CFHT Information Bulletin, number 39, Second Semester 1998



A Conference on Mirror Coating

Barney Magrath, CFHT, magrath@cfht.hawaii.edu.

In November of 1997 CFHT hosted the Technical Conference "1997 Telescope Mirror Coating and Cleaning Conference". This two-day conference was attended by 59 people from around the world. The purpose of the event was to provide a forum where practioners of the "black art" of mirror coating and mirror maintenance could exhange knowledge and inform others of the latest developements in this field. The first day was scheduled for thirteen 20 minute presentations. The abstracts of these talks are presented here. The second day was a tour of five Obervatories on the Mauna Kea summit. Complete proceedings of the conference are available.

Coating and Cleaning at Keck

Mike DiVittorio

A review of the performance of the optical coatings and cleaning at Keck Observatory.

An update on the development and testing of low emissivity silver coatings for use in the Keck 2 telescope. This will include discussion of candidate protected silver coatings for use in the adaptive optics (1-5 um) and interferometry (10 um) systems.

Coating Procedure of Subaru Telescope Optics

Saeko S. Hayashi, Yukiko Kamata, Tomio Kanzawa, Akihiko Miyashit, Masao Nakagiri, Tetsuo Nishimura, Takeshi Noguchi, Kiichi Okita, Norio Oshima, Goro Sasaki, Yasuo Torii, Masami Yutani, Tsuyoshi Ishikawa

For coating large mirrors of Subaru Telescope, we employ conventional evaporation scheme because it is known for uniform coverage. We will report installation and the performance verification of the coating facility of Subaru telescope. The coating facility consists of a washing tower for stripping the old coating from the primary mirror, a large evaporation coating chamber, two trolleys for the primary mirror, and a scissors-like primary mirror lifter.

Tests with large coating chamber at Mauna Kea, as well as with smaller chamber at Mitaka, will be discussed. To supply a large number of filaments with uniform quality, our practical solution is to pre-wet the filaments and keep them in a controlled environment before the evaporation. In the initial test, aluminum film over the large area exceeded the number targeted for the thickness and yet the uniformity turned out to be better than the specification. Reflectivity of the fresh surface was over 90% at visible wavelength. In September 1997, we re-aluminized 1.6m infrared simulator at Mitaka for the first time using pre-wetted filaments. The result verified our coating procedures for the secondary mirror in late 1997 and the 8.3m primary mirror in early in 1998.

In-Situ Cleaning of the Primary Mirror of Subaru Telescope

Yasuo Torii, Saeko S. Hayashi, Masahiro Toda

Since Subaru Telescope adopted a flushing-type enclosure, we considered that fine cinders at the Mauna Kea summit will be a biggest contamination source for the telescope optics. Dry ice (carbon dioxide snow) in-situ cleaning system is one of the candidates for cleaning the large area, and we have conducted experiments to understand why the dry ice cleaning method is better than other possible methods such as dry air or nitrogen gas blowing technique. We used 3.5 inch test mirrors to apply various cleaning schemes and measured the resultant performance. The result clearly confirmed the better performance of the dry ice cleaning. We have further investigated the parameters of this method, such as the shape of the nozzle, distance between the mirror, blowing time interval, and direction with respect to the mirror. We will report the results and implication of these experiments that lead to the design concept of the dry ice in-situ cleaning system for Subaru Telescope.

Mirror Cleaning News at the CFHT

Barney Magrath

Mirror Cleaning and preventative maintenance of front surface reflectors at the CFHT emphasis in-situ reflectivity measurements and their associated reflectivity degradation rates. The process of measuring mirrors and interpreting the data will be explained. In-situ reflectivity data for two separate silver coatings will be presented in addition to a laser cleaning experiment performed by Radiance Services for CFHT. Lastly, in order to remind us that better astronomy is the goal, I will describe a simulation that predicts the effect of scattered light on the well-defined Point Spread Function (PSF).

Operational Strategy in the Research Of Cleaning Techniques for the VLT-Mirrors

Paul Giordano

An optical maintenance program was initiated in 1990 as part of the VLT project. Various analysis were performed to evaluate the new site quality. Measured data will be presented, and the efforts made at ESO since this date to define suitable on-line monitoring and preventive maintenance will be detailed. In-situ cleaning techniques, existing equipment and procedures will be reviewed. Emphasis will be put on the CO2 snow-flake cleaning technique and the experience acquired with the integrated cleaning device of the 3.5 m NTT telescope will be described. The plans for the in-situ cleaning of the VLT mirrors will be explained.

An Overview of the Maui Space Surveillance Site

Tom Glesne

We will discuss the new 3.7m Advanced Electro-Optical System (AEOS) Telescope situated atop Haleakala on the Island of Maui. Maintenance of optical surfaces, sensors and plans for visiting experiments will be discussed.

An Automatic Washing Machine to Remove Aluminum From Astronomical Mirrors

Valentina Zitelli

Cleaning of large astronomical mirrors, before aluminization, required in the past a large amount of manual operations on the surface. With very large mirrors, 8 meter or more, manual operations become time consuming, expensive and often dangerous, both for mirror surface and operators. A fully automated procedure is thus mandatory when handling large mirrors. To this aim we experimented on a small scale (60 cm) an automatic procedure, free from any manual contact with the mirror, capable of removing old aluminum and leaving a clean, wet surface ready for a successful new aluminization. First we manually treated small borosilicate mirrors, obtained from the LBT primary mirror glass batch, with different sequences of chemicals, commonly used to this purpose. These small mirrors were checked with a Wyko interferometer before and after treating, to trace change in roughness of the surface. Quality and stability of the new aluminum deposition after cleaning was also checked.

The washing machine prototype is composed by a water proof box on rigid PVC with a moving arm, a pump and a series of tanks containing the used chemicals. All the adopted components can be used with acids and other corrosive fluids. The machine is designed to hold the mirror in vertical position. An arm with 10 cm spaced nozzles moves up and down in front of the mirror spraying the adopted chemicals in a defined sequence. A pump forces the liquid through the circuit. After the washing, the mirror is left, protected from the dust in the washing machine, for about 2 hours to drip the water, then is moved into the vacuum pump to check the final result of aluminizing the cleaned surface. A homogeneous layer of aluminum follows only after a careful cleaning, otherwise a fast oxidize process, or a inhomogeneous aluminum coating appear. Scaling times, fluxes and costs from this experiment to large size mirrors leads to realistic, affordable figures.

Gran Telescopio Canarias-AO: Current Status of its Mirror Coating and Cleaning Program

Lotti Jochum

A conceptual design of the GTC optical system has been completed in summer 1997. It will be a Ritchey-Chrtien type telescope with a flat tertiary mirror to feed Nasmyth and folded Cassegrain focal stations. The telescope will have a segmented primary mirror and an entrance pupil area equivalent to an aperture of 10m in diameter. The development of the baseline of the mirror coating and cleaning program has mainly been driven by the scientific requirements for reflectivity, IR emissivity and surface degradation rates. The first light configuration will be an aluminum coated primary and tertiary mirror and a silver coated secondary mirror. With a segmented primary mirror, 100% of the mirror surface will be available 100% of the time, there will be no down time for coating renewal, but there will never be a 100% freshly coated primary mirror. Therefore, an efficient in situ mirror cleaning method is especially important. Studies about which cleaning technique might be best to meet the GTC requirements are still in progress.

Stratospheric Observatory for Infrared Astronomy (SOPHIA) Mirror Coating Facility

Ed Austin

The joint US and German project, Stratospheric Observatory for Infrared Astronomy (SOFIA), to develop and operate a 2.5 meter infrared airborne telescope in a Boeing 747-SP began late last year. Universities Space Research Association (USRA), teamed with Raytheon E-Systems and United Airlines, was selected by NASA to develop and operate SOPHIA. The 2.5 meter telescope will be designed and built by a consortium of German companies. The observatory is expected to operate for over 29 years with the first science flights beginning in 2001.

The SOPHIA Observatory will fly at and above 12.5 km, where the telescope will collect radiation in the wavelength range from 0.3 micrometers to a 1.6 millimeters. Universities Space Research Association (USRA) with support from NASA is currently evaluating methods of recoating the primary mirror in preparation for procurement of mirror coating equipment. The decision analysis technique, decision criteria and telescope specifications will be discussed.

Pupil Imaging in the Infrared

Doug Toomey

Pupil imaging in the infrared has proven a powerful technique at the IRTF for monitoring the telescope emissivity and understanding the telescope background. What a pupil image is, how the data is taken, and how it is reduced will be discussed. The recent results of the long term mirror degradation tests will be presented. Results include bare silver mirrors that have been exposed for 5-years.

Residual Gas Analysis: A Technology at a Crossroads

Laura Peters

Residual gas analyzers (RGAs) can be thought of as an answer waiting for the right question. Today they are used most often in three applications: as leak detectors, as analyzers of contamination in sputtering and ion implantation processes, and as troubleshooting tools in vacuum-based systems.

Development of silver coating options for the Gemini 8-M Telescopes Project

Michael Jacobson

Acting as a prime contractor to the Gemini Project (GP), the Optical Data Associates (ODA), with its major subcontractors, BOC Coating Technologies (BOCCT) and Deposition Sciences, Inc. (DSI), developed options for depositing protected silver coatings on the8-M primary mirrors.





Editor: Dr. Timothy M. C. Abbott, tmca@cfht.hawaii.edu
Copyright © 1998, CFHT Corporation. All rights reserved.