In magnetic resonance imaging (MRI), an object within a field-of-view (FOV) is spatially encoded with a broad spectrum of frequency components generating signals that decohere with one another to create a decaying echo with a large peak amplitude. The echo is short and decays at a rapid rate relative to the readout period when performing high resolution imaging of a sizable object where many frequency components are encoded resulting in faster decoherence of the generated signals. This makes it more difficult to resolve fine details as the echo quickly decays down to the quantization limit. Samples collected away from the peak signal, which are required to produce high resolution images, have very low amplitudes and therefore, poor dynamic range. We propose a novel data acquisition system, Calculated Readout in Spectral Parallelism (CRISP), that spectrally separates the radio frequency (RF) signal into multiple narrowband channels before digitization. The frequency bandwidth of each channel is smaller than the FOV and centered over a part of the image with minimal overlap with the other channels. The power of the corresponding temporal signal in each channel is reduced and spread across a broader region in time with a slower decay rate. This allows the signal from each channel to be independently amplified such that a larger portion of the signal is digitized at higher bits. Therefore, the dynamic range of the signal is improved and sensitivity to quantization noise is reduced. We present a realization of CRISP using inexpensive analog filters and preliminary results from high resolution images.
Radial velocity (RV) of stellar targets were measured using a dispersed Fourier transform spectrometer (dFTS). The instrument used a laser based optical metrology system in cooperation with a mechanical metrology system to measure the absolute position of the retro-reflectors in the dFTS with a precision of 0.1 nm. The combined metrology system data allowed stellar RV measurements to be precise at the 10 m/s level, or about 0.1% relative error with respect to the RV amplitude. The dFTS instrument is well suited for precise RV measurements, and is less cumbersome to calibrate and operate than echelle spectrometers - a competing instrument for RV measurements of stellar targets.
We present a technique for calibrating optical long-baseline interferometric observations in which both the calibration corrections and the source characteristics are obtained from the observations of a program star. This calibration can only be applied to certain classes of objects, such as emission-line sources or binary systems, in which the parameters describing the characteristics of the source are orthogonal to the calibration parameters. The technique is applied to observations of γ Cassiopeiae, obtained on four different nights with the Navy Prototype Optical Interferometer, and utilizes measurements obtained simultaneously in many spectral channels covering a wide spectral range, of which only two channels contain a strong signal due to the circumstellar envelope in the Hα emission line. The calibrated observations in Hα show a clearly resolved circumstellar structure. The best-fit elliptical Gaussian model fitted to our observations has ensemble average parameters of 3.67 ± 0.09 mas for the angular size of the major axis, 0.79 ± 0.03 for the axial ratio, and 32° ± 5° for the position angle, all in good agreement with values reported by previous investigations.
Remote sensing from an aerial platform has many similarities to medical imaging. Line, whiskbroom, and pushbroom scanning techniques are compared with scan patterns from medical imaging. Satellite imaging uses a scan mirror or sensor array to achieve across track imagery and uses its procession in orbit to achieve along track movement. Medical imaging technologies, like confocal microscopy and optical coherence tomography, use similar scanning mechanisms for across track imagery, but are not in orbit and must introduce the along track movement with a second galvanometer scan mirror or linear stage. Square, triangle, sinusoidal, and sawtooth waveform inputs to the galvanometer provide the actuation signal to control sweeping patterns across a sample. A tissue handling system for medical applications is introduced for discussion and simulation of scan mechanism implementation. The scan system uses a galvanometer and linear stage combination to provide control over light delivery and sample positioning. The synchronization requirements and efficacy of various scan patterns are examined.
We have combined two epochs of Hubble Space Telescope WFPC2 imaging data with ground-based expansion velocities to determine distances to three planetary nebulae (NGC 6578, NGC 6884, and IC 2448). We used two variants of the expansion parallax technique—a gradient method and a magnification method—to determine the distances. The results from the two methods agree to within the errors. A fourth nebula was included in the study (NGC 6891), but the expansion was too small to determine the distance, and only a lower limit was obtained. This is the second paper in a series that will examine at least 24 nebulae in total.
Tornado Spectral Systems (TSS) has developed the High Throughput Virtual Slit (HTVSTM), robust all-reflective pupil slicing technology capable of replacing the slit in research-, commercial- and MIL-SPEC-grade spectrometer systems. In the simplest configuration, the HTVS allows optical designers to remove the lossy slit from pointsource spectrometers and widen the input slit of long-slit spectrometers, greatly increasing throughput without loss of spectral resolution or cross-dispersion information. The HTVS works by transferring etendue between image plane axes but operating in the pupil domain rather than at a focal plane. While useful for other technologies, this is especially relevant for spectroscopic applications by performing the same spectral narrowing as a slit without throwing away light on the slit aperture. HTVS can be implemented in all-reflective designs and only requires a small number of reflections for significant spectral resolution enhancement–HTVS systems can be efficiently implemented in most wavelength regions. The etendueshifting operation also provides smooth scaling with input spot/image size without requiring reconfiguration for different targets (such as different seeing disk diameters or different fiber core sizes). Like most slicing technologies, HTVS provides throughput increases of several times without resolution loss over equivalent slitbased designs. HTVS technology enables robust slit replacement in point-source spectrometer systems. By virtue of pupilspace operation this technology has several advantages over comparable image-space slicer technology, including the ability to adapt gracefully and linearly to changing source size and better vertical packing of the flux distribution. Additionally, this technology can be implemented with large slicing factors in both fast and slow beams and can easily scale from large, room-sized spectrometers through to small, telescope-mounted devices. Finally, this same technology is directly applicable to multi-fiber spectrometers to achieve similar enhancement. HTVS also provides the ability to anamorphically "stretch" the slit image in long-slit spectrometers, allowing the instrument designer to optimize the plate scale in the dispersion axis and cross-dispersion axes independently without sacrificing spatial information. This allows users to widen the input slit, with the associated gain of throughput and loss of spatial selectivity, while maintaining the spectral resolution of the spectrometer system. This "stretching" places increased requirements on detector focal plane height, as with image slicing techniques, but provides additional degrees of freedom to instrument designers to build the best possible spectrometer systems. We discuss the details of this technology for an astronomical context, covering the applicability from small telescope mounted spectrometers through long-slit imagers and radial-velocity engines. This powerful tool provides additional degrees of freedom when designing a spectrometer, enabling instrument designers to further optimize systems for the required scientific goals.
Abstract We present linear radii for four Cepheid variable stars, spanning a pulsation period range from 3 to 11 d, measured with the Navy Prototype Optical Interferometer (NPOI). We compare these radii to those found using traditional indirect methods, and to various period–radius relations found in the literature.
Remote sensing has moved out of the laboratory and into the real world. Instruments using reflection or Raman imaging modalities become faster, cheaper and more powerful annually. Enabling technologies include virtual slit spectrometer design, high power multimode diode lasers, fast open-loop scanning systems, low-noise IR-sensitive array detectors and low-cost computers with touchscreen interfaces. High-volume manufacturing assembles these components into inexpensive portable or handheld devices that make possible sophisticated decision-making based on robust data analytics. Examples include threat, hazmat and narcotics detection; remote gas sensing; biophotonic screening; environmental remediation and a host of other applications.
We have used the Navy Prototype Optical Interferometer (NPOI) to obtain the first multi-channel optical aperture synthesis images of a star. We observed the spectroscopic binary zeta (1) Ursae Majoris at 6 to 10 milliarcseconds separation during seven nights, using three interferometric baselines and 19 spectral channels (lambda lambda520 - 850 nm) of the NPOI. After editing, a typical 90 sec scan yielded fringe visibilities at 50 spatial frequencies and closure phases at 15 wavelengths. Three to five scans were obtained each night. The separations and position angles are in good agreement with the visual orbit obtained with the Mark III interferometer (Hummel et al.markcite{hum1} 1995 [AJ, 110, 376]) but show small systematic difference that can be used to improve the orbit. The closure phase data provide a sensitive measure of the magnitude difference between the components. These results demonstrate the power of broad-band interferometric observations for fast imaging and the utility of vacuum delay lines for simultaneous observations over a wide band. These observations are the first to produce simultaneous visibilities and closure phases with a separate-aperture optical interferometer, and the second to produce closure phase images, following the results from COAST reported by Baldwin et al.markcite{bal} (1996 [A&A, 306, L13]). The angular resolution here is the highest ever achieved at visual wavelengths, exceeding by an order of magnitude the best thus far achieved by any single-aperture optical telescope. We generated complex visibilities and closure phases (the data types commonly used in radio interferometry) from the optical data and used standard radio interferometry techniques to produce these images. However, the fundamental observables of optical interferometry, the squared visibility amplitude and the closure phase, require the development of new analysis techniques.
