Deep Brain Stimulation (DBS) is FDA-approved for the treatment of Parkinson's disease and essential tremor. Currently, placement of DBS leads is guided through a combination of anatomical targeting and intraoperative microelectrode recordings. The physiological mapping process requires several hours, and each pass of the microelectrode into the brain increases the risk of hemorrhage. Optical Coherence Domain Reflectometry (OCDR) in combination with current methodologies could reduce surgical time and increase accuracy and safety by providing data on structures some distance ahead of the probe.
For this preliminary study, we scanned a rat brain in vitro using polarization-insensitive Optical Coherence Tomography (OCT). For accurate measurement of intensity and attenuation, polarization effects arising from tissue birefringence are removed by polarization diversity detection. A fresh rat brain was sectioned along the coronal plane and immersed in a 5 mm cuvette with saline solution.
OCT images from a 1294 nm light source showed depth profiles up to 2 mm. Light intensity and attenuation rate distinguished various tissue structures such as hippocampus, cortex, external capsule, internal capsule, and optic tract. Attenuation coefficient is determined by linear fitting of the single scattering regime in averaged A-scans where Beer’s law is applicable. Histology showed very good correlation with OCT images. From the preliminary study using OCT, we conclude that OCDR is a promising approach for guiding DBS probe placement.
Corneal hydration plays an essential role in maintaining optimal vision. During laser ablation surgery, corneal hydration varies greatly and is likely to affect the outcome. Quantitative measurements of this interaction may help improve the results of vision correction surgery. In addition, prescreening of corneal hydration could be used to correct the laser surgery procedure for hydration variation in the patient population.
We present a functional extension of Optical Coherence Tomography (OCT) to measure cornea hydration in vitro using two light sources simultaneously, one at 1294 nm (negligible water absorption loss) and another at 1410 nm (large water absorption loss). Measuring the ratio of the intensity profile at these two wavelengths allows us to separate the effect of absorptive attenuation from the reflectivity structure of the sample. We first measured the differential absorption coefficient of a calibration target: a 1 mm cuvette containing controlled mixtures of water (H2O) and heavy water (D2O). The optical properties of heavy water are almost identical with those of water, except that it has negligible absorption near 1410 nm. Next, we scanned in vitro fresh cornea bathed in Optisol. We then scraped off the epithelium and immersed the cornea into Balanced Salt Solution in order to increase the hydration through swelling. Then, the cornea was immersed in a 15% Dextran solution to reverse the swelling. After the OCT scans, the cornea hydration level was evaluated by standard weight measurement.
The result of the calibration experiment showed that a strong correlation exists between measured differential water absorption coefficient and actual water content within the cuvette. We derived the hydration level profile over corneal depth from a least squares fit of the log-intensity ratio. Average hydration from the OCT data agreed with the hydration determined by weight measurement.
During the 2001 observing season, the CHARA Array was in regular operation for a combined program of science, technical development, test, and commissioning. Interferometric science operations were carried out on baselines up to 330 meters -- the maximum available in the six-telescope array. This poster gives sample results obtained with the approximately north-south telescope pair designated S1-E1. At operating wavelengths in the K band, the 330 m baseline is well suited to diameter determinations for angular diameters in the range 0.6 - 1.2 milliarcseconds. This is a good
range for study of a wide range of hot stars. In this poster, angular
diameters for a set of A,B and F stars are compared to results derived from other sources. These confirm CHARA performance in the range 3-10% in visibility. The normal stars follow a normal spectral type - surface brightness relation, and a classical Be star deviates from the norm by an amount consistent with its apparent colors.
The CHARA Array at Mt. Wilson uses a PICNIC array camera for fringe detection, connected to a realtime fringe tracking computer running RTLinux. This paper describes the PC- and RTLinux-based camera controller and software that is used to allow high-speed, deterministic, low-latency readout of frames from the camera, as well as a camera simulator that mimics the behavior of the camera. This camera controller is built from commercial off-the-shelf (COTS) PC hardware and uses software running on the free RTLinux operating system, resulting in a very inexpensive camera controller system. The hardware costs for the system, including the PC (although excluding the costs of analog signal interfaces and power supplies), are less than $2000. The controller is capable of reading out arbitrary subimages from the camera, can quickly switch between different readout patterns, and is capable of controlling either CCD cameras or infrared array cameras. Detailed camera timing can be supplied by and/or tuned by the end user, as desired. In addition, a camera simulator unit has been developed. This camera simulator allows the development of camera interface hardware without the risk of damage to the expensive camera. The camera controller described connects to the Niro camera supplied to CHARA by Mark Shure, and the camera simulator mimics the behavior of this camera.
The CHARA Array is a six element optical and near infrared interferometer built by Georgia State University on Mount Wilson in California. It is currently operating in the K and H bands and has the largest baseline (330 m) in operation of any similar instrument in the world. We expect to begin I band operations in 2002. We will present an update of the status of the instrumentation in the Array and set out our plans for the near term expansion of the system.
The CHARA Array consists of six 1-meter telescopes. The telescopes are at fixed positions laid out in a Y-shaped pattern, where the longest available baseline is 330 meters. The resolving power of this interferometric array operating at visible and short infrared wavelengths is better than one milli-arcsecond. The current infrared beam combination system is capable of combining the light from any two of the six telescopes in the array. With the existing infrared beam combination and detection system, we routinely observe in K and H band, where our magnitude limit is 6.
The CHARA Array employs vacuum light pipes between the telescopes and the beam combination area. The complex terrain of the Mt. Wilson site poses interesting problems, with light pipes both underground and suspended up to 10 meters above ground. Telescope to beam-combination distances are up to about 180 meters. The support scheme and alignment strategy will be described.
Georgia State University's Center for High Angular Resolution Astronomy (CHARA) is building an interferometric array of telescopes for high resolution imaging at optical and infrared wavelengths. The `CHARA Array' consists of six 1-m diameter telescopes arranged in a Y-shaped configuration with a maximum baseline of approximately 350 m. Construction of the facility will be completed during 2000, and the project will enter a phase in which beam combination subsystems will be brought on line concurrently with initial scientific investigations. This paper provides an update on recent progress, including our reaching the significant milestone of `first fringes' in November 1999. An extensive collection of project technical reports and images are available at our website.
A high performance, bias tunable, p-GaAs homojunction interfacial work function internal photoemission far-IR (FIR) detector has been demonstrated. A responsivity of 3.10 +/- 0.05 A/W, a quantum efficiency of 12.5 percent, and a detectivity D* of 5.9 X 1010 cm (root) Hz/W, were obtained at 4.2K, for cutoff wavelengths form 80 to 100 micrometers . The bias dependences of quantum efficiency, detectivity, and cutoff wavelength have been measured and are well explained by the theoretical models. The cutoff wavelength is modeled by a modified high density theory, and the quantum efficiency is predicted by scaling the free carrier absorption coefficient linearly with the doping concentration. The effect of the number of layers on detector performance and the uniformity of the detectors have been discussed. A comparison with Ge:GA photoconductive detectors suggests that a similar or even better performance may be obtainable.
The telescope requirements of optical interferometry are somewhat different from conventional astronomy. The need for multiple units (in the CHARA case initially five, eventually seven) accentuates the importance of cost control, and at the same time provides opportunity for cost savings by careful procurement and production practices. Modern ideas about telescope enclosures offer significantly reduced dome seeing, but it is difficult to capture these benefits at low cost. The CHARA group has followed a series of design and bid procedures intended to optimize the costperformance of the telescope+enclosures. These have led to a compact but massive telescope design, blending modern and classical features, an unusual mirror blank selection process (directly ompeting several mirror blank technologies) , and a novel telescope enclosure concept which allows a continuous trade between wind protection and natural ventilation. This contribution will review and motivate the design decisions and show the resulting equipment and facilities.
The Center for High Angular Resolution Astronomy (CHARA) at Georgia State University is building an
interferometric array of telescopes for high resolution imaging at optical and infrared wavelengths. The "CHARA Array" will initially consist offive 1-rn diameter telescopes arranged in a Y-shaped configuration with a maximum baseline of approximately 350 m. The facility is being constructed on Mt. Wilson, near Pasadena, California, a site noted for stable atmospheric conditions that often gives rise to exceptional image quality. The Array will be capable of submilliarcsecond imaging and will be devoted to a broad program of science aimed at fundamental stellar astrophysics in the visible and the astrophysics of young stellar objects in the infrared (2.2μm) spectral regions. This project is being funded in approximately 50/50% shares by Georgia State University and the National Science Foundation. The CHARA Array is expected to become operational during 1999. This paper presents a project status report. An extensive collection of project reports and images are available at our website (http://www.chara.gsu.edu).
We have just recently commissioned a new 1.0-5.5 micrometers IR array camera for the NASA IR Telescope Facility based upon the Santa Barbara Research Center 256x256 InSb array. The primary features of this new instrument are three user-selected platescales, a variety fo fixed bandpass filters, 1 to 2% spectral resolution circular variable filters, coronagraph masks, polarization imaging capability, an optical guider/imager, and a grism. In this paper we briefly outline the design and performance of the camera system, describe some unique operating modes, and show some recent images.
The design of a multipurpose 1 - 5.5 micrometers infrared camera (NSFCAM) for the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, is described. The camera is built around the new 256 X 256 InSb array manufactured by Santa Barbara Research Center (SBRC) and incorporates a variety of observing modes to fulfill its role as a major facility instrument. These include three remotely-selectable image scales, a selection of fixed bandpass filters, R equals 50 - 100 spectral resolution circularly variable filters, a grism, coronographic masks, and a polarization imaging capability. Through the use of flexible array clocking schemes, driven by programmable digital signal processors (DSPs), we plan to implement several new operating modes, including real-time shift and add for image stabilization, and fast subarray readouts for occultations. Simultaneous optical and infrared imaging of the same field will be possible through the use of a cold dichroic beamsplitter. This feature is primarily intended for use with the IRTF tip-tilt image stabilization system currently being built. Given a suitable guide star, the camera should achieve near-diffraction limited imaging at 2 - 5 micrometers . In this paper we discuss the design of the optics, cryogenic, electronics and software needed to provide the camera with these capabilities.
ProtoCAM, a new infrared array camera, has been built for the NASA 3-m Infrared Telescope Facility (IRTF). The camera is built around a 62 x 58 InSb hybrid array and is sensitive throughout the 1-5-micron atmospheric windows. The camera is equipped with standard astronomical filters as well as a full complement of continuously variable filters providing a spectral resolution down to 1 percent. On the IRTF, the platescale is variable real-time from 0.14 to 0.35 arcsec. The camera, the electronics, the software, and the performances are discussed, and some preliminary astronomical results are presented.