitrogen tank, which has toroidal shape with rectangular cross section, It provides support and cooling for two optical benches, which are located of opposite faces of the vessel itself, and for the radiation shield. It contains about 3 liters of liquid nitrogen. The central hole of torus allows the beam to pass from one optical bench to the other.
In the rear optical bench is the second nitrogen reservoir (about 0.5 l), which is used to cool the detector, through a copper strap. The detector and its cooling reservoir are thermally separated from the surrounding environment, lowering the pressure in this nitrogen container it is possible to cool the detector less than 77 K. The detector has a controller that will assure the stability of the temperature in the range 45-80 K with precision of about 0.1 K.
The grating motion is assured by an external motor via a ferrofluidic feedthrough, two internal stepper motors, modified to operate at cryogenic temperature, drive the filter and slits wheels, allowing their selection,
The internal cold structure is supported by nine low thermal conductivity rods, which are fixed between the internal liquid nitrogen tank and the external vacuum shield, that will be 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 and length of about 60 cm.
The internal vacuum (less than 10e-4 mb) is assured for a sufficiently long time (more than 20 days) by a small amount of active charcoal in thermal contact with the second nitrogen vessel. The regeneration of the charcoal must be carried out once a month in order to mantain the absorption rate high enough. This operation consist of heating the charcoal up to 300 K, while the pressure inside of the dewar is mantained below 10e-1 mb by mean of a rotary vacuum pump. Because the charcoal is cooled by an indipendent cryogenic system the heating of optics and main part of mechanics is not necessary and the operation can be completed in about 4 hours.
To cool and warm the whole instrument in a reasonable times 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 result 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.