**EO-1 Technology Demonstration of Silicon Carbide Polishing**

# 

September 2001

Contract #1205103

SSG Precision Optronics

65 Jonspin Road

Wilmington, MA 01810

1.  **INTRODCTION**

The material properties of Silicon Carbide (SiC) are extremely well
suited for space-based, high precision, visible optical instrumentation.
The combination of high specific stiffness and high thermal stability
enable highly lightweighted mirrors that can withstand the extreme
thermal environments encountered in space.

SSG, in cooperation with MIT/LL, has designed, fabricated and deliver
the flight telescope subsystem for the Advanced Land Imager (ALI). This
system demonstrates the viability of SiC technology for the EO-1 and
future space-based missions. The SiC optical telescope design
incorporates a wide field-of-view (15 degrees in the cross-track
direction) and a flat image plane with low distortion and excellent
image quality. The optics consisted of hot-pressed SiC; polished SiC for
flat and spherical surfaces and Silicon coated SiC for the aspheric
surfaces.

The system performance of the SiC ALI telescope met the specifications
for component level surface figure, reflectivity, field of view, angular
resolution, point spread function, system throughput, image quality over
FOV, distortion map over FOV, focal length, aperture uniformity,
mechanical stability, thermal stability, size and weight

. However, the specification for stray light was not achieved.

SSG has worked with NASA to develop a technology program to optimize the
process for generating low scatter SiC optics and demonstrate the
process on SiC optics both flat and aspheric which satisfy the ALI
flight optics requirements.

Summary of results:

-   BRDF measurements made before and after Denton FSS-99 silver coating
    demonstrate that there is no significant degradation in stray light
    performance for either bare RB SiC or Si clad RB SiC optics from the
    Denton coating.

-   Bare RB SiC flats polished using the optimized polishing process
    resulted in a 10x (@ 10 degrees) improvement over the ALI F1 optics
    and fully satisfied the derived component level BRDF requirements.

-   Si-Clad RB SiC aspheric sample (concave ellipsoid) showed a 10x
    improvement at 10 degrees over ALI M2 asphere and have met the ALI
    specification.

-   A spare hot-pressed Si-clad EO-1 M1 processed with the optimized
    procedure resulted in a 12x improvement over flight EO-1 M1 and
    demonstrates meeting stray light specification for ALI flight
    hardware.

Stray light analysis of the ALI telescope design conducted by two
outside contractors (Lambda Research, Littleton, MA and Computational
Physics Applications, Inc.) has identified the scatter from the M1 as
the major contributor to the system stray light.

The BRDF improvements can be attributed to two critical changes to the
optical fabrication process (1) Use of Si-cladding which is highly
ductile leading to excellent diamond turnability and ease of figuring
and polishing and (2) the addition of a superfinishing step after the
previously utilized polishing process.

**2. Program Objectives**

The primary objective of this technology program is to demonstrate
polishing processes for low-scatter SiC optics. The program has
demonstrated that SiC mirrors both flat and aspheric, can be produced,
in a repeatable and reliable fashion, which meet the requirements of the
ALI flight optics. The overall objective was divided into three
technical objectives:

-   Demonstrate bare SiC (flat) and silicon coated (aspheric) optics
    with finish and figure in line with ALI specifications.

-   Quantify the effects of Denton FSS-99 silver coating on SiC optics
    scatter performance.

-   Incorporate current state-of-the-technology SiC materials into the
    technology program (replacement of hot-pressed substrates with
    reaction bonded substrates).

The program objectives were pursued in two technical tasks: (1) Sample
Development Demonstration and (2) EO-1 M1 Demonstration Mirror
Fabrication and Testing.

The objective of the sample development task is to generate a set of
optimized process flows for the critical process steps in the
fabrication of SiC optics. These steps are:

-   Silicon cladding for aspheric optics

-   Polishing of bare SiC which is consistent with flats and spherical
    optics

-   Polishing of silicon clad aspheric optics.

The optimized flow is then applied to a spare EO-1 M1 mirror.

The technical discussion will detail the background and results for this
program. An overview of the ALI telescope will present the overall
design and performance of the flight hardware. Particular attention will
be given to the stray light requirements and the derivation of a
component level BRDF from system level specifications. The
experimentation and results will be detailed for the bare RB SiC flats,
Si-clad SiC aspheres and the spare EO-1 M1.

0.  **Technical Discussion**

1.  **ALI Telescope Subsystem Overview**

The ALI telescope subsystem specifications are presented in Table 3-1.
The optical design is a four mirror, all reflective, unobscured system
with an Invar 36 metering structure. The focal surface is flat. All
optics are hot-pressed SiC. Figure 3-1 shows the complete as delivered
instrument.

+-----------------------------------+-----------------------------------+
| ## TABLE 3-1.                     |                                   |
|                                   |                                   |
| ## ALI Telescope Specifications   |                                   |
+-----------------------------------+-----------------------------------+
| Aperture                          | 12.5 cm                           |
+-----------------------------------+-----------------------------------+
| Focal Length                      | 94 cm                             |
+-----------------------------------+-----------------------------------+
| Field-of-View                     | 1.0 x 15 degrees                  |
+-----------------------------------+-----------------------------------+
| Wavelength                        | 0.4 -- 2.5 microns                |
+-----------------------------------+-----------------------------------+
| MTF @ 0.6 microns (37.5 lp/mm)    | \>0.5                             |
+-----------------------------------+-----------------------------------+
| Distortion                        | \< 275 microns, \< 250 microns    |
+-----------------------------------+-----------------------------------+

  ----------------------------------------------------------------------------------
  ![](media/image1.png){width="2.3311023622047244in"
  height="2.3401574803149607in"}![](media/image1.png){width="2.3311023622047244in"
  height="2.3401574803149607in"}

  *Figure3-1. SiC ALI Telescope Subsystem.*
  ----------------------------------------------------------------------------------

**3.3.1 ALI Optical Design**

The optical design form is a reflective version of a Cooke Triplet. The
off-axis design provides for the 15 degree wide FOV. The aperture stop
is located on the secondary mirror. Image plan is flat. The ray trace is
shown figure 3-2. Table 3-2 provides the mirror parameters.

  -----------------------------------------------------------------------

  *Figure 3-2. ALI Telescope Raytrace.*
  -----------------------------------------------------------------------

+-----------------------------------------------------------------------+
| ## TABLE 3-2                                                          |
|                                                                       |
| ## ALI Telescope Mirror Parameters                                    |
+-----------------------------------------------------------------------+
| ![](media/image2.wmf){width="4.909722222222222in" height="2.28125in"} |
+-----------------------------------------------------------------------+

The primary and the secondary mirrors are aspheric. (See figure 3-3 for
photographs of these optics). The hot-pressed substrates are machined to
the base radius, coated with a layer of amorphous Si, then diamond
turned to the desired asphere. Final figure and finish were achieved by
hand polishing. The tertiary and fold flat were finished and figured in
the bare hot-pressed SiC. All optics were coated with Denton FSS-99.
Moderate lightweighting was used for the primary mirror.

Invar 36 was chosen as the metering structure and optical bench. Invar
was selected to avoid brittle damage risks associated with conventional
ceramic SiC. The CTE match between Invar 36 and SiC minimizes
bi-material effects, improving the optical performance under varying
thermal loads. SiC composites are available, but were not sufficiently
mature at the time and therefore not considered. MIT/LL machine shop was
responsible for optical bench fabrication.

+-----------------------------------------------------------------------+
| ![](media/image3.wmf){width="3.6666666666666665in"                    |
| height="0.9965277777777778in"}                                        |
+-----------------------------------------------------------------------+
| ###### Figure 3-3. ALI Primary and Secondary Mirrors                  |
+-----------------------------------------------------------------------+

**3.1.2 ALI Optical Performance**

The optical performance of the ALI telescope were characterized by image
quality, distortion and stray light analysis. System level wavefront
error (WFE) and modulation transfer function (MTF) were used to quantify
the image quality.

**3.1.2.1 Image Quality**

The WFE was derived from the MTF specifications using Code V. The system
level requirement at temperature is less than 0.15 λ RMS at 0.63
microns. The measured WFE is shown in figure 3-4. 12 field points were
tested with the WFE at temperature ranging from 0.089 to 0.148 λ RMS.

+-----------------------------------------------------------------------+
| ![](media/image5.png){width="5.875in" height="3.0284722222222222in"}  |
+-----------------------------------------------------------------------+
| ######                                                                |
| Figure 3-4. On-Axis, System Level WFE ( 0.596λ Pk-Valley, 0.089λ RMS) |
+-----------------------------------------------------------------------+

System level MTF performance was projected from wavefront maps using
Code V. Figure 3-5 shows the specification and measured MTF. The
performance meets or exceeds the specificatio at 18.75 lp/mm the 37.5
lp/mm.

  -----------------------------------------------------------------------
  ![](media/image6.wmf){width="4.0in" height="2.5743055555555556in"}

  *Figure 3-5. MTF Specification and Performance.*
  -----------------------------------------------------------------------

2.  **Distortion**

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2.  

Optical performance was also assessed by measuring the system level
distortion. A target with 40 scribed points was used to map the
distortion of the optical system. The uncorrected data (figure 3-6)
shows a maximum distortion vector length of 928 microns. Using a cubic
polynominal (Dr. David Hearn, MIT/LL) the corrected residual distortion
is below 9 microns. This corrected value is much below the system
specification of 250 microns

+-----------------------------------------------------------------------+
| ![](media/image7.png){width="5.375in" height="2.379166666666667in"}   |
+-----------------------------------------------------------------------+
| ![](media/image8.png){width="5.375in" height="2.497916666666667in"}   |
+-----------------------------------------------------------------------+
| ###### Figure 3-6. Distortion map prior to and after correction.      |
+-----------------------------------------------------------------------+

2.  **Stray Light**

The stray light requirement for the ALI flight optics was specified at
the system level. The stray light contribution in any of the bands was
to be less than 2% of the nominal full-scale signal level in the
panchromatic and spectral bands 1-5 and 7. This specification was met in
four of the eight spectral bands, but was out of spec by 2x to 10x in
the other five bands.

Analysis of the stray light performance for the flight optics and the
stray light requirements \[1,2\] has indicated that if the component
level BRDF were at the level of the ALI M2, the system level stray light
requirements would be satisfied. In addition, the analysis indicated
that M1 was the primary contributor to the system stray light, greater
than 70% depending on the spectral band. The M3 was also a significant
factor accounted for up to 27% of the stray light. Similar analysis
conducted at SSG derived a component level BRDF goal, which is shown in
figure 3-7 along with the measured component level BRDF data and the ABg
curve fit to the data. Therefore, the goal of this program is to define
a polishing process, which demonstrates BRDF levels in in-line with
these derived component level specifications. Of primary importance is
the demonstration of the process using a spare EO-1 M1.

  -----------------------------------------------------------------------
  ![](media/image9.wmf){width="5.99375in" height="2.838888888888889in"}

  *Figure 3-7. BRDF Measurements of ALI Telescope optics after coating
  with Denton FSS-99.*
  -----------------------------------------------------------------------

**3.1.3 ALI Performance Summary**

The ALI SiC optical system met or exceeded the majority of the telescope
requirements including the component level surface figure, system
transmission, field-of-view, angular resolution, point spread function,
system throughput and image quality over the FOV. The one exception
where the system performance did not meet the requirement was the stray
light performance. A stray light analysis of the ALI telescope indicated
that the primary contributors to the high BRDF levels were M1 and M3,
with greater than 70% resulting from M1 and up to 27% from M3.

The effort in the present EO-1 Technology program is to demonstrate that
the BRDF issues encountered in the ALI flight optics are a limitation
associated with the specific ALI flight optics, not a fundamental
limitation of SiC optics.

######## 3.2 Program Overview

In the first task a series of sample pieces were used to optimize the
process flow for producing low-scatter optics. Included in this set of
samples were bare, flat RB SiC and silicon clad aspheric optics (see
figure 3-8). RB SiC was chosen rather than hot-pressed SiC (used in the
ALI flight optics) because it represents the state-of-the technology in
lightweight SiC optics. BDRF and surface roughness measurements were
made before and after coating with Denton FSS-99 silver. This allowed
any effects on scatter due to the Denton coating to be isolated. Surface
figure measurements were also made. Table 3-3 summaries the witness
sample set.

+-----------------------------------------------------------------------+
| ## Table 3-3. Summary of Witness Samples                              |
+-----------------------------------------------------------------------+
| 4-2" diameter RB SiC Flats                                            |
+-----------------------------------------------------------------------+
| 2.5" diameter RB SiC/Si coated Convex Hyperboloid                     |
+-----------------------------------------------------------------------+
| 4" diameter RBSiC/Si coated Concave Ellipsoid (rib supported)         |
+-----------------------------------------------------------------------+

  ---------------------------------------------------------------------- --
  ![](media/image10.jpeg){width="1.9131944444444444in"                   
  height="1.679861111111111in"}                                          

  ![](media/image11.wmf){width="2.325in" height="2.1840277777777777in"}  

  *Figure 3-8. (a) Back rib structure of RB SiC Concave Ellipsoid and    
  (b) Si-clad Convex Hyperboloid optic used in EO-1 Technology Program.* 
  ---------------------------------------------------------------------- --

**3.2.1 Reaction Bonded SiC Flat Sample Results**

The results for the RB SiC flats are presented in figures 3-9 to 3-14
and table 3-4. Interferograms were taken before the Denton FSS-99
coating. As the surface roughness and BRDF data indicate there is no
significant degradation from the Denton FSS-99 silver coating evident.
Samples #1 and #2 were polished by an outside vendor using a planetary
lapping and polishing technique. Samples #3 and #4 were worked at
SSGoptics division using a planetary lapping machine and spindle
polishing. All BRDF measurements were made at Schmitt Measurement
Systems (Portland, OR). Three BRDF measurements were made on each sample
for both the before and after Denton coating. The angle of incidence was
5 degrees from normal to mirror.

Measurements were made from 0.01 degrees to 90 degrees in both the
forward and back scatter directions. Figure 3-10 shows the signature
scan and the average BRDF for three spots on sample #1. For small angles
(\< 0.2 degrees) is evident that the background light level, given by
the signature scan, do not allow sufficient signal to noise to provide
valid BRDF data. Typically a SNR of 10 is desired for valid data.
Therefore, only the data for angles greater than 0.2 degrees will be
presented.

  -----------------------------------------------------------------------
  ![](media/image12.png){width="2.4618055555555554in"
  height="1.8868055555555556in"}

  *Figure 3-9. Typical surface figure of RB SiC flats. (0.016λ RMS 0.093λ
  Pk-valley).*
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image13.wmf){width="3.9743055555555555in"
  height="2.547222222222222in"}

  *Figure 3-10. Curve showing low signal to noise ratio (\<1) for BRDF
  measurements less than 0.2 degrees. Data below this angle are assumed
  not valid.*
  -----------------------------------------------------------------------

Table 3-4 presents surface roughness results for before and after Denton
FSS 99 silver coating along with surface figure. Surface roughness
measurements were made at GSFC using a WYKO TOPO-3D at 10x. Between 4
and 6 measurements were made on each sample. The presented results are
the average of these measurements.

In all cases the BRDF measurements made after the Denton coating were
lower than those made before the coating. This result is counter
intuitive, but similar results have been seen on other bare RB SiC
sample sets and may be attributed to the two-phase nature of RB SiC. As
light impinges on the uncoated RB SiC two materials contribute to the
scattering of light. In addition, the interface between the individual
phases will contribute to scatter. When the Denton coating is applied to
the bare RB SiC these contributions are eliminated and the BRDF level
decreases. The surface roughness data indicate no significant
degradation due to the Denton coating.

  -------------- --------------- --------------- -------------- --------------
  Table 3-4.                                                    
  Surface                                                       
  roughness and                                                 
  figure for RB                                                 
  SiC flats.                                                    

                 Sample #1       Sample #2       Sample #3      Sample #4

  Part \#        10350001117-3   10350001117-5   112106         112104

  Surface Figure 0.016λ RMS      0.016λ RMS      \<λ/4 RMS@ 0.6 \<λ/4 RMS@ 0.6
                 \@0.6 μm        \@0.6 μm        μm             μm

  Surface        7.18 Angstroms  8.14 Angstroms  13.5 Angstroms 12.3 Angstroms
  Roughness      RMS             RMS             RMS            RMS
  (prior to                                                     
  Denton                                                        
  coating)                                                      

  Surface        7.30 Angstroms  7.58 Angstroms  15.2 Angstroms 12.6 Angstroms
  Roughness      RMS             RMS             RMS            RMS
  (after Denton                                                 
  coating)                                                      
  -------------- --------------- --------------- -------------- --------------

  -----------------------------------------------------------------------
  ![](media/image14.wmf){width="4.0in" height="2.5618055555555554in"}

  *Figure 3-11. BRDF data for sample #1. Data is the average of 3 points
  on the mirror.*
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image15.wmf){width="4.05in" height="2.591666666666667in"}

  *Figure 3-12. BRDF data for sample #2. Data is the average of 3 points
  on the mirror.*
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image16.wmf){width="4.125in" height="2.634027777777778in"}

  *Figure 3-13. BRDF data for sample #3. Data is the average of 3 points
  on the mirror.*
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image17.wmf){width="3.9583333333333335in"
  height="2.529861111111111in"}

  *Figure 3-14. BRDF data for sample #3. Data is the average of 3 points
  on the mirror.*
  -----------------------------------------------------------------------

Figure 3-15 presents a comparison between the derived ALI BRDF component
specification and the bare RB SiC flats. These results are an
significant improvement from the performance of the ALI flight optics
and meet or beat the BRDF requirements. They also indicate that RB SiC
can out perform hot-pressed SiC for flats and spheres.

  -----------------------------------------------------------------------
  ![](media/image18.wmf){width="5.25in" height="3.8979166666666667in"}

  *Figure 3-15. BRDF results from Bare RB SiC flats compared with ALI
  component level specification.*
  -----------------------------------------------------------------------

**3.2.2 Silicon-coated SiC Asphere Sample Results**

The two asphere witness samples represent optics similar to the ALI
telescope M2. Both of these samples have RB SiC substrates with a
sputtered amorphous Si cladding approximately 0.006" thick. The Si-clad
substrate is then diamond turned and finished by hand. These samples, as
with the RB SiC flats, had surface roughness (2 --3 locations on each
mirror) and BRDF measurements (three locations on each mirror) made
before and after a Denton FSS99 silver coating. The surface roughness
and BRDF measurements were made at GSFC and Schmitt, respectively. As
was the case with the RB SiC flats, the BRDF signature scan indicated
that small angle data was not valid due to unacceptable SNR. In the case
of the aspheres only data at angles larger than 0.5 degrees will be
reported.

Table 3-5 presents the surface figure and surface roughness results. A
slight increase in surface roughness was observed for both samples (6%
for the convex hyperboloid and 13% for the concave ellipsoid). A small
increase is also seen in the BRDF measurements after the Denton coating
(see figures SD and DF). The BRDF data presented in figures 3-16 and
3-17 are the average of the all multple locations on the mirror. One set
of data was dropped from the concave ellipsoid because of the scatter in
the data and a significant difference from the other data taken on the
sample.

+-----------------------+-----------------------+-----------------------+
|                       |                       |                       |
+-----------------------+-----------------------+-----------------------+
|                       |                       |                       |
+-----------------------+-----------------------+-----------------------+
|                       |                       |                       |
+-----------------------+-----------------------+-----------------------+
|                       |                       |                       |
+-----------------------+-----------------------+-----------------------+
| Table 3-5. Surface    |                       |                       |
| figure and roughness  |                       |                       |
| of SiC apsheres       |                       |                       |
+-----------------------+-----------------------+-----------------------+
|                       | Convex Hyperboloid    | Concave Ellipsoid     |
+-----------------------+-----------------------+-----------------------+
| Surface Figure        | 0.0349 λ RMS @ 0.633  | \--                   |
|                       | μm                    |                       |
+-----------------------+-----------------------+-----------------------+
| Surface Roughness     | 27.0 Angstroms RMS    | 13.4 Angstroms RMS    |
|                       |                       |                       |
| (prior to Denton      |                       |                       |
| coating)              |                       |                       |
+-----------------------+-----------------------+-----------------------+
| Surface Roughness     | 28.6 Angstroms RMS    | 15.4 Angstroms RMS    |
|                       |                       |                       |
| (after Denton         |                       |                       |
| coating)              |                       |                       |
+-----------------------+-----------------------+-----------------------+

  -----------------------------------------------------------------------
  ![](media/image19.wmf){width="5.192361111111111in"
  height="3.308333333333333in"}

  *Figure 3-16. BRDF results for Si-clad RB SiC convex hyperboloid before
  and after Denton FSS99 sliver coating.*
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image20.wmf){width="5.072916666666667in"
  height="3.2354166666666666in"}

  *Figure 3-17. BRDF measurements of Si-clad RB SiC concave ellipsoid.
  Values are the average of three points on the mirror.*
  -----------------------------------------------------------------------

Figure 3-18 shows a comparison of the results from this program and the
ALI component specification and the results from the flight ALI M2. The
results indicate that the Si-clad RB SiC concave ellipsoid meets the ALI
specification and the flight M2.

  -----------------------------------------------------------------------
  ![](media/image21.wmf){width="4.75in" height="3.032638888888889in"}

  *Figure 3-18. BRDF results for aspheres compared with ALI M2 and
  component level specification.*
  -----------------------------------------------------------------------

**3.2.3 EO-1 M1 Demonstration**

The optimized process demonstrated on the SiC witness samples was
applied to a spare EO-1 M1 substrate (see figure 3-19 for photo of
finished M1). Two critical improvements have been made in the optical
fabrication process in comparison to the flight ALI M1. First,
modifications were made to the Si sputter coating process to produce a
more ductile Si layer. Improvements in the ductility of the Si-cladding
allowed better diamond turning results and eased the effort in obtaining
final figure and finish (the material was easier to work and took less
time to obtain the requirements). The second critical process
improvement was the addition of a superfinishing step to the polishing
schedule. Scatterometer measurements made prior to the superfinishing
step were 30 -- 40 angstroms, After the superfinishing step the values
were between 5 and 10 angstroms. These process changes produced an M1
with a 12x improvement in comparison to the flight ALI M1 and produced
BRDF results in-line with the derived component level specification.

Figure 3-20 shows the interferogram of the spare M1. The pk-valley is
0.294λ and RMS is 0.035λ.

BRDF measurements were made in SSGs stray light lab. It was anticipated
that Schmitt would do these measurements, but a facility shutdown
prohibited us from following this path. Measurements where made at three
locations on the spare EO-1 M1 at 632.8 nm. The spot size on the mirror
was 0.25" in diameter. Figure 3-21 shows the results along with the ALI
M1 flight optics and the component BRDF specification. The spare EO-1 M1
did not have the Denton FSS99 applied. Therefore, the results presented
in the figure have been scaled to account for the increase in BRDF from
coating being applied. The scaling factor was derived from the aspheric
samples from Task 1 in which before and after Denton coating BRDF
measurements were taken.

Comparison with the ALI flight M1 shows a 12x improvement and
demonstrates BRDF levels, which meet the ALI specification. In addition,
by obtaining a figure within our original goal of λ/2 to λ/5, it has
been demonstrated that both figure and finish to visible quality can be
obtained in SiC optics.

  -----------------------------------------------------------------------
  ![](media/image22.wmf){width="3.1243055555555554in"
  height="2.7305555555555556in"}

  *Figure 3-19. Photograph of Spare EO-1 M1.*
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image23.png){width="4.125in" height="2.2916666666666665in"}

  *Figure 3-20. Interferogram of EO-1 Technology M1. Final figure 0.294λ
  pk-valley, 0.035 λ RMS.*
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image24.wmf){width="5.995833333333334in"
  height="4.002083333333333in"}

  *Figure 3-21. BRDF results for EO-1 M1, ABg model of ALI flight M1and
  the derived ALI component level specificaton.*
  -----------------------------------------------------------------------

####### 4.0 Summary

ALI SiC flight instrument demonstrates excellent image quality, MTF, and
distortion performance over a wide field of view. SSG's continuing SiC
materials development allows new SiC materials to be applied to similar
missions with significant cost savings, weight reduction and improved
material properties.

The 12x improvement in BRDF achieved on the spare EO-1 M1 is the result
of two process changes, the use of a ductile Si-cladding and the
addition of a superfinishing step in the polishing process.

EO-1 Technology Program has demonstrated RB SiC flats and silicon clad
RB SiC aspheres that meet or exceed BRDF-stray light requirements
specified in EO-1 ALI. Spare ALI primary mirror was processed and
demonstrated excellent optical figure and BRDF needed to meet all ALI
requirements.

## 

## REFERENCES

1.  Wang, Xindong, Godden, Gerald and Qiu, Shi-Yue, "The EO-1 Advanced
    Land Imager (ALI) Scattered Light Study Report." Oct. 12, 1998.

2.  MIT Lincoln Laboratory and Lambda Reseach Corp., "EO-1 Stray Light
    Analysis Report No. 3." May 4, 1998.