Engineering topics

Cotton, Daniel M.

Publications and source records attributed to Cotton, Daniel M..

11 records

NASA NTRS1993

Primary results from the Berkeley EUV airglow rocket spectrometer: O I and N2 FUV/EUV dayglow in the thermosphere and lower exosphere

The Berkeley extreme-ultraviolet airglow rocket spectrometer (BEARS) made spectroscopic measurements of far and extreme UV, atomic oxygen emissions from a Black Brandt XII (12.041 WT) sounding rocket launched from Wallops Island, Virginia, on September 30, 1988. BEARS' primary instrument, a near-normal Rowland mount spectrometer, measured several atomix oxygen and molecular nitrogen dayglow features at high spectral resolution (1.5 A): O I (989, 1027, 1304, and 1356 A); and N2 Lyman-Birge-Hopfield (4, 0) and (3, 0) bands at 1325 and 1354 A. The instrument collected over 800 s of data spanning altitudes of 150 - 963 km with look directions between 95 deg and 125 deg from zenith. We have analyzed the data using electrons and radiative transport models in a forward modeling approach. The model and data are generally in good agreement. However, there are some discrepancies, which are discussed in terms of remote sensing capabilities and improvements to the models.

Cotton, Daniel M.↗

NASA NTRS1993

Optically thick cascade to the O I 3s 3S state in the Earth's thermosphere

The electron impact excitation cross section for the O I (1304 A) emission is enhanced in the terrestrial thermosphere owing to an optically thick cascade mechanism. Values for the contribution of the cascade have been calculated and deduced from measured 1304-A airglow emissions to between o and 200%. We have developed a model to calculate a large portion of the optically thick cascade contribution, including contributions from the radiative entrapment of the O I (1040, 1027, and 989 A) lines. High-resolution spectroscopic data from a sounding rocket experiment are analyzed using this model. The optically thick cascade contribution for the three lower lying states was evaluated to be 24.5% and compares well with recent calculations. The data, however, support a total contribution of 40%. The 15.5% difference is likely due to the contribution from higher Rydberg states.

Cotton, Daniel M.↗

NASA NTRS1993

Sounding rocket observation of a hot atomic oxygen geocorona

A sounding rocket measurement of the ultraviolet, atomic oxygen dayglow reveals an excess of emission compared to standard thermospheric model calculations at exospheric altitudes. We explore two explanations for this discrepancy: a breakdown of the radiative transfer model due to nonlocal thermal equilibrium (non-LTE) conditions above the exobase and a hot atomic oxygen geocorona. In particular, the effects of non-LTE on the 3P2, 1, 0 sublevel populations are modeled, and a hot O component in the upper thermosphere and lower exosphere is added to investigate the effects on the modeled emissions. For both cases, the data are reanalyzed and compared with the results using a standard LTE model. A hot O geocorona having a peak density of 10(exp 6)/cc at 550 km and a temperature of 4000 K is consistent with the data and appears to be the most reasonable explanation of the high-altitude enhanced emissions observed in the data.

Cotton, Daniel M.↗

NASA NTRS1992

Gas ionization solar spectral monitor (GISSMO)

We are currently developing an instrument free from optical components to measure the full-disk solar spectrum in the extreme ultraviolet regime covering wavelengths from 75-500 A. The instrument, which will be launched aboard a NASA Black Brant sounding rocket in September 1992, consists of a windowless noble gas ionization cell followed by a toroidal electrostatic analyzer to spatially disperse photoelectrons as a function of their energies. A microchannel plate based position sensitive detector will be used to detect individual electrons, indirectly returning the solar EUV spectrum.

Vickers, James S.↗

NASA NTRS1992

The Coronal Ultraviolet Berkeley Spectrometer (CUBS)

We describe an instrument package to remotely measure thermospheric, exospheric, and plasmaspheric structure and composition. This instrument was flown aboard the second test flight of the Black Brant XII sounding rocket on December 5, 1989, which attained an apogee of 1460 km. The experiment package consisted of a spectrophotometer to measure He I 584 A, O II 834 A, O I 989 A, hydrogen Lyman beta (1025 A), hydrogen Lyman alpha (1216 A), and O I 1304 A transitions, and a photometer to measure the He II 304 A emission. The optical design of the spectrophotometer was identical to that of the Berkeley Extreme Ultraviolet (EUV) Airglow Rocket Spectrometer payload, flown on September 30, 1988 aboard the maiden flight of the Black Brant XII rocket. We present the initial data analysis and describe directions we will go toward the completion of our study.

Bush, Brett C.↗

NASA NTRS1991

High resolution, two-dimensional imaging, microchannel plate detector for use on a sounding rocket experiment

We discuss a high resolution microchannel plate (MCP) imaging detector to be used in measurements of Doppler-shifted hydrogen Lyman-alpha line emission from Jupiter and the interplanetary medium. The detector is housed in a vacuum-tight stainless steel cylinder (to provide shielding from magnetic fields) with a MgF2 window. Operating at nominal voltage, the four plate configuration provides a gain of 1.2 x 10 exp 7 electrons per incident photon. The wedge-and-strip anode has two-dimensional imaging capabilities, with a resolution of 40 microns FWHM over a one centimeter diameter area. The detector has a high quantum efficiency while retaining a low background rate. A KBr photocathode is used to enhance the quantum efficiency of the bare MCPs to a value of 35 percent at Lyman-alpha.

Bush, Brett C.↗

NASA NTRS1991

Rigid lightweight optical bench for a spaceborne FUV spatial heterodyne interferometer

Requirements for spaceflight optical instruments usually dictate that for the structures be rigid, lightweight, and thermally stable. In addition, for interferometric far ultraviolet (FUV) spectrometers, the requirements for torsional deflection are more severe than with conventional spectrometers. To meet the challenge for rigid and lightweight optical instruments, this paper explores the design of a high-stiffness structure for the support of an FUV spatial heterodyne interferometer where the torsional deflection of the instrument is on the order of 10 arc seconds. The structure is based on use of a thin, hollow section beam with weight-relieving between optical elements. The design also uses a modular and self-contained positioning mechanism that is removed after final optical alignment. Several specific material properties are presented as criteria for material selection. The parameters which affect the particular design requirements are identified with respect to the desired material properties and physical design features. Although large thin sections are susceptible to thermal gradients, this could be minimized by a trade-off for weight, where adequate margin exists. This paper describes the preliminary design for the structure and presents an analysis to verify compliance with the requirements.

Tom, James L.↗

NASA NTRS1991

Modular removable precision mechanism for alignment of an FUV spatial heterodyne interferometer

Spaceflight optical instruments have two conflicting requirements. They need to be both rigid and lightweight. In addition, for interferometric far ultraviolet spectrometers, the requirements for precision positioning are more severe than for conventional spectrometers. To meet the challenge of lightweight optical instruments, a modular adjustment mechanism was developed to position two orthogonal axes of a universal three-axis gimbal support system with a positioning accuracy on the order of 10 arc seconds. The mechanism was designed as a self-contained assembly which can be removed after final alignment of a spacebound optical instrument to reduce its in-flight mass. To demonstrate the concept, a number of these assemblies were made and mounted on two of the positioning axes of a far ultraviolet spatial heterodyne interferometer. A shaft clamp was used on each positioning axis to retain the adjusted position. This paper describes the design of the mechanism and presents optical test results.

Tom, James L.↗

NASA NTRS1991

V-groove diffraction grating for use in an FUV spatial heterodyne interferometer

A need has arisen for efficient, blazed, symmetric gratings for use as beam splitters in far and extreme ultraviolet interferometers. In particular, the development of an all-reflection, far ultraviolet spatial heterodyne interferometer can benefit tremendously from such a grating. To fulfill this need, we have manufactured a mechanically ruled grating with a V-groove profile blazed for H Lyman-alpha at 1216 A. We present the grating performance at Lyman-alpha in the context of its application to the spatial heterodyne interferometer.

Cotton, Daniel M.↗

NASA NTRS1991

Instrument design and test results of the new all-reflection spatial heterodyne spectrometer

An all-reflection spatial heterodyne spectrometer (SHS) has been recently developed. The advantages over conventional high-resolution grating spectrometers are that the SHS requires no mechanical scanning, a self-compensating optical design permits easy alignment, and it is much smaller than other spectrometers of comparable resolution. Since all beam-splits and recombinations occur by reflection off of a diffraction grating, the interferometer is capable of operating well into the extreme ultraviolet (EUV) and possibly into the soft X-ray region. A description of the design and the characteristics of the instrument is presented. Also, test results, including sample interferograms as well as their Fourier-transformed spectra, at both visible and UV wavelengths are shown. Finally, we report on future developments and possible applications.

Bush, Brett C.↗

NASA NTRS1989

Calibration of the Berkeley EUV Airglow Rocket Spectrometer

The Berkeley Extreme-ultraviolet Airglow Rocket Spectrometer (BEARS), a multiinstrument sounding rocket payload, made comprehensive measurements of the earth's dayglow. The primary instruments consisted of two near-normal Rowland mount spectrometers: one channel to measure several atomic oxygen features at high spectral resolution (about 1.5 A) in the band passes 980-1040 and 1300-1360 A, and the other to measure EUV dayglow and the solar EUV simultaneously in a much broader bandpass (250-1150 A) at moderate resolution (about 10 A). The payload also included a hydrogen Lyman-alpha photometer to monitor the solar irradiance and goecoronal emissions. The instrument was calibrated at the EUV calibration facility at the University of California at Berkeley, and was subsequently launched successfully on September 30, 1988 aboard a four-stage experimental sounding rocket, Black Brant XII flight 12.041 WT. The calibration procedure and resulting data are presented.

Cotton, Daniel M.↗