The Southern Connecticut Stellar Interferometer (SCSI) is an intensity interferometer that is designed to use correlated photon arrival times to determine the geometry of stars. Originally a low-cost, two-telescope instrument that used a 1-pixel single-photon avalanche diode (SPAD) detector at the focal plane of each telescope to record photon events, it is now being upgraded to include a third telescope. This will allow for the simultaneous detection of the photon correlation at three baselines, and thus the ability to map out the two-dimensional geometry of the source much more efficiently than with the two-telescope arrangement. Recent papers in the literature suggest that it may be possible to derive phase information in the Fourier domain from such triple correlations for the brightest stars, potentially giving SCSI an imaging capability. Prior to investigating this possibility, steps must be taken to maximize the observing efficiency of the SCSI. We present here our latest efforts in achieving better pointing, tracking, and collimation with our telescopes, and we discuss our first modeling results of the three-telescope situation in order to understand under what conditions the upgraded SCSI could retrieve imaging information.
The Quad-camera Wavefront-sensing Six-channel Speckle Interferometer (QWSSI) is a new speckle imaging instrument available on the 4.3-m Lowell Discovery Telescope (LDT). QWSSI is built to efficiently make use of collected photons and available detector area. The instrument images on a single Electron Multiplying CCD (EMCCD) at four wavelengths in the optical (577, 658, 808, and 880nm) with 40nm bandpasses. Longward of 1µm, two imaging wavelengths in the NIR are collected at 1150 and 1570nm on two InGaAs cameras with 50nm bandpasses. All remaining non-imaging visible light is then sent into a wavefront EMCCD. All cameras are operated synchronously via concurrent triggering from a timing module. With the simultaneous wavefront sensing, QWSSI characterizes atmospheric aberrations in the wavefront for each speckle frame. This results in additional data that can be utilized during post-processing, enabling advanced techniques such as Multi-Frame Blind Deconvolution. The design philosophy was optimized for an inexpensive, rapid build; virtually all parts were commercial-off-the-shelf (COTS), and custom parts were fabricated or 3D printed on-site. QWSSI’s unique build and capabilities represent a new frontier in civilian high-resolution speckle imaging.
Binary star systems where one of the stars is an exoplanet host appear to be more common than expected prior to the Kepler mission. The Kepler mission and subsequent ground-based follow-up work have revealed a number of Kepler Objects of Interest (KOIs) that have nearby stellar companions (within ~1 arcsec). KOIs with stellar companions and at least one suspected exoplanet were selected for this work. Recent work on these stars has mainly focused on placing the companions on the H-R diagram and inferring if they are likely to be gravitationally bound based on whether their locations are consistent with a common isochrone. However, we have been observing these KOI double stars with speckle imaging over several years and are now in a position to assess whether these systems have components with a common proper motion, and can be seen as physically associated on that basis. We will give sample results of KOI double stars that are in fact common proper motion pairs. We compare our results with estimates of the multiplicity rate of exoplanet hosts from other methods and comment on the use of our data for constraining orbital parameters at this point, particularly the inclination angle. For transit observations, the inclination of the planetary orbit is already known, and the relationship between planetary and stellar orbital planes could have implications for star and planet formation.
The Southern Connecticut Stellar Interferometer (SCSI) is a portable optical intensity interferometer located on the campus of Southern Connecticut State University in New Haven, Connecticut. Since its completion in 2016, the instrument has been used to take engineering data of bright stars. This paper will discuss the data collection and analysis methods, as well as the progress toward reliably measuring a significant stellar photon correlation. Vega has been the main star chosen for test observations to date because its diameter is well known by other methods, and it is not an extended source for the baselines used. The correlation peak in the processed data is compared to theoretical expectations. Given our expected sensitivity, a significant correlation peak is expected for small baselines (~2 m) to appear after a few hours of observation. So far, the observations indicate that the correlation peak is at the expected time delay, and the signal-to-noise ratio roughly scales as predicted.
Speckle imaging is a well known method to achieve diffraction-limited (DL) imaging from ground-based telescopes. The traditional observing method for speckle has been to observe a single, unresolved, source per telescope pointing over a relatively small field of view (FOV). The need for large DL surveys of targets with high sky density motivates a desire for simultaneous speckle imaging over large FOVs, however it is currently impractical to attain this by covering the entire focal plane with fast readout detectors. An alternative approach is to connect a relatively small number of detector pixels to multiple interesting targets spanning a large FOV through the use of optical fibers, a technique employed in spectroscopy for decades. However, for imaging we require the use of coherent fiber bundles (CFBs). We discuss various design considerations for coherent fiber speckle imaging with an eye toward a multiplexed system using numerous configurable CFBs, and we test its viability with a prototype instrument that uses a single CFB to transport speckle images from the telescope focal plane to a traditionally designed, fast readout speckle camera. Using this device on University of Virginia's Fan Mountain Observatory 40-inch telescope we have for the first time successfully demonstrated speckle imaging through a CFB, using both optical and NIR detectors. Results are presented of DL speckle imaging of well-known close (including subarcsecond) binary stars resolved with this fiber-fed speckle system and compared to both literature results and traditional speckle imaging taken with the same camera directly, with no intervening CFB.
Lowell Observatory and Southern Connecticut State University are currently involved in a joint project to determine the stellar multiplicity rates and the fundamental stellar parameters of M dwarf stars using the Differential Speckle Survey Instrument (DSSI) at Lowell’s Discovery Channel Telescope (DCT). DSSI observes speckle patterns simultaneously at two separate wavelengths, allowing color measurements of the components of a binary system to be made in a single observation. This paper will describe the initial data gathering process, which began in 2016. Since then, over 1000 stars have been observed. We summarize the analysis on these objects so far, and discuss the relevance of these observations for existing and future space missions such as TESS, JWST, and Gaia.
Recent work with the NESSI speckle camera at Kitt Peak and the 'Alopeke speckle camera at Gemini-North indicates that speckle data reduction techniques can be successfully modified to produce high-resolution images over fields that are at least tens of arc seconds across. While these wide-field speckle image reconstructions are not diffraction-limited, the improvement in resolution over the seeing-limited case can be substantial. In this paper, we explore the application of these techniques to data taken with a small (0.6-m) telescope in an urban environment. Many telescopes located in urban communities, such as New Haven, Connecticut, where Southern Connecticut State University resides, have limited use scientifically due to substantial light pollution, poor seeing, poor telescope tracking, and other issues. We will present initial data using our set-up and discuss the potential for this approach for improving the imaging capabilities of small telescopes on our campus and beyond.
The Southern Connecticut Stellar Interferometer (SCSI) is a two-telescope astronomical intensity interferometer that was completed in June 2016 and has been taking photon correlation data since that time. It uses single-photon avalanche diode (SPAD) detectors at the telescope focal plane and a central timing module, which records the signals from both telescopes simultaneously. In the observations taken to date, single-pixel SPADs have been connected to signal cables that stretch from each telescope to the timing module. However, we are now in the process of making the instrument “wireless” by using a separate timing module at each telescope and synchronizing the signals recorded using GPS timing cards. We have also upgraded one of the two stations with an 8-pixel SPAD device, which allows us to achieve higher count rates in a variety of observing conditions. In this paper, we report on the current state of the instrument, including engineering tests made in preparation for wireless operation, and we discuss the expected capabilities in that mode.
The construction of a new prototype visible-light intensity interferometer for use in stellar astronomy is described. The instrument is located in New Haven, Connecticut, at Southern Connecticut State University, but key components of the system are also portable and have been taken to existing research-class telescopes to maximize sensitivity and baseline. The interferometer is currently a two-station instrument, but it is easily expandable to several stations for simultaneous measurement using multiple baselines. The design features single photon avalanche diode (SPAD) arrays, which increase the throughput and signal-to-noise ratio of the instrument. Predicted system performance and preliminary observations will be discussed.
In recent years, speckle imaging has proven very useful for certain problems in single-aperture high-resolution imaging, including searching for faint stellar companions near exoplanet host stars and for satellite imaging. These developments have largely been the result of the availability of electron-multiplying CCD cameras, which allow for greater sensitivity and better photometric linearity when compared with other detectors that have comparable speed. This in turn has led to an increased use for speckle imaging at mid-sized and large telescopes. Some results of these efforts will be discussed, and the outlook for the future of speckle will be given.
Two new instruments are currently being built for the Gemini-North and WIYN telescopes. They are based on the existing DSSI (Differential Speckle Survey Instrument), but the new dual-channel instruments will have both speckle and "wide-field" imaging capabilities. Nearly identical copies of the instrument will be installed as a public access permanent loan at the Gemini-N and WIYN telescopes. Many exoplanet targets will come from the NASA K2 and TESS missions. The faint limiting magnitude, for speckle observations, will remain around 16 to 17th magnitude depending on observing conditions, while wide-field, high speed imaging should be able to go to 21+. For Gemini, the instrument will be remotely operable from either the mid-level facility at Hale Pohaku or the remote operations base in Hilo.
KEYWORDS: Interferometry, Telescopes, Sensors, Interferometers, Stars, Signal to noise ratio, Astronomy, Electronics, Optical interferometry, Global Positioning System
Intensity interferometry, which was first used for obtaining ultra—high-resolution image information in astronomy in the 1960's and 1970's, is now being revived using modern detectors and electronics. This paper explores the possibility of wireless optical interferometry made possible by technological advancements in timing correlation, signal processing, and detector technology. If this can be achieved, then baselines of one to several kilometers may be possible in optical interferometry in the coming years. This would improve the resolution over the current generation of amplitude-based optical interferometers by a factor of at least ten.
A limitation of the current generation of long baseline optical interferometers is the need to make the light interfere prior
to detection. This is unlike the radio regime where signals can be recorded fast enough to use electronics to accomplish
the same result. This paper describes a modern optical intensity interferometer based on electronics with picosecond
timing resolution. The instrument will allow for portable optical interferometry with much larger baselines than currently
possible by using existing large telescopes. With modern electronics, the limiting magnitude of the technique at a 4-m
aperture size becomes competitive with some amplitude-based interferometers. The instrumentation will permit a
wireless mode of operation with GPS clocking technology, extending the work to extremely large baselines. We discuss
the basic observing strategy, a planned observational program at the Lowell Observatory 1.8-m and 1.0-m telescopes,
and the science that can realistically be done with this instrumentation.
Observations using the Starfire Optical Range (SOR) 1.5m telescope, located near Albuquerque, NM, were made during two separate observing runs, one in 1995 and the other in 1996. Image data was collected using a cooled 2K by 2K user provided CCD camera system. During the first observing sessions a standard SOR direct imaging configuration was used where a CCD imager. For the second observing run the configuration was modified to use a mirror with a small on- axis hole that allowed all the light in the central region to be used by the AO system while the remainder of the field was directed to the CCD. The data from these observations were used to investigate a number of issues related to AO observing including: (i) the effect of scattered laser light on image quality; (ii) the photometric accuracy across an AO corrected field; (iii) the PSF variations across an AO corrected field; (iv) the limits to observation of close companions using a mirror with a hole as a coronograph. The result of these observing runs are presented along with representative images obtained using no correction, partial correction, using natural guide stars, and Rayleigh laser beacon configuration.
A design for a new-concept speckle imaging system is presented. The instrument, now under construction at Rochester Institute of Technology (RIT), will be able to use any large- format scientific-grade CCD as the imager. The high quantum efficiency, low noise, and linear response of current CCDs are attractive characteristics when compared with traditional photon-counting speckle imaging systems. The RIT system consists of an optics package, placed between the telescope and the imager, that contains a piezoelectric tip-tilt mirror capable of executing a timed sequence of movements to place many speckle patterns over the active area of the CCD. The system will either (a) place a series of speckle images in a row and then use the CCD electronics to periodically shift charge toward the serial register or, if the CCD electronics do not allow, (b) have the mirror perform a serpentine step- and-scan motion over the entire CCD. When the entire CCD is full of speckle images, the chip is read out as normal. This kind of 'burst mode' speckle data collection effectively uses the large area of the CCD as a memory cache of speckle data frames, allowing large format scientific-grade CCDs that already exist at many observatories to be used efficiently and inexpensively in speckle imaging. The expected performance of the system, which is dependent on the CCD imager, is discussed. CCD speckle observations at the WIYN* 3.5-m telescope and simulation results indicate that, when used with a very low noise CCD, this system could obtain speckle data that are superior to those of even the best photon-counting cameras at the fainter magnitudes where such cameras are currently used.
The Multi-Anode Microchannel Array (MAMA) detector system is a true photon-counting imager which records the position and arrival time of each detected photon for post facto image reconstruction analysis. Imaging by time-tag photon detection with the MAMA is being used for image stabilization on sounding rockets, and for speckle interferometry and speckle image reconstruction at ground-based telescopes using the Stanford University Speckle Interferometer System (SUSIS). This paper describes the construction and mode-of-operation of the MAMA time-tag photon-detection system, including recent improvements to the data- handling system which permit a data-recording rate in excess of 1 M event s-1. The intrinsic time resolution of the MAMA detector system is < 300 ns and the time resolution of the SUSIS used to date is 3.2 microsecond(s) for each detected photon. A number of examples of both laboratory data and visible-light speckle interferometric deconvolutions and two-dimensional speckle image reconstructions are presented.
The multi-anode microchannel array (MAMA) is a microchannel plate based photon counting detector with applications in ground-based and space-based astronomy. The detector electronics decode the position of each photon event, and the decoding algorithm that associates a given event with the appropriate pixel is determined by the geometry of the anode array. The standard MAMA detector has a spatial resolution set by the anode array of 25 microns, but the MCP pore resolution exceeds this. The performance of a new algorithm that halves the pixel spacing and improves the pixel spatial resolution is described. The new algorithm does not degrade the pulse-pair resolution of the detector and does not require any modifications to the detector tube. Measurements of the detector's response demonstrate that high resolution decoding yields a 60% enhancement in spatial resolution. Measurements of the performance of the high resolution algorithm with a 14 micron MAMA detector are also described. The parameters that control high resolution performance are discussed. Results of the application of high resolution decoding to speckle interferometry are presented.
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