NICMOS Instrument Design

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.

		New in the Last 45 Days
		NICMOS ISR 2008-002: Improvements to Calnica This ISR describes a number of improvements to calnica, the STSDAS task that performs routine instrumental calibration of NICMOS raw images. We report two major and a number of minor updates to the code. The major updates consist of changes in the way the count rate is calculated from up-the-ramp fitting and changes in the cosmic rays rejection algorithm. All updates have been extensively tested and analysis shows improvement in the S/N of output images by 6-15% compared to the previous calnica version. A further improvement is that the error extension in the calibrated images now represents the true errors in a more consistent way compared to the old implementation of the software. T. Dahlen 04 Jun 2008 (pdf)