Use of Curved Slits to Increase the Throughput of a Hadamard Spectrometer

The throughput of a Hadamard spectrometer is directly proportional to the length of the slits. However, optical aberrations limit the slit length that can be used. The usable length of curved and straight slits are theoretically and experimentally compared for a typical singly-encoded Hadamard spectrometer that has a Czerny-Turner plane-grating mounting. For this Hadamard spectrometer the optical throughput can be increased by a factor of ten by using properly curved slits

The Sims Technique: An Introduction

This is a tutorial paper on the SIMS. The SIMS is an acronym for the spectrométre inter - férentiel à modulation sélective (selective modulation interference spectrometer), recently introduced by Maréchal and Fortunato. The paper reviews the basic principles of operation, the properties, the implementation configurations, and the current investigations of the SIMS at Utah State University. The following properties make the SIMS a powerful spectroscopic instrument. It has an extremely large optical throughput. For example, the SIMS can be configured with a light gathering capability, throughput, thousands of times larger than that of a conventional slit spectrometer. The SIMS does not require a computer to recover the spectrum; it has moderate resolving power, and it has relatively rugged implementation configuration

Conceptual design study for Infrared Limb Experiment (IRLE)

The phase A engineering design study for the Infrared Limb Experiment (IRLE) instrument, the infrared portion of the Mesosphere-Lower Thermosphere Explorer (MELTER) satellite payload is given. The IRLE instrument is a satellite instrument, based on the heritage of the Limb Infrared Monitor of the Stratosphere (LIMS) program, that will make global measurements of O3, CO2, NO, NO2, H2O, and OH from earth limb emissions. These measurements will be used to provide improved understanding of the photochemistry, radiation, dynamics, energetics, and transport phenomena in the lower thermosphere, mesosphere, and stratosphere. The IRLE instrument is the infrared portion of the MELTER satellite payload. MELTER is being proposed to NASA Goddard by a consortium consisting of the University of Michigan, University of Colorado and NASA Langley. It is proposed that the Space Dynamics Laboratory at Utah State University (SDL/USU) build the IRLE instrument for NASA Langley. MELTER is scheduled for launch in November 1994 into a sun-synchronous, 650-km circular orbit with an inclination angle of 97.8 deg and an ascending node at 3:00 p.m. local time

Selective Modulation Interferometric Spectrometer (SIMS) Technique Applied to Background Suppression

A method of using the SIMS (the Selective Modulation Interferometric Spectrometer) to measure the difference between the spectral content of two optical beams is given. The differ - encing is done optically; that is, the modulated detector signal is directly proportional to the difference between the two spectra being compared. This optical differencing minimizes the dynamic -range requirements of the electronics and requires only a simple modification of the basic cyclic SIMS spectrometer. This technique can be used to suppress background radiation for the enhancement of target detection and tracking. Laboratory measurements demonstrating the application of this technique are reported

Focus Optimization of a Cryogenic Collimater Using Interferometric Measurements and Optical Modeling

Space Dynamics Laboratory at Utah State University (SDL/IJSU) optimized the focus of an off-axis, cryogenically cooled infrared collimator for cryogenic operating temperatures. Historically, collimator focus was optimized at ambient temperatures where interactive focus adjustment and testing coulà be performed. The focus shift that occurred when the optics were cooled was minimized by collimator design, and the change was negligible compared to the spatial resolution of the IR sensor measuring the collimator\u27s simulated point source. However, the focus determined at ambient temperature does not meet the image quality requirements of state-of-the-art sensors. The method used by SDL to determine optimal focus at cryogenic temperatures applies classical optical techniques to the cryogenically cooled environment. System level interferometric measurements are first made to characterize the system wavefront error. These measurements are then applied to an aberration- free optical model to evaluate system focus for a wavelength of 12 tim. The method also uses a knife edge test to refer the interferometric measurements to the aperture located near the focal point of the collimator. This paper discusses the physical test setup, outlines the optical model and analysis procedure, and presents results before and after focus optimization of a multifunction infrared calibrator

Predicting the Focus of Cryogenicaly-Cooled Optical Systems

Results of an experimental study to ascertain how well the focal -plane location of cryogenically -cooled optical systems can be predicted are reported. These results indicate that if the required low- temperature thermal expansion and index -of- refraction data are available, the focal shift caused by cooling to cryogenic temperatures can be accurately predicted by simply computing the shift in the paraxial focus. In this study, the differences between the measured focal shifts and the computed shift in the paraxial focus were less than the diffraction -limited depth -of -focus tolerance. The results of this study also indicate that for off - the -shelf optical systems ray- tracing analysis may not adequately predict the absolute location of the focal plane. Thus, the following method of predicting the focal -plane location of a cryogenically- cooled optical system is suggested: first measure the focal -plane location with the optics at room temperature, and then add the computed paraxial focal shift to the measured location

Multiplexed Dispersive Spectrometers Using Reduced Background Infrared Detectors

The application of multiplex spectrometry to cryogenically cooled LWIR extrinsic photodetectors is limited by system noise. This noise limitation results in a detector NEP that is directly proportional to bandwidth. Therefore, multiplex schemes that require increased bandwidth are not productive of real advantage. However, doubly encoded systems that are based on 2n - 1 or n + N - 1 measurements have the potential to provide a real throughput gain proportional to the number of elements used on the throughput matrix

Tunable Optical Filter Using an Interferometer for Selective Modulation

Using the selective modulation interferometric spectrometer (SIMS) as a tunable filter is proposed. This tunable filter can have a large optical throughput and a resolving power on the order of a few thousand. A basic explanation of the operation of this filter is given with an emphasis on the similarities and differences between it and a Fourier spectrometer. Several equations that have been found to be particularly useful in designing, operating, and calibrating this filter are presented. The construction and operation of a tunable filter prototype are reported

WIRE Instrument Description: Focal Planes, Optics, Electronics

An elegantly simple cryogenic instrument has been proposed to measure far infrared radiation from starburst galaxies. The experiment—known as WIRE—employs a Cassegrain telescope with diamond-turned mirrors to provide a light-weight optical system for photon collection. A dichroic beamsplitter and filter separate the light into two broad, well-defined bands of interest. Two 128- x 128-pixel arsenic-doped silicon focal plane arrays spatially sample the incoming photons. These arrays feature exceptionally low dark current and low read noise, which allows the coaddition of thousands of images. The entire optical section and focal plane arrays are cooled to 12 Kelvin and 7.5 Kelvin, respectively, by a two-stage, solid-hydrogen cryostat. An uncomplicated electronics package provides some on-board coaddition of images, accepts the simple commands required by the WIRE instrument, and interfaces the data signals to the SMEX spacecraft for telemetry to the ground