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 as well as an open field-of-view during ground performance testing within a cryogenic dewar. In order to achieve the light attenuation requirements when closed as well as provide a open field of view for the detectors within the cryogenic dewar, a light shield mechanism was designed. This paper discusses the details of the light shield mechanism design, development, and test results

Sharma, Rajeev K.

NASA Technical Reports Server

Source of Acquisition 
NASA Goddard Space Flight Center 
ABSTRACT 
Rajeev Sharma (I)  
(I)  NASA Goddard Space Flight Center, Greenbelt, Maryland, US.A 
N&rv K.,Charmn ’@I(I.su. yov 
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 as well as an open field-of-view during 
ground performance testing within a cryogenic dewar. 
In order to achieve the light attenuation requirements 
when closed as well as provide a open field of view for 
the detectors within the cryogenic dewar, a light shield 
mechanism was designed. This paper discusses the 
details of the light shield mechanism design, 
development, and test results. 
1. 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 degrees Kelvin 
(-253OC). 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. 
2. 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 (01 1 in x 6 in). The shield 
must be able to open or close within about 60 seconds, 
operate in a vacuum (1 O 6  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. 
light shield door range of rotation is about 124 degrees 
from open to close. 
3. SHUTTER 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. 
1 ma 
Figure I .  Light Shield Door 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. 
When open, the shield door is to remain outside the 76 
degree cone angle of the detector field of view. The 
https://ntrs.nasa.gov/search.jsp?R=20070032656 2019-08-31T01:10:55+00:00Z
4.1 Torque 
Figure 2. Shutter Door manufacturing process. 
After rough-cut fabrication, the door is taken and 
soaked in liquid nitrogen to determine ifpart 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 operations. Next, the dewar 
bulkhead plate is fabricated. Aluminum 6061- T65 1 is 
again used to minimize stress. Figure 3 shows the 
exploded isometric view of the mechanism. 
Calibration LED’s , 
Dewar Bulkhead 
Figure 3. Light Shield Mechanism Isometric View. 
4. MOTOR SELECTION 
One cryogenic compatible stepper motor is used to 
drive the light shield door mechanism. The motor was 
procured from CDA Intercorp 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. 
Analysis indicates the light shield mechanism requires, 
with margin, a maximum driving torque of 
0.19 Nom (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 N*m 
(5.3 in-lb) value. The procured motor provides a 
measured detent torque of 0.75 N*m (6.67 inglb). 
4.2 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: 1 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. 
4.3 Cryogenic 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 M0S2 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 MoSz 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 0 3  with Gold (p <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. MoS2 coating thickness measured along 
width of gear tooth near tooth t@ of a representative 
motor gear part. 
Nominally, Hohman plates parts to a thickness of 3000- 
10000 Angstroms (0.3 to 1 pm or 1.18 x l o 5  to 3.93 x 
l o 5  in) over 50 minutes. Considering the low life cycle 
requirements of the mechanism, the associated 
mechanical loads it would undergo, and margin, a MoSz 
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. However, through the coating of several sample 
gears, analysis shows 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 
(2x105 to 1.5 x 10“‘ 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. 
. e  
o 1~ D m m tm IS 110 ?m im a0 
le”. I. 
Dmum 
Figure 5. MoS2 Coating Thickness vs. Application 
Time on Time for a typical motor gear part. 
The bearings in the motor are also dry lubricated with 
MoS2. Lubrication thickness is the same as that for all 
other motor parts, 1.27 pm (0.00005 in). The support 
bearing on the opposite side from the motor on the 
bulkhead is a radial bearing and is not sputter coated. 
Instead, the support bearing is a “BarTemp” bearing 
from The Barden Corporation. “BarTemp” bearings 
have a cage made of a Teflon-coated, highly 
compressed material with very fine glass fibers and 
molybdenum disulfide impregnation. During operation, 
the rotation of the bearing causes the balls to rub off 
small amounts of the cage which coats the raceways 
with a thin lubrication layer. 
5. 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:l. 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 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. 
6. 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. 
7. CRYOGENIC TESTING AND OPERATIONS 
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 1 80° 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. 
Special thanks goes to Mike Dube and Mark McLendon 
of the NASA-GSFC Materials Group for their analysis 
and insight in evaluating the coatings of the actuator 
parts. 


JWST NIRSpec Cryogenic Light Shield Mechanism

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