Shortly after launch, orbital testing of NICMOS revealed that expansion of solid
nitrogen deformed the dewar leading to thermal contact between the instrument cold well and the inner vapor-cooled shield. Debris lying on top of the detectors are believed to be bits of anti-reflective paint scraped from when the optical baffles when they were pushed against each other. These paint flecks, now referred to as grot, can be found in each of the cameras and are characterized by small areas of reduced sensitivity. The largest example, now known as the "battleship" feature can be seen in camera 1 and affects approximately 35 pixels.

Scrape tests conducted shortly after NICMOS was launched reveal that the paint flecks can range in size from 25micrometers to greater than 100micrometers. Since NICMOS pixels are 40micrometers on a side, this means there is potential for grot smaller than 1 pixel. However, pieces such as these do not represent any significant decrease in pixel sensitivity. Grot affects approximately 250 pixels in each of the cameras, or less than 1% of the total number of pixels in each detector.

Detection at all thresholds reveals no wavelength dependence. Grot present in all the filters does not disappear or change to any great extent over the examined time scale, and the amount of grot since December 1997 has remained constant.Dithering observations is the best way to guard against grot. However, masks are still useful and at the very least provide a road-map for pixels about which one should be wary.

NICMOS Cosmetics

Type	NIC1	NIC2	NIC3
Hot Pixles	10	11	3
Cold Pixles	68	94	17
Pixles affected by Grot	284	227	288

Below are grot masks for each of the cameras, in FITS format.
Image headers can be ignored, they contain information from
prior steps in the processing:

Camera 1 use for data taken 8/97 to 11/97

Camera 1 use for data taken 12/97 to 1/99

Camera 2 use for all NIC2 data

Camera 3 use for all NIC3 data

Grot masks for data taken after March 2002
may be found at this website POST NCS Cosmetics

For more information on NICMOS anomalies, see the NICMOS Anomalies Web Page


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