Future space telescopes with coronagraph instruments will use a wavefront sensor (WFS) to measure and correct for phase errors and stabilize the stellar intensity in high-contrast images. The HabEx and LUVOIR mission concepts baseline a Zernike wavefront sensor (ZWFS), which uses Zernike’s phase contrast method to convert phase in the pupil into intensity at the WFS detector. In preparation for these potential future missions, we experimentally demonstrate a ZWFS in a coronagraph instrument on the Decadal Survey Testbed in the High Contrast Imaging Testbed facility at NASA’s Jet Propulsion Laboratory. We validate that the ZWFS can measure low- and mid-spatial frequency aberrations up to the control limit of the deformable mirror (DM), with surface height sensitivity as small as 1 pm, using a configuration similar to the HabEx and LUVOIR concepts. Furthermore, we demonstrate closed-loop control, resolving an individual DM actuator, with residuals consistent with theoretical models. In addition, we predict the expected performance of a ZWFS on future space telescopes using natural starlight from a variety of spectral types. The most challenging scenarios require ∼1 h of integration time to achieve picometer sensitivity. This timescale may be drastically reduced by using internal or external laser sources for sensing purposes. The experimental results and theoretical predictions presented here advance the WFS technology in the context of the next generation of space telescopes with coronagraph instruments.
Debris disks around nearby stars are tracers of the planet formation process, and they are a key element of our understanding of the formation and evolution of extrasolar planetary systems. With multi-color images of a significant number of disks, we can probe important questions: can we learn about planetary system evolution; what materials are the disks made of; and can they reveal the presence of planets? Most disks are known to exist only through their infrared flux excesses as measured by the Spitzer Space Telescope, and through images measured by Herschel. The brightest, most extended disks have been imaged with HST, and a few, such as Fomalhaut, can be observed using ground-based telescopes. But the number of good images is still very small, and there are none of disks with densities as low as the disk associated with the asteroid belt and Edgeworth Kuiper belt in our own Solar System.
Direct imaging of disks is a major observational challenge, demanding high angular resolution and extremely high dynamic range close to the parent star. The ultimate experiment requires a space-based platform, but demonstrating much of the needed technology, mitigating the technical risks of a space-based coronagraph, and performing valuable measurements of circumstellar debris disks, can be done from a high-altitude balloon platform. In this paper we present a balloon-borne telescope concept based on the Zodiac II design that could undertake compelling studies of a sample of debris disks.
Zodiac II is a proposed balloon-borne science investigation of debris disks around nearby stars. Debris disks are
analogs of the Asteroid Belt (mainly rocky) and Kuiper Belt (mainly icy) in our Solar System. Zodiac II will
measure the size, shape, brightness, and color of a statistically significant sample of disks. These measurements
will enable us to probe these fundamental questions: what do debris disks tell us about the evolution of planetary
systems; how are debris disks produced; how are debris disks shaped by planets; what materials are debris disks
made of; how much dust do debris disks make as they grind down; and how long do debris disks live? In addition,
Zodiac II will observe hot, young exoplanets as targets of opportunity.
The Zodiac II instrument is a 1.1-m diameter SiC telescope and an imaging coronagraph on a gondola carried
by a stratospheric balloon. Its data product is a set of images of each targeted debris disk in four broad visiblewavelength
bands. Zodiac II will address its science questions by taking high-resolution, multi-wavelength images
of the debris disks around tens of nearby stars. Mid-latitude flights are considered: overnight test flights within
the United States followed by half-global flights in the Southern Hemisphere. These longer flights are required to
fully explore the set of known debris disks accessible only to Zodiac II. On these targets, it will be 100 times more
sensitive than the Hubble Space Telescope's Advanced Camera for Surveys (HST/ACS); no existing telescope
can match the Zodiac II contrast and resolution performance. A second objective of Zodiac II is to use the
near-space environment to raise the Technology Readiness Level (TRL) of SiC mirrors, internal coronagraphs,
deformable mirrors, and wavefront sensing and control, all potentially needed for a future space-based telescope
for high-contrast exoplanet imaging.
Less than 20 years after the discovery of the first extrasolar planet, exoplanetology is rapidly growing with more than
one discovery every week on average since 2007. An important step in exoplanetology is the chemical characterization
of exoplanet atmospheres. It has recently been shown that molecular signatures of transiting exoplanets can be studied
from the ground. To advance this idea and prepare more ambitious missions such as THESIS, a dedicated spectrometer
named the New Mexico Tech Extrasolar Spectroscopic Survey Instrument (NESSI) is being built at New Mexico Tech
in collaboration with the NASA Jet Propulsion Laboratory. NESSI is a purpose-built multi-object spectrograph that
operates in the J, H, and K-bands with a resolution of R = 1000 in each, as well as a lower resolution of R = 250 across
the entire J/H/K region.
Direct detection of mature exoplanets is possible using a visible-wavelength telescope and coronagraph in the
stratosphere. We analyze two sources of dynamic wavefront perturbations: turbulence in the free atmosphere and locally
generated turbulence. We find that they are expected to have relatively small effects on the wavefront. We find that
neither source should limit observations at 10-9 contrast levels for planet-star separations of 0.5 arcsec. On this basis, we
expect that it is feasible to image and characterize several known radial-velocity exoplanets.
The development of widely tunable coherent frequency sources for application as local oscillators or simply as test equipment above 1 THz remains an impediment in receiver development and characterization. Photomixer sources have demonstrated sufficient power to pump SIS mixers to over 600 GHz and have demonstrated over 2.5 THz of bandwidth in a single device. First generation photomixer system solved the problem of frequency calibration, but failed to fully address the needed spectral purity required for heterodyne applications. A number of improved laser technologies are greatly simplifying the implementation and improving the spectral purity of photomixer systems, however a full system demonstration in the THz frequency range remains elusive. The current state of the art for photomixer based sources is explored in light of heterodyne local oscillator and coherent tests sources for antenna and component characterization at THz frequencies.
We developed a tunable, cavity-locked diode laser source at 850 nm for difference-frequency generation of coherent THz- waves. The difference frequency is synthesized by three fiber-coupled external-cavity diode lasers, where tow of the lasers are locked to adjacent modes of an ultra-stable Fabry-Perot cavity and the third laser is offset-phase- locked to the second cavity-locked laser using a tunable microwave oscillator. The first cavity-locked laser and the offset-locked laser produces the difference frequency, whose value is precisely determined by sum of integer multiple of free spectral range of the Fabry-Perot cavity and the offset frequency. The difference-frequency signal is amplified to 500 mW by the master oscillator power amplifier technique, simultaneous two-frequency injection-seeding with a single semiconductor optical amplifier. Here we demonstrate the difference-frequency generation of THz waves with the low- temperature-grown GaAs photomixers and its application to high-resolution spectroscopy of simple molecules. An absolute frequency calibration was carried out with an accuracy of approximately 10-7 using CO lines in the THz region.