Many of the actual characteristics of the NCS such as power consumption, lowest possible operating temperature, or temperature stability, are still uncertain and have to await on-orbit testing. The numbers stated here should be regarded as estimates only.

The cryocooler, manufactured by Creare, Inc., is a reverse-Brayton cycle turbine design. The compression and subsequent expansion of the Neon gas results in a net cooling which can be used to remove heat from the NICMOS dewar via the heat exchanger to the circulator loop ( see Figure 2.5 in the Instrument Handbook).

The Creare design has several major advantages for application on HST. First, the closed loop system operates at very high speed -- about 7000 revolution per second -- which limits the probability of mechanical coupling to the HST structure, thus minimizing the risk of spacecraft jitter. Second, the system is capable of providing large cooling power. In order to maintain NICMOS at an operating temperature around 75-86 K, an estimated cooling power of 400 mW is needed. Since the parasitic losses due to the flex lines and bayonet couplings (see below) are rather large, delivering 400 mW of cooling power to NICMOS requires about 8 W from the cooler. Finally, the Creare design is compact enough to fit into the previously unused space between the NICMOS enclosure and the HST aft end bulkhead.

The cryocooler dissipates between 250 and 400 W of energy which needs to be removed from the aft shroud. This is achieved via a Capillary Pumped Loop (CPL) that thermally connects the housing of the compressor pump with an external radiator. The continuous heat flow through the (passive) CPL lines is maintained by a set of heaters that are controlled by the ESM micro controller. The heat from inside NICMOS is transported to the cryocooler via a set of flex lines. The flex lines connect to the inside of the dewar via bayonet fittings at an interface plate outside of the dewar which is accessible to the astronauts during SM3B.

After installation, this loop -- the circulator loop -- will be filled with Neon gas from high pressure storage bottles. The Neon gas, driven by a high speed circulator pump and cooled via the heat exchanger to the cryocooler loop, circulates through the cooling coil at the aft end of the NICMOS dewar, thus cooling the entire instrument. The exact temperature of the Neon gas in the circulator loop - and thus the temperatures reached by the NICMOS detectors - will depend on the nature and amount of the parasitic heat loads to the flex lines and the NICMOS dewar. Current best estimates are 62 K at the heat exchanger, with about 400 mW of parasitic heat loads, this should result in detector temperatures between 75 and 86 K.

The ESM will use the telemetry readings of a number of temperature sensors inside the NICMOS dewar in an active control law in order to regulate the cooling power of the NCS, and thus provide stability of the operating temperature of the NICMOS detectors. Because of the sensitive dependence of a number of detector characteristics on temperature, the temperature stability is a crucial requirement on NCS performance. The stability specification of 0.1 K/orbit - combined with a setpoint repeatability of 0.5 K - should allow stable calibration of NICMOS data. All components of the NCS are combined into a common enclosure, the NICMOS CryoCooler (NCC). The system has been successfully tested in space during the HST Orbital Systems Test (HOST) shuttle mission in fall 1998.

Remaining uncertainties about the NCS performance stem from the natural lack of tests with the actual NICMOS dewar. In ground tests, as well as during HOST, a NICMOS simulator was used to mimic the expected parasitic heat loads from, and the mechanical connections to the actual NICMOS. How closely this simulator resembles the NICMOS dewar is somewhat uncertain, hence we expect a range of possible operating temperatures for the revived NICMOS detectors.