The Large Millimeter Telescope Alfonso Serrano (LMT) is a 50m-diameter radio telescope for millimeter-wave astronomy. The performance of large radio telescopes like the LMT is often limited by their response to deformations caused by thermal gradients within the antenna structure. In this paper, we describe a development project to build a real time metrology system for the LMT. The Large Aperture Surface Error Recovery System (LASERS) will measure, on time scales of a minute in real time, the shape of the antenna's primary mirror and the location of its secondary mirror to high accuracy. The LMT's existing active systems may then be used to maintain precise alignment of the telescope’s primary surface and its secondary.
The Large Millimeter Telescope (LMT) Alfonso Serrano is a bi-national (Mexico and USA) telescope facility constructed on the summit of Sierra Negra, at an altitude of 4600m, in the Mexican state of Puebla. The LMT is a 50-m diameter single-dish telescope, with an active surface control-system to correct gravitational and thermal deformations of the primary reflector, designed and optimized to conduct scientific observations using heterodyne and continuum receivers, as well as VLBI observations, at frequencies between ~70 and 350 GHz. We describe the current status and technical performance of the recently commissioned LMT 50-m, the instrumentation development program, and future engineering upgrades that will optimize the optical efficiency of the telescope and increase its scientific productivity.
The TolTEC camera is a next generation three-band imaging polarimeter for the Large Millimeter Telescope. With 7514 lumped element kinetic inductance detectors across three simultaneously observing passbands at 1.1 mm, 1.4 mm, and 2.0 mm, TolTEC has diffraction-limited beams with FWHM of 5, 7, and 11 arcsec, respectively. Herein, we cover a brief overview of the instrument along with the first quantitative measures of TolTEC’s performance at the LMT. We also provide initial reductions of commissioning targets - demonstrating TolTEC's ability to detect both faint and extended structures over a wide dynamic range of flux and angular scales.
The Large Millimeter Telescope (LMT) Alfonso Serrano is a 50m-diameter single-dish radio telescope constructed at an altitude of 4600 meters on the summit of Volcan Sierra Negra, an extinct volcano in the Mexican state of Puebla. The LMT is a bi-national scientific collaboration between Mexico and the USA, led by the Instituto Nacional de Astrofisica, Optica y Electronica (INAOE) and the University of Massachusetts at Amherst. The telescope currently operates at wavelengths from 4mm to 1mm, and during the dry winter months the LMT site provides the highest levels of atmospheric transmission and potential future access to submillimeter observing windows. This paper describes the current status and scientific performance of the LMT, the suite of scientific instrumentation and future engineering upgrades that will optimize the optical efficiency of the telescope and increase its scientific productivity.
TolTEC is a three-band imaging polarimeter for the Large Millimeter Telescope. Simultaneously observing with passbands at 1.1mm, 1.4mm and 2.0mm, TolTEC has diffraction-limited beams with FWHM of 5, 7, and 11 arcsec, respectively. Over the coming decade, TolTEC will perform a combination of PI-led and Open-access Legacy Survey projects. Herein we provide an overview of the instrument and give the first quantitative measures of its performance in the lab prior to shipping to the telescope in 2021.
The Large Millimeter Telescope Alfonso Serrano (LMT) is a 50m-diameter radio telescope for millimeter-wave astronomy. In this paper we describe a number of initiatives underway to upgrade the antenna systems and permit scientific observations during daylight hours. We summarize recent efforts to characterize the thermal gradients that occur within the LMT structure and to identify important modes of surface deformation. The mitigation program involves use of the LMT's active surface to counteract the effects of measured thermal gradients within the antenna structure. It also includes active measures such as the installation of a ventilation system in the antenna backup structure. Prospects for additional active metrology measurements of the antenna surface for real-time surface corrections are also discussed.
The mm-wavelength sky reveals the initial phase of structure formation, at all spatial scales, over the entire observable history of the Universe. Over the past 20 years, advances in mm-wavelength detectors and camera systems have allowed the field to take enormous strides forward – particularly in the study of the Cosmic Microwave Background – but limitations in mapping speeds, sensitivity and resolution have plagued studies of astrophysical phenomena. In fact, limitations due to inherent biases in the ground-based mm-wavelength surveys conducted over the last 2 decades continue to motivate the need for deeper and wider-area maps made with increased angular resolution. TolTEC is a new camera that will fill the focal plane of the 50m diameter Large Millimeter Telescope (LMT) and provide simultaneous, polarization-sensitive imaging at 2.0, 1.4, and 1.1mm wavelengths. The instrument, now under construction, is a cryogenically cooled receiver housing three separate kilo-pixel arrays of Kinetic Inductance Detectors (KIDs) that are coupled to the telescope through a series of silicon lenses and dichroic splitters. TolTEC will be installed and commissioned on the LMT in early 2019 where it will become both a facility instrument and also perform a series of 100 hour “Legacy Surveys” whose data will be publicly available. The initial four surveys in this series: the Clouds to Cores Legacy Survey, the Fields in Filaments Legacy Survey, the Ultra-Deep Legacy Survey and the Large Scale Structure Survey are currently being defined in public working groups of astronomers coordinated by TolTEC Science Team members. Data collection for these surveys will begin in late 2019 with data releases planned for late 2020 and 2021. Herein we describe the instrument concept, provide performance data for key subsystems, and provide an overview of the science, schedule and plans for the initial four Legacy Survey concepts.
The Large Millimeter Telescope (LMT) Alfonso Serrano is a bi-national (Mexico and USA) telescope facility operated by the Instituto Nacional de Astrofisica, Optica y Electronica (INAOE) and the University of Massachusetts. The LMT is designed as a 50-m diameter single-dish millimeter-wavelength telescope that is optimized to conduct scientific observations at frequencies between ~70 and 350 GHz. The LMT is constructed on the summit of Sierra Negra at an altitude of 4600m in the Mexican state of Puebla. The site offers excellent mm-wavelength atmospheric transparency all-year round, and the opportunity to conduct submillimeter wavelength observations during the winter months. Following first-light observations in mid-2011, the LMT began regular scientific operations in 2014 with a shared-risk Early Science observing program using the inner 32-m diameter of the primary reflector with an active surface control system. The LMT has already performed successful VLBI observations at 3mm with the High Sensitivity Array and also at 1.3mm as part of the Event Horizon Telescope. Since early 2018 the LMT has begun full scientific operations as a 50-m diameter telescope, making the LMT 50-m the world´s largest single-dish telescope operating at 1.1mm. I will describe the current status of the telescope project, including the early scientific results from the LMT 50-m, as well the instrumentation development program, the plan to improve the overall performance of the telescope, and the on-going transition towards the formation of the LMT Observatory to support the scientific community in their use of the LMT to study the formation and evolution of structure at all cosmic epochs.
The Large Millimeter Telescope (LMT), located in central Mexico, saw completion of the final construction phase in 2017 with the installation of the full 50-meter primary reflector, following three years of operation as a 32-meter facility. The task was accomplished by adding two more concentric rings of surface segments to the existing three inner rings. Various techniques have been used previously to measure and align the 32-meter surface, including multiple laser trackers and far-field phased holography. Whilst the former method is time-consuming, requiring a full night to obtain a single surface map, holography provides low spatial resolution and requires removal of the secondary reflector for installation of the twin-horn receiver at the primary focus.
Photogrammetry has been used as an alternative measurement technique for the 32-m primary since 20151, and has gradually replaced our use of holography and laser trackers for this task2 during recent years. Once the object has been targeted, photogrammetry maps may be obtained in around one hour. The technique does not require the installation of special equipment on the antenna, and has the advantage of allowing surface maps to be taken at any chosen elevation. The main drawbacks for the LMT application are environmental, since the antenna operates without an enclosure; strong winds may prevent use of the site tower crane for image taking, while the formation of condensation and frost on the reflector surface will "switch off" the reflective targets.
In this paper we discuss comparative measurements taken as the first outer segments were installed, and the use of photogrammetry to carry out the alignment of the fully installed 50-meter surface. At the time of writing this activity is still in progress, however full-surface alignment to the order of just over 100 microns was achieved quite quickly, with multiple elevation maps allowing the development of a usable 50-m active surface model for compensation of gravitational distortions.
The Large Millimeter Telescope relies on an active primary surface to achieve its specified surface accuracy. The active primary has two functions: (1) it provides a means to correct the surface for gravitational deformations with changing elevation; and (2) it provides a capability to improve the shape of the surface in real time due to transient effects of thermal gradients within the structure. At LMT, our development work has addressed both problems and in this paper we describe the derivation of the gravity deformation model and the schemes developed to measure and improve the antenna surface during regular scientific observations.
The Large Millimeter Telescope (LMT) makes extensive use of 12 GHz holography during maintenance periods to finetune the alignment of primary reflector segments to the best-fit design parabola. Tracker measurements have also been used for this task, however the technique is severely limited by environmental noise and large data collection times, on the order of many hours for a single map. In 2015 we started photogrammetry trials as a complimentary measurement technique. Photogrammetry can offer reduced mapping times compared with laser trackers, and like holography, allows maps to be made at arbitrary elevation angles. Depending on the placement of reflecting targets, the technique can also provide higher spatial resolution than currently achieved using our holography system.
Accurate photogrammetry requires a robust strategy for the incorporation of multiple camera stations, a task complicated by the size of the antenna, obstructions of the surface by the sub-reflector and tetrapod legs, and the practicability of using the site tower crane as a moving camera platform. Image scaling is also a major consideration, since photogrammetry lacks any inherent distance reference. Therefore appropriate scale bars must be fabricated and located within the camera field of view. Additional considerations relate to the size and placement of reflective targets, and the optimization of camera settings. In this paper we present some initial comparisons of laser tracker, holography and photogrammetry measurements taken in 2015, showing clearly the status of alignment for distinct zones of the currently operating 32.5 m primary collecting area.
The Large Millimeter Telescope observatory is extending its night time operation to the day time. A sun avoidance strategy was therefore implemented in the control system in real-time to avoid excessive heating and damage to the secondary mirror and the prime focus.
The LMT uses an ”on-the-fly” trajectory generator that receives as input the target location of the telescope and in turn outputs a commanded position to the servo system. The sun avoidance strategy is also implemented ”on-the-fly” where it intercepts the input to the trajectory generator and alters that input to avoid the sun. Two sun avoidance strategies were explored. The first strategy uses a potential field approach where the sun is represented as a high-potential obstacle in the telescope’s workspace and the target location is represented as a low-potential goal. The potential field is repeatedly calculated as the sun and the telescope move and the telescope follows the induced force by this field. The second strategy is based on path planning using visibility graphs where the sun is represented as a polygonal obstacle and the telescope follows the shortest path from its actual position to the target location via the vertices of the sun’s polygon.
The visibility graph approach was chosen as the favorable strategy due to the efficiency of its algorithm and the simplicity of its computation.
One of the fundamental design principles of the LMT is that its segmented primary surface must be active: the position and orientation of each of the segments must be moved in order to maintain the precise parabolic surface that is required by the specifications. Consequently, a system of actuators, one at the corner of each segment, is used to move the segments to counteract surface deformations attributed to gravity or thermal effects.
A new control system was designed and built within the project to implement an active surface at the LMT. The technical concept for the active surface control system is to provide a set of bus boxes with built-in control and I/O capabilities to run four actuators each. Bus boxes read the LVDT sensor position and limit switch status for each actuator and use this information to drive the actuator’s DC motor, closing the position loop. Each bus box contains a DC power supply for the electronics, a second DC power supply for the motors, an embedded controller with I/O to close the position loop, and a custom printed circuit board to condition the LVDT signals and drive the motors. An interface printed circuit board resides in each actuator providing a single connector access to the LVDT, the motor, and the limit switches. During the fall of 2013, 84 bus boxes were commissioned to control the 336 actuators of the inner three rings of the telescope. The surface correction model was determined using holography measurements and the active surface system has been in regular use during the scientific observation at the LMT.
This paper describes the current status of the Large Millimeter Telescope (LMT), the near-term plans for the telescope
and the initial suite of instrumentation. The LMT is a bi-national collaboration between Mexico and the USA, led by the
Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE) and the University of Massachusetts at Amherst, to
construct, commission and operate a 50m-diameter millimeter-wave radio telescope. Construction activities are nearly
complete at the 4600m LMT site on the summit of Volcán Sierra Negra, an extinct volcano in the Mexican state of
Puebla. Full movement of the telescope, under computer control in both azimuth and elevation, has been achieved. The
commissioning and scientific operation of the LMT is divided into two major phases. As part of phase 1, the installation
of precision surface segments for millimeter-wave operation within the inner 32m-diameter of the LMT surface is now
complete. The alignment of these surface segments is underway. The telescope (in its 32-m diameter format) will be
commissioned later this year with first-light scientific observations at 1mm and 3mm expected in early 2011. In phase 2,
we will continue the installation and alignment of the remainder of the reflector surface, following which the final
commissioning of the full 50-m LMT will take place. The LMT antenna, outfitted with its initial complement of
scientific instruments, will be a world-leading scientific research facility for millimeter-wave astronomy.
This paper, presented on behalf of the Large Millimeter Telescope (LMT) project team, describes the status
and near-term plans for the telescope and its initial instrumentation. The LMT is a bi-national collaboration
between Mexico and the USA, led by the Instituto Nacional de Astrofísica, Optica y Electronica (INAOE) and the
University of Massachusetts at Amherst, to construct, commission and operate a 50m-diameter millimeter-wave
radio telescope. Construction activities are nearly complete at the 4600m LMT site on the summit of Sierra
Negra, an extinct volcano in the Mexican state of Puebla. Full movement of the telescope, under computer
control in both azimuth and elevation, has been achieved.
First-light at centimeter wavelengths on astronomical
sources was obtained in November 2006. Installation of precision surface segments for millimeter-wave operation
is underway, with the inner 32m-diameter of the surface now complete and ready to be used to obtain first light
at millimeter wavelengths in 2008. Installation of the remainder of the reflector will continue during the next
year and be completed in 2009 for final commissioning of the antenna. The full LMT antenna, outfitted with its
initial complement of scientific instruments, will be a
world-leading scientific research facility for millimeter-wave
astronomy.
We present a summary of the Large Millimeter Telescope (LMT) Project and its current status. The LMT is a joint project of the University of Massachusetts (UMass) in the USA and the Instituto Nacional de Astrofisica, Optica y Electronica (INAOE) in Mexico to build a 50m-diameter millimeter-wave telescope. The LMT site is at an altitude of 4600 m atop Volcan Sierra Negra, an extinct volcanic peak in the state of Puebla, Mexico, approximately 100 km east of the city of Puebla. Construction of the antenna steel structure has been completed and the antenna drive system has been installed. Fabrication of the reflector surface is underway. The telescope is expected to be completed in 2008.
We present a brief review of recent scientific and technical advances at the Infrared Optical Telescope Array (IOTA). IOTA is a long-baseline interferometer located atop Mount Hopkins, Arizona. Recent work has emphasized the use of the three-telescope interferometer completed in 2002. We report on results obtained on a range of scientific targets, including AGB stars, Herbig AeBe Stars, binary stars, and the recent outburst of the recurrent nova RS Oph. We report the completion of a new spectrometer which allows visibility measurements at several high spectral resolution channels simultaneously. Finally, it is our sad duty to report that IOTA will be closed this year.
The tip-tilt correction system at the Infrared Optical Telescope Array (IOTA) has been upgraded with a new star tracker camera.
The camera features a backside-illuminated CCD chip offering doubled overall quantum efficiency and a four times higher system gain compared to the previous system. Tests carried out to characterize the new system showed a higher system gain with a lower read-out noise electron level. Shorter read-out cycle times now allow to compensate tip-tilt fluctuations so that their error imposed on visibility measurements becomes comparable to, and even smaller than, that of higher-order aberrations.
We describe the fringe-packet tracking software installed at the infrared optical telescope array (IOTA). Three independently developed fringe-packet tracking algorithms can be used to equalise the optical path lengths at the interferometer. We compare the performance of these three algorithms and show results obtained tracking fringes for three independent baselines on the sky.
We present near infrared aperture synthesis maps of the well known binary star Capella (alpha Aur) produced with the upgraded IOTA interferometer on top of Mount Hopkins, Arizona. Michelson interferograms were obtained simultaneously on three interferometer baselines in the H-band between 2002 November 12 and 16. The simultaneous observation of fringes on three baselines permitted a single phase closure to be estimated along with three visibility amplitudes. Hybrid Mapping techniques were then used to reconstruct the source brightness distribution with a beam size of 5.4 x 2.6 mas, which allows for the resolution of the stellar surfaces of the Capella giants. The maps provide the first demonstration of imaging with phase closure on the IOTA instrument.
We are working towards imaging the surfaces and circumstellar envelopes of Mira stars in the near-infrared, using the IOTA interferometer and the IONIC integrated-optics 3-beam combiner. In order to study atmospheric structures of these stars, we installed 3 narrow-band filters that subdivide H-band into 3 roughly equal-width sub-bands - a central one for continuum, and 2 adjacent ones to sample Mira star's (mostly water) absorption-bands. We present here our characterization of the IOTA 3-Telescope interferometer for closure-phase measurements with broad and narrow-band filters in the H atmospheric window. This includes characterizing the stability, chromaticity, and polarization effects of the present IOTA optics with the IONIC beam-combiner, and characterizing the accuracy of our closure phase measurements.
Closure-phase science and technology are dominant features of the recent activity at IOTA.
Our science projects include imaging several spectroscopic binary stars, imaging YSOs including Herbig AeBe stars, detecting asymmetries in a large sample of Mira stars, and measuring water shells around Miras.
Many technology projects were pursued in order to make these science observations possible. These include installation of a third-generation integrated-optics 3-beam combiner (IONIC), completion of the real-time control system software, installation of fringe-packet tracking software, use of narrow sub-H band filters, validation of
the phase-closure operation, development of CPLD control of the science camera (PICNIC) and star-tracker camera (LLiST), installation of a new star-tracker camera, expansion of the observing facility, and installation of new semi-automated optical alignment tools.
We present a summary of the Large Millimeter Telescope Project and its
present status. The Large Millimeter Telescope (LMT) is a joint project of the University of Massachusetts (UMass) in the USA and the Instituto Nacional de Astrofisica, Optica y Electronica (INAOE) in Mexico to build a 50m-diameter millimeter-wave telescope. The LMT is being built at an altitude of 4600 m atop Volcan Sierra Negra, an extinct volcanic peak in the state of Puebla, Mexico, approximately 100 km east of the city of Puebla. Construction of the antenna is now well underway. The basic structure with a limited number of surface panels is expected to be completed in 2005. Engineering acceptance and telescope commissioning are expected to be completed in 2007.
The IOTA (Infrared Optical Telescope Array) has been routinely
operating with two-telescopes since 1994, a mode destined to become
obsolete following its recent conversion to a three-telescope
array. In two-telescope mode, the IOTA has made numerous
scientific and technical contributions, see e.g. our list of
publications at http://cfa-www.harvard.edu/cfa/oir/IOTA/PUBLI/publications.html.
We present preliminary results on three different topics using recent
data from the two-telescope IOTA: (1) measurements of Mira star
diameters simultaneously in three different near-infrared spectral
bands, (2) measurement of the characteristic size and shape of the
source of near-infared emission in the x-ray binary system CI Cam, and (3) aperture synthesis of the Carbon star V Hydrae combining data from the IOTA and from aperture masking at the Keck-I telescope.
New beam combination techniques, using two and three telescopes, have been the focus of activity at IOTA during the past two years since our last update. In particular, we have added a third telescope, made closure-phase measurements, demonstrated two- and three-beam combination with integrated optics combiners, demonstrated two-beam combination with an asymmetric coupler, and made simultaneous JHK visibility measurements with an image-plane combiner.
We report the first long-baseline interferometric observations of R CrB. The observations were carried out at the Infrared Optical Telescope Array (IOTA), using our new JHK beam combiner which enables us to record fringes simultaneously in the J-, H-, and K-bands. The circumstellar envelope of R CrB is resolved at a baseline of 21 m, and the K-band visibility is derived to be 0.61 ± 0.03 along a position angle of about 170 degrees. The visibility obtained with IOTA, as well as speckle visibilities with baselines up to 6 m and the spectral energy distribution (SED), are fitted with 2-component models consisting of the central star and an optically thin dust shell. The K-band visibilities predicted by the models are about 10% smaller than the visibility obtained with IOTA. However, given the simplifications adopted in our models and the complex nature of the object, this can be regarded as rough agreement. As a hypothesis to explain the small discrepancy, we propose that there might be a group of newly formed dust clouds, which might appear as a third visibility component.
Our new IOTA JHK-band beam combiner allows the simultaneous recording of spectrally dispersed J-, H- and K-band Michelson interferograms. In this paper we present our IOTA observations of the Mira star T Cep with this beam combiner (observations in June 2001; four baselines in the range of 14 m to 27 m). The beam combiner optics consists of an anamorphic cylindrical lens system and a prism. From the interferograms of T Cep we derive the visibilities and the J-, H-, and K-band uniform-disk diameters of 14.0 ± 0.6 mas, 13.7 ± 0.6 mas and 15.0 ± 0.6 mas, respectively. Angular stellar filter radii and Rosseland radii are derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of different Mira star models. The available HIPPARCOS parallax (4.76 ± 0.75 mas) of T Cep allows us to determine linear radii. For example, from the K-band visibility we derive a Rosseland radius of 329-50/+70 solar radii if we use the CLVs of the M-models as fit functions. This radius is in good agreement with the theoretical M-model Rosseland radius of 315 solar radii. The comparison of measured stellar parameters (e.g. diameters, effective temperature, visibility shape) with theoretical parameters indicates whether any of the models is a fair representation of T Cep.
The ratios of visibilities of different spectral channels can be measured with higher precision than absolute visibilities. Therefore, we use the visibility ratios V(λ1)/V(λ2) to investigate the wavelength dependence of the stellar diameter. We find that the 2.03 μm uniform-disk diameter of T Cep is about 1.26 times larger than the 2.26 μm uniform-disk diameter.
We present observations of the symbiotic star CH Cyg with a new JHK-band beam combiner mounted to the IOTA interferometer. The new beam combiner consists of an anamorphic cylindrical lens system and a grism, and allows the simultaneous recording of spectrally dispersed J-, H- and K-band Michelson interferograms. The observations of CH Cyg were conducted on 5, 6, 8 and 11 June 2001 using baselines of 17m to 25m. From the interferograms of CH Cyg, J-, H-, and K-band visibility functions can be determined. Uniform-disk fits to the visibilities give, e.g., stellar diameters of (7.8 ± 0.6) mas and (8.7 ± 0.8) mas in H and K, respectively. Angular stellar filter radii and Rosseland radii are derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of Mira star models. The available HIPPARCOS parallax of CH Cyg allows us to determine linear radii. For example, on the basis of the K-band visibility, Rosseland radii in the range of 214 to 243 solar radii can be derived utilizing CLVs of different fundamental mode Mira models as fit functions. These radii agree well within the error bars with the corresponding theoretical model Rosseland radii of 230 to 282 solar radii. Models of first overtone pulsators are not in good agreement with the observations. The wavelength dependence of the stellar diameter can be well studied by using visibility ratios V(λ1)/V(λ2) since ratios of visibilities of different spectral channels can be measured with higher precision than absolute visibilities. We found that the 2.03 μm uniform disk diameter of CH Cyg is approximately 1.1
times larger than the 2.15 μm and 2.26 μm uniform-disk diameter.
We describe the new control system for the PICNIC near-infrared camera and the visible star tracker, implemented at the IOTA interferometer, based on the ALTERA Complex Programmable Logic Device (CPLD) technology. These digital components provide an adaptive interface between the control system and the cameras used at IOTA, allowing flexibility when connecting very different devices. In particular the clocking and processing circuits used for the PICNIC camera can be changed in milliseconds during normal operation. The camera can then switch between full quadrant readout mode used for alignment and diagnostics, and a N pixel readout mode used for science operation.
We present a summary of the Large Millimeter Telescope Project and its present status. The Large Millimeter Telescope (LMT) is a joint project of the University of Massachusetts (UMass) in the USA and the Instituto Nacional de Astrofisica, Optica y Electronica (INAOE) in Mexico to build a 50m-diameter millimeter-wave telescope. The LMT is being built at an altitude of 4600 m atop Volcan Sierra Negra, an extinct volcanic peak in the state of Puebla, Mexico, approximately 100 km east of the city of Puebla. Construction of the antenna is now well underway, and it is expected to be completed in 2004.
The third telescope project to enable phase-closure observations at the IOTA interferometer is well underway, and is anticipated to be completed later this year. For this project, we present the main technical improvements which we have already made or expect to make, including a new VxWorks control system, improved star acquisition cameras, improved siderostat and primary mirror supports, five-axis control of the telescope secondary mirrors, automated control of the long delay line, trihedral retroreflectors, three-beam combination, the PICNIC camera, and fringe packet tracking.
We have conducted the first systematic study of Herbig Ae/Be stars using the technique of long baseline stellar interferometry. The Infrared Optical Telescope Array resolves the source of the near-infrared excess flux characteristic of these systems in 11 of the 15 stars observed. A close companion to MWC 361-A (18 mas separation) has been detected interferometrically for the first time. The visibility data has been interpreted within the context of four models which represent the range of plausible representations for the brightness of the excess emission: a Gaussian distribution, a narrow uniform ring, an accretion disk, and an infrared companion. We find that the large sizes measured by the interferometer, 0.5 - 5.9 AU, essentially invalidate accretion disk models that had been previously used to explain the spectroscopic observations. Although a unique model can not be determined for each source due to limited spatial frequency coverage, the observed symmetry of the sources favors, for the ensemble of the data, models in which the circumstellar dust is distributed in spherical envelopes.
We summarize the characteristics and performance of a recent instrument for the detection of interference fringes in the near IR at the IR Optical Telescope Array (IOTA). The instrument had its first test run and saw 'first fringes' in Spring 1997, and has been regularly used since by several groups at the IOTA for a variety of high resolution investigations in the near IR. We present preliminary results from our group's campaign to study Mira variables and the circumstellar environments of Herbig AeBe stars.
The first two telescopes of the Infrared-Optical Telescope Array (IOTA) project are now in place and yielding data at the Smithsonian Institution's F. L. Whipple Observatory on Mt. Hopkins, near Tucson, Arizona. The IOTA collectors are 45 cm in diameter, and may be moved to various stations in an L-shaped configuration with a maximum baseline of 38 m. A third collector will be added as soon as funding permits. Each light-collector assembly consists of a siderostat feeding a stationary afocal Cassegrain telescope that produces a 10-X reduced parallel beam, which is in turn directed vertically downward by a piezo-driven active mirror that stabilizes the ultimate image position. The reduced beams enter an evacuated envelope and proceed to the corner of the array, where they are turned back along one arm for path compensation. The delay line, in one beam, consists of two parts: one dihedral reflector positioned in a slew-and-clamp mode to give the major part of the desired delay; and a second dihedral mounted on an air-bearing carriage to provide the variable delay that is needed. After delay, the beams exit from the vacuum and are directed by dichroic mirrors into the infrared beam-combination and detection system. The visible light passes on to another area, to the image-tracker detectors and the visible-light combination and detection system. The beams are combined in pupil-plane mode on beam splitters. The combined IR beams are conveyed to two cooled single-element InSb detectors. The combined visible-light beams are focussed by lenslet arrays onto multimode optical fibers that lead to the slit of a specially-designed prism spectrometer. For the visible mode, the delay line is run at several wavelengths on one side of the zero- path point, so that several cycles of interference occur across the spectrum. First results were obtained with the IR system, giving visibilities for several K and M stars, using 2.2 micrometers radiation on a N-S baseline of 21.2 m. From these measurements we obtained preliminary estimates of effective stellar diameters in the K band.
Considerable experience has been gained in the construction of images from ground-based interferometry data. A brief introduction to this work is presented, and some lessons learned from this experience that are relevant to space-based interferometers are discussed.
Hybrid mapping techniques originally developed for radio aperture synthesis imaging are applied to the case of image reconstruction in optical interferometry. It is found that the use of redundant baselines with hybrid mapping can increase the fraction of phase information that is obtained in a realistic optical experiment to approach that obtained in a typical multistation radio experiment. However, this additional information does not greatly improve the quality of the resulting maps in cases with good coverage of the u-v plane. Thus, in constructing an experiment with redundant baselines, the adequacy of the u-v coverage should be considered along with any desire for redundancy. The use of phase differences between two colors in a dispersed fringe for hybrid mapping experiments are investigated and it is found that maps may be made of comparable quality to hybrid maps with phase closure.
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