This article describes the modeling of the generic spatial heterodyne spectrometer, which resembles a somewhat modified Michelson interferometer. The power spectrum of the input source is determined by performing a one dimensional Fourier transform on the output intensity profile captured by a linear array detector. In order to illustrate the techniques employed in the modeling, an example of a heterodyne interferometer is developed and a comparison undertaken between its performance and that of a conventional spectrometer. Unlike the traditional FTIR system, the heterodyne spectrometer has the very desirable feature of having no moving components.
In this paper, we provide an overview of the adaptive optics (AO) program for the Thirty Meter Telescope (TMT) project, including an update on requirements; the philosophical approach to developing an overall AO system architecture; the recently completed conceptual designs for facility and instrument AO systems; anticipated first light capabilities and upgrade options; and the hardware, software, and controls interfaces with the remainder of the observatory. Supporting work in AO component development, lab and field tests, and simulation and analysis is also discussed. Further detail on all of these subjects may be found in additional papers in this conference.
Although many of the instruments planned for the TMT (Thirty Meter Telescope) have their own closely-coupled adaptive
optics systems, TMT will also have a facility Adaptive Optics (AO) system, NFIRAOS, feeding three instruments
on the Nasmyth platform. This Narrow-Field Infrared Adaptive Optics System, employs conventional deformable mirrors
with large diameters of about 300 mm. The requirements for NFIRAOS include 1.0-2.5 microns wavelength range,
30 arcsecond diameter science field of view (FOV), excellent sky coverage, and diffraction-limited atmospheric turbulence
compensation (specified at 133 nm RMS including residual telescope and science instrument errors.) The reference
design for NFIRAOS includes six sodium laser guide stars over a 70 arcsecond FOV, and multiple infrared tip/tilt sensors
and a natural guide star focus sensor within instruments. Larger telescopes require greater deformable mirror (DM)
stroke. Although initially NFIRAOS will correct a 10 arcsecond science field, it uses two deformable mirrors in series,
partly to provide sufficient stroke for atmospheric correction over the 30 m telescope aperture, but mainly to improve
sky coverage by sharpening near-IR natural guide stars over a 2 arcminute diameter "technical" field. The planned upgrade
to full performance includes replacing the ground-conjugated DM with a higher actuator density, and using a deformable
telescope secondary mirror as a "woofer." NFIRAOS feeds three live instruments: a near-Infrared integral field
Imaging spectrograph, a near-infrared echelle spectrograph, and after upgrading NFIRAOS to full multi-conjugation, a
wide field (30 arcsecond) infrared camera.
Although many of the instruments planned for the TMT (Thirty Meter Telescope) have their own closely-coupled adaptive optics systems, TMT will also have a facility Adaptive Optics (AO) system feeding three instruments on the Nasmyth platform. For this Narrow-Field Infrared Adaptive Optics System, NFIRAOS (pronounced nefarious), the TMT project considered two architectures. One, described in this paper, employs conventional deformable mirrors with large diameters of about 300 mm and this is the reference design adopted by the TMT project. An alternative design based on MEMS was also studied, and is being presented separately in this conference. The requirements for NFIRAOS include 0.8-5 microns wavelength range, 30 arcsecond diameter output field of view (FOV), excellent sky coverage, and diffraction-
limited atmospheric turbulence compensation (specified at 133 nm RMS including residual telescope and science instrument errors.) The reference design for NFIRAOS includes multiple sodium laser guide stars over a 70 arcsecond FOV, and an infrared tip/tilt/focus/astigmatism natural guide star sensor within instruments. Larger telescopes require greater deformable mirror (DM) stroke. Although initially NFIRAOS will correct a 10 arcsecond science field, it uses two deformable mirrors in series, partly to provide sufficient stroke for atmospheric correction over the 30 m telescope aperture, but mainly to partially correct a 2 arcminute diameter "technical" field to sharpen near-IR natural guide stars and improve sky coverage. The planned upgrade to full performance includes replacing the groundconjugated DM with a higher actuator density, and using a deformable telescope secondary mirror as a "woofer." NFIRAOS incorporates an instrument rotator and selection of three live instruments: a near-Infrared integral field Imaging
spectrograph, a near-infrared echelle spectrograph, and after upgrading NFIRAOS to full multi-conjugation, a wide field (30 arcsecond) infrared camera.
A wide-field low-resolution multi-object optical spectrograph suitable for a 30-m F/15 telescope is described. The effort to build a 30-m class telescope is gaining momentum. Many science cases for such a telescope make the need for a wide-field seeing-limited spectrograph a high priority. Our concept comprises four identical instruments placed symmetrically around the optical axis of the telescope, this allows smaller dimensions for the spectrographs and their components. Each instrument is placed in one quadrant of the telescope focal plane; a space at the center of the field is free for other instrumentation. Using a dichroic beam-splitter each instrument feeds a "red" and "blue" camera. The total field is 81 square arcmin, the wavelength range covers simultaneously 310 nm to 1000 nm and the spectral resolution (R) is 300 to 5000. The instruments are designed for vertical mounting at a Nasmyth focus to avoid gravity vector changes and reducing mechanical flexure problems during observation. The layout also allows access to internal components for maintenance. The design offers advantages for the location of a slit mask and filters. The instruments can also be used for imaging. Optical and opto-mechanical models and analyses are presented with specifications and expected performance.
We describe the basic principles behind the design of a zoom ophthalmoscope. The purpose of such a device is to replace two instruments which are currently used to examine the retina; the small field but high magnification direct ophthalmoscope and the large field but low magnification indirect ophthalmoscope. The desired field-of-view and zoom ratio cover the fields-of-view and magnifications of these two instruments.
An instrument used for measuring the length of cable as it passes form one reel to another is described. The optical portion of the instrument comprises two distinct components a beam delivery unit and a collector assembly. The package is capable of performing length measurements on cables having an assortment of shapes, including those with corrugations.
KEYWORDS: Sensors, Signal to noise ratio, Short wave infrared radiation, Spectrographs, Spectroscopy, Calibration, Data acquisition, Minerals, Detector arrays, Spectral resolution
The SWIR full spectrum imager is an imaging spectrometer covering the short-wave infrared from 1220 to 2420 nm, which has been developed for remote sensing from an airborne platform. The sensor has been designed to acquire the full spectrum at high spectral resolution and the full image swath at high spatial resolution simultaneously. The instrument utilizes a 2D detector array, refractive optics and a transmission grating. The fore-optics and spectrograph are f/1.8, and the angular field-of-view is 9.4 degrees. The detector is a 488 line by 512 pixel PtSi Schottky barrier photodiode array. A VME bus computer communicates with the array controller, performs the data acquisition and provides the operator interface. The optical design and sensor system are described: calibration methods and results are presented. Post flight data processing procedures are described and the spectral signal-to-noise ratio is calculated from in-flight data. A sample single-band image from data collected on the JUne 1995 Nevada mission is displayed, spectra of minerals and trees are extracted, and a classification of this image is shown.
This article describes a technique employed in the development of anti-vignetting filters to be incorporated in wide angle lens systems. The technique requires no knowledge of the optical prescription, but would include lens parameters such as entrance pupil location, type of vignetting and intensity distribution across the image plane. Results are given for the application of this technique to an 85 degree camera objective.
The SWIR full spectrum imager (SFSI) is an imaging spectrometer, covering the short-wave infrared (SWIR) from 1200 to 2400 nm, which has been developed for remote sensing from an airborne platform. The sensor has been designed to acquire the full spectrum at high spectral resolution (10 nm) and the full image swath at high spatial resolution (50 cm) simultaneously. The instrument utilizes a platinum silicide (PtSi) detector array, refractive optics, and a transmission grating. A VME bus computer communicates with the array controller, performs the data acquisition, and provides the operator interface. The camera and data acquisition subsystems have been completed and test flown. The fore-optics, spectrograph, and sensor housing have been fabricated. Integration of the camera, spectrograph, and auxiliary components is scheduled for July 1994 followed by laboratory testing and calibration. Our goal is to obtain pilot project data by the end of autumn 1994. Here we describe the optical design, the sensor system, early test flight image data, and expected sensor performance based on laboratory testing. The objectives and procedures for the spectral, geometric, and radiometric calibration of this sensor are also discussed.
Optical design software is developed that includes the capability to model illumination systems. The software enables predicting the intensity distribution of radiation at a plane located at some distance from the source. An illumination system comprising an ellipsoidal reflector and a spherical source is modeled and the results of the analysis are compared to those achieved experimentally.
As part of an ongoing investigation of airglow emissions of the upper atmosphere, an intensified CCD imaging spectrograph has been developed for a sounding rocket project called GEMINI (general excitation mechanisms in nightglow). The instrument, known as LISA (limb-imaging spectrograph for airglow), will be used to measure the limb profiles of some important nighttime airglow emission features. The observed limb profiles will be analyzed to provide atmospheric temperatures and density profiles of excited atomic and molecular species of interest to specific modelling problems in the mesopause and lower thermosphere. The GEMINI rocket is to be launched from White Sands Missile Range, New Mexico, in late 1993 or early 1994. The payload will be three-axis stabilized and absolute pointing will be derived from a star video camera. We describe the design capabilities of the LISA instrument, which include a spectral range of 310 to 390 nm, a wavelength resolution of ~0.3 nm, a height resolution of 1 km, and a theoretical count rate of 0.04 count R-1 s-1, where R represents rayleighs. The imager design is discussed and we present the results of some laboratory tests performed by means of an artificial source of the oxygen nightglow emission.
Optical design software has been developed which includes the capability to model illumination systems. The software allows one to predict the intensity distribution of radiation at a plane located at some distance from the source. An illumination system comprising an ellipsoidal reflector and a spherical source was modeled and the results of the analysis compared to those achieved experimentally.
As part of an ongoing investigation of airglow emissions from space, we have developed an intensified CCD imaging spectrograph for a sounding rocket project called General Excitation Mechanisms In Nightglow (GEMINI). The instrument, known as Limb Imaging Spectrograph for Airglow (LISA) will be used to measure the limb profiles of some important nighttime airglow emission features. The GEMINI rocket is to be launched from White Sands Missile Range, New Mexico, in early 1993. The payload will be three-axis stabilized and absolute pointing will be derived from a star video camera. In this paper the imager design is discussed and we present the results of some laboratory tests performed using an artificial source of the oxygen nightglow emission.
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