NICMOS is an axial bay instrument which replaced the Faint Object Spectrograph (FOS) in the HST aft shroud during the Second HST Servicing Mission in February 1997. Its enclosure contains four major elements: a graphite epoxy bench, the dewar, the fore-optics bench, and the electronics boxes. The large bench serves to establish the alignment and dimensional stability between the HST optics (via the latches or fittings), the room temperature fore optics bench, and the cryogenic optics and detectors mounted inside the dewar. The instrument has three adjacent but not contiguous cameras, designed to operate independently and simultaneousely. The NICMOS dewar was designed to use solid nitrogen as a cryogen. Cold gas vented from the dewar was used to cool the vapor cooled shield (VCS) which provides a cold environment for both the dewar and the transmissive optical elements (i.e., the filters, polarizers, and grisms). The VCS is itself enclosed within two layers of thermal-electrically cooled shells (TECs). The dewar was built with a design lifetime of approximately 4.5 ± 0.5 years. However, the thermal short developed by NICMOS after installation on HST caused Camera 3 to be no longer parfocal with the other two cameras (Camera 1 and 2 are still close to parfocality); thus, operating Camer 3 simultaneousely with the other two cameras is not scientifically viable as a re-focussing of the instrument is involved. View a schematic of the NICMOS instrument.
The external plumbing at the dewar aft end, which was used for the periodical recooling of the solid nitrogen during ground testing, will now form the interface to the NCS. During SM3B, the NCS was connected to the bayonet fittings of the NICMOS interface plate. This allows the NCS to circulate cryogenic Neon gas through the cooling coils in the dewar, thus providing the cooling power to bring the instrument into the temperature range required for operation.