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The Michelson interferometer system on the NASA Lear Jet Airborne Observatory is described as well as the data reduction procedures. The objects observed (standard stars, M stars, a nebula, planets, and the moon) are discussed and the observing parameters are listed for each flight date. The spectra obtained from these data flights are presented, grouped by class of object.
Measured data obtained by Michelson interferometer spectrometer were stored in a computer file and smoothed by being passed forward and backward through a digital four-pole low pass filter. Infrared spectra of the 10 lunar samples are presented in the format of brightness temperature versus frequency. The mol % of feldspar, pyroxene, olivine, ilmenite and ferromagnetic silicate in each sample is presented in tables. The reflectance spectra of ilmenite and enstatite are shown in graphs.
An approximate formula is derived for the spectrum ghosts caused by periodic drive speed variations in a Michelson interferometer. The solution represents the case of fringe-controlled sampling and is applicable when the reference fringes are delayed to compensate for the delay introduced by the electrical filter in the signal channel. Numerical results are worked out for several common low-pass filters. It is shown that the maximum relative ghost amplitude over the range of frequencies corresponding to the lower half of the filter band is typically 20 times smaller than the relative zero-to-peak velocity error, when delayed sampling is used. In the lowest quarter of the filter band it is more than 100 times smaller than the relative velocity error. These values are ten and forty times smaller, respectively, than they would be without delay compensation if the filter is a 6-pole Butterworth.
A Michelson interferometer feed monitor system developed for Spacelab is described. The device is helium cooled and is to be used to measure far infrared radiation sources in space. Performance data and development sequence are presented.
An atlas of 1339 methane absorption lines in the range 1120 to 1800 reciprocal centimeters, including the nu(4) and nu(2) bands, is presented. Laboratory spectra were obtained by a Nicolet Fourier transform Michelson interferometer with a resolution of approximately 0.06 reciprocal cm and a path length of 6.35 m of 0.98, 4.86 and 19.97 torr. Observed spectra are also compared with spectral intensities calculated line-by-line on the basis of tabulated intensities of the observed spectral lines.
The Voyager IR investigation is described, which uses a Michelson interferometer in the 180-2500/cm range, and a single-channel radiometer for the visible and near-IR, sharing a 50-cm diameter telescope. Emphasis is placed on the differences between the Voyager and the previous designs, including reductions in the field of view and in the noise equivalent spectral radiance of the instrument. Attention is given to the optical layout, the electronics module, power supply placement, thermal control heaters and flash heaters, data reduction, and calibration. A sample spectrum of Jupiter is also discussed.
Measurements of the far-infrared spectra of the powerful H II regions W51-IRS 2 and W49 NW from 65 to 345 per cm with about 9 per cm resolution were obtained by using an airborne Michelson interferometer. The most remarkable feature of the far-infrared spectra of the two regions is the smoothness of the continuum; no evidence is found in the spectra for features of H2O ice at 45 and 62 microns. The spectrum of W51 is well fitted by a 70 K blackbody with a diameter of 14 arc sec, but the spectrum of W49 NW is narrower than a blackbody. The implications of the apparently high peak optical depths of these sources are discussed.
The in flight calibration of the radiometer and the Michelson interferometer of the Voyager 1 infrared instrument is discussed. The calibrated full disk measurements are applied to derive values of the albedo, the thermal emission and the global energy balance of Jupiter.
Full disk measurements recorded 31 days before the Voyager 1 encounter with Jupiter by the radiometer of the infrared instrument, IRIS, indicate a geometric albedo of 0.274 + or - 0.013. Combining this measurement with the Pioneer derived phase integral of 1.25 and our error estimate of 0.1 yields a Jovian Bond albedo of 0.343 + or - 0.032. Infrared spectra recorded at the same time by the Michelson interferometer, along with a model extrapolation to low wave numbers not covered by the instrument, yield a thermal emission of (1.359 + or - 0.014) .001 W cm to the (-2) power. As in the case of the albedo measurement, the quoted errors in the emission measurement reflect estimates of systematic effects and are uncertain while the random component is negligible. From these measurements the internal heat flux of Jupiter is estimated to be (5.444 + or - 0.425) .0001 W cm to the (-2) power, and the energy balance defined as the ratio of emitted thermal to absorbed solar energy is 1.668 + or - 0.085.
Measurements of continuous absorption near 2400 per cm by N2 and CO2 over long path lengths in the lower stratosphere are presented. The continua were measured in a stratospheric solar spectrum obtained during sunset with a balloon-borne Michelson interferometer in the 2380-2500 per cm region, and transmittances were calculated by ratioing the amplitudes to those of a high-sun spectrum in order to eliminate the wavelength dependence of the measured flux. Comparison of the measured transmittances with those calculated for a multilayered atmospheric model using laboratory absorption measurements results in a fair agreement, and reveals the primary component of the absorption throughout most of the range to be N2, with the CO2 contribution equal to that of N2 only at the CO2 band head. In this region, the shape of the continuum is very sensitive to the sub-Lorentzian line shape assumed in the calculations, and so, if the shape of the N2 continuum at low temperatures can be determined through laboratory measurements, may be used to infer air-broadened far-wing CO2 line shape.
The mechanisms giving rise to certain airglow and auroral emissions and their height distribution are briefly surveyed. The basic principles of operation of scanning Fabry-Perot spectrometers are outlined in order to illustrate the calibration and use of such instruments to measure Doppler broadening and shift of atomic emission lines in the spectrum of the airglow and aurora and hence to infer atmospheric temperatures and wind velocities. Technical details of some specific instruments are given. Brief mention is made of the use of wide angle Michelson interferometers for the same application. Emphasis is given to the future importance of imaging Fabry-Perot spectrometers to permit the detailed mapping of temperature and wind velocity, especially in the auroral regions.
A birefringent filter with a large field of view and no additional polarization is discussed. It plays an important role in observing the solar monochromatic image and the solar vector magnetic field. It has only one channel. For simultaneous multichannel observations, the solar spectrograph is better than the birefringent filter. A suggestion was proposed to try to obtain a multichannel birefringent filter which will be used in a new telescope at the Huairou reservoir station of Beijing Observatory. By means of N polarizing beam splitters, (N+1) channels can be divided. In principle, any number of limitless channels can be obtained, thereby subdividing the whole solar spectrum. But since the space in a telescope is limited, the channels to be used are also limited. For the new telescope, 5 and 9 channels are being considered, and the spectral range is from lambda 3800A to lambda 7000A. Many lines are included in this range, for example, H, K, H beta, lambda lambda 5324A, 5250A, 6302A, H alpha, etc., and some of the lines are suited to measure solar velocity fields. According to the character of these lines, the half width of each channel is determined. Moreover, in some channels the solid polarizing Michelson interferometer is considered for measuring velocity field with a lm/s accuracy. The advantages of the filter and problems to be solved are listed.
Spectra obtained by balloon-borne Michelson interferometers over South Australia in 1977 and New Mexico in 1978 have been analyzed to obtain HF mixing ratios using a recently developed, sensitive analysis method for Fourier transform spectroscopy. The stratospheric HF mixing ratio appears to be significantly larger in the northern hemisphere than in the southern hemisphere for similar respective latitudes with the profiles having a similar vertical shape: the HF mixing ratio increases from 20 to 25 km, stays approximately constant between 25 and 35 km, and increases upward from 35 km. A brief discussion is given of possible causes for HF profile shape.
An evaluation is made of the potential usefulness of far-IR thermal emission measurements for simultaneous retrieval of temperature and constituent distributions. Attention is given to the design and performance of a high resolution Michelson interferometer which is currently used in balloon-borne far-IR stratospheric studies. Numerical results based on synthetic limb radiance data for model atmospheres are presented. Formal inversion techniques, together with a radiative transfer model that is based on line-by-line transmittance calculations, are used. Numerical results based on synthetic limb radiance data are given in order to assess the retrieval accuracies of the temperature profiles and the vertical mixing ratio profiles of O3, HCl, and OH.
The process of creating a major observatory, with emphasis on the configurations, costs, and site involved, is examined. The premise that cost optimization starts with a modestly priced building block, such as a 7.5-m telescope, is considered. The telescope unit and the configurations in which it is used, ranging from a single telescope or a set of separate telescopes, to an MMT in which the light of all of the telescopes can be phased when desired, are described. It is shown that options exist for using elements in a nonredundant linear array or Michelson interferometer. A means of estimating costs of different facilities, all on the same basis, is provided. The importance of placing the observatory on a site, such as Mt. Graham in Arizona, which maximizes the output per unit cost of the facility is discussed.
A fiber-optic thermometer is described which uses a thick-film SiC sensing etalon. The etalon's temperature-dependent phase shift is determined by analyzing its spectral reflectance, using an LED and a tunable Michelson interferometer. Temperatures from 20 to 1000 C are measured with better than 0.5 deg C resolution.
A method for generating tunable far-infrared radiation is described. The apparatus includes a Schottky-barrier diode which has one side coupled through a conductor to a waveguide that carries a tunable microwave frequency; the diode has an opposite side which is coupled through a radiating whisker to a bias source. Infrared light is directed at the diode, and infrared light with tunable sidebands is radiated by the whisker through an open space to a reflector. The original infrared is separated from a tunable infrared sideband by a polarizing Michelson interferometer.
The IR limb emission of the lower stratosphere has been measured using a balloon-borne liquid nitrogen-cooled Michelson interferometer with liquid helium-cooled Si:Ga detectors. Portions of the thermal emission spectrum have been recorded between 650 and 2000/cm with an unapodized spectral resolution of 0.03/cm. This is the highest spectral resolution limb emission thus far obtained. A preliminary description is given of these data along with a discussion of the significant features. Species identified to date include CO2, O3, CFCl3, CF2Cl2, H2O, CH4, HNO3, N2O, NO2, and ClONO2. A tentative identification is made for NO, representing the first direct spectroscopic detection of NO in emission.