Primary Mirror Successfully Aluminized

January, 28, 2004

The freshly Coated SOAR primary mirror seen inside the coating Chamber at Gemini South

The SOAR primary mirror was successfully aluminized in the Gemini South coating plant on January 28, 2004. Following months or preparation, the coating process itself took only 43 minutes. Subsequent measurements confirm that a good quality coating was achieved with reflectivity at 470nm in excess of 91% over the majority of the clear aperture.

The SOAR Primary mirror, 4.3m (14 feet) in diameter, but only 10cm (4 inches) thin, weighs 3,200kg (7000 lbs) and is made of ULE a special, low expansion, glass produced by Corning Inc. at their Canton NY. Plant. The mirror was polished to a perfect aspheric shape to within 17 nanometers RMS. (0.00000067 inches RMS), by the Electro Optical Systems Division of the Goodrich Corporation in Danbury Connecticut. After five years of development the mirror finally delivered to Cerro Pachón on January, 9 2004.

However, as delivered the polished glass is not very shiny, reflecting only about 3% of the light which falls on it. To make it reflective a very thin metallic film must be deposited on the bare glass. Aluminum was selected for the coating of the SOAR mirror because it gives good reflectivity over the entire optical and near IR wavelength region. The thickness of this film must be precisely controlled (it needs to be about 90nm or 0.000003 inch thick), and must be uniform over the entire surface of the mirror so as not to degrade its very precise figure. It has been said that an astronomical telescope with its sophisticated mount and optics (costing US$ 30M and weiging 100 Tons in the case of SOAR) is no more than a means to hold this thin film of aluminium (worth a few cents and weighing less than 5g) exactly where it needs to be.

This process was carried out in the coating facility at Gemini South. The mirror was first very carefully washed since even the tiniest traces of grease or dust on the glass result in a coating with poor adhesion. Then on the mirror was installed on a giant turntable enclosed within a vacuum vessel. All the air was pumped out of the chamber over night to produce a near perfect vacuum. Then a device, known as a magnetron, was used to create an electric discharge which stripped Aluminum atoms off a target, "sputtering" them onto the surface of the glass at a carefully controlled rate. In this way the magnetron deposits aluminum along a narrow radial strip across the mirror, which is slowly rotated on the turntable, until the whole surface is coated. This whole coating process took only 43 minutes. Measurements taken immediately on opening the chamber confirmed that an excellent had been achieved, with a reflectivity in excess of 91% at 470nm, close to the theoretical maximum for freshly deposited aluminum, over the entire clear aperture.

Why was this done at Gemini South? The coating chamber is a big ticket item in the budget of any new telescope, and is seldom used, since telescope mirrors typically only need to be coated every one to two years. Fortunately Gemini South already had a state-of-the-art coating facility, and where willing to share. SOAR was thus able to economize in this area. It was, however.,necessary to make some special fixtures to allow the SOAR mirror to be properly positioned in Gemini's chamber designed to hold their much larger 8.1m diameter mirror, and careful checks had to be performed to ensure that the magnetron would not collide with the fragile mirror when used in this very different configuration.

The success of this operation required the efforts of many talented people: Eduardo Serrano and Rosano Rivera for the design of the fixtures to hold the SOAR mirror in the Gemini chamber which were fabricated by local contractor IMMAR; the AOSS carpentry and maintenance shops for the dummy mirrors; the combined SOAR/CTIO mirror wrangling and washing crew - Eduardo Aguirre, Jorge Briones, Gerado Gomez, Eduardo Huanchicay, Wilson Muñoz , Esteban Parkes, Oscar Saa, Eduardo Serrano and Roberto Tighe; George Cleveland of Goodrich for on-site support and training; our friends at Gemini-South: including Maxime Boccas, Christian Aguilar, Claudio Araya, Pablo Diaz, Carlos Figueroa, Diego Maltes, and Tomislav Vucino; and last but not least Victor Krabbendam who lifts US$7m worth of glass without batting an eyelid.

More Images of the Coating Process

click on the images below to see a larger version

	This special hardware was required to mount the SOAR mirror inside the Gemini South Coating chamber
	A wooden dummy mirror fabricated in the AOSS carpentry shop was needed to ensure the mirror would fit, without colliding with the magnetron Eduardo Serrano seems pleased that it fits
	Another dummy mirror, this one a test mass, simulating the weight and moment of inertia of the real glass was used to tune the servo control mechanism of the turntable
	The SOAR Primary mirror is meticulously cleaned prior to installation in the coating chamber
	The partially coated mirror reflects the purple discharge from the magnetron. (photo credit : Maxime Boccas, Gemini)
	The freshly coated Primary Mirror inside the Gemini South Coating chamber (photo credit : Maxime Boccas, Gemini)