The Navy Precision Optical Interferometer (NPOI) has been in operation since 1996 and has built a significant data archive of interferometric observations over a range of visible wavelengths. Here we present a catalog of ~600 objects with NPOI data including the year(s) of observation(s) and how many scans were obtained for each target, as well as statistics on the targets' properties. The sample includes a wide range of single stars and binary systems, some of which has been published previously. Our intent in this paper is to let the community know exactly when we observed a given target, and how many observations were obtained each year. We welcome collaboration with researchers interested in discovering if NPOI data can help their programs.
As a part of regular operations, the Navy Precision Optical Interferometer (NPOI) uses Narrow Angle Trackers (NAT) for atmospheric tip-tilt correction. This correction is done using a quad cell array for each station, and is based on the error signals measured by these arrays. The relationship between error signals and atmospheric seeing error is analyzed in detail, including modeling of quad cell behavior and optical modeling of propagation of seeing errors from receiving siderostats to quad cell displacements in the laboratory. This investigation updates a previous investigation1 and demonstrates direct mapping of atmospheric seeing to jitter error measurements. Limitations of this technique and future improvements are also discussed.
After initial development at the Intelligence Advanced Research Projects Activity (IARPA) of the Office of the Director of National Intelligence, the development of a prototype hypertelescope was transitioned to the site of the Navy Precision Optical Interferometer (NPOI). Though all hardware was purchased and all subsystems were demonstrated before transition, a planned final Open Sky demonstration of the hypertelescope was prevented, in large part, by operational restrictions experienced during the government shutdown of 2020. This presentation provides a brief overview of the hypertelescope design and previous accomplishments and details the recently revived effort to achieve Open Sky collection with this direct-imaging system.
Aperture masking is a technique to transform a filled-aperture telescope into an interferometer. One reason masks help boost sensitivity is that they facilitate the measurement of closure phases. Closure phases are insensitive to differential piston in the wavefront of light captured by each subaperture, so are a precise observable encoding the structure of the observed object. Spatial structure in the wavefront of light over each subaperture biases visibilities and closure phases. All extant aperture masking data sets show residual closure phase dispersion after calibration larger than estimates based on photon-noise alone, suggesting time variable substructure in the wavefront is playing a role in limiting closure phase precision. We are developing a technique harnessing the capabilities of holographic aperture masks to multiplex subapertures to provide for simultaneous focal-plane wavefront sensing of each subaperture. This device can be used to measure the spatial structure of the wavefront, facilitating self-calibrating closure phases. We will present the results of simulations demonstrating the concept and describe a prototype instrument design.
We will present the status of the Navy Precision Optical Interferometer. We will discuss upgrades that occurred over the course of the last couple of years, their related scientific achievements, ongoing and future work. We will discuss the improvements done to the infrastructure of the site, the return to observations with 6 telescopes simultaneously and the results of these observations. We will discuss the deployment of new capabilities, such as an infrared beam combiner, siderostat controllers and a new angle tracker. We will also present the deployment of the Amon Hen hypertelescope experiment and changes done to the inner room in order to accommodate the use of both systems without the need of large rearrangements of the optics.
KEYWORDS: Switches, Control systems, Safety, Precision optics, Interferometers, Analog electronics, Computer programming, Stepper motor drivers, Logic, Power supplies
The Navy Precision Optical Interferometer (NPOI) is approaching 30 years of operational life. During that time it has carried out many observations of single and multiple star systems, and has seen upgrades to subsystems and expansion of capabilities. For the last couple of years there has been a major ongoing effort to expand the capabilities of the NPOI with new telescopes, new instruments, and completion of the bootstrapping and imaging capability that was part of the original NPOI design. As part of this upgrade we are also upgrading and replacing some electronic systems. Some technologies which were state-of-the art, e.g. VxWorks, have over the years been overtaken by inexpensive systems such as embedded microcontroller, consumer-grade compact system such as Raspberry Pi1 and Arduino,2 and inexpensive manufacture of simple and powerful custom PCBs. This makes it possible to incorporate remote controls of actuators which will be a major convenience compared to the existing system of manual controls.
The Navy Precision Optical Interferometer (NPOI) has been in operation since 1996, building a substantial data archive. This provided the opportunity to determine how many data points are needed for a single star’s angular diameter fit to become stable as more data points were included. In an iterative process, we calculated the diameters for an ever-increasing number of data points for 31 stars. We found that at approximately 1,000 data points, the scatter in the diameter fits fell below 2%. Assuming a 3-telescope triangle and using 15 channels across a range of wavelengths, 1,000 data points equates to 22 to 25 bracketed observations, which can usually be accomplished in 2 to 4 nights. This will be a useful rule-of-thumb when planning observations and gathering data on single, symmetrical stars.
We have been progressing on our comprehensive program of improving high-resolution imaging at the Navy Precision Optical Interferometer (NPOI) hosted at Lowell Observatory’s Anderson Mesa site, for the purpose of spatially resolved observations of faint objects at scales down to less than 1 milliarcsecond. The ‘PALANTIR’ upgrade of NPOI has commenced with individually operating 1 meter PlaneWave PW1000 telescopes at the site, with integration of those telescopes into the array with interferometric operations expected in the near-term. These telescopes are housed in mobile domes for rapid relocation around the array, and are being augmented with adaptive optics. Another notable recent milestone has been the re-activation of full six-way on-sky operations with siderostat feeds during the summer of 2021. Additionally, our ‘NPOI Plus-Up’ plan will implement sweeping infrastructure updates, improving and streamlining its operations. Upcoming Plus-Up work taking place over the next few years includes expansion of the operating infrastructure to the array’s longest physical baselines at 432 meters, adding a near-infrared beam combiner, rehabilitation of the VISION visible combiner, modernization of the fast delay line control system, and implementation of the long delay lines in the framework of a beam train auto-aligner.
We have been pursuing a comprehensive program of improving high-resolution imaging at the Navy Precision Optical Interferometer (NPOI) hosted at Lowell Observatory’s Anderson Mesa site, for the purpose of spatially resolved observations of faint objects at scales down to less than 1 milliarcsecond. This activity at NPOI is being implemented with two primary phases. First, the ‘PALANTIR’ upgrade of NPOI is augmenting the existing telescope array with three 1-meter PlaneWave PW1000 telescopes. These telescopes are housed in mobile domes for rapid relocation around the array, and are being augmented with adaptive optics. Second, we are implementing a ‘NPOI Plus-Up’ plan which is modernizing the array infrastructure and streamlining its operations. All of these activities are being carried out as our current operations are continuing.
We present an approach to upgrading the beam transport system at the Navy Precision Optical Interferometer. These upgrades together will provide consistent beam transport, improve fringe contrast by preserving beam wavefront, reduce tracking errors by increasing the frequency response of the tracker, and automatically realign the entire transport train after thermal drift over the course of nightly observations. The beam transport system passively redirects stellar light from the telescope output to the fast delay line through a train of flat mirrors. This multi-mirror transport train reduces wavefront preservation due to stack-up of surface flatness errors. We demonstrated previously by using a contour-conformable mirror instead of one of the flats in the train that a 63% improvement in wavefront flatness is achievable. Also, the 25 Hz tracker is replaced by a 100 Hz tracker to further stabilize the trajectory during observations. Finally, we include an auto-aligner to systematically realign the entire beam transport system from thermal drifts. This is necessary for long baseline interferometry with short drift time constants. The beam transport system is common to all front ends (telescopes and siderostats), beam delay, and back-ends (beam combiners and detectors). These three upgrades expand the utility of the NPOI from a relatively short 97 m baseline interferometer to its full reconfigurable 437 m baselines and allow consistent beam transport with various potential experimental telescope front ends and beam combiner back-ends. In this paper, we describe our three-pronged upgrade approach, experimental method and results, and recommendations.
We present a multi-synchronous architecture for controlling multiple telescope or siderostat stations simultaneously from a central computer. This computer synchronously processes and distributes commands to six net- worked stations. The stations, comprised of a telescope or siderostat, a narrow angle tracker and a star-acquisition camera, are useful in the optical and infrared wavelengths. The central computer sends signed packets that include instructions for a particular station through the network at a rate of 600 Hz. A microcomputer at each station accepts its appropriate packets and parses the embedded data into control signals. These signals are then converted into appropriate outputs for motor controllers and piezoelectric actuators. The motors steer the siderostats while the piezoelectric actuators finely point the narrow angle tracker. A closed feedback loop couples the siderostat pointing with the narrow angle tracker to dynamically steer and maintain stable image positions of a stellar object. Two major components comprise this feedback loop: a wide-angle star acquisition camera inserted between a narrow angle tracker and siderostat, and a photon counter located near the end of the light path. The central computer utilizes positional error data from the camera to intricately align a siderostat. Simultaneously the computer processes pointing error derived from photon counts to finely correct a narrow angle tracker. This synchronized feedback and control system allows for precise, concurrent tracking over a wide range of stellar objects of interest. In this paper, we describe the parallelized software architecture, control hardware, experimental results, conclusions and recommendations.
KEYWORDS: Antennas, Calibration, Receivers, Polarization, Electromagnetic coupling, Radio astronomy, Analytical research, Signal to noise ratio, Radio interferometry, Standards development
The National Radio Astronomy Observatory's 27 antenna Karl G. Jansky Very Large Array (NRAO VLA) is host to a commensal low frequency system called the VLA Low-band Ionosphere and Transient Experiment (VLITE). This system currently records data from 16 of the primary focus 330 MHz feeds during nearly all observing programs that use one of the eight Cassegrain receivers. This type of commensal (or piggy-back) system provides a powerful opportunity to increase the scientific capabilities of an instrument, yet it is accompanied by numerous complexities resulting from lack of control over the observational setup. In this paper we specifically investigate the stability of the instrumental bandpass response as recorded by VLITE. We demonstrate that the bandpass for each antenna is stable for long periods of time. This allows the use of a global bandpass derived from high signal-to-noise ratio observations of reliable calibrators, which may be applied to any dataset. This method avoids the loss of data when a bandpass cannot be calculated because the appropriate calibrators are not observed, not observed for long enough, or when the observations are severely impacted by radio frequency interference. We also demonstrate that monitoring the behavior of the bandpass solutions over time can be a powerful tool to determine intermittent equipment issues, as well as long term changes to the system.
The Navy Precision Optical Interferometer (NPOI) is currently undergoing a fundamental renaissance in its functionality and capabilities. Operationally, its fast delay line (FDL) infrastructure is completing its upgrade from a VME/VxWorks foundation to a modern PC/RTLinux core. The Classic beam combiner is being upgraded with the New Classic FPGA-based backend, and the VISION beam combiner has been upgraded over this past summer with low-noise EMCCD cameras, resulting in substantial gains in sensitivity. Building on those infrastructure improvements, substantial upgrades are also in progress. Three 1-meter PlaneWave CDK1000 telescopes are being delivered to the site, along with their relocatable enclosure-transporters, and stations are being commissioned for those telescopes with baselines ranging from 8 meters to 432 meters. Baseline-wavelength bootstrapping will be implemented on the facility back-end with a near-infrared beam combiner under development. Collectively, these improvements mark substantial progress in taking the facility towards realizing its full intrinsic potential.
Detecting exoplanets and characterizing their orbital properties is a difficult task, given the proximity of these objects relative to their host stars, as well as their brightness ratios. We present an interferometric fringe nulling technique, aimed at solving these issues. This technique uses baseline phases and takes advantage of the strong phase fluctuations, due to the presence of an exoplanet, that can be observed at spatial frequencies adjacent to the null crossing. We present initial results based on observations of the multiple stellar system η Aql, obtained with the Navy Precision Optical Interferometer, which indicate the presence of a Δm~5 mag close to the brightest star in this system.
We recently used archival and newly obtained data from the Navy Precision Optical Interferometer to measure the fundamental properties of 87 stars. The sample consisted of 5 dwarfs, 3 subgiants, 69 giants, 3 bright giants, and 7 supergiants, and spanned a wide range of spectral classes from B to M. We combined our angular diameters with photometric and distance information from the literature to determine each star’s physical radius, effective temperature, bolometric flux, luminosity, mass, and age. Several dozen of the stars have visibility curves sampled down to the first null, where the visibilities drop to zero. Here we present preliminary results showing limb-darkening fits for the five zero crossing stars that have the best coverage of the second lobe.
Since 1994, the Navy Precision Optical Interferometer (NPOI) has operated at visual wavelengths (450 to 850 nm). Its primary Classic backend is a pupil-plane combiner that disperses the light at a resolution R ≈ 50, uses avalanche photo-diodes as photon-counting detectors, and scans interference fringes by modulating the delay at 1 kHz. The newer NPOI image-plane combiner, VISION (Tennessee State University), which is similar to CHARA’s MIRC and is currently being upgraded, dispenses with delay modulation. We are now developing a third backend to expand into the near infrared. Its primary purpose will be to stabilize the NPOI for high-resolution observations by bootstrapping from the infrared to visual wavelengths.
We present the results of multi band optical photometry (B, V, R and I) for 66% of the sample of geosynchronous satellites observable from Flagstaff, AZ. Observations were done while the targets were at solar phase angle smaller than ±5°, to probe the period when they are expected to reach maximum brightness. We discuss the distribution of magnitudes and colors of the sample, how these properties correlate with satellite age, and the how the color information can be used to differentiate between satellite buses.
One of the main problems faced by the Space Situational Awareness community is the detection and characterization of faint objects around geosats. Independent of the origin of these objects, whether they are debris or controlled spacecraft, they can potentially harm these assets and contaminate the geobelt environment. The challenge of detecting these companion objects comes from their proximity and brightness ratio relative to geosats. Here we present a novel interferometric fringe nulling technique, aimed at solving these issues. This technique takes advantage of the fact that the presence of companions introduces large phase fluctuations in the fringe phase observed by an interferometer, when the interferometer is observing a target at spatial frequencies where the fringe amplitude is near zero. We describe the ongoing development of this technique at the Navy Precision Optical Interferometer, and the results of simulations of interferometric observations of satellites with companions. We also present the current state of the NPOI and related SSA work being done with this interferometer, as well as undergoing upgrades to the system.
We describe the current status of the Navy Precision Optical Interferometer (NPOI), including developments since the last SPIE meeting. The NPOI group has added stations as far as 250m from the array center and added numerous infrastructure improvements. Science programs include stellar diameters and limb darkening, binary orbits, Be star disks, exoplanet host stars, and progress toward high-resolution stellar surface imaging. Technical and infrastructure projects include on-sky demonstrations of baseline bootstrapping with six array elements and of the VISION beam combiner, control system updates, integration of the long delay lines, and updated firmware for the Classic beam combiner. Our plans to add up to four 1.8 m telescopes are no longer viable, but we have recently acquired separate funding for adding three 1 m AO-equipped telescopes and an infrared beam combiner to the array.
The New Classic instrument was built as a electronics and computer upgrade to the existing Classic beam combiner at the Navy Precision Optical Interferometer (NPOI). The classic beam combiner is able to record 32 of 96 available channels and has a data throughput limitation which results in a low duty cycle. Additionally the computing power of the Classic system limited the amount of fringe tracking that was possible. The New Classic system implements a high-throughput data acquisition system which is capable of recording all 96 channels continuously. It also has a modern high-speed computer for data management and data processing. The computer is sufficiently powerful to implement more sophisticated fringe-tracking algorithms than the Classic system, including multi-baseline bootstrapping. In this paper we described the New Classic hardware and software, including the fringe-tracking algorithm, performance, and the user interface. We also show some initial results from the first 5-station, 4-baseline bootstrapping carried out in January 2015.
Imaging with optical interferometers requires fringe measurements on baseline long enough to resolve the target. These long baselines typically have low fringe contrast. Phasing them requires fringe tracking on shorter baselines which typically have greater fringe contrast and combining the fringe-tracking signals on the short baselines to phase the long baselines in a baseline bootstrapping configuration. On long resolving baselines coherent integration also becomes necessary in order to shorten the integration time. This paper addresses both the baseline bootstrapping and the coherent integration. The Navy Precision Optical Interferometer (NPOI) is laid out in a way which permits long-baseline phasing from shorter baselines in a multi-baseline scheme. The New Classic instrument for NPOI was designed specifically to implement the multi-baseline bootstrapping capability and multi-baseline observations can now be carried out routinely at the NPOI. This paper provides details about the bootstrapping scheme at NPOI and shows some initial results. We also discuss the bootstrapping error budget, describe our new Bayesian coherent integration algorithm and compare its performance to theory.
As a part of regular operations, the Navy Precision Optical Interferometer (NPOI) uses Narrow Angle Trackers (NAT) for atmospheric tip-tilt correction. This correction is done using a quad cell array for each station, and is based on the error signals measured by these arrays. We compiled NPOI NAT jitter information for the period of 2005 to 2014. Here we investigate the correlation of the NAT jitter between different NPOI stations, and determine a correction for shot-noise induced jitter. We present initial results from the correlation between NAT jitter and quasi simultaneous seeing measurements done with the Lowell Observatory 31" telescope, separated by 500 m. We also discuss some limitations of this technique and future improvements.
We plan to measure the angular diameters of a sample of Penn State-Torun Planet Search (PTPS) giant exoplanet host star candidates using the Navy Precision Optical Interferometer. The radii of evolved giant stars obtained using spectroscopy are usually ill-defined because of the method’s indirect nature and evolutionary model dependency. The star’s radius is a critical parameter used to calculate luminosity and mass, which are often not well known for giant stars. Therefore, this problem also affects the orbital period, mass, and surface temperature of the planet. Our interferometric observations will significantly decrease the errors for these parameters. We present preliminary results from NPOI observations of six stars in the PTPS sample.
We describe multi-baseline observations of a geostationary satellite using the Navy Precision Optical Interferometer (NPOI) during the glint season of March 2015. We succeeded in detecting DirecTV-7S with an interferometer baseline length of 8.8 m on two nights, with a brief simultaneous detection at 9.8 m baseline length on the second night. These baseline lengths correspond to a resolution of ~4 m at geostationary altitude. This is the first multiple-baseline interferometric detection of a satellite.
We describe multi-baseline observations of a geostationary satellite using the Navy Precision Optical Interferometer (NPOI) during the glint season of March 2015. We succeeded in detecting DirecTV-7S with an interferometer baseline length of 8.8 m on two nights, with a brief simultaneous detection at 9.8 m baseline length on the second night. These baseline lengths correspond to a resolution of ~4 m at geostationary altitude.
This is the first multiple-baseline interferometric detection of a satellite.
The Navy Precision Optical Interferometer is an astronomical optical interferometer operating near Flagstaff, Arizona. A
joint program between the United States Naval Observatory, the Naval Research Laboratory and Lowell Observatory, it
has historically been involved in space imagery and astrometry. More recent work has pushed for the addition of more
baselines. It is currently capable of co-phasing 6 elements, so the commissioning of additional baselines requires ease of
use and reconfigurability. At the time of this publication, a seventh station has been added and the final commissioning
work on an eighth and ninth station are being completed. These last two stations will increase the longest baseline to 435
meters. This paper discusses the work to date on adding these stations and provides details on increased capabilities.
We present progress on the stellar surface imaging project recently funded by the U. S. National Science Foun- dation. With the unique layout of the Navy Precision Optical Interferometer (NPOI) in combination with data acquisition and fringe-tracking upgrades we expect to be able to substantially exceed the imaging fidelity and resolution of any other interferometer in operation. The project combines several existing advances and infras- tructure at NPOI with modest enhancements. For optimal imaging there are several requirements that should be fulfilled. The observatory should be capable of measuring visibilities on a wide range of baseline lengths and orientations, providing complete Fourier (UV) coverage in a short period of time. It should measure visibility amplitudes with good SNR on all baselines as critical imaging information is often contained in low-amplitude visibilities. It should measure the visibility phase on all baselines. The technologies which can achieve this are the NPOI Y-shaped array with (nearly) equal spacing between telescopes and an ability for rapid configuration. Placing 6-telescopes in a row makes it possible to measure visibilities into the 4th lobe of the visibility function. By arranging the 12 available telescopes carefully we can switch, every few days, between 6 different 6-station chains which provide symmetric coverage in the Fourier plane without moving any telescopes, only by moving beam relay mirrors. The 6-station chains are important to achieve the highest imaging resolution, and switching rapidly between station chains provides uniform coverage. Coherent integration techniques can be used to obtain good SNR on very small visibilities. Coherently integrated visibilities can be used for imaging with standard radio imaging packages such as AIPS. The commissioning of one additional station, the use of new data acqui- sition hardware and fringe tracking algorithms are the enhancements which are making this project a reality. The New Classic data acquisition system, based on a powerful Stratix FPGA and fast Direct Memory Access module, upgrades the existing Classic beam combiner to allow for continuous data recording across all baselines available with 6 telescopes. It also provides the computing power and software environment necessary for im- plementing the 6-station, 5-baseline fringe-tracking algorithms. In separate papers we discuss the New Classic data acquisition system and the fringe-tracking algorithms in greater detail. In this paper we will focus on an overview of the project. We will describe the observation planning, logistics of the observations, and discuss the current status of the project including preliminary results and simulations of expected future results.
The Navy Precision Optical Interferometer (NPOI) has been recording astronomical observations for nearly two decades, at this point with hundreds of thousands of individual observations recorded to date for a total data volume of many terabytes. To make maximum use of the NPOI data it is necessary to organize them in an easily searchable manner and be able to extract essential diagnostic information from the data to allow users to quickly gauge data quality and suitability for a specific science investigation. This sets the motivation for creating a comprehensive database of observation metadata as well as, at least, reduced data products. The NPOI database is implemented in MySQL using standard database tools and interfaces. The use of standard database tools allows us to focus on top-level database and interface implementation and take advantage of standard features such as backup, remote access, mirroring, and complex queries which would otherwise be time-consuming to implement. A website was created in order to give scientists a user friendly interface for searching the database. It allows the user to select various metadata to search for and also allows them to decide how and what results are displayed. This streamlines the searches, making it easier and quicker for scientists to find the information they are looking for. The website has multiple browser and device support. In this paper we present the design of the NPOI database and website, and give examples of its use.
We observed 85 stars using the Navy Precision Optical Interferometer in order to determine their angular diameters. Here we present preliminary uniform disk fits for the stars. Many of the targets have measurements through the first zero crossing and onto the second lobe of the visibility curve. We will use these observations to test limb darkening laws, namely the effectiveness of plane parallel versus spherically symmetric models. These results have important implications for the accuracy with which we can determine the limb darkening of stars used as calibrators on long baselines being implemented in the near future on the NPOI, which will almost certainly have to be at least semi-resolved. The validation or exposure of systematics in the limb darkening laws can also be applied to any number of stars observed interferometrically.
The Navy Precision Optical Interferometer (NPOI) has a station layout which makes it uniquely suited for imaging. Stellar surface imaging requires a variety of baseline lengths and in particular long baselines with resolution much smaller than the diameter of the target star. Because the fringe signal-to-noise ratio (SNR) is generally low on such long baselines, fringe-tracking cannot be carried out on those baselines directly. Instead, baseline bootstrapping must be employed in which the long baseline is composed of a number of connected shorter baselines. When fringes are tracked on all the shorter baselines fringes are also present on the long baseline. For compact sources, such as stellar disks, the shorter baselines generally have higher SNR and making them short enough that the source is unresolved by them is ideal. Thus, the resolution, or number of pixels across a stellar disk, is roughly equal to the ratio of the length of the long baseline to the length of the short baselines. The more bootstrapped baselines, the better the images produced. If there is also a wide wavelength coverage, wavelength bootstrapping can also be used under some circumstances to increase the resolution further. The NPOI is unique in that it allows 6-station, 5-baseline bootstrapping, the most of any currently operating interferometer. Furthermore, the NPOI Classic beam combiner has wavelength coverage from 450 nm to 850 nm. However, until now, this capability has not been fully exploited. The stellar surface imaging project which was recently funded by the National Science Foundation is exploiting this capability. The New Classic data acquisition system, reported separately, is the hardware which delivers the data to the fringe-tracking algorithm. In this paper we report on the development of the fringe-tracking capability with the New Classic data acquisition system. We discuss the design of the fringe tracking algorithm and present performance results from simulations and on sky observation.
We present the results of a study done with data from the Navy Precision Optical Interferometer (NPOI). We use data from the Narrow Angle Trackers to perform a photometric calibration of the visibilities. We describe the method and preliminary results on improvements to the precision of the visibility amplitude calibration.
The New Classic data acquisition system is an important portion of a new project of stellar surface imaging with the NPOI, funded by the National Science Foundation, and enables the data acquisition necessary for the project. The NPOI can simultaneously deliver beams from 6 telescopes to the beam combining facility, and in the Classic beam combiner these are combined 4 at a time on 3 separate spectrographs with all 15 possible baselines observed. The Classic data acquisition system is limited to 16 of 32 wavelength channels on two spectrographs and limited to 30 s integrations followed by a pause to ush data. Classic also has some limitations in its fringe-tracking capability. These factors, and the fact that Classic incorporates 1990s technology which cannot be easily replaced are motivation for upgrading the data acquisition system. The New Classic data acquisition system is based around modern electronics, including a high-end Stratix FPGA, a 200 MB/s Direct Memory Access card, and a fast modern Linux computer. These allow for continuous recording of all 96 channels across three spectrographs, increasing the total amount of data recorded by a an estimated order of magnitude. The additional computing power on the data acquisition system also allows for the implementation of more sophisticated fringe-tracking algorithms which are needed for the Stellar Surface Imaging project. In this paper we describe the New Classic system design and implementation, describe the background and motivation for the system as well as show some initial results from using it.
The Navy Precision Optical Interferometer (NPOI) was designed from the beginning to support baseline boot- strapping with equally-spaced array elements. The motivation was the desire to image the surfaces of resolved stars with the maximum resolution possible with a six-element array. Bootstrapping two baselines together to track fringes on a third baseline has been used at the NPOI for many years, but the capabilities of the fringe tracking software did not permit us to bootstrap three or more baselines together. Recently, both a new backend (VISION; Tennessee State Univ.) and new hardware and firmware (AZ Embedded Systems and New Mexico Tech, respectively) for the current hybrid backend have made multi-baseline bootstrapping possible.
A Michelson optical interferometer, such as an upgraded version of the Navy Precision Optical Interferometer, could image geosynchronous satellites (geosats) with resolution of roughly 1 m. Baselines that sample features as small as 0.2 m can be built, however, the fringes would be swamped by the resolved component. Recent observations have shown that small glints known as “glintchen,” aside from being a nuisance, serve to isolate and highlight the signal from these structures. Imaging of geosats during glintchen events can determine the dimensions of these structures and can also play a critical role in determining if these glintchen are due to a previously undetected companion satellite. An approach for performing this glint-aided imaging of geosats and the wealth of detail it would yield, is discussed.
This paper describes two designs for beam combiners which measures all of the Stokes parameters. The first is the
combination of a optical polarimeter for a single telescope with a simple beam combiner. The second approach
forms 2N beams by splitting the light from each of the N telescopes into orthogonal, linear polarizations. Then,
those 2N beams are fed into a single, multi-beam combiner. We present optical designs for both systems. We
claim the second design is simpler and has the advantage of being easier to calibrate.
We simulate the observations of a red supergiant star and an asteroid with an optical interferometer mounted on
a boom. This instrument has an advantage over more traditional interferometers because it significantly reduces
the number of reflections and surfaces, thus allowing one to combine a larger number of telescopes without a
significant loss of sensitivity. We investigate two telescope arrays distributed on a hexagonal pattern, one that
produces a non redundant coverage of the uv-plane and one that produces a redundant coverage of the uv-plane.
These simulated observations are combined with traditional aperture synthesis techniques to reconstruct images
and determine the accuracy of these images relative to the original ones.
Optical interferometry and polarimetry have separately provided new insights into stellar astronomy, especially
in the fields of fundamental parameters and atmospheric models. Optical interferometers will eventually add
full-Stokes polarization measuring capabilities, thus combining both techniques. In this paper, we: 1) list the
observables, calibration quantities, and data acquisition strategies for both limited and full optical interferometric
polarimetry (OIP); 2) describe the masking interferometer AMASING and its polarization measuring
enhancement called AMASING-POL; 3) show how a radio interferometry imaging package, CASA, can be used
for optical interferometry data reduction; and 4) present imaging simulations for Be stars.
Optical interferometry is the only means of directly measuring the sizes of stars. The most precise angular
diameter measurements, however, depend on measuring complex fringe visibilities V at spatial frequencies where
Re(V ) crosses zero. We can then use the spatial frequency B⊥/λ0 of the zero crossing as a measure of the stellar diameter via θUD,0 ≈ 1.22λ0/B⊥, where λ0 and is the wavelength at which Re(V ) = 0 when observed
with a baseline length B⊥ projected toward the star, and θUD,0 is the equivalent uniform disk diameter. The variation in limb darkening with wavelength leads to a corresponding variation in θUD,0 with λ, even at fixed B,
which allows us to measure the limb darkening in detail and probe the structure of the atmosphere. However, in
order to take meaningful data at those spatial frequencies, we need some form of bootstrapping, in wavelength,
baseline length, or both. Reduction of these bootstrapped data benefits greatly from the increase in SNR offered
by coherent averaging. We demonstrate the effect of limb darkening on θUD,0(λ) with simulated observations based on model atmospheres, and compare them to coherently averaged NOI data.
Geostationary satellites are generally too small to image at high resolution with conventional single-dish tele-
scopes. An alternative to a 100+ m diameter telescope is to use an optical/infrared interferometer consisting
of multiple smaller telescopes in an array configuration. In this paper we focus on what is required to achieve
the required signal-to-noise ratio to image. We will look at the signal-to-noise ratio required to track fringes on
satellites on multiple baselines, a pre-requisite to imaging.We will also look at how to achieve the required signal-
to-noise required for image reconstruction. We compare these performance specifications to the performance of
existing interferometers as well as that of a new interferometer concept optimized for satellite imaging.
Optical Interferometry has long been limited by low SNR making it nearly impossible to measure the small
visibilities required to make resolved images. Although the SNR exists in the raw data, much SNR is lost in
the conventional squared-visibility processing. In modern interferometers fringes are recorded simultaneously at
many wavelengths and baselines. This makes phase-referencing possible, which is the key to coherent integration,
which in turns can greatly improve the SNR of measurements, making small-amplitude resolving measurements
possible. In this paper we will detail the theory of coherent integration. We will also explain why coherent
integration should, in most cases, be carried out during post-processing in software rather than in real-time in
hardware. We will then compare it to conventional processing approaches for some data from the Navy Optical
Interferometer. We will demonstrate how coherent integration can improve the accuracy of observations.
We present the results of the fifth Interferometric Imaging Beauty Contest. The contest consists in blind imaging of test data sets derived from model sources and distributed in the OIFITS format. Two scenarios of imaging with CHARA/MIRC-6T were offered for reconstruction: imaging a T Tauri disc and imaging a spotted red supergiant. There were eight different teams competing this time: Monnier with the software package MACIM; Hofmann, Schertl and Weigelt with IRS; Thiebaut and Soulez with MiRA ; Young with BSMEM; Mary and Vannier with MIROIRS; Millour and Vannier with independent BSMEM and MiRA entries; Rengaswamy with an original method; and Elias with the radio-astronomy package CASA. The contest model images, the data delivered to the contestants and the rules are described as well as the results of the image reconstruction obtained by each method. These results are discussed as well as the strengths and limitations of each algorithm.
Coherent integration is an analysis approach which, can greatly increase the SNR of optical interferometric
visibilities compared to those computed by the traditional squared visibility power spectrum technique. Co-
herent integration relies on phase-referencing, optimally through post-processing fringe-tracking, to effectively
create long coherent integrations of the fringe. At the Navy Precision Optical Interferometer (NPOI) this phase-
referencing is achieved by a combination of wavelength bootstrapping and baseline bootstrapping. The result
is that the complex visibility with full phase information is retrieved and that the poor noise associated with
the power spectrum approach is greatly reduced. For small visibilities, which are most important in resolving
objects, the SNR can be improved sometimes by orders of magnitudes, sometimes making the difference between
easy and practically impossible observations. The fringe-tracking portion of coherent integration is limited by the
SNR of the tracking signal and the noise of that causes some fringe smearing which must be calibrated. In this
paper we develop a theoretical model of the resulting fringe smearing and its correction. We then demonstrated
its validity through simulation and on observations from the NPOI.
Even the longest geosatellite, at 40 m, subtends only 0.2 arcsec (1 microradian). Determining structure and
orientation with 10 cm resolution requires a 90 m telescope at visual wavelengths, or an interferometer. We de-
scribe the application of optical interferometry to observations of complex extended targets such as geosatellites,
and discuss some of its challenges. We brie
y describe our Navy Optical Interferometer (NOI) group's eorts
toward interferometric observations of geosatellites, including the rst interferometric detection of a geosatellite.
The NOI observes in 16 spectral channels (550{850 nm) using up to six 12-cm apertures, with baselines (separa-
tions between apertures) of 16 to 79 m. We detected the geosatellite DirecTV-9S during glint seasons in March
2008 and March 2009, using a single 16 m baseline (resolution 1:6 m). Fringes on a longer baseline were too
weak because the large-scale structure was over-resolved. The fringe strengths are consistent with a combination
of two size scales, 1:3 m and & 3:5 m. Our near term NOI work is directed toward observing geosatellites with
three or more 10 to 15 m baselines, using closure phase measurements to remove atmospheric turbulence eects
and coherent data averaging to increase the SNR. Beyond the two- to three-year time frame, we plan to install
larger apertures (1.4 and 1.8 m), allowing observations outside glint season, and to develop baseline bootstrap-
ping, building long baselines from chains of short baselines, to avoid over-resolution while increasing maximum
resolution. Our ultimate goal is to develop the design parameters for dedicated satellite imaging interferometry.
The increase in the number of satellites and space debris in low Earth orbit (LEO) makes tracking these objects
and avoiding collisions a major endeavor. A particularly important issue is the determination of the altitude
of these objects, which in many cases is not known with a precision better than 1 km. Here we present the
idea of using simultaneous observations by 2 optical telescopes, separated by a few hundred km, to refine the
altitude measurement of these objects to a precision of 10 m. We discuss the requirements for such a system,
like aperture, timing precision, and the precision to which one needs to know the positions of the telescopes and
background stars.
We simulate observations of geostationary satellites using different optical interferometer array configurations.
We test several array designs, including the typical Y shaped array, a couple of circular arrays, telescopes mounted
on a linear movable boom, and a couple of arrays of 30 telescopes on a non redundant and a redundant hexagonal
grid. We use aperture synthesis techniques to reconstruct images from the simulated observations. We compared
the performance and reliability of the different arrays, and find that the image quality increases with the number
of telescopes being used. We also find that short baselines, with lengths of ~2m are needed in order to recover the
large scale structure of the satellite. Some of the best results are produced by the non redundant and redundant
arrays on a hexagonal grid. Considering that the satellite appearance changes with illumination, the boom array
is not a good design, since it requires too much time to observe at different angles.
KEYWORDS: Interferometers, Satellites, Telescopes, Optical fibers, Wavefront sensors, Signal to noise ratio, Space telescopes, Sensors, Interferometry, Imaging systems
This paper presents the results of a study designed to test the feasibility of imaging satellites in geostationary
orbit from the ground. We argue that the instrument should be an interferometer consisting of > 30 telescopes
mounted on a common, steerable boom. Light from the telescopes is fed to the beam combiner with optical
fibers. The delays are equalized by steering the boom and stretching the fibers. The feed system and delay lines
are replaced with single mode fibers.
This system should be better throughput than the optical interferometers in use today and should be able
to reach the sensitivity needed to image these targets with meter-scale telescopes. Calculations supporting this
claim and a system design are presented.
Intensity interferometry, in which intensity fluctuations at separate apertures are measured and then correlated,
is an attractive technique for high angular resolution measurements because of its simplicity. There is no
need to transport light beams from the telescopes of the interferometer array to a beam combiner, and the
telescope optics need not be precise. Michelson interferometry, in which light beams are brought together
and the interference pattern is measured, is significantly more difficult, requiring precision optics and precise
pathlength control, but it has a great advantage in sensitivity, requiring milliseconds to make a detection that
might require hours with an intensity interferometer. However, for interferometry with a large number of array
elements, the sensitivity of Michelson interferometry suffers from the fact that the light beams must be shared
among many correlations, thereby reducing the sensitivity of each measurement. We explore these and other
influences on the relative sensitivities of these techniques to determine under what circumstances, if any, their
sensitivities become comparable.
We report a tentative interferometric detection of an earth-orbiting artificial satellite using optical interferometry. We
targeted four geosynchronous communications satellites with the Navy Prototype Optical Interferometer (NPOI) near
Flagstaff, AZ, and obtained interferometric fringes on one of them, DIRECTV-9S. We used an east-west 15.9-meter
baseline of the NPOI and took data in 16 spectral channels covering the 500-850 nm wavelength range. Observations
took place during the "glint season" of 28 February to 3 March 2008, when the geometry of the solar panel arrays and the
Sun's position creates glints as bright as 2nd magnitude of a few minutes' duration each night. We detected fringes on
the satellite at approximately the 2 σ level on 1 March at magnitude 4.5. Subsequent analysis shows that the fringe
amplitudes are consistent with a size scale of 2 meters (50 nanoradians at geosynchronous orbit) in an east-west
direction. This detection shows that interferometric detection of satellites at visual wavelengths is possible, and suggests
that a multi-baseline interferometer array tailored to the angular size and brightness of geosynchronous satellites could
lead to images of these satellites.
We present the results of Navy Prototype Optical Interferometer observations of the binary stars θ2 Tauri and
HR7955. These data are reduced using standard methods, as well as coherent integration, and were fitted using
three different methods to measure the separation and position angle of the components, and their magnitude
differences. We used the traditional technique of fitting the V2's, triple amplitudes and triple phases, we also
fitted the baseline phases obtained through coherent integration, and measured the separation of the components
directly on images reconstructed using complex visibilities and phase self calibration. We find that fitting baseline
phases produces the highest precision. The results obtained from imaging are similar to these, although with
higher uncertainties, while the traditional method has the lowest precision. We attribute this result to the fact
that the traditional method combines multiple measurements, e.g. triple phases, thus increasing the errors and
reducing the amount of information that can be fitted. We also obtain a preliminary fit to the orbit of HR7955.
In this paper we will discuss the current status of coherent integration with the Navy Prototype Optical Interferometer
(NPOI). Coherent integration relies on being able to phase reference interferometric measurements, which in turn relies on making measurements at multiple wavelengths. We first discuss the generalized group-delay approach, then the meaning of the resulting complex visibilities and then demonstrate how coherent integration
can be used to perform very precision measurement of stellar diameters. The phase of the complex visibility is
particularly attractive as a data product because it is not biased in the same way as visibility amplitudes. We
discuss the relative SNR of triple-product phases and single-baseline phases. We then demonstrate how singlebaseline
phases can be used to make accurate measurements of magnitude differences and separations of binary stars.
We present the angular diameters, physical radii, and effective temperatures of five stars observed using both
the Palomar Testbed Interferometer (PTI) and the Navy Prototype Optical Interferometer (NPOI). These preliminary
results are part of a larger project focused on measuring the angular diameters of the 62 stars that have
been observed with both instruments. We plan to achieve diameter accuracies of 0.5% or better through the
combination of infrared- and visible-wavelength data from PTI and the NPOI, respectively. The difference in
limb-darkening effects between the two wavelength regimes, together with other external parameters, will allow
us to test the atmospheric models on which the limb-darkening corrections are based. High quality angular diameters
for these stars will also lead to more accurate physical diameter measurements and effective temperature determinations.
We have detected a satellite via optical interferometry for the first time, using a 16 m baseline of the Navy
Prototype Optical Interferometer (NPOI) to observe the geostationary communications satellite DirecTV-9S
during the "glint" seasons of February-March 2008 and 2009 when the sun-satellite-NPOI geometry was favorable
for causing specular reflections from geostationary satellites. We used the US Naval Observatory Flagstaff
Station 1 m telescope to generate accurate positions for steering the NPOI. Stars are the easiest targets for
optical/infrared interferometers because of their high surface brightness. Low surface brightness targets are
more difficult: if they are small enough not to be resolved out by typical baselines, they are likely to be too faint
to produce detectable fringes in an atmospheric coherence time. The 16 m NPOI baseline, the shortest available
at the time of our observations, resolves out structures larger than ~ 1.5 m at the geostationary distance, while
a typical size for the solar panel arrays is 2 m x 30 m. Our detection indicates that a small fraction of the
satellite glinted, not surprising given that the solar panels are not accurately flat. Our fringe data are consistent
with a two-component image consisting of a 1 to 1.3 m higher surface brightness component and a significantly
larger lower surface brightness component. The brightness of the glints (2.m 4 and ~ 1.m 5 for the two detections in March 2009) and the size scale suggest that the compact component has an albedo of 0.06 to 0.13, while the
larger-scale component is much darker, if circular geometry is assumed.
We report an interferometric detection of an earth-orbiting artificial satellite using optical interferometry. We targeted
four geosynchronous communications satellites with the Navy Prototype Optical Interferometer (NPOI) near Flagstaff,
AZ, and obtained interferometric fringes on one of them, DIRECTV-9S. We used an east-west 15.9-meter baseline of
the NPOI and took data in 16 spectral channels covering the 500-850 nm wavelength range. Observations took place
during the "glint season" of 28 February to 3 March 2008, and then again in February - March 2009, when the geometry
of the solar panel arrays and the Sun's position creates glints as bright as 2nd magnitude of a few minutes' duration each
night. We detected fringes on the satellite at approximately the 2 sigma level on 1 March at magnitude 4.5. Subsequent
analysis shows that the fringe amplitudes are consistent with a size scale of 2 meters (50 nanoradians at GEO) in an east-west
direction. This detection shows that interferometric detection of satellites at visual wavelengths is possible, and
suggests that a multi-baseline interferometer array tailored to the angular size and brightness of geosynchronous
satellites could lead to images of these satellites.
The instrumental status of the Navy Prototype Optical Interferometer (NPOI) since the last SPIE meeting in 2006 is
summarized, along with the results of the current science programs. The commissioning of new stations and plans for
greatly increased telescope apertures are discussed, along with other instrumentation upgrades. Recent results in the
areas of wide-angle astrometry, binary stars, physical modeling of the circumstellar disks of early-type stars,
improvements in coherent averaging, and phase-reference imaging are also reviewed.
We present the results of the third Optical/IR Interferometry Imaging Beauty Contest. A formal comparison is
presented of the performance of algorithms used for imaging data from optical/infrared long-baseline interferometers.
The contest consists of blind imaging of test data sets derived from model sources and distributed in the
OI-FITS format. The test data consisted of datasets on two objects each "observed" in J, H, and K bands. The
majority of the entries produced accurate reconstructions of the initial models. Each of the methods presented
is discussed.
In this paper we will discuss the current status of coherent integration with the Navy Prototype Optical Interferometer
(NPOI).1 Coherent integration relies on being able to phase reference interferometric measurements,
which in turn relies on making measurements at multiple wavelengths.We first discuss the generalized group-delay
approach, then the meaning of the resulting complex visibilities and then demonstrate how coherent integration
can be used to perform very precision measurement of stellar properties. For example, we demonstrate how we
can measure the diameter of a star to a precision of one part in 350, and measure properties of binary stars. The
complex phase is particularly attractive as a data product because it is not biased in the same way as visibility
amplitudes.
In this paper we use coherently integrated visibilities (see separate paper in these proceedings1) to measure the
properties of binary stars. We use only the phase of the complex visibility and not the amplitude. The reason
for this is that amplitudes suffer from the calibration effect (the same for coherent and incoherent averages) and
thus effectively provide lower accuracy measurements. We demonstrate that the baseline phase alone can be used
to measure the separation, orientation and brightness ratio of a binary star, as a function of wavelength.
Recovering images from optical interferometric observations is one of the major challenges in the field. Unlike
the case of observations at radio wavelengths, in the optical the atmospheric turbulence changes the phases on
a very short time scale, which results in corrupted phase measurements. In order to overcome these limitations,
several groups developed image reconstruction techniques based only on squared visibility and closure phase
information, which are unaffected by atmospheric turbulence. We present the results of two techniques used by
our group, which employed coherently integrated data from the Navy Prototype Optical Interferometer. Based
on these techniques we were able to recover complex visibilities for several sources and image them using standard
radio imaging software. We describe these techniques, the corrections applied to the data, present the images of
a few sources, and discuss the implications of these results.
We report on the results of an experiment to characterize the fringe scanning stroke on the Navy Prototype
Optical Interferometer (NPOI) Fast Delay Line (FDL) strokes. The measurements were carried out during three
days April 11-13, 2005 at the NPOI site near Flagstaff, AZ. The NPOI uses a heterodyne metrology laser system
in its operations. It consists of a HeNe laser with a 2 MHz heterodyne component generated by an Acousto-Optic
Modulator (AOM). One polarization is used as the 2 MHz clock, and the other is sent through the feed system
twice and bounces off the piezo stroke modulators. We sampled both signals at 50 MHz, and obtained stroke
and cart combined motion at the frequency of the stroke modulated 2 MHz heterodyne signal. By counting
zero-crossings in the reference and feed system signals, a rough position (to a wavelength) can be obtained. This
can be further refined to the few-nanometer level by measuring the relative phases of the reference and feed
system signals. This results in approximately 4000 positions measurements per 2 ms stroke with a precision of
approximately 1 nm. We recorded stroke positions for approximately 500 strokes (1 s), for all but one of the six
FDLs, under a variety of conditions: different stroke amplitudes, different cart speeds, and different cart positions
in the FDLs. We then analyzed these data from a total of 100 tests to understand the deviation of the actual
stroke from the ideal stroke. We found that the mean stroke differs from the ideal stroke, and that consecutive
strokes differ from each other. We computed the effect of the non-ideal stroke on the science data. A non-ideal
stroke results in leakage of fringe power between fringe frequencies. This leakage is not significant during most
normal operations of the NPOI. However, when the squared visibilities of baselines on the same spectrograph
differ by large amounts (a factor of 10), care should be taken. Ideally, High- and low-visibility baselines should
be placed on different spectrographs.
We demonstrate a new calibration technique that can be applied to multi-spectral interferometric observations.
The technique measures a fixed-pattern in squared visibility measurements across the spectral channels of each
baseline. Because the fixed-pattern appears to be stable on time scales longer than one night, nightly or weekly
averages can be calculated based on observations of calibrator stars. The averaged fixed-pattern values can then
be removed from data of target stars. We demonstrate the performance of the calibration technique on actual
observations obtained with the Navy Prototype Optical Interferometer and show that the fixed-pattern effects
can be suppressed by up to an order of magnitude.
Visibility measurements with Michelson interferometers, particularly the measurement of fringe contrast, are
affected by various atmospheric and instrumental effects, all of which reduce the measured contrast. To compensate
for this, stars with known or predictable diameters (calibrators) are observed so that the overall reduction
in the visibility can be measured. Objects with the smallest possible diameters are preferred as calibrators, since
the predicted visibilities become less sensitive to any uncertainties. Therefore, unreddened, early type stars are
usually chosen if they are available because they are relatively bright for a given angular diameter. However,
early type stars bring additional complications. Rapid rotation, common with these stars can cause variations
in the visibility amplitudes due to oblateness and surface brightness asymmetries that are larger than implied by
the usual error estimates. In addition, rotation can introduce significant phase offsets. Using Roche models, von
Zeipel theory, and the observed constraints of V, B-V, and v sin i, it is possible to put limits on the size of these
effects and even estimate the distribution of possible visibilities. To make this easily available to the community,
we are in the process of creating a catalog of possible calibrators, including histograms of the visibilities, calculated
for configurations used at a number of observatories. We show the examples of several early type stars
which are potential calibrators using parameters appropriate for the Navy Prototype Optical Interferometer.
We have developed an approach for systematically investigating the optical throughput performance of the different segments of a Michelson stellar interferometer, and applied it to the characterization of the Navy Prototype Optical Interferometer (NPOI). We report the results of the first phase of throughput measurements on NPOI, as well as some of the lessons learned.
Since the current generation of ground-based optical interferometers all suffers from varying degree of throughput degradation while the dominant causes for throughput loss are expected to vary for each individual instrument, the methodologies and approaches developed here could be of general use for the quantitative characterization of the throughput performance of the different optical interferometers, a prerequisite for its ultimate improvement.
We present the results of differential phase experiments done with data from the Navy Prototype Optical Interferometer (NPOI). We take advantage of the fact that this instrument simultaneously records 16 spectral channels in the wavelength range 550-850nm, for multiple baselines. We discuss the corrections applied to the data, and show the results obtained for Vega and the Be star β Lyrae.
We report on experiments in multi-wavelength phase referencing using the Navy Prototype Optical Interferometer (NPOI). In these experiments we use the unique capability of the NPOI to simultaneously observe 16 spectral channels covering 512-850 nm on multiple baselines simultaneously. We present observations of the well-known Be star ζ Tauri using custom filters which allow us to isolate the Hα line in a single spectral channel while the other channels observe the stellar continuum. Since the central star is unresolved, we can use the data in the continuum channels to calibrate the spectral line data. Using the phase information recovered in this way, it is possible for the first time to use standard techniques to construct simple images of the line-emitting region around the star.
Atmospheric turbulence is a major impediment to ground-based optical interferometry. It causes fringes to move
on ms time-scales, forcing very short exposures. Because of the semi-random phase shifts, the traditional approach
averages exposure power spectra to build signal-to-noise ratio (SNR). This incoherent average has two problems:
(1) A bias of correlated noise is introduced which must be subtracted. The smaller the visibility/the fainter the
target star, the more diffcult bias subtraction becomes. SNR builds only slowly in this case. Unfortunately, these
most difficult small visibility baselines contain most of the image information. (2) Baseline phase information is
discarded. These are serious challenges to imaging with ground based optical interferometers. But if we were able
to determine fringe phase, we could shift and integrate all the short exposures. We would then eliminate the bias
problem, improve the SNR, and we would have preserved most of the phase information. This coherent averaging
becomes possible with multi-spectral measurements. The group delay presents one option for determining phase.
A more accurate approach is to use a time-dependent model of the fringe. For the most interesting low-visibility
baselines, the atmospheric phase information can be bootstrapped from phase determinations on high-visibility
baselines using the closure relation. The NPOI, with 32 spectral channels and a bootstrapping configuration,
is well-suited for these approaches. We will illustrate how the fringe modeling approach works, compare it to
the group-delay approach, and show how these approaches can be used to derive bias-free visibility amplitude
and phase information. Coherent integration provides the highest signal-to-noise (SNR) improvement precisely
in the situations where SNR builds most slowly using incoherent averaging. Coherent integration also produces
high-SNR phase measurements which are calibration-free and thus have high real uncertainties as well. In this
paper we will show how to coherently integration on NPOI data, and how to use baseline visibilities and calibrate coherently integrated visibility amplitudes.
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