JWST NlRSpec Cryogenic Light Shield Mechanism  Kathleen Hale’ and Rajeev Sharma’  Abstract  The focal plane detectors for the Near-Infrared Spectrometer (NIRSpec) instrument on the James Webb  Space Telescope (JWST) require a light tight cover for calibration along with an open field-of-view during  ground performance testing within a cryogenic dewar. In order to meet the light attenuation requirements  and provide open and closed fields of view without breaking vacuum, a light shield mechanism was  designed. This paper describes the details of the light shield mechanism design and test results. Included  is information on the labyrinth light path design, motor capability and performance, dry film lubrication,  mechanism control, and mechanism cryogenic performance results.  Background  A light tight cover mechanism design and development is discussed for ground testing the Near Infrared  Spectrometer Instrument’s focal plane detectors on the James Webb Space Telescope. The NIRSpec  focal plane detectors require a light tight cover for calibration along with an open field-of-view during  ground performance testing within a cryogenic dewar. These tests include exposing the detectors to  infrared light for calibration in “darkness” while under vacuum at approximately 20 Kelvin  (-253°C). The darkness requirement is to maintain light levels less than 0.001 electron/sec/pixel at the  detectors. In order to provide this low light level during calibration and also allow the detectors to be  exposed to the required light sources when needed, the light shield mechanism was developed.  Requirements  The light shield mechanism is required to attenuate light in the chamber to a level less than 0.001  electronlseclpixel before it reaches the detectors. Volume space allotted to the mechanism is 27.9 cm in  diameter by 15.2 cm high (01 1 in x 6 in). The shield must be able to open or close within about 60  seconds, operate in a vacuum Torr) at 20 Kelvin, and in any gravitational orientation. The lifetime  requirement for the shield is a few thousand cycles where one cycle is open and close.  When open, the shield door is to remain outside the 76-degree cone angle of the detectors’ field of view.  The light shield door range of rotation is about 124 degrees from open to close.  Shutter Light Path Design  A labyrinth light path is created, Figure 1, by the aligning of teeth-like protrusions on the shutter door with  teeth cut outs into the plate covering the detector housing, hereafter referred to as the dewar bulkhead.  The door and dewar bulkhead protrusions are separated by a gap of 0.64 mm (0.025 in) to prevent any  contact debris from being created during the opening and closing operation of the door. This gap allows  light entrance to the path, however, the labyrinth design combined with the proper material selection and  treatment forces any light entering to bounce many times off the path’s walls and thus be absorbed to  levels below the requirement. Aluminum 6061, which is bead blasted and black anodized, makes up both  the door and dewar bulkhead.  “NASA Goddard Space Flight Center, Greenbelt, MD  Proceedings of the 38 Aerospace Mechanisms Symposium, Langley Research Center, May 15- 17,200G  223