Optical innovations: high reflectance mirrors for the James Webb space telescope

When Galileo pointed his telescope to the sky over 400 years ago, revolutionary scientific discoveries has been made and our look to the natural world has been forever changed. Telescopes have since then replaced the naked eye for observing and discovering the universe. In the following centuries, more powerful and complex telescopes have been introduced. In the early 20s, astronomer Edwin Hubble used the largest telescope of his day to observe galaxies beyond our own at the Mt. Wilson Observatory near Pasadena in California. In April 1990, a telescope of his name was launched from Kennedy Space Center in Florida. The Hubble telescope was the first telescope to be launched in space and allowed since its servicing in 1990 to perform 1.3 million observations and provide valuable data for more than 15 000 scientific papers.

 

hst-sm4
Figure 1: photograph of Hubble Space Telescope taken on the fourth servicing mission to the observatory in 2009, Credits to NASA

 

The Hubble telescope will be soon (Spring 2019) replaced by The James Webb Space Telescope (JWST). NASA Goddard space flight Center, the headquarters of this telescope, has built along the way collaboration with the European and Canadian Space agencies, five more aerospace companies and uses test facilities from several NASA agencies in order to bring this telescope to the world.

 

Originally known as the Next Generation Space Telescope, the biggest asset of the James Webb over the Hubble is its ability to operate in the infrared range only, allowing catching photons from the earliest days of the universe (from 13.5 billion years ago).  Figure 1 helps seeing the design and the structure of the James web telescope: it is a three-mirror anastigmatic telescope. Very close to the Hubble, a Cassegrain telescope, the James Webb telescope is composed of a primary mirror with an opening in its middle which gathers the light and bounces it off a secondary mirror in front of it. The secondary mirror will focus the light into the aft optical subsystem, which contains the tertiary mirror and a fine-steering mirror that helps to stabilize the image.

 

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Figure 2: JWST’s subsystems, credit to STSci (NASA, Goddard Space flight Center)

 

The honeycomb primary mirror has a diameter of 6.5m (the width of a tennis court) allowing a light collecting area of 25m2, 6 times greater than the Hubble’s. The primary mirror is composed of 18 hexagonal segments. Each segment is around 1.3m wide. The mirror substrate is made out of Beryllium instead of glass in order to reduce the weight of the mirror. Beryllium (Be) is a light weight material with a low thermal expansion coefficient allowing the mirror to hold its shape in cryogenic temperatures. Beryllium is mined in Utah and purified by Brush Wellman in Ohio. It starts as a fine powder pressed to a flat shape. The block is then cut to blanks put together to form a segment and sent to Axsys Technologies. This company provides the final shape of the Beryllium substrate by cutting away most of the back side and leaving a thin rib structure (Figure 3b). Each rib is 1mm thick. This process allows reducing the substrate weight down to 20kg.

 

After the mirror has been shaped, the front surface is perfectly polished and smoothed out. A cryogenic testing is performed by Ball Aerospace and NASA. The substrates are cooled down to 30K in order to ensure that the material will hold its shape in the space. Corrections are made to the mirrors shape. The mirrors are then sent for gold coating. This coating is performed by Quantum Coatings Inc. in Moorestown.  A 100nm thick gold coating is evaporated on the surface of the substrates with 10nm uniformity over 1.5m wide substrate. The gold reflectance is estimated to 99% over a range from 0.8 to 26μm. The gold coating is very pure and soft. In order to protect the surface from scratches and contaminants, a SiO2 coating is applied. Several tests are performed to check the stress, the reflectance and the roughness of the gold coatings. The mirrors undergo another series of cryogenic testings at NASA for holding it shape.

 

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Figure 3:  a. top photograph: a segment of the primary mirror being controlled by NASA Engineers, b: bottom photograph: back of Beryllium substrate.

 

This article gives an overview of all the process steps, challenges and qualifications for large area mirrors destined to space applications. The James Webb telescope has however more technical challenges related to its numerous components such as NIRSpec, the near infrared camera spectrograph and the Mid Infrared instrument (MIRI).

The James Webb telescope will be launched in Spring 2019 on an Arianne A5 vehicle in French Guiana.

 

By Najwa Sidqi, early stage research at Helia Photonics, Edinburgh.

 

References:

[1] James Webb Space Telescope, Goddard Space Flight Center:

https://jwst.nasa.gov/whatsNext.html

 

[2] NASA: Hubble Space telescope

https://www.nasa.gov/mission_pages/hubble/story/index.html

 

[3] Gold Mirror Coatings for James Webb Space Telescope (JWST)

http://www.quantumcoating.com/projects/

 

[4] James Webb Space Telescope Successor TO Hubble

http://www.ball.com/aerospace/programs/jwst

 

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