Characterization of Silver and Aluminum custom mirror coatings for the MRO interferometric telescopes

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1 Characterization of Silver and Aluminum custom mirror coatings for the MRO interferometric telescopes Krista M. McCord a, Dan A. Klinglesmith a, Colby A. Jurgenson a, Eric J. Bakker a Reed A. Schmell b, Rodney A. Schmell b, Darren Gartner b, Anthony Jaramillo c, Kelly Romero b, Andres Rael b, Jeff Lewis b a New Mexico Tech/ Magdalena Ridge Observatory, 101 East Road, Socorro, NM, b Optical Surface Technologies, LLC, 2801 Broadbent Pkwy, Ste B, Albuquerque, NM, c W.M. Keck Observatory, Mamalahoa Highway, Kamuela, HI ABSTRACT We report on the design, application, and testing of custom protected silver and aluminum coatings for use on the Magdalena Ridge Observatory Interferometers (MROI) unit telescopes. The coatings were designed by Optical Surface Technologies (OST), and tested under normal observational conditions on Magdalena Ridge. Mirror coating samples fabricated by OST were given to MRO, and then placed in an insulated automated enclosure at the observatory site. Within the enclosure, environmental conditions such as temperature and humidity were continuously monitored. The automated enclosure was instructed to open during the night dependent upon weather conditions matching those that would occur under normal operations of the interferometer. This paper tracks the affect of the Magdalena Ridge environment on the performance of the coatings, specifically with regards to reflectivity. Keywords: telescopes, mirror coatings, interferometry, reflectivity tests, astronomy 1. INTRODUCTION The Magdalena Ridge Observatory Interferometer 1 will be a ten telescope optical interferometric array. It will be located on the top of Magdalena Ridge at an elevation of 3,185 meters and will operate in different photometric band passes in a wavelength range from 0.6 μm to 2.4 μm. The 1.4-meter unit telescopes will be arranged in a Y shape configuration. Advanced Mechanical and Optical Systems (AMOS) in Belgium is designing and constructing the telescopes. Each unit telescope will contain a primary, secondary, and tertiary mirror to direct the incoming light into the beam relay pipes. The primary mirror will have an aluminum coating and the secondary and tertiary mirrors will be coated with silver. The telescopes will be individually housed in a transportable enclosure. Fig. 1. The image above is an actual aerial photo of the Magdalena Ridge with an overlaid CAD drawing of the unit telescope pads and enclosures done by the architect M3.

2 Fig. 2. The above figure shows the design of the unit telescopes and indicates where the primary, secondary and tertiary mirrors will be placed. Courtesy of AMOS. Optical Surface Technologies (OST) will design and fabricate the mirrors and coatings for each unit telescope. OST has provided MROI with a number of different silver and aluminum coated mirror samples. These samples were exposed to the environment on Magdalena Ridge to determine how the weather would affect their reflectivity. An automated enclosure 2 was built and programmed to withstand the environment on Magdalena Ridge to test these coating samples. The enclosure was placed near the array arms and the coating samples were exposed to the environment at night during normal operating weather conditions. The purpose of this experiment was to see how the weather would affect the coatings. This would show the durability of the coatings and the differences in the change of reflectivity. This paper will describe in detail the experimental setup, coatings that were tested, and results. 2. MIRROR COATING REQUIREMENTS There are 4 major considerations for the quality of the unit telescope coatings: reflectivity, polarization, scattering and durability. The interferometer has 12 mirrors between the sky and the science instruments. The first 3 mirrors (primary, secondary and tertiary) are exposed to the outside environment. The remaining mirrors and other optical components are either in vacuum tubes or within a thermally controlled environment. Therefore the highest possible reflectivity is required for each mirror surface. In order to obtain fringes we need to maintain the polarization aspect for each mirror in the chain. The facility is located at high elevation where recoating of a large optical surface is difficult.

3 As a result of these considerations the MROI project has specified that our primary mirror will be coated with aluminum and have a SiO 2 overcoat for durability. The secondary and tertiary mirrors will be coated with silver and have an overcoat that will minimize the polarization effects. The reflectivity requirements for the silver and aluminum coatings are listed below. Silver Coatings Aluminum coatings nm 92% nm 85% nm 97% nm 85% nm 95% Also the mirrors must be able to undergo frequent cleaning with a soap and water solution and/or with CO 2 snow. coatings must maintain their reflectivity for at least 12 months. The 3. RATIONALE FOR COATING CHOICES Since we are looking for high reflectivity and long durability, we have chosen aluminum 2 with a protective overcoat of SiO 2 for our primary mirror. In this study we were looking at the effects of the thickness of the SiO 2 overcoat for reflectivity and durability. The higher durability of the protected aluminum coating was desired for the large primary mirror because recoating would require removing the mirror from the telescope and transporting it off our high mountain observatory. CO 2 snow cleaning will be used to remove dust. For the smaller secondary and tertiary mirrors the most important aspect is that of reflectivity, as long as a reasonable durability can be maintained. Hence we chose silver as the reflecting surface along with a SiO 2 overcoat for durability. These smaller mirrors can reasonably be cleaned with CO 2 snow in place without danger to the primary mirror. Table 1 details the parameters of the different batches that were decided upon for our initial testing. Table 1: Batch parameters for sample witness mirrors Layer 1 Layer 2 Layer 3 coating run samples not standard cleaned T ( ⁰C) depth (nm) base depth (nm) over coat depth (nm) over coat P011209c1 E42 E Ag 31 SiO 2 P012109c1 A44 G Ag 181 SiO HfO 2 D030907c1 B46 D Al 30 SiO 2 D030409c1 A Al 25 SiO 2 D032309c1 323A 323B Al 25 SiO 2 The table includes the batch number which includes the date (mmddyy) that the batch was produced. For each batch samples were created. One, standard, was kept in a desiccator at the MROI office and the other, not cleaned, was exposed to environment on the Ridge as described below. It was not cleaned over the period of this experiment. The

4 substrate temperature and the deposition depth for each layer are also given. For the silver batches there was an undercoat of chromium used to allow the silver to adhere to the mirror substrate. The aluminum was applied using a magnetron sputtering technique. The silver was applied to the substrate by a resistance evaporation technique. The SiO 2 overcoats were applied by magnetron sputtering. 4. EXPERIMENTAL SETUP 4.1 Hardware The test enclosure is a modified metal trash can 3 containing a high torque motor that lifts the lid of the trash can to expose the witness mirrors. The high torque motor was purchased from LinEngineering along with a driver so that the motor could be computer controlled. A spool was attached to the shaft of the high torque motor to pull a wire that is connected to the lid. A pulley system is attached to the back side of the trash can to increase the torque when opening the lid. To reduce condensation inside the test enclosure, foam insulation was installed on the inside walls of the trash can. The actual unit telescope enclosures will have humidity and temperature control. To simulate this with the test enclosure, desiccant was placed inside the test enclosure to control the humidity. Lights were installed around the sides of the enclosure to prevent ice from forming around the lid and stop the motor from opening the test enclosure. Fig. 3. The image above shows the automated enclosure with the mirror coating samples inside. The samples are sitting on foam padding near the opening of the enclosure. 4.2 Software A LabView program was written to control the automated enclosure. The requirements for the program were: The enclosure must open at night during specific weather conditions and be closed during the day. The program must read a weather data file from a nearby weather station.

5 The program must be able to close the enclosure immediately if the weather changes suddenly or if the weather station goes down. The program must know if the enclosure is open or closed. The program first compares the computer time with a specified opening time interval. If the computer time does not fall within this interval the enclosure will automatically close if it is open or will remain closed. If the computer time is within this interval, the program then compares the computer time with the time collected from the weather station nearby. This is to verify that the weather station is up and running. In a situation where the weather station is inoperable, the enclosure will be told by the program to close or remain closed. If the weather station is in operation, the program will read the humidity and wind speed from a data file provided by the weather station. If the humidity from the weather file is less than or equal to 60% and/or the wind speed is less than or equal to 10 m/s, the enclosure will open. If these values are higher than 60% or 10 m/s the enclosure will immediately close. This program is based on loops therefore the program continuously checks and compares times and weather data. Fig. 4. The above flow chart describes the protocol that the program for the automated enclosure follows. The computer Loop executes every 3 minutes to make sure the weather has not drastically changed. 5. RESULTS In this section we present the results of a 6 month study of the reflectivity and durability of the witness mirror samples. The results for both the aluminum and silver coating are presented. 5.1 Silver coating results: Two different batches of silver coatings were tested. The batches each had two mirrors; where one was kept in a dessicator as a standard and the other was placed up on the Ridge. Once a month, the mirrors were taken off of the Ridge and reflectivity tests were made at OST using a Cary 5000 UV-NIR-infrared spectrophotometer at a 0⁰ incidence angle. These mirrors have undergone testing for a period of at least six months. Figure 5 shows a typical scan of a silver coating. The sample was scanned from 200nm 2400nm but only the region from 400nm 2400nm is shown as that is the region of interest for MROI. Three lines are shown, the curved line is the actual reflectivity measurements from the

6 Cary The step function line shows the MROI reflectivity requirements. The boxes at the bottom of the graph are the wavelength regions that are used to measure the average reflectivities that are used to determine if the sample meets the requirements. Fig. 5. A typical standard silver scan. Shown here is the actual scan, the MROI required reflectivity and the band passes used for measuring the degradation. The degradation results are shown in Figure 6. The plots on the left side of Figure 6 are for the standard samples and the plots of the right side are for the samples that have been exposed to Ridge environment. Fig. 6. Reflectivity as a function of time for the silver coatings. The horizontal lines at 97% and 92% are the MROI reflectivity requirements. The + symbol lines are for the short wavelength range ( nm) and the * are for the long wavelength range ( nm).

7 The standard samples have maintained more than enough reflectivity and have been stable for at least 6 months. The samples that have been exposed to the Ridge environment have lost reflectivity to the point that they are now below the MROI reflectivity requirements. 5.2 Aluminum coating results: Two aluminum coating batches were tested. For each batch one mirror was placed in a dessicator to be the standard and the other mirror was exposed to the Ridge environment. As with the silver batches, these mirrors had reflectivity tests done once a month at OST. A typical reflectivity scan is shown in figure 7. Fig. 7. A typical standard aluminum scan. The graph shows the actual reflectivity as measured from the Cary 5000, the MROI requirements and the band passes used for computing an average reflectivity. Fig. 8. Reflectivity as a function of time for the aluminum coatings. The horizontal lines at 95% and 85% are the MROI reflectivity requirements for the aluminum coatings. The + line is for the nm range, the * line is for the nm range and the simple line is for the nm range.

8 As shown in Figure 8, the aluminum standard samples are holding up well above the MROI reflectivity requirements over the 5 months that they have been tested. On the other hand the samples that have been exposed to the ridge environment and have not been cleaned are losing reflectivity. The visible and near infrared band passes are still above the MROI requirements while the infrared band have deteriorated below the MROI requirements. 5.3 Fitting degradation rates We have attempted to fit the silver and aluminum degradation rates. The silver degradation can be fit with a Gaussian function: R = R 0 * e -1/2(t/t 0 )2. (1) Whereas the aluminum degradation rates were better fit with simple exponential function: R = R 0 * e -t/t 0. (2) Where R 0 is the first measured reflectivity and t and t 0 are in days. The results for the fitting are shown in table 2. Table 2: Fitting coefficients for coating degradation coating Sample ID Wavelength nm R 0 % T 0 days function Silver E % 320 Gaussian Silver E Gaussian Silver G Gaussian Silver G Gaussian Aluminum D Exponential Aluminum D Exponential Aluminum D Exponential 6. CONCLUSION Further testing is needed to choose a proper coating for the unit telescope optics. The next step is to have OST make new mirror coating samples based on the coating properties above. With these samples two cleaning processes will be added. This cleaning will be done on a monthly basis. The first cleaning process will be that one of the mirrors on the ridge will be washed with Orvus. The second cleaning process will be to use CO 2 snow. As before there will be a standard sample left in the desiccators at the research office building and one sample that is exposed to the ridge observing conditions but not cleaned. The reflectivity of the mirrors will be tested for reflectivity each month and a prolifometer will be used to measure the surface profile of the mirror samples. Based on the results presented here we have decided that the next set of batches will constructed as shown in Table 3. OST will prepare 6 silver batches and 3 aluminum batches. The substrate will be 1.5-inch diameter Zerodur blanks polished to the same specifications as the telescope mirrors.

9 Table 3 Future batches Batch Layer 1 Layer 2 Layer 3 temperature 1 180nm Ag 200nm SiO 2 100⁰C 2 180nm Ag 180nm SiO NbO 5 100⁰C 3 180nm Ag 30nm SiO 2 100⁰C 4 180nm Ag 200nm SiO 2 75⁰C 5 180nm Ag 180nm SiO NbO 5 75⁰C 6 180nm Ag 30nm SiO 2 75⁰C 7 120nm Al 200nm SiO 2 50⁰C 8 120nm Al 55nm SiO 2 50⁰C 9 120nm Al 30nm SiO 2 50⁰C The coating methods will be the same as the previous batches and there will be 70nm layer of chromium under the silver layer. 7. ACKNOWLEDGEMENTS The Magdalena Ridge Observatory is funded by Agreement No. N C902 with the Naval Research Laboratory (NRL). MROI is hosted by the New Mexico Institute of Mining and Technology (NMT) at Socorro, NM, USA, in collaboration with the University of Cambridge (UK). Our collaborators at the University of Cambridge wish to also acknowledge their funding via STFC (formerly PPARC) in the UK. [1] 8. REFERENCES Creech-Eakman, M.J, Romero, V., Westpfahl D., Cormier, C., Haniff, C., Buscher D., Bakker E., Berger L., Block; E., Coleman T., Festler P., Jurgenson C., King R., Klinglesmith D., McCord K., Olivares A., Parameswariah C., Payne I., Paz T., Ryan E., Salcido C., Santoro F., Selina R., Shtromberg A., Steenson J., Baron F., Boysen R., Coyne J., Fisher M., Seneta E., Sun X., Thureau N., Wilson D., Young J., Magdalena Ridge Observatory interferometer: progress towards first light, Proc. SPIE 7013, (2008) [2] Willey, R. R., [Practical Design and Production of Optical Thin Films], Marcel Dekker, New York, 294- [3] 298 (2002) McCord, K. M., Jurgenson, C., Klinglesmith, D. A., Schmell, R., Jaramillo, A. Mirror Coating Verification Tests at the Magdalena Ridge Observatory Interferometer, BAAS 41, 432 (2008)