HET Mirror Coating Durability Study

Data Archive

HET Mirror Coating Durability Study

Prepared by: Richard Schuler

October 7, 2002

1. Introduction

The following is a plan for monitoring the durability of six mirror coatings over a time period of one year. The manufacturers and coating types for the study are listed in Table 1. The procedure will expose the coatings to the environment in the HET and 107” domes, and monitor the condition of the coatings over time.

The coatings in the HET dome will be exposed to two environmental variables and two cleaning variables. In addition to the HET dome, one set of samples will be placed in the 107” dome, and control samples will be placed in a dessicator box for the duration of the study. Data collected will include: descriptions of coating condition, photographic documentation and reflectometer measurements.

The Denton and Doss coatings have been stored in their original protective wrapping for roughly 1.5 years. Visual inspection of selected samples indicates that they are still in very good condition. There is no evidence of the coating degradation seen on the HET primary mirror.

The HET samples will be secured to four metal trays which are attached to the interior side of the cat walk railing. Three sample trays will be uncovered and contain one sample set each (See Table 2). The fourth sample set will be placed in a covered tray. A fifth sample set will be uncovered and secured to a handrail in the 107” dome.

Please try to avoid disturbing the samples during the test. Be especially careful not to touch the samples as skin oils will degrade the coatings and seriously compromise the test.

Table 1. Manufacturers and coating types used in the study

Manufacturer	Coating	Approximate Coating Date
Denton	FSS99-UV (HET Primary) FSS99-500 (HET SAC) FSS99xSiO₂ X1-Silver	(03/01)
Dave Doss	Aluminum (bare)	(04/01)
DSI	Protected Silver	(07/02)

1.1 Environmental Variables

The primary environmental conditions in the HET dome include those present in West Texas (dust, humidity, bugs etc.), and gasses generated during use of the JLG Manlift. In order to quantify the effect of the particulates and gasses in the HET dome, two exposure conditions will be evaluated. The sample sets in the three uncovered trays will be completely open to the environment, and therefore exposed to both the particulates and gasses present in the dome. The remaining tray will be covered on the top and sides, with holes drilled in the bottom, thus this sample set will only be exposed to the gaseous environment of the dome.

In addition to placing samples in the HET dome, one tray will be placed in the 107” dome. These samples will be uncovered, and determine if the coating degradation is specific to the HET environment.

1.2 Cleaning Variables

The effects of particulates sitting on a coated surface in the HET dome will be evaluated with two cleaning variables: monthly and quarterly cleaning with the CO₂ snow cleaning process. The samples in two of the three uncovered trays in the HET dome will be cleaned monthly, and the samples in the third uncovered tray will be cleaned quarterly. The samples in the covered tray (HET dome) will be cleaned only if particulates are observed on the surface. The samples in the 107” dome will be cleaned monthly.

Note: The covered samples near the JLG (HET dome) and in the 107” dome in Table 2 do not include a DSI sample (protected silver) because only four coating samples were provided by the company.

1.3 Data

Prior to deployment in the domes or dessicator, all samples will be: examined visually, photographed, and tested for reflectivity. The samples in the 107” dome and selected samples throughout the HET dome will be CO₂ snow cleaned, visually examined and photographed monthly. All samples in the HET dome will be visually examined, photographed and reflectivity tested quarterly after the CO₂ snow cleaning. The reflectivity testing data will be gathered using the TMA μScan™ scatterometer which uses a 670 nm laser diode light source.

2. Procedure

Install the fixtures for the four sample trays on the HET dome catwalk. Locate two trays (one covered and one uncovered) above the JLG and two (uncovered) trays roughly across the dome. Both locations should be accessible to the CO₂ snow wand and out of the direct stream of the dome A/C ventilators.

Install the fixture for one uncovered sample tray on the handrail of the 107”. The tray should be accessible to the CO₂ snow wand and out of the direct stream of the dome A/C ventilators.

Label the back side of each sample. Visually inspect and measure the reflection efficiency of each sample.

Install samples on the trays using polyamide tape. Label trays adjacent to each sample.

Photograph each tray and the control samples.

Install trays on fixtures in domes. Place control samples in dessicator box.

Inspect samples initially once a week to look for onset of surface damage. Record observations.

After one month, clean (CO₂ snow) the samples on the appropriate two uncovered trays in the HET dome and the tray in the 107” dome. Visually inspect each sample, and photograph all four trays.

After two months, clean (CO₂ snow) the samples in the appropriate two uncovered trays in the HET dome and the tray in the 107” dome. Visually inspect each sample and photograph all four trays.

After three months, clean (CO₂ snow) the samples in all four uncovered trays and clean samples in the covered tray if required. Visually inspect each sample. Photograph each tray and the control samples. Take reflectometer measurements of each sample (including control samples).

Repeat steps 8 through 10 for the remaining three quarters of the year.

Submit a brief report each quarter and a final report at the completion of the study.

Table 2. Mirror coating durability study test matrix

Coating Durability Study Matrix
Tray No.	Sample No.	Sample Type	Location	Exposure Type	Maintenance
1	1 2 3 4 5 6	FSS99-UV FSS99-500 FSS99xSiO₂ X1-Silver Aluminum DSI Protected Silver	HET near JLG	Gasses/ Particulates	Monthly Cleaning (CO₂ Snow)
2	7 8 9 10 11	FSS99-UV FSS99-500 FSS99xSiO₂ X1-Silver Aluminum	HET near JLG	Gasses only (covered)	Quarterly Cleaning (if required) (CO₂ Snow)
3	12 13 14 15 16 17	FSS99-UV FSS99-500 FSS99xSiO₂ X1-Silver Aluminum DSI Protected Silver	HET opposite JLG	Gasses/ Particulates	Monthly Cleaning (CO₂ Snow)
4	18 19 20 21 22 23	FSS99-UV FSS99-500 FSS99xSiO₂ X1-Silver Aluminum DSI Protected Silver	HET opposite JLG	Gasses/ Particulates	Quarterly Cleaning (CO₂ Snow)
5	24 25 26 27 28	FSS99-UV FSS99-500 FSS99xSiO₂ X1-Silver Aluminum	107"	Gasses/ Particulates	Monthly Cleaning (CO₂ Snow)
N/A	29 30 31 32 33 34	FSS99-UV FSS99-500 FSS99xSiO₂ X1-Silver Aluminum DSI Protected Silver	Dessicator Box	Control	None

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Last updated: 01-Oct-2003