[Top] [Prev] [Next] [Bottom]

NICMOS Status Summary

Early on-orbit tests of NICMOS revealed dynamic changes to its characteristics and performance. On-orbit expansion of the solid nitrogen in the dewar (used to cool the detectors) has deformed the dewar leading to a thermal contact between the instrument cold well (which contains the three detectors inside the solid nitrogen dewar) and the inner vapor-cooled shield and also causing changes in focus for all three NICMOS cameras. The expected lifetime of the NICMOS cryogen has been significantly reduced as a result. Best estimates are that, if the current rate of cryogen loss continues, NICMOS will remain operational until late 1998 or early 1999. If the thermal contact breaks and the cryogen loss rate is consequently decreased, that time could be longer. NICMOS camera 3 (NIC3) has suffered the largest performance loss. Cameras 1 and 2 (NIC1, NIC2) are performing well and can meet most observational requirements. Based on our best estimates of the current situation, we are working from the following set of assumptions:

Lifetime:
- The useful lifetime of NICMOS will be reduced; we are planning for NICMOS observations to end by about November 1998.
Focus Issues:
Thermal Background:
- Thermal backgrounds are much lower than estimated in Version 1.0 of the NICMOS Instrument Handbook (see Chapter 3).
Coronograph:
- The coronograph performance in NIC2 is slightly degraded as the coronographic hole is no longer confocal with the detector (a compromise focus is presently being explored and results should be available on the WWW by August 1, 1997).
- The position of the coronographic hole is changing with time. A flight software modification is being developed in time for Cycle 7-NICMOS which will locate the hole prior to a coronographic acquisition with minimal overhead cost.
Detector Issues:
- A small percentage of pixels in each camera have reduced throughput, probably from debris on the detectors. The pattern of affected pixels is time variable and therefore difficult to fully calibrate.
- The bias level exhibits enhanced signal after periods of non-use which presently cannot be fully calibrated out. Excess signal levels of ~10 DN are not uncommon. Laboratory testing is ongoing to validate an improved detector operation method which may reduce or eliminate this effect.

[Top] [Prev] [Next] [Bottom]

stevens@stsci.edu

Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 07/24/97 15:31:22