Nancy Roman Telescope’s primary 2.4-meter mirror is ready


The Nancy Roman telescope has reached another milestone in its development. NASA has announced that the primary mirror of the space telescope is now complete. The 2.4 m (7.9 ft) mirror took less time to develop than other mirrors because it was not made from scratch. It is a re-shaped and re-exposed mirror that came from the National Reconnaissance Office.

The Nancy Grace Roman Space Telescope was initially named the WFIRST (Wide Field Infrared Space Telescope). The telescope project was approved in February 2016, and in May 2020, NASA announced a name change. WFIRT became the Nancy Grace Roman Space Telescope in honor of NASA’s first major astronomer, which passed in 2018. The telescope is sometimes called the Roman Space Telescope, or RST.

Dr. Nancy Grace Roman at NASA’s Goddard Space Flight Center, 1972 Circuit. Image credit: by NASA / ESA – CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid-88649875

The primary mirror is the heart of a telescope. It is responsible for collecting light that can later be directed to various devices. The primary mirror of the RST is similar to the size of the Hubble, but is too light for technological advances. The RST is also a wider area than Hubble, in fact 100 times greater. It will use its power and wide area to examine cosmic objects from near and far.

The RST, like the James Webb Space Telescope (JWST), is an infrared observatory. The primary mission of JWST is to look back in as much time as possible and see the first light of the Universe. But RST is different. Its wide area means that it is the primary concern, studying dark energy and exoplanet. And now with its primary mirror, its one step closer to launch, scheduled for some time in 2025.

“It is very exciting to achieve this feat,” said Scott Smith, Roman telescope manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Success depends on a team in which everyone is doing their part, and this is especially true in our current challenging environment. Everyone plays a part in collecting that first image and answering inspiring questions. ”

Telescope mirrors are coated with different materials depending on the wavelength of the light it is designed for. Hubble was designed to be viewed in infrared, ultraviolet and optical, so this mirror was coated in layers of aluminum and magnesium fluoride. The JWST mirror is coated with gold as it appears in the infrared wavelength for a long time.

The primary mirror of the Roman Space Telescope shows the American flag.  Image Credit: L3Harris Technologies
The primary mirror of the Roman Space Telescope shows the American flag. Image Credit: L3Harris Technologies

The Roman Space Telescope’s mirror is coated with an exceptionally thin layer of silver, it is used because of its ability to reflect infrared light. It is less than 400 nanometers thick, 200 times thinner than a human hair. Like all advanced telescope mirrors, it is meticulously polished. The average bump on the mirror surface is only 1.2 nanometers high, which NASA says is twice as much as Mississippi operations require. If the mirror were of the size of the earth, the longest bump would have been only 1/4 inch long.

Since the mirror is twice as smooth as the design called for, it should provide better science results than predicted. “The mirror was perfect for the optical prescription of the Roman Space Telescope,” said Bonnie Patterson, program manager at L3Harris Technologies in Rochester, New York. Patterson said in a press release, “Since it is more sleek than necessary, it will provide more scientific benefits than originally planned.”

Once the primary mirror collects infrared light, the light is sent to two instruments of the telescope: the Coronograph Instrument and the Wide Field Instrument, which is the primary instrument of the RST.

Camilo Mejia Prada, an optical engineer at NASA's Jet Propulsion Laboratory in Pasadena, California, shines a light on the interior of a test for RST's Coronagraph Instruments.  Image Credit: NASA / JPL-Caltech / Matthew Lum
Camilo Mejia Prada, an optical engineer at NASA’s Jet Propulsion Laboratory in Pasadena, California, shines a light on the interior of a test for RST’s Coronagraph Instruments. Image Credit: NASA / JPL-Caltech / Matthew Lum

The Coronagraph instrument allows RST to study exoplanet by ejecting light from its star. While this will not be the first telescope to use a coronograph, (Hubble has one, but very weak) RST should allow the telescope to see planets that are a billion times farther than their stars. If it works as intended.

Coronographs emit light from a star, making it easier to study things near the star.  This is an image of the Sun from NASA's Solar and Heliospheric Observatory (SOHO).  The coronograph allowed SOHO to observe three coronal mass evictions despite light indiscriminately from the sun.  Image Credit: NASA / GSFC / SOHO
Coronographs emit light from a star, making it easier to study things near the star. This is an image of the Sun from NASA’s Solar and Heliospheric Observatory (SOHO). The coronograph allowed SOHO to observe three coronal mass evictions despite light indiscriminately from the sun. Image Credit: NASA / GSFC / SOHO

The Wide Field Instrument (WFI) is basically a huge 300 megapixel camera. While it has an angular resolution similar to that of Hubble, its field of view is about 100 times wider than that of Hubble. This will give the power to map the distribution and structure of dark energy in the universe. It will also help researchers to understand how the universe has evolved over time.

“We’re going to try to discover the fate of the universe,” said Jeff Crook of Goddard, project scientist at the Nancy Grace Roman Space Telescope. “The expansion of the universe is accelerating, and one of the things in the Wide Field Instrument will help us detect whether acceleration is increasing or slowing down,” Crook said in a press release.

The expansion rate of the universe is one of the permanent questions in astronomy. It is difficult to reduce the rate of expansion – called the Hubble Constant – and different researchers keep coming up with different values. In recent years, there have been differences between about 67 and 77 (km / s) / MCP in the measurement of expansion rates. Dark energy is the name given to the force driving expansion, and the Roman Space Telescope will examine that rate using three techniques: baron acoustic oscillations, observations of distant supernovae, and weak gravitational lensing.

The RST will also complete a census of exoplanets, which Kepler will pick up on mission work. It will be able to examine the distant, giant exoplanet thanks to its coronagraph. The RST will also be able to find rogue planets, planets flowing through space without being bound to a single planet. Right now we know about those handful of planets, but RST will help us find more. Some scientists think that there may be up to one trillion of these flows in the Milky Way. Current estimates of rogue planet numbers lack accuracy, but the Roman Space Telescope should provide an estimate that is 10 times more accurate.

Earth-sized rogue planet contact with a star.  Credit: Christine Pulliam (CfA)
Earth-sized rogue planet contact with a star. Credit: Christine Pulliam (CfA)

Now that it is complete, the primary mirror will undergo more testing. Of particular concern is the experience of how the mirror will react to temperature changes. The mirror is made of special glass that resists expansion and contraction. Since expansion and contraction can distort the shape of the mirror, a lot of it will be for distorted images.

While the mirror has been tested for temperature extremes during development, future testing will test not only the mirror, but also its support structure.

“Roman’s primary mirror is complete, yet our work is not finished,” Smith said. “We are excited to see this mission launch and beyond, and look forward to seeing the surprise.”

The RST is scheduled to launch in the year 2025 on a commercial launch vehicle from Cape Canaveral. It will travel to the Sun-Earth Lagrange 2 point, where it will orbit a halo. It has a planned duration of five years.

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