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 g round 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 designe d. This paper describes the details of the light shield mechanism design and test results. Included is information on the labyrinth light p ath design, motor capability and performance, dry film lubrication, m echanism control, and mechanism cryogenic performance results

Hale, Kathleen

Sharma, Rajeev

NASA Technical Reports Server

Source of Acquisition 
NASA Goddard Space Flight Center 
JWST NlRSpec Cryogenic Light Shield Mechanism 
Kathleen Hale’ and Rajeev Sharma’ 
Abstract 
The focal plane detectors for the Near-Infrared Spectrometer (NlRSpec) 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 NlRSpec 
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 degrees Kelvin 
(-253°C). The darkness requirement is to maintain light levels less than 0.001 electrons/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 
electrons/sec/pixel before it reaches the detectors. Volume space allotted to the mechanism is 27.9 cm in 
diameter by 15.2 cm high (011 in 5 6 in). The shield must be able to open or close within about 60 
seconds, operate in a vacuum (1 0- 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 I ,  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‘h Aerospace Mechanisms Symposium, Langley Research Center, May 15-1 7, 2006 
I 
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https://ntrs.nasa.gov/search.jsp?R=20060020064 2019-08-31T00:33:11+00:00Z
Door 
Figure 1. Light- Path Labyrinth Cross-Section View 
Fabrication of parts was completed on the Micron 600U 5-axis high-speed milling machine as shown in 
Figure 2. The small sizes and angles of the light path protrusions make it necessary to use this machine. 
An additional advantage of the high speed machine is the extremely low amount of stress that is added to 
the part by the machining as opposed to the level normally added by typical machines. Stress is a 
concern due to the fact that cryogenic temperatures can cause the part's stress to increase and warp the 
movable door. Therefore, as well as using the high speed machine, Aluminum 6061-T651 is used to 
minimize stress. Aluminum 6061 -T651 is heat treated to stress relieve the material. 
Figure 2. Picture of door being cut on Micron machine 
After rough-cut fabrication, the door is taken and soaked in liquid nitrogen to determine if part warping at 
-cryogenic temperatures is an issue. Key measurements'taken before and after this process are 
compared and it is found that the door's critical dimensions do not permanently move any significant 
amount. Therefore, it is determined that the cryogenic environment will not warp the door and affect its 
2 
operations. Next, the dewar bulkhead plate is fabricated. Aluminum 6061- T651 is again used to 
minimize stress. Figure 3 shows the exploded isometric view of the mechanism. 
Light Box for 
P~l:l...-m+;m... \ 
\.I 9 
Light Shield Door 
rnrn rnrn Stepper Motor 
End of Travel 
0279 rnrn (I 1 .O in) End of Travel 
Switch 
Figure 3. Exploded Isometric View of Light Shield Mechanism 
Motor Selection 
One cryogenic compatible stepper motor is used to drive the light shield door mechanism. The motor was 
procured from CDA lntercorp with a right angle gear head, a planetary gear train, dry film Molybdenum Di- 
Sulfide (MoS2) coated parts, a motor shaft mounted resolver for position feedback, and a keyed motor 
shaft interface to the door. Motor design parameters are discussed below. 
Toruue 
Analysis indicates the light shield mechanism requires, with margin, a maximum driving torque of 
0.19 N*m (1.67 in-lb) in the worst case gravity orientation. In this orientation, the door is required to be 
held open or closed against gravity by the motor detent alone as power must be disabled during detector 
tests. This motor detent torque with margin turns out to be the driving design parameter and calculates to 
0.1 Nom (0.88 in-lb). A healthy factor of safety of 6 is used to bring the motor output detent torque to a 
0.6 Nom (5.3 in-lb) value. The procured motor provides a measured detent torque of 0.75 Nom (6.67 
in4b). 
Gearing 
A right angle gear-head is essential for this mechanism design as the volume space available to mount 
the motor and mechanism to the dewar bulkhead is limited. The total motor gear ratio is 187:l implying 
one motor step equals the door motion of 0.16 degrees. This small step size is required so that if during 
door motion the motor bounces back or misses a step near the fully closed position, the gap left “open” 
does not allow a significant amount of light to enter the light path. Analysis confirms the gap is not large 
enough to degrade the light shield capability of the mechanism. 
3 
Crvosenic Lubrication 
Dry lubrication on the motor components is required for proper cryogenic temperature operation. A 
sputtered Molybdenum Disulfide (MoS2) coating was used on the motor gears, bearings, and moving 
parts. MoS2 was chosen for this mechanism based on the ability to provide any thickness needed and 
also its heritage on numerous previous flight programs such as COBE, Cassini, SIRTF, and others. 
Drawbacks in using many dry film lubricants, including MoS2 are that the coating should be operated in a 
dry oxygen and water vapor free environment. Operation should not occur at humidity levels above about 
40% due to the reactivity of the coating with Oxygen and water vapor. 
The MoS2 dry film lubricant sputtering process was provided by Hohman Plating Inc. In a vacuum, at 
149°C (300°F), an electron emitter breaks up the lubricant material into atomic size particles which 
bombard the parts. The coating particles adhere to the parts’ surfaces and build up thickness over time. 
Coatings can be co-sputtered with different elements to improve crystalline structures and lower the 
friction coefficient. The majority of coatings are co-sputtered with Nickel giving a 0.02 friction coefficient. 
More advanced co-sputter options are Antimony O3 (p = 0.01) or Antimony O3 with Gold (p c 0.01). This 
project co-sputtered with Nickel as the coefficient of friction meets the required amount and is less 
expensive than the other co-sputter options. Figure 4 shows the dry film thickness along the tooth of a 
motor gear part as seen in a cross-section view. 
Figure 4. MoSl coating thickness measured along width of gear tooth near tooth tip of a 
representative motor gear part. Thickness measured from mounting adhesive to gap. 
Nominallyg Hohman plates parts to a thickness of 3000-10000 Angstroms (0.3 to 1 pm or 1 .I 8 x I 0-5 to 
3.93 x 10- in) over 50 minutes. Considering the low life cycle requirements of the mechanism, the 
associated mechanical loads it would undergo, and margin, a MoS2 coating thickness of 2.5 pm (0.0001 
in) was considered. Initially, the relationship of lubricant coating buildup over time with the sputtering 
process was thought to be linear. 1-iowever, through the coating of several sample gears, analysis shows , 
4 
it to not be so. At first the thickness and time were extrapolated linearly to that for 2.5 pm (0.0001 in) 
which resulted in a 3 hour long application. Destructive analysis was performed on a sample gear by the 
materials group at NASA Goddard, and the average thickness of the coating was measured to be 
17.8 pm (0.0007 in). Another gear sample was then coated at a lesser time of 70 minutes and the 
measured thickness of the coating ranged from 0.5 to 3.8 pm ( ~ x I O - ~  to 1.5 x 1 O4 in). Based on the 
gathered data shown in Figure 5 it was decided that an application duration time of 80 minutes would be 
sufficient for the light shield mechanism motor parts. All parts are coated for the 80 minute duration 
resulting in a thickness of 1.2 pm (0.00005 in). The final dry lube coating applied to the motor parts is 
based on the sample gear data and the minimum motor assembly tolerances for nominal fit. After the 
coating process was complete, all motor parts were successfully reassembled and tested. 
1 .OOE-05 - 
9.00E-06 - 
8.OOE-06 - 
7.00E-06 - 
6.OOE-06 - 
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5 OOE-06 - 
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L: 
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4.OOE-06 
3.00E-06 - 
2.OOE-06 - 
1 .OOE-O6 - 
O.OOE+OO 
MoS2 Coating Thickness vs. Time 
t 
t 
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t . 
* t  
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Mechanism Control 
The light shield mechanism motor is a two phase DC stepper motor with 30 degree steps and a total right 
-angle gear ratio of 187: 1. Nominal operation occurs at 24 VDC. Since the mechanism must operate at 
room temperature and at 20 Kelvin, a current controlled electrical drive system is used for maintaining 
proper performance margins under all operating conditions. A Newport 300 Series current control 
amplifier drives the light shield mechanism motor. Step pulse rate and phase current levels are settable 
, 
5 
quantities through a front panel or macro driver software which controls the mechanism to open or close 
the light shield door in approximately one minute. Software time out and TTL limit switch interfaces allow 
electronically disabling motor power redundantly with this controller. Motor resolver analog signal is read 
and converted to a TTL quadrature signal that the Newport controller can read. Motor power is disabled 
when contact is made with one of the end of travel switches in either the open or closed position of the 
light shield door. The motor shaft resolver is used as a method of determining the door position. 
Over Travel Analysis 
As the light shield reaches its end of travel in either the open or close direction, it comes into contact with 
an end-of-travel switch. This switch then cuts off power to the motor and the door stops its motion. In the 
close direction there is a hard stop that does not allow the door to over travel. Travel in the open 
direction, however, does not have a hard stop. Therefore, stress analysis is performed on the tab that 
comes into contact with the switch to determine the maximum amount of force that can be handled. 
These calculations determine that the load felt on the tab is 20 times smaller than the yield load of the 
door material, including margin. Therefore, at the slow speed that the motor rotates, the tab contacting 
the switch will not yield if the motor attempts to continue to rotate, and the shield door will not over travel. 
Cryogenic Testing 
Before delivery to the project, the light shield mechanism is tested in a dewar at 20K in the operational 
configuration. The dewar bulkhead is attached to an interface plate to mate the mechanism to the 
chamber. Once assembly is complete, the chamber is closed and the then rotated 180" to orient the 
mechanism to its operational configuration. This places the mechanism so that gravity is acting to open 
the door from its closed position. 
Tesf resulfs are forthcoming and will be presenfed af fhe conference. 
Special thanks goes to Mike Dube and Mark Mclendon of the Materials Group at Goddard for their 
destructive analysis and insight in evaluating the dry film coating of motor parts. 
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JWST NIRSpec Cryogenic Light Shield Mechanism

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