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Mirror Segment Washing Study
November 22, 2002

1.0 Introduction

The goal of this study was to compare the effectiveness of CO2 snow cleaning and water washing on a coated mirror segment. Since it would be very difficult to control the water if a mirror in the primary array was water washed, a spare HET mirror segment was used for the study. Scatterometer data was taken before cleaning, after CO2 snow cleaning and finally after washing with soap and water.


2. Procedure

All measurements were taken with the µScan Scatterometer. The unit measures specular reflection using a 670 nm light source and a detector with an angle of incidence and reflection of 25 degrees. In addition to specular reflection, the scatterometer measures scattered light with two detectors located 25 degrees on either side of the specular reflection (i.e. "reflection angle" of 0.0 and 50 degrees). The unit gives the Bidirectional Scatter Distribution Function (BSDF) for each detector, and calculates the RMS surface roughness based on the BSDF data and specular reflection.

The scatterometer reflection measurements are calibrated against two reference mirrors of known reflection efficiency. One is an Edmund Scientific enhanced aluminum mirror (87.4% @ 670 nm), and the other is an FSS99 coating provided by Denton Vacuum (98.6% @ 670 nm). The absolute reflectivity of each mirror was measured in June 1999 by Optical Data Associates. The reported reflectivity is scaled by the results of this calibration. The BSDF and surface roughness values reported in this document have not been scaled.

The spare mirror segment used in this study had been stored in a shipping crate in the mezzanine of the HET receiving area for some time. Prior to performing any cleaning operations, the segment was measured at twenty points with the µScan Scatterometer. The approximate locations for the measurements were twelve locations around the perimeter, six locations in the interior and two locations near the center.

After the segment was measured in the “pre-cleaned” condition, it was CO2 snow cleaned in the same manner as the mirrors in the primary array are cleaned. This consisted of making roughly twenty passes of the wand in and up / down motion across the mirror, and twenty passes in a left / right motion. After the CO2 snow cleaning was completed, twenty scatterometer data points were taken.

Finally, the segment was cleaned with soap and water. The mirror was first sprayed with a generous amount of distilled water. Next, a solution of distilled water and Orvis soap was sprayed on the mirror. Two mechanical cleaning techniques were used on the soapy mirror. Roughly one-half of the mirror was cleaned by gently “dragging” a soft cotton cloth over the surface, and the other half was cleaned by “patting” the surface with the same cloth (patting minimizes the mechanical action between the cloth and the mirror). Next, the segment was rinsed with distilled water, blown with clean, ionized air, and patted dry. Ten measurements were taken on the “patted side” and another ten were taken on the “dragged side.”


3.0 Results

The condition of the segment coating was consistent with the mirrors in the array in that the coating had visibly degraded. The test segment did vary from the array segments in that it did not have a significant layer of dust prior to cleaning.

Table 1: Mirror Segment Washing Data

Statistic

Reflectivity
Surface Roughness
(Ǻ)
BSDF
(0 Degrees)
(sr-1)
BSDF
(50 degrees)
(sr-1)
Pre-Clean
Average
Std. Dev.
Max.
Min.
0.920
0.044
0.953
0.768
81.7
22.0
152.7
59.2
1.37E-2
7.4E-3
3.88E-2
6.66E-3
6.03E-3
3.33E-3
1.65E-2
2.07E-3
Post-CO2Clean
Average
Std. Dev.
Max.
Min.
0.964
0.012
0.977
0.927
51.3
12.3
81.2
36.9
5.02E-3
2.04E-3
9.44E-3
2.58E-3
1.48E-3
6.1E-4
2.90E-3
7.3E-4
Post Water Wash

“Patted”
Average
Std. Dev.
Max.
Min.

“Dragged”
Average
Std. Dev.
Max.
Min.


0.933
0.058
0.969
0.806


0.913
0.088
0.972
0.753


67.0
35.9
133.1
31.0


76.0
45.3
178.9
29.3


9.22E-3
8.68E-3
2.45E-2
1.94E-3


1.14E-2
1.29E-2
4.50E-2
1.8E-3


2.65E-3
2.35E-3
8.12E-3
8.2E-4


3.10E-3
3.38E-3
1.19E-2
8.4E-4


The results of the scatterometer measurements are included in Table 1. There is not a significant difference between any of the conditions (pre-cleaned, post CO2 and post water wash) for the measured reflectivity. The surface roughness and BDSF values showed a slight improvement for the cleaned conditions (as compared to the pre-cleaned condition), but there was not a significant difference between the cleaning methods.

Images of the mirror segment in each of the three aforementioned conditions are included in Images 1-3. The blue reflection seen in the mirrors is the receiving bay door.


Image 1. Mirror in pre-cleaned condition

mirror -pre clean


Image 2. Mirror after CO2 snow cleaning

mirror - after CO2 clean

Image 3. Mirror after water wash

mirror -after water wash


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