Surface scattering measurement techniques can be adopted for use in spacecraft contamination control programs. A witness mirror measurement program was developed and implemented for the NASA Cosmic Background Explorer (COBE) spacecraft. Scattering measurements were obtained before and after a qualification level vibration test for several COBE prototype instruments. The measurements indicate that a significant amount of particle redistribution occurred during the vibration test. The methodology used and the results obtained from this monitoring program are discussed

Carosso, Nancy J. P.

Carosso, Paolo A.

English

NASA Technical Reports Server

SURFACE SCATTERING CHARACTERISTICS 
AND 
THE MEASUREMENT OF PARTICULATE CONTAMINATION 
Paolo A. Carosso 
TS Infosystems, Inc. 
and 
Nancy J. P. Carosso 
NASA Goddard Space Flight Center 
ABSTRACT 
Surface scattering measurement techniques can be adopted for use in 
spacecraft contamination control programs. A witness mirror measurement 
program was developed and implemented for the NASA Cosmic Background Explorer 
(COBE) spacecraft. Scattering measurements were obtained before and after a 
qualification level vibration test for several COBE prototype instruments. 
The measurements indicate a significant amount of particle redistribution 
occurred during the vibration test. This paper summarizes the methodology 
used and the results obtained from this monitoring program. 
INTRODUCTION 
Contamination deposited on optical surfaces can greatly reduce instrument 
performance, therefore impairing the achievement of mission scientific 
goaXs. Particulates such as fibers, dust or remains of fabrication 
processes or chemical species, outgassed from a variety of materials or 
generated during virtually any mission phase and subsequently adsorbed 
or deposited on critical surfaces, degrade surface characteristics. Among 
the principal effects of the presence of contaminants are surface obscuration, 
scattering and absorption of incident radiation, loss of throughput, and 
increase of surface absorptivity. Particles in the instrument field of view 
have an obvious negative influence in the process of data collection. 
383 
https://ntrs.nasa.gov/search.jsp?R=19880001480 2020-03-20T08:53:48+00:00Z
The instruments installed on the Cosmic Background Explorer (COBE), are 
highly sensitive to contamination. 
under control the amount of contamination on all sensitive surfaces. Such 
an effort must be undertaken from the early fabrication, assembly and testing 
phases, and continue through all subsequent phases of the COBE mission. All 
requirements, procedures and monitoring plans necessary to achieve this 
goal are identified in the COBE Contamination Control Plan (CCP) [l]. 
It is therefore very important to keep 
The COBE CCP includes the implementation of a contamination 
monitoring program for the Thermal Structural Unit (TSU). The TSU is a set 
of prototype instruments which were used for test and demonstration. The COBE 
instruments are located in an element known as the Cryogenic Optical Assembly 
(COA). The TSU consisted of a prototype COA which contained a Diffuse 
Infrared Radiation Experiment (DIRBE) and a prototype Far Infrared Radiation 
Spectrometer (FIRAS), two of the most contamination-critical instruments on 
the COBE. Figure 1 depicts the COA with the two instruments. The primary 
objective of the contamination monitoring program was to monitor the presence 
of contaminants during all phases of the TSU testing. 
This paper describes the results of the initial phase of the program 
developed to monitor contamination during the vibration test performed on 
the flight-like TSU integrated COA (Cryogenic Optical Assembly). 
The vibration test was conducted March of 1985. The TSU COA was flight-like 
except there was no contamination cover over the DIRBE baffle aperture. 
Prior to test, all TSU COA parts were cleaned to a level LOOA per MIL STD 
12466 [2], and assembled in a Class 10,000 clean tent. Surface cleanliness 
measurements taken before transportation to the vibration cell, revealed 
external COA surfaces to be level 200-300 per MIL STD 12468. Prior to 
transport to the vibration test, the TSU was double bagged in cleaned RCAS 
2400 bagging material. 
The integrated flight-like TSU COA was originally instrumented with 
15 contamination witness mirrors. 
the DIRBE mirror mounts, two witness mirrors were taken off the TSU 
prior to the vibration test. The location of the witness mirrors are 
illustrated in Figure 1 . 
Due to concerns about overstressing two of 
The contamination witness mirror program consists of performing before 
and after testing characterization on each witness mirror, including: 
1)- BRDF characterization; 
2)- Surface cleanliness determination' per ASTM F24-65 standard 
[3] ;  and 
3)- Surface morphology examination by optical microscopy, with 
photographic documentation. 
Bidirectional Reflectance Distribution Function (BRDF) measurement is a 
particularly interesting technique, as it provides information about the 
scattering properties of optical surfaces. The DIRBE performance 
requirement [l] is stated in terms of percent change of mirror BRDF. 
3 84 
BRDF measurements 
after the vibration test. The comparison of these data show percent 
change in BRDF characteristics for the witness mirrors, which were 
exposed to the same contamination environment as the TSU COA surfaces. 
were performed on all witness mirrors before and 
RESULTS 
BRDF CHARACTERIZATION 
A complete description of the BRDF characterization technique adopted 
in this program is reported in Reference [4]. 
Each witness mirror was measured before and after the vibration test. In 
order to obtain a complete description of the mirror characteristics, three 
areas were measured on each mirror. The BRDF curves obtained for mirrors 11, 
VI, are reported in Figures 2 to 13. In these figures, the curves labeled B1, 
B2 and B3 are relative to the mirror areas as measured before the vibration 
test. Analogously, curves Al, A2 and A3 refer to the after vibration test 
BRDF measurements. As it can be seen from the figures, the after test 
BRDF values are generally higher than the before test ones. The 
differences are certainly above the experimental error, and represent, 
therefore, an effect of the contamination accumulated on the mirrors during 
the vibration test. In some cases, the contamination effect is 
strong enough to cause dramatic changes in the BRDF values, up to two 
orders of magnitude 
In general, as already mentioned, the BRDF values obtained in the after test 
measurements are higher in modulus. Also, the curves tend to level off 
at high scattering angles, which represents a diffuse scatter component due 
to the presence of contaminants. In other words, the specular behaviour 
typical of the clean mirror (straight BRDF line with negative slope), 
poisoned by a component typical of a diffuse scatterer, in this case the 
contaminant particulates. 
is 
In Table 1, representative values of the BRDF curves are reported for each 
witness mirror. The BRDF values obtained at the scattering angles of 30 and 
60 degrees on the central areas of each mirror were arbitrarily 
chosen to be representative comparison data. (curves #2 in the Figures). 
The percent change of BRDF based on these measurements are also reported in 
Table 1. It is noteworthy to remember at this time that the current 
DIRBE requirements allow a maximum of 50% BRDF change due to mirror 
contamination. 
showing that, in some cases, a very significant amount of contamination 
occurred during the vibration test. 
The reported BRDF percent changes vary from 8% up to 600%, 
385 
SURFACE CLEANLINESS LEVELS 
The presence of contamination on the after test mirrors detected by the 
BRDF measurements is confirmed by the surface cleanliness level data, also 
reported in Table 1. In most cases, the after test cleanliness levels 
are higher (higher level of contamination) than the before test levels 
(which were levels 25-100 on the mirrors). Nine out of the twelve mirrors 
show an after test cleanliness level higher than the COA cleanliness 
requirement (level 300). 
OPTICAL MICROSCOPY EXAMINATION 
A l l  witness mirrors were examined via optical microscopy to 
evaluation of the nature and morphology of contaminants and to evaluate the 
mirror cleanliness levels. Photographical records of contaminants 
were taken at magnifications varying between 45X and 1OOX. Cleanliness 
levels were obtained following the ASTM procedure [ 3 ] .  
In Table 2, the most interesting observed features are reported for each 
mirror. Fifteen mirrors were planned to be installed in the COBE TSU; 
Mirrors VI1 and IX were not installed because of potential stress on 
mirror mounts. Mirror XIV was not retrieved after the vibration test; it 
will be removed later in the program. 
obtain a visual 
386 
I 
SUMMARY AND CONCLUSIONS 
The data obtained from the contamination witness mirrors after the TSU COA 
first vibration test show that the TSU became significantly more contaminated 
during the course of the test. Because the TSU was bagged (except for the 
bottom of the bag which was taped to the attachment fixture in the 
vibration cell), the contamination could not 
have come from the vibration cell. It is deduced that the increase in 
contamination levels on the mirrors is a result of particle 
within and around the COA. In essence, the TSU COA was the source 
of contamination for itself. Unfortunately, because of TSU schedule 
constraints, some contamination control measures could not be implemented. 
For example, the FIRAS was not disassembled, cleaned, and reassembled in a 
clean room as originally planned; it was assembled 'dirty', and then cleaned 
as a completely assembled instrument, which is considered a substantially 
less effective cleaning method. Last minute drilling operations on the 
COA had to be performed in the COA clean room, and although measures were 
taken to clean-up any generated contamination, it is possible that some 
particulate matter was missed. 
redistribution 
Particle redistribution on relatively "clean" hardware does occur. The 
result is further contamination of virtually all surfaces. The TSU COA 
vibration test witness mirror results presented in this report show 
that, on 9 out of 12 mirrors, contamination levels after the vibration test 
exceed the overall COA cleanliness requirement (level 300). And, 
specifically the DIRBE primary mirror, which has a cleanliness requirement 
level 100, showed unacceptable increases in contamination (level 800 and 
level 1000 on the two witness mirrors located on the dummy primary in the 
TSU). Considering the BRDF data presented, one of the DIRBE primary 
witness mirrors had a 44% change in BRDF at 30 degrees, which falls 
close to the 50% requirement; and the other DIRBE primary witness mirror 
had a 77% change in BRDF, which exceeds the requirement. 
of 
FUTURE WORK 
Using witness mirrors and the scattering measurement technique as a means to 
monitor contamination has proven to be an effective and useful tool. This 
technique can be used to monitor contamination in a number of different 
scenarios. For example, monitoring of cleanrooms and particle fallout, or 
monitoring of surface cleanliness during assembly, integration, test, 
transport etc., could be accomplished using this technique. 
387 
Table 11. OPTICAL MICROSCOPY EXAMINATION DATA 
Mirror  d - Locat ion 
I 
11 
111 
IV 
V 
V I  
V I 1 1  
X 
X I  
X I  I 
X I 1 1  
xv 
FIRAS PICKOFF 
MIRROR 
FIRAS PICKOFF 
MIRROR 
APCO 
APCO 
APCO 
DIRBE PRIMARY 
APCO 
APCO 
COA SIDE PANEL 
APC 0 
DIRBE PRIMARY 
- Contamination f e a t u r e s  
Small p a r t i c l e s  observed. No l a r g e  p a r t i c l e s  
o r  f i b e r s .  
Very large f i b e r s  observed,  (175813264 urn, 
1055x55 urn, 530x70 urn). 
Sharp edge metal p a r t i c l e s  p re sen t ,  130x60 urn. 
Seve ra l  p a r t i c u l a t e s  up t o  W 100 urn diam. 
One l a r g e  f i b e r  (1385x176 urn). Round particles 
up t o  180 urn diarn. i n  c e n t r a l  area. Small 
dus t  p a r t i c l e s  elsewhere. 
W 
Large metal particles with golden appearance,  
approximately round (198x3176 urn, 176x99 urn). 
One e l m g a t e d  f i b e r  (circa 2000 urn), metal 
p a r t i c l e s  (285X110), d u s t .  
Elongated f i b e r  (770x33 urn), smaller f i b e r s ,  
y e l l o r r e d d i s h  p a r t i c l e s  (oxide  l i k e ) ,  150 urn 
max diam. Metal particles (probably A l ) .  
Seve ra l  smaller d u s t  p a r t i c l e s .  
Dust p a r t i c l e s  observed (240x110 urn mx). 
Large f i b e r  (505x420 urn). Dust p a r t i c l e s ,  
max diam. circa 110 urn. 
Very e longated ,  fo lded  f i b e r ,  593x374 urn. 
Round p a r t i c u l a t e s  up t o  circa 260 urn. 
F i b e r s  (530x310 urn, 44013350 urn). Poss ib l e  
ceramic-type p a r t i c l e  with i n c l u s i o n s ,  
370x242 urn. 
Metal s p l i n t e r  (poss ib ly  A l )  950x130 um. 
Small diameter  dus t  p a r t i c l e s .  
COA SIDE PANEL Dust p a r t i c l e s  observed on t h e  whole su r face .  
388 
REFERENCES 
[1]- N.J.Puge1, "Cosmic Background Explorer Contamination 
Control Plan. Phase 11", COBE-PL-732-1703-01, September 1984. 
[2]- MIL-STD 1246A, "Product Cleanliness Levels and 
Contamination Control Plan". 
[3]- ASTM Standard P24-65 (Reapproved 19831, "Measuring and 
Counting Particulate Contamination on Surfaces". 
[4]- P.A.Carosso and N.J.P.Carosso, "The Role of Scattering 
Distribution Functions in Spacecraft Contamination Control 
Practices", Appl.Opt., Vol 25, page 1230, 1986. 
c 
0 
Ld u 
0 
.3 U 
4 
390 
0 
log BRDF 
I 
1 1.5 
Figure 2. Mirror I 
2 .  
39 1 
O l  
log BRDE' 
- I  - 
-2 - 
-3 
- 4  
1 .  
Figure 3. Mirror I1 
1 .r 
log 
2. 
392 
log BRDl 
- I  
-2 
- 3  a 
-4  - 
o m  
‘t 
L 4 2  
1 .  
I 
1 .s 
Figure 4. Mirror I11 
393 
2 .  
2. 
log 8 
Figure 5 .  Mirror IV 
394 
0 -  
log BRDF 
-I - 
-3  - 
a 
2 .  
Figure 6. Mirror V 
395 
0 -  
log BRDF 
-I . 
- 3  
-4 
396 
log BRDF 
-4  - _  
-I 
- 2  
- 3  
I 
1.5 
log 9 
2. 
Figure 8. Mirror VIII 
397 
c) 
log BRDF 
- I  
-2 
2. 
Figure 9. Mirror X 
398 
0 
log BRDF' 
- 1  
- 2  
- 3  - 
-4  - 
1. 1 .F 
log $. 
83 
I 
Figure 10. Mirror XI 
399 
- 1  
-3 
-4 
# 
I 
2. 
Figure 12. Mirror XI11 
400 
0 
log BRDF 
- I  
-3 - 
- 4 -  
1 .  I 1.5 
log 9 
Figure 1 1 .  Mirror XI1 
2 .  
40 1 
0 
log BRDF 
- I  
-2  
- 3  
- 4  
8 
I 
1 .  1.5 
log 0- 
2 .  
Figure 13. Mirror XV 
402 


Surface scattering characteristics and the measurement of particulate contamination

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server