Engineering topics

Harwit, M.

Publications and source records attributed to Harwit, M..

At least 73 records · Page 4

NASA NTRS1976

Fourier and Hadamard transform spectrometers - A limited comparison

An encoding figure of merit is established for a detector-noise limited Fourier transform spectrometer (FTS) and compared to the comparable figure for a Hadamard transform spectrometer (HTS). The limitation of the Fourier system is partly that it does not truly Fourier analyze the radiation. Instead a cosine squared modulation is imposed on the different spectral frequencies. An additional difficulty is that neither the cosine nor the cosine squared functions form an orthonormal set. This makes the Fellgett's advantage (root-mean-squared figure of merit) for a single detector Michelson interferometer a factor of the square root of (N/8) greater than for a conventional grating instrument - rather than the square root of (N/2). The theoretical limit would be the square root of N.

Tai, M. H.↗

NASA NTRS1976

Hadamard transform imager and imaging spectrometer

An imager and a spectrometric imager, which achieve multiplexing by the use of binary optical encoding masks, have been built and tested. The masks are based on orthogonal, pseudorandom digital codes derived from Hadamard matrices. The spatial (and/or spectral) data are therefore obtained in the form of a Hadamard transform of the spatial (and/or spectral) scene. Computer algorithms are used to decode the data and reconstruct images of the original scene. The hardware, algorithms processing and display facility are described. A number of spatial and spatial/spectral images, obtained in the laboratory, are presented.

Swift, R. D.↗

NASA NTRS1976

Far infrared spectral observations of Venus, Mars and Jupiter

Far-IR spectra of Venus, Mars and Jupiter between 46 and 115 microns have been obtained at a resolving power of about 10, observing from the NASA Lear Jet at an altitude of 13.7 km. The results are calibrated with lunar observations and show Mars and Venus to have relatively constant brightness temperatures over this wavelength region, with Venus appearing somewhat warmer at longer wavelengths. The brightness temperature of Jupiter decreases slightly toward longer wavelengths.

Ward, D. B.↗

NASA NTRS1976

Far-infrared spectral observations of M42 and M17

Results are reported for coarse-resolution far-IR spectral observations of the H II regions M17 and M41 in the wavelength range from 42 to 115 microns. The observations were made at an altitude of 13.7 km with the 30-cm telescope of the NASA Lear jet. The spectrum of M42 is shown to be quite smooth and to fit a diluted 100-K blackbody very well. It is found that the emission from M17 falls steeply from 40 to 63 microns, then rises again to 80 microns before falling off once more toward longer wavelengths. The best fit of this spectrum for an inverse-square-wavelength emissivity law is found to require a two-temperature model with temperatures below 30 K for the cold dust component. It is concluded that the spectrum of M17 seems to require the presence of a material with a better far-IR emissivity than those for which detailed laboratory measurements have been made.

Ward, D. B.↗

NASA NTRS1976

Development and evaluation of a Hadamard transform imaging spectrometer and a Hadamard transform thermal imager

A spectrometric imager and a thermal imager, which achieve multiplexing by the use of binary optical encoding masks, were developed. The masks are based on orthogonal, pseudorandom digital codes derived from Hadamard matrices. Spatial and/or spectral data is obtained in the form of a Hadamard transform of the spatial and/or spectral scene; computer algorithms are then used to decode the data and reconstruct images of the original scene. The hardware, algorithms and processing/display facility are described. A number of spatial and spatial/spectral images are presented. The achievement of a signal-to-noise improvement due to the signal multiplexing was also demonstrated. An analysis of the results indicates both the situations for which the multiplex advantage may be gained, and the limitations of the technique. A number of potential applications of the spectrometric imager are discussed.

Harwit, M.↗

NASA NTRS1976

Masks for Hadamard transform optics, and weighing designs

This paper gives a brief survey of the design of masks for Hadamard spectrometers and image scanners. Three different criteria are described for judging a mask, as well as techniques for choosing masks that are not too far from the optimum.

Sloane, N. J. A.↗

NASA NTRS1975

Detection of the O III 88.16-micron forbidden line in M17

This paper reports the detection of the fine-structure forbidden line of O III at 88.16 microns in emission from M 17 (the Omega Nebula), which is an optically bright galactic H II region and a strong thermal radio source. The observations were made using an airborne 30-cm telescope and a liquid helium-cooled grating spectrometer with Ge:Ga photoconductive detectors. The line intensity is found to be 2.2 (+1.0, -0.7) by 10 to the -15th power W/sq cm. The results are compared with those of a search for the present line in M 42 as well as with theoretical predictions of the line intensity.

Ward, D. B.↗

NASA NTRS1975

Errors in Hadamard spectroscopy or imaging caused by imperfect masks

An analysis is given of the errors in Hadamard spectroscopy that are caused by transparent slits in the mask being systematically wider or narrower than they should be. It is shown that if the input spectrum consists of a single line, the distorted spectrum that is actually calculated consists of this line plus four small blips. When the transparent slits are too wide, these blips are of equal height and the same sign with one pair surrounding the line and another pair 5isplaced a certain distance from it. When the slits are too narrow, the displaced blips have the same amplitude, but are negative. The response to an arbitrary input spectrum is then determined from this. The same method of analysis may also be used to handle other types of errors.

Tai, M. H.↗

NASA NTRS1975

Practical multi-spectrum Hadamard transform spectrometer

Specifications of a new Hadamard transform spectrometer for infrared astronomical applications are presented. The instrument has 15 entrance slits and 255 exit slits. Data can be gathered at the rate of up to five readings per second so that the entire spectral run lasts some thirteen minutes. The data is processed in real time by a minicomputer. At the end of the thirteen minute run the spectrum is available for immediate viewing on a CRT display. The spectra can be punched and stored on paper tape for later processing.

Tai, M. H.↗

NASA NTRS1975

Infrared galaxies - Evolutionary stages of massive star formation

We cite evidence which indicates that infrared galaxies may represent evolutionary stages during which a large number of massive stars are being formed. The lifetimes of these stars would be rather short (1-10 million years), and the resulting supernova explosions could account for the level of nonthermal activity which often accompanies the thermal infrared emission.

Harwit, M.↗

NASA NTRS1975

Uncertainties in the far-infrared absorption of interstellar clouds

The observed absorption of far-infrared radiation by an interstellar dust cloud is a complicated function of the material properties of grains, of their column density, and of the extent of stimulated emission. At long wavelengths, neglect of stimulated emission would lead to an underestimate of the amount of absorbing material observed along a line of sight. However, if the complex index of refraction for grains is measured at the low temperatures in question, a classical analysis will be correct since the measured refractive index correctly accounts for stimulated emission. Since the grain temperatures may strongly affect the refractive index of a grain, typical interstellar grain temperature fluctuations are analyzed in a final section.

Harwit, M.↗

NASA NTRS1974

Observations of the Orion nebula at 100 microns

The Orion nebula spectrum was obtained using an airborne cooled grating spectrometer in the spectral range from 80 to 125 microns at a resolution of 8 microns. The spectrum, corrected for instrument profile and atmospheric water vapor absorption, could be fit by the 60 and 100 K black body curves. The results are in good agreement with those of Erickson et al. (1973) and are consistent with a thermal emission spectrum produced by cool interstellar grains radiating at a wavelength much longer than the grain size.

Ward, D. B.↗

NASA NTRS1974

Infrared astronomy and the Shuttle

The infrared spectral range extends approximately from 1 micron to 1000 microns. Observations in infrared astronomy are made with the aid of ground-based telescopes and telescopes flown in aircraft, balloons, or rockets. Advantages and drawbacks of infrared studies conducted by different approaches are discussed. Infrared astronomers are looking forward to observations made from orbiting spacecraft. Present plans call for placing a large telescope aboard the Space Shuttle Orbiter for short missions lasting from seven days to three weeks several times a year. However, the occurrence of a pollution of the spacecraft environment by dust particles and gases coming from the spacecraft might present a problem for the observations. Possible approaches for solving this problem are discussed.

Harwit, M.↗

NASA NTRS1971

Doubly multiplexing dispersive spectrometer.

Description of a spectrometer that incorporates 19 entrances and 19 exit slots. The instrument's dual capability to perform either one-dimensional imaging or alternately to give a spectrum with a high SNR over the entire source is shown. The instrument operates in the Littrow mode at f/8. The slot width and length for each mask are 0.625 and 3.5 mm, respectively, giving a total aperture length of 12.1 mm. The corresponding resolving power at 1.7 microns is 230.

Phillips, P. G.↗

NASA NTRS1971