Gas correlation imagers are important instruments for remotely detecting effluent emissions. However, making a
functional design for field testing is non-trivial given the range of environmental conditions the system may be operated under and the required matched imaging performance for both channels. We present a dual channel 7 degree full field of view f/2.5 athermal optical design athermalized from 0 to 50 degrees C that operates in the wavelength range of 2.0 to 2.5 microns suitable for methane imaging. We present the optical design, tolerance budget, and alignment plan used for the system. Predicted and as-built performance data including interferometric and ensquared energy measurements for both imaging channels are also shown.
A discrete Fourier transform (DFT) or the closely related discrete cosine transform (DCT) is often employed as part of a
data compression scheme. This paper presents a fast partial Fourier transform (FPFT) algorithm that is useful for
calculating a subset of M Fourier transform coefficients for a data set comprised of N points (M < N). This algorithm
reduces to the standard DFT when M = 1 and it reduces to the radix-2, decimation-in-time FFT when M = N and N is a
power of 2. The DFT requires on the order of MN complex floating point multiplications to calculate M coefficients for
N data points, a complete FFT requires on the order of (N/2)log2N multiplications independent of M, and the new FPFT
algorithm requires on the order of (N/2)log2M + N multiplications. The FPFT algorithm introduced in this paper could
be readily adapted to parallel processing. In addition to data compression, the FPFT algorithm described in this paper
might be useful for very narrow band filter operations that pass only a small number of non-zero frequency coefficients
such that M << N.
The use of modern microstreolithography (MSL) technology gives optics developers the freedom to integrate mounting and positioning structures directly into an optical mask structure. We have created Hadamard spectrometer masks with increments of 150 μm and less using the Sony SCS-6000 microstereolithography apparatus (MSLA). Due to laser over cure and other parameters, adjustments were made iteration by iteration until appropriate mask tolerances were met. A mounting structure was integrated with the mask for testing and application. At the computer aided design (CAD) model level, the mounting geometry can be adjusted to fit any specific mounting apparatus. By using the MSLA, features as small as 75 μm and larger than 300 mm can be created in the same build. Additionally, the conceptual design of an entire positioning system constructed using layer additive MSLA microfabrication is underway. This positioning system may be built as an integrated assembly, encapsulating necessary components. Optical characterization results are presented.
Phase diversity imaging is an established technique for deriving optical phase information from measurements of intensity. Knowledge of the optical phase can then be used to provide real time feedback control to adaptive optics or to perform post-detection image restoration. The work presented in this paper concerns the use of phase diversity to improve image restoration for aberrated optical systems. All previous implementations of phase diversity imaging of which the author is aware use defocus to introduce phase diversity. Defocus is rotationally symmetric with respect to the optical axis (no q dependence) and has an approximately quadratic dependence (ρ^2) on the radial coordinate in the exit pupil plane. Because of this rotational symmetry, defocus diversity is effective in improving the quality of image restorations only for even-order aberrations such as spherical (ρ4) and astigmatism (ρ2•cos2θ). However, simple defocus diversity fails to improve the quality of image restorations for odd-order aberrations such as coma (ρ3•cosθ). This paper explores the use of non-quadratic and rotationally non-symmetric phase diversity versus standard defocus diversity. Simulated image restorations are presented for a hypothetical system that would use an adaptive optic to introduce generalized phase diversity to remediate the impact of residual aberrations.
KEYWORDS: Signal to noise ratio, Modulation transfer functions, Diffraction, Composites, Spatial frequencies, Imaging systems, Monochromatic aberrations, Active optics, Point spread functions, Electronic filtering
Aperture size imposes, by way of diffraction, a fundamental limit on the spatial resolution that can be attained with an imaging system that operates at a given distance from a target. It is therefore natural to seek to improve image resolution by increasing the size of the collection aperture of a remote sensing system. On the other hand, as aperture size increases it becomes technically more difficult, and financially much more expensive, to maintain a figure quality that holds geometrical aberrations to a level that is negligible compared to diffraction effects. This paper presents an analysis of an approach that combines active optics with what has been called a post-detection phase diversity technique [R. A. Gonsalves, "Phase retrieval and diversity in adaptive optics," Opt. Eng. 21, 829-832 (1982)]. The basic concept is to allow for variable focus as an inherent system capability, in order to acquire multiple defocused images, each one of which has slightly different, and complementary, spatial frequency content. The proper deconvolution and merging of these images produces a composite image that is superior to the image that can be acquired at any single focal setting. This paper presents the theoretical basis of this approach, and numerical simulations that include the effects of noise and various levels of third order spherical aberration.
A familiar concept in imaging spectrometry is that of the three dimensional data cube, with one spectral and two spatial dimensions. However, available detectors have at most two dimensions, which generally leads to the introduction of either scanning or multiplexing techniques for imaging spectrometers. For situations in which noise increases less rapidly than as the square root of the signal, multiplexing techniques have the potential to provide superior signal-to-noise ratios. This paper presents a theoretical description and numerical simulations for a new and simple type of Hadamard transform multiplexed imaging spectrometer. Compared to previous types of spatially encoded imaging spectrometers, it increases etendue by eliminating the need for anamorphically compressed re-imaging onto the entrance aperture of a monochromator or spectrophotometer. Compared to previous types of spectrally encoded imaging spectrometers, it increases end-to-end transmittance by eliminating the need for spectral re-combining optics. These simplifications are attained by treating the pixels of a digital mirror array as virtual entrance slits and the pixels of a 2-D array detector as virtual exit slits of an imaging spectrometer, and by applying a novel signal processing technique.
The measurements of Pollution in the Troposphere (MOPITT) instrument aboard the Earth Observing System (EOS) Terra spacecraft measures tropospheric CO and CH4 by use of a nadir-viewing geometry. MOPITT cloud algorithm detects and removes measurements contaminated by clouds before retrieving CO profiles and CO and CH4 total columns. The collocation between MOPITT and MODIS is also established and MODIS cloud mask will be used in the MOPITT cloud algorithm. The cloud detection results in the use of MOPITT data alone agree with MODIS cloud mask for more than 80% of the tested cases.
The MOPITT Airborne Test Radiometer (MATR) uses gas filter correlation radiometry to measure tropospheric carbon monoxide (CO) with three optical channels or methane (CH4) with one channel. MATR uses the same gas correlation techniques as does the MOPITT satellite instrument, namely length modulation and pressure modulation MATR data serves to test retrieval techniques for converting infrared radiometric data into atmospheric CO, or CH4 amounts. MATR will also be applied to MOPITT data validation. This paper gives an overview of the MATR instrument design; it discusses the results of laboratory testing and calibration; and it presents results from recent flights.
The primary objective for the MOPITT algorithm test radiometer (MATR) is to support the pre-launch testing of data retrieval algorithms for the MOPITT satellite instrument. Particular areas of concern in the retrieval are the effects of variable ground reflectance, the operation of the PMR, the accuracy of the CH4 spectral data in the HITRAN data base, and the calculated interference from water vapor. The review panel for the MOPITT algorithm theoretical basis document strongly encouraged a ground-air field effort to obtain measurements of the real atmosphere with prototype instruments. The plans for MATR include three detection channels. Channel one will use a length modulator cell (LMC) followed by a detector system with a spectral bandpass near 2.3 micrometer. This LMC will be filled with CO (or alternatively with CH4) to make total column measurements that are strongly weighted near the surface. Channel two will use the same length modulator cell (LMC) as channel one, but it will use a detector system with a spectral bandpass near 4.6 micrometer. This channel will be sensitive to CO primarily in the free troposphere. Channel three will use a pressure modulator cell (PMC) followed by a detector system with a spectral bandpass also near 4.6 micrometer. This channel will be sensitive to CO primarily in the upper troposphere and lower stratosphere. A first round of laboratory, ground-based atmospheric, and airborne measurements have been completed to date using the 2.3 micrometer CH4 channel. The current status of MATR will be presented, along with results obtained to date.
A portable shortwave infrared (SWIR) spectrometer has been developed that covers the range from 1.05 to 2.45 μm. The spectral sampling interval is 1.37 nm, and the spectral resolution can be varied from about 5 nm to more than 100 nm. A single spectrum can be acquired in as little as 1 s. The signal-to-noise ratio (SNR) for a single 1-s scan is about 100 at a wavelength of 2.2 μm for a lambertian surface of 100% reflectance illuminated by the sun at normal incidence with 14-nm spectral resolution. The SNR at 1.25 μm is about 900 for the same conditions. The estimated 1o uncertainty in the absolute radiometric calibration of the instrument is 2% to 3%. Field-of-view defining optics are coupled by a flexible fiber optic bundle to the spectrometer, which consists of a nonscanning concave holographic diffraction grating with a flat focal field imaged onto a 1024-element liquid nitrogen cooled PtSi linear array detector. The primary use for the instrument is the collection of ground reflectance and radiance data for the radiometric calibration of operational and proposed high spectral resolution remote sensing systems.
The design of a three-channel solar radiometer for obtaining total columnar water vapor using solar transmittance and differential absorption is presented. Water vapor transmittance is determined using a modified Langley approach and converted to columnar water vapor using a band model developed at the University of Arizona. Several cases are presented in which columnar water vapor amounts determined using the current instrument and method are compared to sounding balloon results. Tests using simulated data indicate that columnar water vapor may be retrieved with an uncertainty less than 10%.
The design, initial calibration, and performance evaluations of a portable short wave infrared (SWIR) spectroradiometer are described. The spectroradiometer covers the range from 1.1 to 2.5 microns with a spectral resolution that may be varied from less than 10 nm to more than 100 nm. A single spectrum is acquired in about 2 sec. The SNR is about 230 at a wavelength of 2.2 microns for a Lambertian surface of 90-percent reflectance illuminated by the sun at normal incidence with 14.8-nm resolution, a 25 C background temperature, and no atmospheric attenuation. FOV-defining optics are coupled by a flexible fiber-optics bundle to the spectroradiometer, which consists of a concave holographic diffraction grating with a flat focal field imaged onto a 1024-element platinum silicide linear-array detector.
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