James W. Bilbro

Airborne Doppler lidar measurements.

This paper describes recent measurements using coherent Doppler lidars operating at a wavelength of 10.6 microm aboard the NASA Ames Convair 990. The purpose of the measurements was to obtain data on the atmospheric wind fields and the distribution of the backscatter coefficient at 10.6 microm. A description of the instruments is provided detailing the modifications incorporated following the 1981 test flights of the systems. The measurement program is outlined, and preliminary results are discussed.

Remote measurement of the transverse wind velocity component using a laser Doppler velocimeter

The wind speed transverse to the line-of-sight of a laser Doppler radar has been measured using the intensity fluctuations of the returned signal. These measurements were made at a range of 100 m with a CO(2) cw laser Doppler velocimeter, which was simultaneously performing its design function of determining the radial velocity component. The transverse component measurements are compared with those obtained using a u, v, w Gill propeller anemometer.

Remote intensity fluctuation measurements with a laser Doppler radar

A coaxial focused CW scanning laser Doppler velocimeter (SLDV) radar equipment applying heterodyne detection at 10.6 microns can measure intensity fluctuations under field conditions. The set includes a 20 W CO2 laser, a coaxial Cassegrainian telescope, standard heterodyne equipment, and a SAW spectrum analyzer with 100 kHz signal resolution. Operation of the equipment and techniques for taking remote measurements are described briefly. Applications to remote measurements of transverse component of wind speed, as a complement to the traditional Doppler method of determining axial velocity, are under study. SLDV equipment has been used in detection, tracking, and measurements of atmospheric turbulence associated with aircraft wing-tip vortices or with dust devils, and in measurement of general atmospheric wind profiles.

Coherent focal volume mapping of a cw CO2 Doppler lidar.

In this paper we determine the coherent response to a 100-μm diam spinning wire as a function of its position within the focal volume of a 10.6-μm coherent Doppler lidar and generate SNR contours both transverse to and along the system line of sight. Application of the contours to single-particle scattering is discussed.

Predicted and preliminary evaluation of the X-ray performance of the AXAF Technology Mirror Assembly

The fabrication of the Technology Mirror Assembly (TMA) is complete, and performance predictions were made based upon mechanical and visible light measurements of the surface properties. An X-ray calibration program has been executed, and a preliminary analysis of a portion of the data is presented. The X-ray image distribution results are in reasonable agreement with the performance predictions which were calculated prior the start of the X-ray tests. The measured X-ray imaging performance approaches that expected for the Advanced X-ray Astrophysics Facility (AXAF).

Design and calibration of a coherent lidar for measurement of atmospheric backscatter

This paper presents a discussion of the background requirements for, and the design and calibration of a coherent Doppler lidar which is oriented specifically toward the measurement of low values of atmospheric backscatter, β (π), at a wavelength of 10.6μm. The lidar to be described is a compact, continuous wave system which can operate in two modes--multiple particle scattering and single particle scattering. In the multiple scattering mode, the lidar employs an extended focal volume and utilizes a technique similar to Dicke switching to achieve measurement of the volume backscatter. In the single scattering mode, the focal volume is reduced to ensure a high probability of single particle scattering. Measure-ment of the single particle backscatter as a function of time leads to the formation of signal histogram from which the volume backscatter can be inferred. In addition to providing the atmospheric backscatter value, appropriate data processing algorithms and focal volume calibration allow the single particle mode to yield information on the atmospheric aerosol scattering cross-section distribution. The system hardware and signal processing are described in this paper along with the algorithms used to calculate the backscatter, (7). Calibration techniques described include the use of known targets such as spinning disks and wires.

Grazing-incidence x-ray reflectivity: studies for the AXAF Observatory

The energy bandwidth and total throughput of a grazing incidence optics system is a strong function of the X-ray reflectivity of the surface coating. In support of the Advanced X-ray Astrophysics Facility (AXAF), studies are underway to evaluate and characterize the reflectivity of potential AXAF coatings. Here we report on results obtained for Au, Ir, and Ni coatings produced by electron-beam evaporation, evaporation with ion-assist, and sputtering. Effects of coating thickness and deposition angle have been evaluated at 6.4 and 8.1 keV; the highest reflectivities are those of the thinner, about 200 A vs about 700 A, coatings. While considerable variations exist, the best Ir samples have higher reflectivity than any of the Au coatings. Data results have been compared with models for theoretical reflectivity, particularly with regard to the effective density of the coatings. Independent measurements of the coating densities have been carried out for comparison with the reflectivity results.

Measurement Of The Effects Of Particulate Contamination On X-Ray Reflectivity

Because particles of sizes larger than a few tenths microns adversely affect hight resolution X-ray telescopes by scattering and absorbing X-rays, we are investigating the clean-liness required to maintain the ~1% overall calibration precision desired for the Advanced X-ray Astrophysics Facility (AXAF). At the grazing angles used for the AXAF mirrors, each particle shadows a surface area ~ 102 times its geometric area, necessitating glass occlusion specifications much more stringent than typically stipulated for visible-light particulate contamination. On test fiats coated with gold, we have deposited controlled levels of contamination spanning the range from 5 x 10-5 to 5 x 10-3 fractional area covered and have measured the absorption component of extinction over a range of grazing angles and X-ray energies to verify the predicted effects of particulate contamination. Further tests are planned to examine a wider range of energies and to take into account sample flatness and distortions introduced by holding fixtures.

Image Analysis of the AXAF VETA-1 X-ray Mirror

Initial core scan data of the VETA-1 X-ray mirror proved disappointing, showing considerable unpredicted image structure and poor measured FWHM. 2-D core scans were performed, providing important insight into the nature of the distortion. Image deconvolutions using a raytraced model PSF was performed successfully to reinforce our conclusion regarding the origin of the astigmatism. A mechanical correction was made to the optical structure, and the minor was tested successfully (FWHM 0.22 arcsec) as a result.

Coherent laser radar: introduction by the feature editor

This brief article introduces some of the papers presented at the OSA Topical Meeting on Coherent Laser Radar: Technology and Applications (July 1987). Plans for the next conference are also discussed.