SPHEREx, the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and ices Explorer, is a NASA MIDEX mission planned for launch in 2024. SPHEREx will carry out the first all-sky spectral survey at wavelengths between 0.75µm and 5µm with spectral resolving power ~40 between 0.75 and 3.8µm and ~120 between 3.8 and 5µm At the end of its two-year mission, SPHEREx will provide 0.75-to-5µm spectra of each 6.”2x6.”2 pixel on the sky - 14 billion spectra in all. This paper updates an earlier description of SPHEREx presenting changes made during the mission's Preliminary Design Phase, including a discussion of instrument integration and test ow and a summary of the data processing, analysis, and distribution plans.
Despite promising astrometric signals, to date there has been no success in direct imaging of a hypothesized third member of the Sirius system. Using the Clio instrument and MagAO adaptive optics system on the Magellan Clay 6.5 m telescope, we have obtained extensive imagery of Sirius through a vector apodizing phase plate (vAPP) coronagraph in a narrowband filter at 3.9 microns. The vAPP coronagraph and MagAO allow us to be sensitive to planets much less massive than the limits set by previous non-detections. However, analysis of these data presents challenges due to the target’s brightness and unique characteristics of the instrument. We present a comparison of dimensionality reduction techniques to construct background illumination maps for the whole detector using the areas of the detector that are not dominated by starlight. Additionally, we describe a procedure for sub-pixel alignment of vAPP data using a physical-optics-based model of the coronagraphic PSF.
SPHEREx, a mission in NASA’s Medium Explorer (MIDEX) program recently selected for Phase-A implementation, is an all-sky survey satellite that will produce a near-infrared spectrum for every 6 arcsecond pixel on the sky. SPHEREx has a simple, high-heritage design with large optical throughput to maximize spectral mapping speed. While the legacy data products will provide a rich archive of spectra for the entire astronomical community to mine, the instrument is optimized for three specific scientific goals: to probe inflation through the imprint primordial non-Gaussianity left on today’s large-scale cosmological structure; to survey the Galactic plane for water and other biogenic ices through absorption line studies; and to constrain the history of galaxy formation through power spectra of background fluctuations as measured in deep regions near the ecliptic poles. The aluminum telescope consists of a heavily baffled, wide-field off-axis reflective triplet design. The focal plane is imaged simultaneously by two mosaics of H2RG detector arrays separated by a dichroic beamsplitter. SPHEREx assembles spectra through the use of mass and volume efficient linear variable filters (LVFs) included in the focal plane assemblies, eliminating the need for any dispersive or moving elements. Instead, spectra are constructed through a series of small steps in the spacecraft attitude across the sky, modulating the location of an object within the FOV and varying the observation wavelength in each exposure. The spectra will cover the wavelength range between 0.75 and 5.0 µm at spectral resolutions ranging between R=35 and R=130. The entire telescope is cooled passively by a series of three V-groove radiators below 80K. An additional stage of radiative cooling is included to reduce the long wavelength focal plane temperature below 60K, controlling the dark current. As a whole, SPHEREx requires no new technologies and carries large technical and resource margins on every aspect of the design.
One of the primary goals of exoplanet science is to find and characterize habitable planets, and direct imaging will play a key role in this effort. Though imaging a true Earth analog is likely out of reach from the ground, the coming generation of giant telescopes will find and characterize many planets in and near the habitable zones (HZs) of nearby stars. Radial velocity and transit searches indicate that such planets are common, but imaging them will require achieving extreme contrasts at very small angular separations, posing many challenges for adaptive optics (AO) system design. Giant planets in the HZ may even be within reach with the latest generation of high-contrast imagers for a handful of very nearby stars. Here we will review the definition of the HZ, and the characteristics of detectable planets there. We then review some of the ways that direct imaging in the HZ will be different from the typical exoplanet imaging survey today. Finally, we present preliminary results from our observations of the HZ of α Centauri A with the Magellan AO system’s VisAO and Clio2 cameras.
The Thermal Infrared imager for the GMT which provides Extreme contrast and Resolution (TIGER) is intended as a
small-scale, targeted instrument capable of detecting and characterizing exoplanets and circumstellar disks, around both
young systems in formation, and more mature systems in the solar neighborhood. TIGER can also provide general
purpose infrared imaging at wavelengths from 1.5-14 μm. The instrument will utilize the facility adaptive optics (AO)
system. With its operation at NIR to MIR wavelengths (where good image quality is easier to achieve), and much of the
high-impact science using modestly bright guide stars, the instrument can be used early in the operation of the GMT.
The TIGER concept is a dual channel imager and low resolution spectrometer, with high contrast modes of observations
to fulfill the above science goals. A long wavelength channel (LWC) will cover 7-14 μm wavelength, while a short
wavelength channel (SWC) will cover the 1.5-5 μm wavelength region. Both channels will have a 30° FOV. In addition
to imaging, low-resolution spectroscopy (R=300) is possible with TIGER for both the SWC and LWC, using insertable
Three of the recently completed NASA Astrophysics Strategic Mission Concept (ASMC) studies addressed the
feasibility of using a Visible Nulling Coronagraph (VNC) as the prime instrument for exoplanet science. The VNC
approach is one of the few approaches that works with filled, segmented and sparse or diluted aperture telescope systems
and thus spans the space of potential ASMC exoplanet missions. NASA/Goddard Space Flight Center (GSFC) has a
well-established effort to develop VNC technologies and has developed an incremental sequence of VNC testbeds to
advance the this approach and the technologies associated with it. Herein we report on the continued development of the
vacuum Visible Nulling Coronagraph testbed (VNT). The VNT is an ultra-stable vibration isolated testbed that operates
under high bandwidth closed-loop control within a vacuum chamber. It will be used to achieve an incremental sequence
of three visible light nulling milestones of sequentially higher contrasts of 108, 109 and 1010 at an inner working angle of
2*λ/D and ultimately culminate in spectrally broadband (>20%) high contrast imaging. Each of the milestones, one per
year, is traceable to one or more of the ASMC studies. The VNT uses a modified Mach-Zehnder nulling interferometer,
modified with a modified "W" configuration to accommodate a hex-packed MEMS based deformable mirror, a coherent
fiber bundle and achromatic phase shifters. Discussed will be the optical configuration laboratory results, critical
technologies and the null sensing and control approach.
We report on our recent laboratory results with the NASA/Goddard Space Flight Center (GSFC) Visible Nulling
Coronagraph (VNC) testbed. We have experimentally achieved focal plane contrasts of 1 x 108 and approaching 109 at
inner working angles of 2 * wavelength/D and 4 * wavelength/D respectively where D is the aperture diameter. The
result was obtained using a broadband source with a narrowband spectral filter of width 10 nm centered on 630 nm. To
date this is the deepest nulling result with a visible nulling coronagraph yet obtained. Developed also is a Null Control
Breadboard (NCB) to assess and quantify MEMS based segmented deformable mirror technology and develop and
assess closed-loop null sensing and control algorithm performance from both the pupil and focal planes. We have
demonstrated closed-loop control at 27 Hz in the laboratory environment. Efforts are underway to first bring the contrast
to > 109 necessary for the direct detection and characterization of jovian (Jupiter-like) and then to > 1010 necessary for
terrestrial (Earth-like) exosolar planets. Short term advancements are expected to both broaden the spectral passband
from 10 nm to 100 nm and to increase both the long-term stability to > 2 hours and the extent of the null out to a ~ 10 *
wavelength / D via the use of MEMS based segmented deformable mirror technology, a coherent fiber bundle,
achromatic phase shifters, all in a vacuum chamber at the GSFC VNC facility. Additionally an extreme stability
textbook sized compact VNC is under development.
We are developing the ability for Focused Ion Beam (FIB) machining of occulting masks for use in coronagraphs.
These masks will be used as soft-edged Lyot stops to suppress light from stars and allow direct imaging of
extrasolar planets. The FIB approach is attractive because it has the potential for higher precision than mechanical
machining and for larger volumes than electron-beam lithography. The mask fabrication process is trifold: 1) a
transparent material-currently, poly(methyl methacrylate) (PMMA)-is doped with dyes; 2) the mask shape is FIB
milled into the material; and 3) the mask is coated with another layer of index-matching transparent absorber. Using
a Zeiss NVision 40 FIB system, we have fabricated conical-shaped masks of various slopes in dye-doped PMMA.
Inherent in this process is the advantage of control of the features through programming the ion beam track. We
have also optically characterized these masks as well as the dye-doped absorbing material. We have found that the
dye-doped PMMA has a very high absorbance, >1 OD.
The Extrasolar Planetary Imaging Coronagraph (EPIC) is a NASA Astrophysics Strategic Mission Concept
under study for the upcoming Exoplanet Probe. EPIC's mission would be to image and characterize
extrasolar giant planets, and potential super-Earths, in orbits with semi-major axes between 2 and 10 AU.
EPIC will provide insights into the physical nature of a variety of planets in other solar systems
complimenting radial velocity (RV) and astrometric planet searches. It will detect and characterize the
atmospheres of planets identified by radial velocity surveys and potentially some transits, determine orbital
inclinations and masses, characterize the atmospheres of gas giants around A and F stars, observed the
inner spatial structure and colors of inner Spitzer selected debris disks. EPIC would be launched into a
heliocentric Earth trailing drift-away orbit, with a 3-year mission lifetime (5 year goal) and will revisit
planets at least three times.
The starlight suppression approach consists of a visible nulling coronagraph (VNC) that enables high order
starlight suppression in broadband light. To demonstrate the VNC approach and advance it's technology
readiness the NASA/Goddard Space Flight Center and Lockheed-Martin have developed a laboratory VNC
and have demonstrated white light nulling. We will discuss our ongoing VNC work and show the latest
results from the VNC testbed.
The Extrasolar Planetary Imaging Coronagraph (EPIC) is a proposed NASA Discovery mission to image
and characterize extrasolar giant planets in orbits with semi-major axes between 2 and 10 AU. EPIC will
provide insights into the physical nature of a variety of planets in other solar systems complimenting radial
velocity (RV) and astrometric planet searches. It will detect and characterize the atmospheres of planets
identified by radial velocity surveys, determine orbital inclinations and masses, characterize the
atmospheres around A and F type stars which cannot be found with RV techniques, and observe the inner
spatial structure and colors of debris disks. EPIC has a proposed launch date of 2012 to heliocentric Earth
trailing drift-away orbit, with a 3 year mission lifetime (5 year goal), and will revisit planets at least three
times at intervals of 9 months. The robust mission design is simple and flexible ensuring mission success
while minimizing cost and risk. The science payload consists of a heritage optical telescope assembly
(OTA), and visible nulling coronagraph (VNC) instrument. The instrument achieves a contrast ratio of 109
over a 4.84 arcsecond field-of-view with an unprecedented inner working angle of 0.14 arcseconds over the
spectral range of 440-880 nm, with spectral resolutions from 10 - 150. The telescope is a 1.5 meter offaxis
Cassegrain with an OTA wavefront error of λ/9, which when coupled to the VNC greatly reduces the
requirements on the large scale optics, compressing them to stability requirements within the relatively
compact VNC optical chain. The VNC features two integrated modular nullers, a spatial filter array (SFA),
and an E2V-L3 photon counting CCD. Direct null control is accomplished from the science focal
mitigating against complex wavefront and amplitude sensing and control strategies.
The CorECam Instrument Concept Study (ICS) addressed the requirements and science program for the
Terrestrial Planet Finder Coronagraph's (TPF-C) primary camera. CorECam provides a simple interface to
TPF-C's Starlight Suppression System (SSS) which would be provided by the TPF-C Program, and
comprises camera modules providing visible, and near-infrared (NIR) camera focal plane imaging. In its
primary operating mode, CorECam will conduct the core science program of TPF-C, detecting terrestrial
planets at visible wavelengths. CorECam additionally provides the imaging capabilities to characterize
terrestrial planets, and conduct an extended science program focused on investigating the nature of the
exosolar systems in which terrestrial planets are detected. In order to evaluate the performance of CorECam
we developed a comprehensive, end-to-end model using OSCAR which has provided a number of key
conclusions on the robustness of the TPF-C baseline design, and allows investigation of alternative
techniques for wavefront sensing and control. CorECam recommends photon counting detectors be
baselined for imaging with TPF-C since they provide mitigations against the background radiation
environment, improved sensitivity and facilitate alternative WFSC approaches.
Smithsonian Astrophysical Observatory (SAO) has set up a program to study coronagraphic techniques. The program consists of the development of new fabrication methods of occulter masks, characterization of the manufactured masks, and application of the masks to study speckle reduction technique. Our occulter mask fabrication development utilizes a focused ion beam system to directly shape mask profiles from absorber material. Initial milling trials show that we can shape nearly Gaussian-shaped mask profiles. Part of this development is the characterization of absorber materials, poly(methyl methacrylate) doped with light-stable chromophores. For the characterization of the masks we have built a mask scanner enabling us to scan the transmission function of occulter masks. The real mask transmission profile is retrieved applying the maximum entropy method to deconvolve the mask transmission function from the beam profile of the test laser. Finally, our test bed for studying coronagraphic techniques is nearing completion. The optical setup is currently configured as a classical coronagraph and can easily be re-configured for studying speckle reduction techniques. The development of the test bed control software is under way. This paper we will give an update of the status of the individual program elements.
The nulling coronagraph is one of 5 instrument concepts selected by NASA for study for potential use in the TPF-C
mission. This concept for extreme starlight suppression has two major components, a nulling interferometer to suppress
the starlight to ~10-10 per airy spot within 2 λ/D of the star, and a calibration interferometer to measure the residual
scattered starlight. The ability to work at 2 λ/D dramatically improves the science throughput of a space based
coronagraph like TPF-C. The calibration interferometer is an equally important part of the starlight suppression system.
It measures the measures the wavefront of the scattered starlight with very high SNR, to 0.05nm in less than 5 minutes
on a 5mag star. In addition, the post coronagraph wavefront sensor will be used to measure the residual scattered light
after the coronagraph and subtract it in post processing to 1~2x10-11 to enable detection of an Earthlike planet with a
SNR of 5~10.
SAO has set up a testbed to study coronagraphic techniques, starting with Labeyrie's multi-step speckle reduction technique. This technique expands the general concept of a coronagraph by incorporating a speckle corrector (phase and/or amplitude) in combination with a second occulter for speckle light suppression. Here we are describing the initial testbed configuration. In addition, the testbed will be used to test a new approach of the phase diversity method to retrieve the speckle phase and amplitude. This method requires measurements of the speckle pattern in the focal plane and slightly out-of-focus. Then we will calculate a phase of the wave from which we can derive a correction function for the speckle corrector. Furthermore we report results from a parallel program which studies new manufacturing methods of soft-edge occulter masks. Masks were manufactured using the spherical caps method. Since the results were not satisfying we also investigated the method of ion beam milling of masks. Here we will present the outline of this method. Masks manufactured with both methods will be fully characterized in our mask tester before their use in the testbed.
A possible system for imaging an exo-planet uses a telescope, coronagraphic mask, and Lyot stop. However, the optics must be nearly flawless for a planet to be imaged, since imperfections cause the image to have speckles which mask the planet. The speckle pattern can be suppressed by removing its phase with an adaptive optic and by reimaging it through a lens with a small, central obscuration. We show how phase diversity can be used to determine the phase.
The Extrasolar Planetary Imaging Coronagraph (EPIC) will provide the first direct measurements of a broad range of fundamental physical characteristics of giant planets in other solar systems. These characteristics include orbital inclination, mass, brightness, color, the presence (or absence) of CH4 and H2O, and orbital or rotational-driven variability. EPIC utilizes a 1.5 meter telescope coupled to a Visible Nulling Coronagraph to achieve these science goals. EPIC has been proposed as a Discovery Mission.
The present status of AOS development at KOSMA is discussed. A study of a new generation of AOS using the new Bragg-cell material "Rutil" is on the way, which is supposed to lead to spectrometers in the range of 4 GHz total bandwidth at an resolution of 2-3 MHz. A second alternative for a 4 GHz bandwidth spectrometer has been developed as an engineering model for the HIFI instrument aboard the ESA cornerstone mission "Herschel". It consists of an array-AOS with 1 GHz bandwidth of each of the four AOS bands at a resolution of 1 MHz. A hybrid system for an input between 4 and 8 GHz is setup, and various laboratory tests have demonstrated that this system is well suited for large bandwidth applications like with HIFI. For eventual future demand of even larger bandwidth, details of a new optical method for Rf-analysis are discussed. It consists of a modulated laser with one or two Fabry-Perot etalons to analyze the frequency distribution of the resulting laser sidebands. A bandwidth of several 10 GHz at moderate resolution can be achieved.
On December 5, 1998, the Submillimeter Wave Astronomy Satellite has been launched with a PEGASUS carrier after more than 3 years delay. SWAS is observing molecular line signals (H2O, 13CO, Cl, O2 and H218O) from astronomical sources at frequencies between 487 and 557 GHz. SWAS is the first sub-millimeter heterodyne space mission, and, for the spectral analysis of the received signals, it carries the first acousto-optical spectrometer (AOS) in space. The AOS has been built at University of Cologne, and it covers 1.4 GHz bandwidth with approximately 1400 frequency channels. The total weight is 7.5 kg and the power consumption is 5.5 Watts only. The very stable temperature conditions on SWAS allow longtime integrations at total observing times far above 100 hours still with radiometric performance. So far, the AOS- spectra have not been degraded by particle hits, particularly the CCD detector of the AOS does not exhibit any visible effect due to cosmic rays. SWAS has already observed many interstellar sources in our galaxy. Emission of water is seen to be very abundant, while signals of molecular oxygen seem to be too weak to be observable.
The first fully space qualified acousto-optical spectrometer (AOS) is described. It is built for the Submillimeter Wave Astronomy Satellite (SWAS) to be launched in July 1995. It has a very large bandwidth from 1400 to 2800 MHz covered by 1365 channels. This corresponds to a nearly 1 MHz channel spacing. The design is optimized for very high stability, which is demonstrated by means of Allan variance stability test. The Allan plot minimum time was found well above 800 seconds. The AOS can operate within a temperature range from -5 to +30 degree(s)C (+5 to +25 degree(s)C nominal) and with temperature variations of up to 2 degree(s)C/h. The performance was verified also after environmental testing such as random vibration (10.2 G rms) and thermal cycling of -30 to +50 degree(s)C. The lightweight mechanical design resulted in a total weight of 7.2 kg including electronics. A detailed optical design study was performed in order to achieve diffraction limited channel resolution, high efficiency and low sensitivity to mechanical distortion. The RF input power needed for full scale is 11 mW. The power consumption is 5.4 Watts (including data pre-averaging and DC-DC converter losses). The development has shown that AOSs are well suited for spaceborne applications.
The Submillimeter Wave Astronomy Satellite (SWAS) mission will study galactic star formation and interstellar chemistry. To carry out this mission, SWAS will survey dense (nH2 > 103 cm-3) molecular clouds within our galaxy in either the ground-state or a low- lying transition of five astrophysically important species: H2O, H218O, O2, CI, and 13CO. By observing these lines SWAS will: (1) test long-standing theories that predict that these species are the dominate coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supply heretofore missing information about the abundance of key species central to the chemical models of dense interstellar gas. During its two-year mission, SWAS will observe giant and dark cloud cores with the goal of detecting to setting an upper limit on the water abundance of 3 X 10-6 (relative to H2) and on the molecular oxygen abundance of 2 X 10-6 (relative to H2). SWAS is designed to carry all elements of a ground based radiotelescope. The telescope is a highly efficient 54 X 68-cm off-axis Cassegrain antenna with an aggregate surface error less than or equal to 11 micrometers rms. The receiver system consists of two independent heterodyne receivers with second harmonic Schottky diode mixers, passively cooled to approximately equals 150 K. The spectrometer is a single acousto-optical spectrometer (AOS) with 1400 1-MHz channels enabling simultaneous observations of the H2O, O2, CI, and 13CO lines.