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.
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.
An Inverse Synthetic Aperture LADAR (ISAL) system is capable of providing high resolution surface mapping of near Earth objects which is an ability that has gained significant interest for both exploration and hazard assessment. The use of an ISAL system over these long distances often presents the need to operate the optical system in photon-starved conditions. This leads to a necessity to understand the implications of photon and detector noise in the system. Here a Carrier-to-Noise Ratio is derived which is similar to other optical imaging CNR definitions. The CNR value is compared to the quality of experimentally captured images recovered using the Phase Gradient Autofocus technique both with and without the presence of atmospheric turbulence. A minimum return signal CNR for the PGA to work is observed.
The SIM Lite Astrometric Observatory is to perform narrow angle astrometry to search for Earth-like planets, and global
astrometry for a broad astrophysics program, for example, mapping the distribution of dark matter in the Galaxy. The
new SIM Lite consists of two Michelson interferometers and one star tracking telescope. The main six-meter baseline
science interferometer observes a target star and a set of reference stars. The four-meter baseline interferometer (guide-1)
monitors the attitude of the instrument in the direction of a target star. The Guide-2 telescope (G2T) tracks a bright star
to monitor the attitude of the instrument in the other two orthogonal directions. A testbed has been built to demonstrate
star-tracking capability of the G2T concept using a new interferometric angle metrology system. In the presence of
simulated 0.2 arcsecond level of expected spacecraft attitude control system perturbations, the measured star-tracking
capability of the G2T testbed system is less than 43 micro-arcsecond during single narrow angle observation.
Main brassboard Michelson interferometer components have been recently developed for the future flight phase
implementations of SIM Lite mission. These brassboard components include two fine steering mirrors, pathlength
modulation and cyclic averaging optics and astrometric beam combiner assembly. Field-independent performance tests
will be performed in a vacuum chamber using two siderostats in retro-reflecting positions and a white light stimulus. The
brightness and color dependence of the angle and fringe tracking performance will be measured. The performance of
filtering algorithms will be tested in a simulated spacecraft attitude control system perturbation. To demonstrate
capability of a dim star observation, the angle and fringe tracking CCD sensors are cooled to -110 C using a cold diode
heat pipe system. The new feed-forward control (angle and path-length) algorithms for the dim star observation will be
tested as well. In this paper, we will report the recent progress toward the integration and performance tests of the
brassboard interferometer.
The Guide-2 telescope (G2T) is an important subsystem of the new SIM Lite Astrometric Observatory. In this paper we present system identification experiments, design and implementation of the G2T stellar pointing loop that achieves milliarcsecond resolution of spacecraft attitude. Special emphasis was placed on characterization and modeling of PZT hysteresis since this nonlinearity plays an important part in the control loop performance. Power spectral densities of the star image centroids were use to evaluate the pointing loop performance with and with out the presence of simulated ACS disturbances injected via a fast steering mirror (FSM).
The Space Interferometry Mission Light (SIM-Lite) is a new mission concept to perform a micro-arcsecond narrow-angle
astrometry to search approximately 50 nearby stars for Earth-like planets, and to perform a global astrometry with
an accuracy of six micro-arcsecond position and parallax measurements. The SIM-Lite consists of two Michelson
interferometers and one telescope. The main six-meter baseline science interferometer observes a target star and a set of
reference stars. The four-meter baseline interferometer (guide-1) monitors the attitude of the instrument in the direction
of a target star. A Guide-2 telescope (G2T) tracks a bright star to monitor the attitude of the instrument in the other two
orthogonal directions. To demonstrate the concept of the G2T, we have developed a testbed using brassboard optics
built for the SIM project. The G2T testbed consists of a 35 cm siderostat, a beam compressor, and a fast steering mirror
(FSM) in closed loop with a CCD based pointing sensor. A heterodyne laser angle metrology system is used to monitor
angular positions of the FSM with required accuracy of 20 micro-arcsecond during SIM-Lite narrow-angle observation
time. We present the concept of the testbed architecture and preliminary test results of the angular metrology (aMet)
system.
Future space-based optical interferometers, such as the Space Interferometer Mission Planet Quest (SIM), require thermal stability of the optical wavefront to the level of picometers in order to produce astrometric data at the micro-arc-second level. In SIM, the internal path of the interferometer will be measured with a small metrology beam whereas the starlight fringe position is estimated from a large concentric annular beam. To achieve the micro-arc-second observation goal for SIM, it is necessary to maintain the optical path difference between the central and the outer annulus portions of the wavefront of the front-end telescope optics to a few tens of picometers. The Thermo-Opto-Mechanical testbed (TOM3) was developed at the Jet Propulsion Laboratory to measure thermally induced optical deformations of a full-size flight-like beam compressor and siderostat, the two largest optics on SIM, in flight-like thermal environments. A Common Path Heterodyne Interferometer (COPHI) developed at JPL was used for the fine optical path difference measurement as the metrology sensor. The system was integrated inside a large vacuum chamber in order to mitigate the atmospheric and thermal disturbances. The siderostat was installed in a temperature-controlled thermal shroud inside the vacuum chamber, creating a flight-like thermal environment. Detailed thermal and structural models of the test articles (siderostat and compressor) were also developed for model prediction and correlation of the thermal deformations. Experimental data shows SIM required thermal stability of the test articles and good agreement with the model predictions.
Recent advances in material science have lead to the development of new, very low temperature actuators of Terbium and Dysoprosium with hitherto unavailable stroke, energy efficiency and force. The test instrument described here was developed to investigate the performance characteristics of these new materials and the devices based upon them. The instrument, referred to as a cryogenic dilatometer, is designed for measuring linear displacements, at accuracies of 0.1 micron, in a material, actuator or sensor operating at low temperatures including that of liquid helium. The instrument, just completed, maintains the sample at a known temperature between 4.2 and 77 K, subjects the actuator material to a known and variable magnetic field of up to 1,500 gauss, places a specific and variable preload of 750 N maximum against the actuator or sample, and measures the resulting actuator displacement. A secondary capability is to provide a reference measurement for calibrating commercial capacitance, eddy current, linear variable differential transformers and other displacement gages at low temperatures. The primary linear displacement measurement tool is a fringe counting interferometer. A liquid helium cooled probe provides the sample test environment.
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