Optical interferometry is a powerful technique to achieve high angular resolution. However, its main issue is its lack of sensitivity, compared to other observation techniques. Efforts have been made in the previous decade to improve the sensitivity of optical interferometry, with instruments such as PIONIER and GRAVITY at VLTI, or MIRC-X and MYSTIC at CHARA. While those instruments pushed on sensitivity, their design focus was not the sensitivity but relative astrometric accuracy, imaging capability, or spectral resolution. Our goal is to build an instrument specifically designed to optimize for sensitivity. This meant focusing our design efforts on different parts of the instrument and investigating new technologies and techniques. First, we make use of the low noise C-RED One camera using e-APD technology and provided by First Light Imaging, already used in the improvement of sensitivity in recent new instruments. We forego the use of single-mode fibers but still favor an image plane design that offers more sensitivity than a pupil plane layout. We also use a minimum number of optical elements to maximize the throughput of the design, using a long focal length cylindrical mirror. We chose to limit our design to 3 beams, to have the capability to obtain closure phases, but not dilute the incoming flux in more beam combinations. We also use in our design an edge filter to have the capability to observe Hand K-band at the same time. We use a low spectral resolution, allowing for group delay fringe tracking but maximizing the SNR of the fringes for each spectral channel. All these elements will lead to a typical limiting magnitude between 10 and 11 in both H- and K-bands.
The Center for High Angular Resolution Astronomy (CHARA) Array is a six-element interferometer with baselines ranging from 34 to 331 m. Three new beam combiners are entering operation: MYSTIC is a 6-telescope combiner for K-band; SPICA is a 6-telescope combiner for the visible R-band; and SILMARIL is a 3-telescope combiner for high sensitivity in H and K-bands. A seventh, portable telescope will use fiber optics for beam transport and will increase the baselines to 1 km. Observing time is available through a program funded by NSF. The programs are solicited and peer-reviewed by NSF’s National Optical-Infrared Astronomy Research Laboratory. The open community access has significantly expanded the range of astronomical investigations of stars and their environments. Here we summarize the scientific work and the on-going technical advances of the CHARA Array.
Much research has been done to show the possibilities of using long transport fibers in optical interferometry. The CHARA Michelson Array Pathfinder will extend the spatial coverage of the CHARA Array by adding a mobile 1-meter telescope connected by optical fibers. The pathfinder will operate in H-band and will explore baselines up to approximately 1 km, giving an angular resolution of 0.2 mas. The new telescope will be placed at short baselines to image the surfaces of large stars and at long baselines to resolve small stars. Here we describe the project and our progress on various subsystems.
The Magdalena Ridge Observatory Interferometer (MROI) is designed to operate 10 1.4m telescopes simultaneously, with baselines ranging from 7.8-347 m and limiting infrared fringe-tracking magnitudes of 14 – it is arguably the most ambitious optical/infrared imaging interferometer under construction today. In this paper we had intended to present an update of activities since the 2018 SPIE meeting as we approached a demonstration of first fringes with the facility originally anticipated for the fall of 2020. However, due to the global pandemic and a loss of funding for our project via AFRL, we have been unable to make the progress we intended. In this paper, we present results up through March, 2020 and a brief discussion of the path forward for the facility.
The first unit telescope of Ridge Observatory Interferometer is integrated on the array and starlight has been observed in the Beam Combining Area for the first time. From the telescope, the beam travels in vacuum over a path of >50m, including a beam relay system and delay line. This feat was made possible by a prototype version of the Automated Alignment System that we are developing for minimising fringe visibility loss due to misalignment. We present results of on-site validation of UTLIS, a reference light source at the unit telescope acting as a proxy for starlight, and BEASST, a Shack-Hartmann sensor that simultaneously detects beam angle and position.
We present the design and testing of FOURIER, the first generation science beam combiner for the MROI. FOURIER is a three-way, J, H and K band image plane combiner which is designed primarily for observations at faint limiting magnitudes. We outline the main science requirements and discuss how the design of FOURIER contributes to meeting these requirements. We present the first laboratory characterisation of the instrument including validation of the PSF profile, demonstration of high contrast fringes, and the spectral resolution of the instrument, all of which show promising results. We conclude by discussing the path to deployment of FOURIER at the MROI ahead of the first science observation at the array.
Xenomai1 is a hard real-time operating system suitable for many low-latency tasks encountered in astronomical instruments. It is open source, has microsecond-level response time and coexists with the Linux kernel, thereby facilitating the execution of hard real time code on Linux systems. This presentation presents experience coding systems with Xenomai for the Magdalena Ridge Observatory Interferometer. Firstly an overview of Xenomai is given, focusing on how it achieves hard real time performance and how it can be used to interact with hardware using Linux-like device drivers. Secondly, a generic outline of the development process is given, including the mindset needed, general pitfalls to be avoided, and strategies that can be employed depending on how open the hardware and any existing source code is. Two specific case studies from the Magdalena Ridge Observatory are then presented: Firstly, the fast tip-tilt system, which must read out a 32x32 subframe from an EMCCD camera, determine a stellar image centroid and send a correction voltage to a tip-tilt mirror at up to 1kHz. Secondly, the MROI delay line metrology system, which must read laser metrology position data for ten delay line trolleys and send correction voltages to their cat’s eyes at 5kHz. Finally, some future challenges to development with Xenomai and other hard real time operating systems are discussed: processors with functionality such as system management interrupts that are beyond operating system control, and the trend towards buffered or closed interfaces between computers and hardware.
The Unit Telescope (UT) for the Magdalena Ridge Observatory (MROI) is composed of four major hardware components: The Unit Telescope Mount (UTM), Enclosure, Optics and the Fast Tip Tilt System (FTTS). Integration of the UT started in 2016 when the UTM arrived and its Assembly, Integration and Verification activities began. Critical activities included: installation at the Maintenance Facility, integration and alignment of the Optics and Wave Front Sensor (WFS) and finally the complete optical alignment. End-to-end UTM Site Acceptance Tests (SAT) were performed. Subsequent activities included receiving and integrating the FTTS. With the arrival and assembly of the Enclosure, the last component of the UT was ready for integration on a dedicated concrete pier. Specialized equipment will be used for the final integration of the UT, and for transportation to its final location on the array where SAT for the UT will take place.
The Magdalena Ridge Observatory Interferometer (MROI) has been under development for almost two decades. Initial funding for the facility started before the year 2000 under the Army and then Navy, and continues today through the Air Force Research Laboratory. With a projected total cost of substantially less than $200M, it represents the least expensive way to produce sub-milliarcsecond optical/near-infrared images that the astronomical community could invest in during the modern era, as compared, for instance, to extremely large telescopes or space interferometers. The MROI, when completed, will be comprised of 10 x1.4m diameter telescopes distributed on a Y-shaped array such that it will have access to spatial scales ranging from about 40 milliarcseconds down to less than 0.5 milliarcseconds. While this type of resolution is not unprecedented in the astronomical community, the ability to track fringes on and produce images of complex targets approximately 5 magnitudes fainter than is done today represents a substantial step forward. All this will be accomplished using a variety of approaches detailed in several papers from our team over the years. Together, these two factors, multiple telescopes deployed over very long-baselines coupled with fainter limiting magnitudes, will allow MROI to conduct science on a wide range and statistically meaningful samples of targets. These include pulsating and rapidly rotating stars, mass-loss via accretion and mass-transfer in interacting systems, and the highly-active environments surrounding black holes at the centers of more than 100 external galaxies. This represents a subsample of what is sure to be a tremendous and serendipitous list of science cases as we move ahead into the era of new space telescopes and synoptic surveys. Additional investigations into imaging man-made objects will be undertaken, which are of particular interest to the defense and space-industry communities as more human endeavors are moved into the space environment.
In 2016 the first MROI telescope was delivered and deployed at Magdalena Ridge in the maintenance facility. Having undergone initial check-out and fitting the system with optics and a fast tip-tilt system, we eagerly anticipate installing the telescope enclosure in 2018. The telescope and enclosure will be integrated at the facility and moved to the center of the interferometric array by late summer of 2018 with a demonstration of the performance of an entire beamline from telescope to beam combiner table shortly thereafter. At this point, deploying two more telescopes and demonstrating fringe-tracking, bootstrapping and limiting magnitudes for the facility will prove the full promise of MROI. A complete status update of all subsystems follows in the paper, as well as discussions of potential collaborative initiatives.
The ICoNN (Infrared Coherencing Nearest Neighbors) fringe tracker system is the heart of the Magdalena Ridge Observatory Interferometer (MROI). It operates in the near-infrared at H or Ks in such a way that the light being used by the fringe tracker can phase up the interferometric array, but not steal photons from the scientific instruments of the interferometer system. It is capable of performing either in group delay tracking or fringe phase tracking modes, depending on the needs of the scientific observations. The spectrograph for the MROI beam combiner was originally designed for the Teledyne PICNIC array. Developments in detector technology have allowed for an alternative to the original choice of infrared array to finally become available – in particular, the SAPHIRA detector made by Selex. Very low read noise and very fast readout rates are significant reasons for adopting these new detectors, traits which also allow relaxation of some of the opto-mechanical requirements that were needed for the PICNIC chip to achieve marginal sensitivity. This paper will discuss the opto-mechanical advantages and challenges of using the SAPHIRA detector with the pre-existing hardware. In addition to a design for supporting the new detector, alignment of optical components and initial testing as a system are reported herein.
Stable beam alignment of an optical interferometer is crucial for maintaining a usable signal-to-noise ratio during science measurements on faint astronomical targets. The Magdalena Ridge Observatory Interferometer will use an Automated Alignment System (AAS) that performs a start-of-night alignment procedure and subsequent alignment corrections in between observations, all without the need for human intervention. Its design has recently been updated in line with a revised error budget for MROI requiring that two axis drifts during science operations should not exceed 15 milliarcseconds in tilt, referred to the sky, nor 1% of the beam diameter in shear. For each beam line, the AAS provides two reference light beams, a pair of quad cells to monitor coarse alignment, and a tilt and shear detector for tracking fine drifts. The tilt and shear detector is a novel application of a Shack-Hartmann array that permits the simultaneous measurement tilt and shear well within requirements for MROI. Results of laboratory testing and simulations are presented here.
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.
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.
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.
The Keck Interferometer combines the two 10 m Keck telescopes as a long baseline interferometer, funded by
NASA, as a joint development among the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the
Michelson Science Center. Since 2004, it has offered an H- and K-band fringe visibility mode through the Keck
TAC process. Recently this mode has been upgraded with the addition of a grism for higher spectral resolution.
The 10 um nulling mode, for which first nulling data were collected in 2005, completed the bulk of its engineering
development in 2007. At the end of 2007, three teams were chosen in response to a nuller key science call to
perform a survey of nearby stars for exozodiacal dust. This key science observation program began in Feb. 2008.
Under NSF funding, Keck Observatory is leading development of ASTRA, a project to add dual-star capability for
high sensitivity observations and dual-star astrometry. We review recent activity at the Keck Interferometer, with an
emphasis on the nuller development.
The Keck Interferometer combines the two 10m diameter Keck telescopes for near-infrared fringe visibility, and mid-infrared
nulling observations. We report on recent progress with an emphasis on new visibility observing capabilities,
operations improvements for visibility and nulling, and on recent visibility science. New visibility observing capabilities
include a grism spectrometer for higher spectral resolution. Recent improvements include a new AO output dichroic for
increased infrared light throughput, and the installation of new wave-front controllers on both Keck telescopes. We also
report on recent visibility results in several areas including (1) young stars and their circumstellar disks, (2) pre-main
sequence star masses, and (3) Circumstellar environment of evolved stars. Details on nuller instrument and nuller science
results, and the ASTRA phase referencing and astrometry upgrade, are presented in more detail elsewhere in this
conference.
The Keck Interferometer Nuller (KIN) is now largely in place at the Keck Observatory, and functionalities and
performance are increasing with time. The main goal of the KIN is to examine nearby stars for the presence of exozodiacal
emission, but other sources of circumstellar emission, such as disks around young stars, and hot exoplanets are
also potential targets. To observe with the KIN in nulling mode, knowledge of the intrinsic source spectrum is essential,
because of the wide variety of wavelengths involved in the various control loops - the AO system operates at visible
wavelengths, the pointing loops use the J-band, the high-speed fringe tracker operates in the K-band, and the nulling
observations take place in the N-band. Thus, brightness constraints apply at all of these wavelengths. In addition, source
structure plays a role at both K-band and N-band, through the visibility. In this talk, the operation of the KIN is first
briefly described, and then the sensitivity and performance of the KIN is summarized, with the aim of presenting an
overview of the parameter space accessible to the nuller. Finally, some of the initial observations obtained with the KIN
are described.
We describe the results of laboratory experiments, using a mock-up stellar interferometer equipped with specialized hardware, undertaken to measure differential-phase to considerable precision (0.1 mrad) over an octave of bandwidth in the infrared. Differential-phase is a precision technique that can detect subtle temporal changes in the relative (color-dependent) photocenter of an astronomical target - making it useful for direct detection of some hot-Jupiter planets from the ground. The set up described herein was built as part of the Keck Interferometer project.
In this paper we report on progress at the Keck Interferometer since the 2004 SPIE meeting with an emphasis on the operations improvements for visibility science.
Mid-infrared (8-13μm) nulling is a key observing mode planned for the NASA-funded Keck Interferometer at the Keck Observatory on the summit of Mauna Kea in Hawaii. By destructively interfering and thereby canceling the on-axis light from nearby stars, this observing mode will enable the characterization of the faint emission from exo-zodiacal dust surrounding these stellar systems. We report here the null leakage error budget and pre-ship results obtained in the laboratory after integration of the nulling beam combiner with its mid-infrared camera and key components of the Keck Interferometer. The mid-infrared nuller utilizes a dual-polarization, modified Mach-Zehnder (MMZ) beam combiner in conjunction with an atmospheric dispersion corrector to achieve broadband achromatic nulling.
The Keck Interferometer Nuller (KIN) will be used to examine nearby stellar systems for the presence of circumstellar exozodiacal emission. A successful pre-ship review was held for the KIN in June 2004, after which the KIN was shipped to the Keck Observatory. The integration of the KIN's many sub-systems on the summit of Mauna Kea, and initial on-sky testing of the system, has occupied the better part of the past year. This paper describes the KIN system-level configuration, from both the hardware and control points of view, as well as the current state of integration of the system and the measurement approach to be used. During the most recent on-sky engineering runs in May and July 2005, all of the sub-systems necessary to measure a narrowband null were installed and operational, and the full nulling measurement cycle was carried out on a star for the first time.
The first high-dynamic-range interferometric mode planned to come on line at the Keck Observatory is mid-infrared nulling. This observational mode, which is based on the cancellation of the on-axis starlight arriving at the twin Keck telescopes, will be used to examine nearby stellar systems for the presence of circumstellar exozodiacal emission. This paper describes the system level layout of the Keck Interferometer Nuller (KIN), as well as the final performance levels demonstrated in the laboratory integration and test phase at the Jet Propulsion Laboratory prior to shipment of the nuller hardware to the Keck Observatory in mid-June 2004. On-sky testing and observation with the mid-infrared nuller are slated to begin in August 2004.
One of the science goals of NASA's Navigator program is ground-based narrow-angle astrometry for extra-solar planet detection, which could be done as part of the proposed Outrigger Telescopes Project. The narrow-angle measurement process, which would use the outrigger telescopes, starts with the determination of the conventional interferometer astrometric baseline, determined from wide-angle astrometry of Hipparcos stars. A baseline monitor system would be employed at each outrigger telescope. This system monitors the pivot point of each telescope - the end point of the astrometric baseline - to measure telescope imperfections that would cause the baseline to vary with telescope rotation. The baseline monitor includes azimuth and elevation cameras that monitor runout along the azimuth and elevation axes of the telescopes. In conjunction with the baseline monitor system, a pivot monitor camera in the dual-star module is used to register the laser metrology corner-cube reflector to the telescope pivot, tying the narrow-angle baseline, which applies to the narrow-angle astrometric measurement, to the wide-angle baseline. In this paper we present the proposed designs for the baseline monitor and pivot-point camera.
A key thrust of NASA's Origins program is the development of
astronomical interferometers. Pursuing this goal in a cost-effective and expedient manner from the ground has led NASA to develop the Keck Interferometer, which saw first fringes between the twin 10m Keck telescopes in March of 2001. In order to enhance the imaging potential of this facility, and to add astrometric capabilities for the detection of giant planets about nearby stars, four 1.8 m 'outrigger' telescopes may be added to the interferometer. Robust performance of the multi-aperture instrument will require precise alignment of the large number of optical elements found in the six optical beamtrains spread about the observatory site. The requirement for timely and reliable alignments dictated the development of an automatic alignment system for the Keck Interferometer. The autoaligner consists of swing-arm actuators that insert light-emitting diodes on the optical axis at the location of each optical element, which are viewed by a simple fixed-focus CCD camera at the end of the beamtrain. Sub-pixel centroiding is performed upon the slightly out-of-focus target spots using images provided by a frame grabber, providing steering information to the two-axis actuated optical elements. Resulting mirror-to-mirror alignments are good to within 2 arcseconds, and trimming the alignment of a full beamtrain is designed to take place between observations, within a telescope repointing time. The interactions of the autoaligner with the interferometer delay lines and coude trains are discussed in detail. The overall design of the interferometer's autoaligner system is presented, examining the design philosophy, system sequencing, optical element actuation, and subsystem co-alignment, within the context of satisfying performance requirements and cost constraints.
KEYWORDS: Interferometers, Cameras, Stars, Signal to noise ratio, Control systems, Servomechanisms, Electronics, Staring arrays, Adaptive optics, K band
The fringe detection and tracking system of the Keck Interferometer, Fatcat, has been operational ever since first fringes at Keck, albeit not in full capacity. At present it supports single baseline (Keck-Keck) operations only. We briefly discuss the instrument design from a hardware and controls standpoint. We also show some recent data from the instrument and summarize some performance limits.
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