In the search for exoplanets, one of the most sought-after goals from the outset has been to survey nearby systems for Earth-mass planets in their habitable zones. Because of their relatively low mass and size, and their separation from their host stars, these most prized targets have been elusive so far. High precision astrometry is an indirect method that is complementary in its reach to those of the Doppler and photometric transit methods. The Micro-arcsecond Astrometry Small Satellite (MASS), is a concept astrometric mission utilizing a small space telescope and ultra-precise focal plane and field distortion calibration. In one version, recently proposed as a NASA mission of opportunity, MASS's performance was evaluated with a 35 cm primary mirror and a 150 Mpix scientific CMOS sensor, and was estimated to achieve a single-look accuracy of 6 µas. Even with this modest telescope, MASS would have the sensitivity needed to find a 1 Earth-mass planet at 1 AU orbit (scaled to solar luminosity) around the ~5 nearest FGK stars and 2 Earth-mass planets around an additional ~15 nearest stars. MASS would be able not only to measure masses of these exo-Earths but also characterize their orbits for follow- on direct detection missions such as HabEx or LUVOIR. In this presentation we describe the proposed mission, the precision astrometric technique, and the results from testbed demonstration of the technique.
KEYWORDS: Telescopes, Sun, Satellites, Large Synoptic Survey Telescope, Signal to noise ratio, Device simulation, Space telescopes, Large telescopes, Solar system, Photons
Large or even medium sized asteroids impacting the Earth can cause damage on a global scale. Existing and planned concepts for finding near-Earth objects (NEOs) with diameter of 140 m or larger would take ~15-20 years of observation to find ~90% of them. This includes both ground and space based projects. For smaller NEOs (~50-70 m in diameter), the time scale is many decades. The reason it takes so long to detect these objects is because most of the NEOs have highly elliptical orbits that bring them into the inner solar system once per orbit. If these objects cross the Earth's orbit when the Earth is on the other side of the Sun, they will not be detected by facilities on or around the Erath. A constellation of MicroSats in orbit around the Sun can dramatically reduce the time needed to find 90% of NEOs ~100-140 m in diameter.
Spacecraft carrying optical communication lasers can be treated as artificial stars, whose relative astrometry to Gaia reference stars provides spacecraft positions in the plane-of-sky for optical navigation. To be comparable to current Deep Space Network delta-Differential One-way Ranging measurements, thus sufficient for navigation, nanoradian optical astrometry is required. Here we describe our error budget, techniques for achieving nanoradian level ground-base astrometry, and preliminary results from a 1 m telescope. We discuss also how these spacecraft may serve as artificial reference stars for adaptive optics, high precision astrometry to detect exoplanets, and tying reference frames defined by radio and optical measurements.
Theia is an astrometric mission proposed to ESA in 2014 for which one of the scientific objectives is detecting
Earth-like exoplanets in the habitable zone of nearby solar-type stars. This objective requires the capability
to measure stellar centroids at the precision of 1x10-5 pixel. Current state-of-the-art methods for centroid
estimation have reached a precision of about 3x10-5 pixel at two times Nyquist sampling, this was shown at
the JPL by the VESTA experiment. A metrology system was used to calibrate intra and inter pixel quantum
efficiency variations in order to correct pixelation errors. The Theia consortium is operating a testbed in vacuum
in order to achieve 1x10-5 pixel precision for the centroid estimation. The goal is to provide a proof of concept
for the precision requirement of the Theia spacecraft.
The testbed consists of two main sub-systems. The first one produces pseudo stars: a blackbody source is
fed into a large core fiber and lights-up a pinhole mask in the object plane, which is imaged by a mirror on the
CCD. The second sub-system is the metrology, it projects young fringes on the CCD. The fringes are created by
two single mode fibers facing the CCD and fixed on the mirror. In this paper we present the latest experiments
conducted and the results obtained after a series of upgrades on the testbed was completed. The calibration
system yielded the pixel positions to an accuracy estimated at 4x10-4 pixel. After including the pixel position
information, an astrometric accuracy of 6 x 10-5 pixel was obtained, for a PSF motion over more than 5 pixels.
In the static mode (small jitter motion of less than 1 x 10-3 pixel), a photon noise limited precision of 3x10-5
pixel was reached.
High quality linear laser frequency chirp of high chirp rate is critical to many laser ranging applications. In this paper, we describe a cost-effective chirp linearization approach implemented on our Inverse synthetic Aperture LADAR (ISAL) imaging testbed. Our approach uses a COTS PZT for external cavity laser frequency tuning and a common self-heterodyne fiber interferometer as a frequency monitor, with a two-step hardware and software chirp linearization procedure to achieve high quality chirp. First, the nominal triangle waveform input to PZT drive is modified through an iterative process prior to ISAL imaging acquisition. Several waveforms with chirp rates between 1 and 4THz/s have been acquired with residual chirp rate error ~ +/-2% in usable region. This process generally needs to be done only once for a typical PZT that has excellent repeatability but poor linearity. The modified waveform is then used during ISAL imaging acquisition without active control while the imperfection in transmitted frequency is monitored. The received imaging data is resampled digitally based on frequency error calculated from the frequency monitor data, effectively reduce chirp nonlinearity to ~+/- 0.2% in chirp rate error. The measured system impulse response from return signal shows near designed range resolution of a few mm, demonstrating the effectiveness of this approach.
Palomar’s Project 1640 (P1640) is the first stellar coronagraph to regularly use active coronagraphic wavefront control (CWFC). For this it has a hierarchy of offset wavefront sensors (WFS), the most important of which is the higher-order WFS (called CAL), which tracks quasi-static modes between 2-35 cycles-per-aperture. The wavefront is measured in the coronagraph at 0.01 Hz rates, providing slope targets to the upstream Palm 3000 adaptive optics (AO) system. The CWFC handles all non-common path distortions up to the coronagraphic focal plane mask, but does not sense second order modes between the WFSs and the science integral field unit (IFU); these modes determine the system’s current limit. We have two CWFC operating modes: (1) P-mode, where we only control phases, generating double-sided darkholes by correcting to the largest controllable spatial frequencies, and (2) E-mode, where we can control amplitudes and phases, generating single-sided dark-holes in specified regions-of-interest. We describe the performance and limitations of both these modes, and discuss the improvements we are considering going forward.
P1640 high contrast imaging system on the Palomar 200 inch Telescope consists of an apodized-pupil Lyot coronagraph, the PALM-3000 adaptive optics (P3K-AO), and P1640 Calibrator (CAL). Science images are recorded by an integral field spectrograph covering J-H bands for detecting and characterizing stellar companions. With aberrations from atmosphere corrected by the P3K-AO, instrument performance is limited mainly by the quasi-static speckles due to noncommon path wavefront aberrations for the light to propagate to the P3K-AO wavefront sensor and to the coronagraph mask. The non-common path wavefront aberrations are sensed by CAL, which measures the post-coronagraph E-field using interferometry, and can be effectively corrected by offsetting the P3K-AO deformable mirror target position accordingly. Previously, we have demonstrated using CAL measurements to correct high order wavefront aberrations, which is directly connected to the static speckles in the image plane. Low order wavefront, on the other hand, usually of larger amplitudes, causes light to leak through the coronagraph making the whole image plane brighter. Knowledge error in low order wavefront aberrations can also affect the estimation of the high order wavefront. Even though, CAL is designed to sense efficiently high order wavefront aberrations, the low order wavefront front can be inferred with less sensitivity. Here, we describe our method for estimating both low and high order wavefront aberrations using CAL measurements by propagating the post-coronagraph E-field to a pupil before the coronagraph. We present the results from applying this method to both simulated and experiment data.
The Project 1640 instrument on the 200-inch Hale telescope at Palomar Observatory is a coronagraphic instru- ment with an integral eld spectrograph at the back end, designed to nd young, self-luminous planets around nearby stars. To reach the necessary contrast for this, the PALM-3000 adaptive optics system corrects for fast atmospheric speckles, while CAL, a phase-shifting interferometer in a Mach-Zehnder con guration, measures the quasistatic components of the complex electric eld in the pupil plane following the coronagraphic stop. Two additional sensors measure and control low-order modes. These eld measurements may then be combined with a system model and data taken separately using a white-light source internal to the AO system to correct for both phase and amplitude aberrations. Here, we discuss and demonstrate the procedure to maintain a half-plane dark hole in the image plane while the spectrograph is taking data, including initial on-sky performance.
Project 1640, a high-contrast spectral-imaging effort involving a coordinated set of instrumentation and software, built at
AMNH, JPL, Cambridge and Caltech, has been commissioned and is fully operational. This novel suite of
instrumentation includes a 3388+241-actuator adaptive optics system, an optimized apodized pupil Lyot coronagraph, an
integral field spectrograph, and an interferometric calibration wave front sensor. Project 1640 is the first of its kind of
instrumentation, designed to image and characterize planetary systems around nearby stars, employing a variety of
techniques to break the speckle-noise barrier. It is operational roughly one year before any similar project, with the goal
of reaching a contrast of 10-7 at 1 arcsecond separation. We describe the instrument, highlight recent results, and
document on-sky performance at the start of a 3-year, 99-night survey at the Palomar 5-m Hale telescope.
P1640 calibrator is a wavefront sensor working with the P1640 coronagraph and the Palomar 3000 actuator
adaptive optics system (P3K) at the Palomar 200 inch Hale telescope. It measures the wavefront by interfering
post-coronagraph light with a reference beam formed by low-pass filtering the blocked light from the coronagraph
focal plane mask. The P1640 instrument has a similar architecture to the Gemini Planet Imager (GPI) and its
performance is currently limited by the quasi-static speckles due to non-common path wavefront errors, which
comes from the non-common path for the light to arrive at the AO wavefront sensor and the coronagraph mask.
By measuring the wavefront after the coronagraph mask, the non-common path wavefront error can be estimated
and corrected by feeding back the error signal to the deformable mirror (DM) of the P3K AO system. Here, we
present a first order wavefront estimation algorithm and an instrument calibration scheme used in experiments
done recently at Palomar observatory. We calibrate the P1640 calibrator by measuring its responses to poking
DM actuators with a sparse checkerboard pattern at different amplitudes. The calibration yields a complex
normalization factor for wavefront estimation and establishes the registration of the DM actuators at the pupil
camera of the P1640 calibrator, necessary for wavefront correction. Improvement of imaging quality after feeding
back the wavefront correction to the AO system demonstrated the efficacy of the algorithm.
The search for Earth-mass planets in the habitable zones of nearby Sun-like stars is an important goal of astrophysics.
This search is not feasible with the current slate of astronomical instruments. We propose a new concept for microarcsecond
astrometry which uses a simplified instrument and hence promises to be low cost. The concept employs a
telescope with only a primary, laser metrology applied to the focal plane array, and new algorithms for measuring image
position and displacement on the focal plane. The required level of accuracy in both the metrology and image position
sensing is at a few micro-pixels. We have begun a detailed investigation of the feasibility of our approach using
simulations and a micro-pixel image position sensing testbed called MCT. So far we have been able to demonstrate that
the pixel-to-pixel distances in a focal plane can be measured with a precision of 20 micro-pixels and image-to-image
distances with a precision of 30 micro-pixels. We have also shown using simulations that our image position algorithm
can achieve accuracy of 4 micro-pixels in the presence of λ/20 wavefront errors.
NEAT, Nearby Exo-Earth Astrometric Telescope is a medium-small telescope ~ 1m in diameter that is designed to
make ultra precise < 1 uas (microarcsec) astrometric measurements of nearby stars in a ~ 1hr observation. Four
major error sources prevent normal space telescopes from obtaining accuracies close to 1 uas. Even with a small 1m
telescope, photon noise is usually not a problem for the bright nearby target stars. But in general, the reference stars
are much fainter. Typically a field of view of ~0.5 deg dia is needed to obtain enough bright reference stars. The
NEAT concept uses a very simple but unusual design to avoid optically induced astrometric errors. The third source
of error is the accuracy and stability of the focal plane. A 1uas error over a ~2000 arcsec field of view implies the
focal plane is accurate or at least stable to 5 parts in 1010 over the lifetime of the mission (~5yrs). The 4th class of
error has to do with our knowledge of the PSF and how that PSF is sampled by an imperfect detector. A Nyquist
sampled focal plane would have > 2 pixels per λ/D, and centroiding to 1uas means centroiding to 10-5 pixels. This
paper describes the mission concept, and an overview of the technology needed to perform 1uas astrometry with a
small telescope, and how we overcome problems 1 and 2. A companion paper will describe the technical progress
we've made in solving problems 3 and 4.
The most stringent astrometric performance requirements on NASA's SIM(Space Interferometer
Mission)-Lite mission will come from the so-called Narrow-Angle (NA) observing scenario,
aimed at finding Earth-like exoplanets, where the interferometer chops between the target star
and several nearby reference stars multiple times over the course of a single visit. Previously,
about 20 pm NA error with various shifts was reported1. Since then, investigation has been under
way to understand the mechanisms that give rise to these shifts. In this paper we report our
findings, the adopted mitigation strategies, and the resulting testbed performance.
SIM-Lite missions will perform astrometry at microarcsecond accuracy using star light interferometry. For typical
baselines that are shorter than 10 meters, this requires to measure optical path difference (OPD) accurate to tens of
picometers calling for highly accurate calibration. A major challenge is to calibrate the star spectral dependency
in fringe measurements - the spectral calibration. Previously, we have developed a spectral calibration and
estimation scheme achieving picometer level accuracy. In this paper, we present the improvements regarding the
application of this scheme from sensitivity studies. Data from the SIM Spectral Calibration Development Unit
(SCDU) test facility shows that the fringe OPD is very sensitive to pointings of both beams from the two arms of
the interferometer. This sensitivity coupled with a systematic pointing error provides a mechanism to explain the
bias changes in 2007. Improving system alignment can effectively reduce this sensitivity and thus errors due to
pointing errors. Modeling this sensitivity can lead to further improvement in data processing. We then investigate
the sensitivity to a model parameter, the bandwidth used in the fringe model, which presents an interesting trade
between systematic and random errors. Finally we show the mitigation of calibration errors due to system drifts
by interpolating instrument calibrations. These improvements enable us to use SCDU data to demonstrate that SIM-Lite missions can meet the 1pm noise floor requirement for detecting earth-like exoplanets.
The SIM-Lite astrometric interferometer will search for Earth-size planets in the habitable zones of nearby stars. In this
search the interferometer will monitor the astrometric position of candidate stars relative to nearby reference stars over
the course of a 5 year mission. The elemental measurement is the angle between a target star and a reference star. This is
a two-step process, in which the interferometer will each time need to use its controllable optics to align the starlight in
the two arms with each other and with the metrology beams. The sensor for this alignment is an angle tracking CCD
camera. Various constraints in the design of the camera subject it to systematic alignment errors when observing a star of
one spectrum compared with a start of a different spectrum. This effect is called a Color Dependent Centroid Shift
(CDCS) and has been studied extensively with SIM-Lite's SCDU testbed. Here we describe results from the simulation
and testing of this error in the SCDU testbed, as well as effective ways that it can be reduced to acceptable levels.
SIM Lite is a space-borne stellar interferometer capable of searching for Earth-size planets in the habitable zones of
nearby stars. This search will require measurement of astrometric angles with sub micro-arcsecond accuracy and optical
pathlength differences to 1 picometer by the end of the five-year mission. One of the most significant technical risks in
achieving this level of accuracy is from systematic errors that arise from spectral differences between candidate stars and
nearby reference stars. The Spectral Calibration Development Unit (SCDU), in operation since 2007, has been used to
explore this effect and demonstrate performance meeting SIM goals. In this paper we present the status of this testbed
and recent results.
KEYWORDS: MATLAB, Computing systems, Cameras, Data storage, Data processing, Data acquisition, Distributed computing, Human-machine interfaces, Control systems, Real-time computing
In the course of fulfilling its mandate, the Spectral Calibration Development Unit (SCDU) testbed for SIM-Lite produces
copious amounts of raw data. To effectively spend time attempting to understand the science driving the data, the team
devised computerized automations to limit the time spent bringing the testbed to a healthy state and commanding it,
and instead focus on analyzing the processed results. We developed a multi-layered scripting language that emphasized
the scientific experiments we conducted, which drastically shortened our experiment scripts, improved their readability,
and all-but-eliminated testbed operator errors. In addition to scientific experiment functions, we also developed a set of
automated alignments that bring the testbed up to a well-aligned state with little more than the push of a button. These
scripts were written in the scripting language, and in Matlab via an interface library, allowing all members of the team to
augment the existing scripting language with complex analysis scripts. To keep track of these results, we created an easilyparseable
state log in which we logged both the state of the testbed and relevant metadata. Finally, we designed a distributed
processing system that allowed us to farm lengthy analyses to a collection of client computers which reported their results
in a central log. Since these logs were parseable, we wrote query scripts that gave us an effortless way to compare results
collected under different conditions. This paper serves as a case-study, detailing the motivating requirements for the
decisions we made and explaining the implementation process.
SCDU (Spectral Calibration Development Unit) is a vacuum test bed that was built and operated for the SIM-Planetquest
Mission and has successfully demonstrated the calibration of spectral instrument error to an accuracy of
better than 20 picometers. This performance is consistent with the 1 micro-arc second goal of SIM. The calibration
procedure demonstrated in the test bed is traceable to the SIM flight instrument. This article is a review of all aspects of
the design and operation of the hardware as well as the methodology for spectral calibration. Spectral calibration to
better than 20 picometers and implications for flight are discussed.
This paper will present the analysis results taken from a well-designed interferometer SCDU. The
objective is to deliver picometer performance to meet the allocated astrometric error budget from SIM
PlanetQuest mission. It will describe the validation of optical designs and analysis procedures to achieve
high accuracy of the tip-tilt and shear alignments. Then it will enumerate environmental factors essential to
the SCDU performances. Finally it will report color-independent 3 picometer Narrow Angle (NA)
performance and all-in-one 17 picometer NA performance. The all-in-one pico-performance will require
spectral calibration modeling to remove delay differential induced by color.
KEYWORDS: Calibration, Spectral calibration, Data modeling, Fourier transforms, Wavefronts, Charge-coupled devices, Optical filters, Monte Carlo methods, Modulation, Signal detection
The SIM PlanetQuest Mission will perform astrometry to one microarcsecond accuracy using optical interferometers
requiring optical path delay difference (OPD) measurements accurate to tens of picometers. Success
relies on very precise calibration. Spectral Calibration Development Unit (SCDU) has been built to demonstrate
the capability of calibrating spectral dependency of the white light fringe OPD to accuracy better than 20pm.
In this article, we present the spectral calibration modeling work for SCDU to achieve the SIM PlanetQuest
Engineering Milestone 4. SCDU data analysis shows that the wave front aberrations cause the instrument phase
dispersions to vary by tens of nanometers over the bandwidth of a CCD pixel making the previous model inadequate.
We include the effect of the wave front aberrations in the white light fringe model and develop a
procedure for calibrating the corresponding model parameters using long stroke fringe data based on Discrete
Fourier Transform. We make the calibration procedure flight traceable by dividing the whole calibration into the
instrument calibration and the source spectral calibration. End-to-end simulations are used to quantify both
the systematic and random errors in spectral calibration. The efficacy of the calibration scheme is demonstrated
using the SCDU experimental data.
Precision white light interferometry performed at the picometer class level is an extremely challenging endeavor.
Over the past several years a combination of analysis, experiment, and reconciliation of the two has yielded
continued improvements and refinements of the process to bring this technology to fruition. This paper provides
an overview of several of the refinements of the interference models and algorithms developed for calibration and
fringe estimation that have evolved over this period.
KEYWORDS: Stars, Interferometers, Charge-coupled devices, Visualization, Superposition, Target detection, Photons, Signal detection, Error analysis, Signal to noise ratio
SIM PlanetQuest will measure star positions to an accuracy of a few microarcseconds using precise white light
fringe measurements. One challenge for the SIM observation scenario is "star confusion," where multiple stars
are present in the instrument field of view. This is especially relevant for observing dim science targets because
the density of number of stars increases rapidly with star magnitude. We study the effect of star confusion on
the SIM astrometric performance due to systematic fringe errors caused by the extra photons from the confusion
star(s). Since star confusion from multiple stars may be analyzed as a linear superposition of the effect from single
star confusion, we quantify the astrometric errors due to single star confusion surveying over many spectral types,
including A0V, F0V, K5III, and M0V, and for various visual magnitude differences. To the leading order, the
star confusion effect is characterized by the magnitude difference, spectral difference, and the angular separation
between the target and confusion stars.
Strategies for dealing with star confusion are presented. For example, since the presence of additional sources
in the field of view leads to inconsistent delay estimates from different channels, with sufficient signal to noise
ratio, the star confusion can be detected using chi-square statistics of fringe measurements from multiple spectral
channels. An interesting result is that the star confusion can be detected even though the interferometer cannot
resolve the separation between the target and confusion stars when their spectra are sufficiently different. Other
strategies for mitigating the star confusion effect are also discussed.
SIM PlanetQuest will be the first space-based interferometer and will allow astrometric measurements that are
several hundred times more accurate than the previous missions operating at optical wavelengths. SIM promises
to achieve microarcsecond accuracy for astrometry on objects as faint as 20th visual magnitude. One of the
challenges is to achieve this accuracy at these dim levels, in the presence of even dimmer stars inside the SIM's
FOV. Therefore it is important to investigate the effects of "confusion" fields on astrometric performance for
SIM. This study will look at effects of Angle Tracking Camera performance for SIM in presence of a crowded
or confusion field near a target star. This will lead to a study that predicts SIM astrometric performance in
a crowded field. Centroid displacements due to any perturbation including a crowded field cause a shift in the
fringe and reduce visibility and performance. In this work we will devise an estimator to estimate the bias in
the centroid of SIM Angle Tracking camera in presence of a crowded field. This analysis will examine pointing
accuracy performance by estimating Angle Tracking centroid under different parameter variations that affect
performance. These parameters are number of neighboring stars, stellar types, and angular separation as well
as their relative brightness.
The Space Interferometry Mission(SIM) will measure optical path differences (OPDs) with an accuracy of tens of picometers, requiring precise calibration of the instrument. In this article, we present a calibration approach based on fitting star light interference fringes in the interferometer using a least-squares algorithm. The algorithm is first analyzed for the case of a monochromatic light source with a monochromatic fringe model. Using fringe data measured on the Micro-Arcsecond Metrology(MAM) testbed with a laser source, the error in the determination of the wavelength is shown to be less than 10pm. By using a quasi-monochromatic fringe model, the algorithm can be extended to the case of a white light source with a narrow detection bandwidth. In SIM, because of the finite bandwidth of each CCD pixel, the effect of the fringe envelope can not be neglected, especially for the larger optical path difference range favored for the wavelength calibration. We eliminate the fringe envelope effect by "projecting away" the fringe envelope, i.e. working in a subspace orthogonal to the envelope signal. The resulting fringe envelope parameters are needed for subsequent OPD estimation in SIM. We show the sensitivities to various errors. The algorithm is validated using both simulation and the fringe data obtained on the MAM test bed.
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