PROJECT:
Deformable Mirror Technology
development
SNAPSHOT
Deformable mirrors enable direct
imaging of exoplanets by correcting imperfections or shape changes in a space
telescope down to subatomic scales.
Credit: NASA Jet Propulsion
Laboratory.
Finding and studying Earth-like planets orbiting nearby stars is
critical to understand whether we are alone in the universe. To study such
planets and assess if they can sustain life, it is necessary to directly image
them. However, these planets are difficult to observe, since light from the
host star hides them with its glare. A coronagraph instrument can be used to
remove the glare light from the host star, enabling reflected light from the
planet to be collected. A deformable mirror is an essential component of
a coronagraph, as it can correct the tiniest of imperfections in the telescope
and remove any remaining starlight contamination.
Detecting an Earth-like planet poses significant challenges as the
planet is approximately 10 billion times fainter than its parent star. The main
challenge is to block nearly all of the star's light so that the faint light
reflected from the planet can be collected. A coronagraph can block the
starlight, however, any instability in the telescope’s optics—such as
misalignment between mirrors or a change in the mirror’s shape—can result in
starlight leakage, causing glare that hides the planet. Therefore, detecting an
Earth-like planet using a coronagraph requires precise control of both the
telescope and the instrument’s optical quality, or wavefront, to an
extraordinary level of 10s of picometers (pm), which is approximately on the
order of the size of a hydrogen atom.
Deformable mirrors will enable future space coronagraphs to achieve this
level of control. These devices will be demonstrated in space on a coronagraph
technology demonstration instrument on NASA’s Roman Space Telescope, which will
launch by May 2027. This technology will also be critical to enable a future
flagship mission after Roman recommended by the 2020 Decadal Survey in
Astronomy and Astrophysics, provisionally called the “Habitable Worlds
Observatory” (HWO).
What is a deformable mirror and how do
they work?
Deformable Mirrors (DM) are devices that can adjust the optical path of
incoming light by changing the shape of a reflective mirror using precisely
controlled piston-like actuators. By adjusting the shape of the mirror, it is
possible to correct the wavefront that is perturbated by optical aberrations
upstream and downstream of the DM. These aberrations can be caused by external
perturbations, like atmospheric turbulence, or by optical misalignments or
defects internal to the telescope.
DM technology originated to enable adaptive optics (AO) in ground-based
telescopes, where the primary goal is to correct the aberrations caused by
atmospheric turbulence. The main characteristics of a DM are: 1) the number of
actuators, which is proportional to the correctable field of view; 2) the
actuators’ maximum stroke – i.e., how far they can move; 3) the DM speed, or
time required to modify the DM surface; 4) the surface height resolution that
defines the smallest wavefront control step, and (5) the stability of the DM
surface.
Ground-based deformable mirrors have set the state-of-the-art in
performance, but to lay the groundwork to eventually achieve ambitious goals
like the Habitable Worlds Observatory, further development of DMs for use in
space is underway.
For a space telescope, DMs do not need to correct for the atmosphere,
but instead must correct the very small optical perturbations that slowly occur
as the space telescope and instrument heat up and cool down in orbit. Contrast
goals (the brightness difference between the planet and the star) for DMs in
space are on the order of 10-10 which
is 1000 times deeper than the contrast goals of ground-based counterparts. For
space applications total stroke requirements are usually less than a
micrometer; however, DM surface height resolution of ~10 pm and DM surface
stability of ~10 pm/hour are the key and driving requirements.
Another key aspect is the increased number of actuators needed for both
space- and ground-based applications. Each actuator requires a high
voltage connection (on the order of 100V) and fabricating a large number of
connections creates an additional challenge.
Deformable Mirror State-of-the-Art
Two main DM actuator technologies are currently being considered for
space missions. The first is electrostrictive technology, in which an actuator
is mechanically connected to the DM’s reflective surface. When a voltage is
applied to the actuator, it contracts and modifies the mirror surface. The
second technology is the electrostatically-forced Micro Electro-Mechanical
System (MEMS) DM. In this case, the mirror surface is deformed by an
electrostatic force between an electrode and the mirror.
Several NASA-sponsored contractor teams are working on advancing the DM
performance required to meet the requirements of future NASA missions, which
are much more stringent than most commercial applications, and thus, have a
limited market application. Some examples of those efforts include improving
the mirror’s surface quality or developing more advanced DM electronics.
MEMS DMs manufactured by Boston Micromachines Corporation (BMC) have been tested in vacuum conditions and have undergone launch vibration testing. The largest space-qualified BMC device is the 2k DM (shown in Fig. 2), which has 50 actuators across its diameter (2040 actuators in total). Each actuator is only 400 microns across. The largest MEMS DM produced by BMC is the 4k DM, which has 64 actuators across its diameter (4096 actuators in total) and is used in the coronagraph instrument for the Gemini ground-based observatory. However, the 4k DM has not been qualified for space flight.
Fig. 2: The Boston Micromachines Corporation 2k DM that has 2040 actuators with 400 um pitch. Credit: Dr. Eduardo Bendek
Electrostrictive DMs manufactured by AOA Xinetics (AOX) have also been validated in vacuum and qualified for space flight. The AOX 2k DM has a 48 x 48 actuator grid (2304 actuators) with a 1 mm pitch. Two of these AOX 2k DMs will be used in the Roman Space Telescope Coronagraph (Fig. 3) to demonstrate the DM technology for high-contrast imaging in space. AOX has also manufactured larger devices, including a 64 x 64 actuator unit tested at JPL.
Fig. 3: The Roman Space Telescope Coronagraph during assembly of the static optics at NASA’s Jet Propulsion Laboratory Credit: NASA
Preparing the technology for the
Habitable Worlds Observatory
Deformable Mirror technology has
advanced rapidly, and a version of this technology will be demonstrated in
space on the Roman Space Telescope. However, it is anticipated that for
wavefront control for missions like the HWO, even larger DMs with up to ~10,000
actuators would be required, such as 96 x 96 arrays. Providing a high-voltage
connection to each of the actuators is a challenge that will require a new
design.
The HWO would also involve
unprecedented wavefront control requirements, such as a resolution step size
down to single-digit picometers, and a stability of ~10 pm/hr. These
requirements will not only drive the DM design, but also the electronics that
control the DMs, since the resolution and stability are largely defined by the
command signals sent by the controller, which require the implementation of
filters to remove any noise the electronics could introduce.
NASA’s Astrophysics Division
investments in DM technologies have advanced DMs for space flight onboard the
Roman Space Telescope Coronagraph, and the Division is preparing a Technology
Roadmap to further advance the DM performance to enable the HWO.
Author: Eduardo Bendek, Ph.D. Jet
Propulsion Laboratory, California Institute of Technology.
The research was carried out at the
Jet Propulsion Laboratory, California Institute of Technology, under a contract
with the National Aeronautics and Space Administration (80NM0018D0004).
ACTIVITY LEADS
Dr. Eduardo Bendek (JPL) and Dr.
Tyler Groff (GSFC), Co-chairs of DM Technology Roadmap working group; Paul
Bierden (BMC); Kevin King (AOX).
SPONSORING ORGANIZATION
Astrophysics Division Strategic
Astrophysics Technology (SAT) Program, and the NASA Small Business Innovation
Research (SBIR) Program
Source: Deformable Mirrors in Space: Key Technology to Directly Image Earth Twins - NASA Science
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