The MUSIC (Multiple Signal Classification) algorithm uses the phase difference at different antenna elements of a receiving antenna array to determine the azimuth and elevation angles of a source. This algorithm can be extended to determine the range of multiple sources as well as their azimuth and elevation angles. In this report a generalized MUSIC algorithm is presented that accomplishes this task when the receiving antenna consists of a planar, rectangular, array of receiving elements. Greater range accuracies can be achieved by increasing the signal to noise ratio, increasing the number of PRIs per CPI, and searching for a solution over range space with a finer mesh. The mesh employed in this study had a range gate size that was 10% of the range space searched. An increase in range accuracy gained by the latter two methods comes at the price of increased processing time.
It is a common practice to give cleaned mirror substrates, and other optical surfaces, a final "dusting" with high pressure Nz, or air, just prior to deposition of thin reflective coatings. The object of this procedure is to remove any particles that may have settled on the substrate since chemical cleaning and washing. Some types of "guns" used for blowing dust off a surface work well. However, other types, such as those containing a radioactive polonium source, can charge the substrate so that it attracts even more dust. In this
report, the connection between electrostatic charge on a substrate and its dust "gettering" (dust collecting) ability is established. Also, test results are presented for the charging and discharging capabilities of various types, and manufacturers, of dusting guns.
There have been some questions in the astronomical community concerning the quality of silver coatings deposited on substrates that have been cleaned with carbon dioxide snow. These questions center around the possible existence of carbonate ions left behind on the substrate by CO2. Such carbonate ions could react with deposited silver to produce insoluble silver carbonate, thereby reducing film adhesion and reflectivity. Carbonate ions could be produced from CO2 via the following mechanism. First, during CO2 snow cleaning, a small amount of moisture can condense on a surface. This is especially true if the jet of CO2 is allowed to dwell on one spot. CO2 gas can dissolve in this moisture, producing carbonic acid, which can undergo two acid dissociations to form carbonate ions. In reality, it is highly unlikely that charged carbonate ions will remain stable on a substrate for very long. As condensed water evaporates, Le Chatelier's principle will shift the equilibrium of the chain of reactions that produced carbonate back to CO2 gas. Furthermore, the hydration of CO2 reaction of CO2 with H20) is an extremely slow process, and the total dehydrogenation of carbonic acid is not favored. Living tissues that must carry out the equilibration of carbonic acid and CO2 use the enzyme carbonic anhydrase to speed up the reaction by a factor of one million. But no such enzymatic action is present on a clean mirror substrate. In short, the worst case analysis presented below shows that the ratio of silver atoms to carbonate radicals must be at least 500 million to one. The results of chemical tests presented here support this view. Furthermore, film lift-off tests, also presented in this report, show that silver film adhesion to fused silica substrates is actually enhanced by CO2 snow cleaning.
The concept of a Sun occulting screen alows a large space telescope to be shielded from solar UV, visible, and IR radiation. Telescope operation in the screen's shadow reduces background radiation from scattering off telescope structures, and reduces expansion and contraction of the telescopes physical dimensions due to changes in solar heating. It is possible to design a non-conducting Sun occulting screen that travels in tandem with a large space telescope such that both the screen and the telescope travel in Sun synchronous orbits that do not cross. The constant separation distance between the screen and the telescope is on the order of 10 Km (far enough from the screen to minimize any radiated IR from the warm screen). The focus of the space telescope's elliptical (or circular) orbit lies at the Earth's center. However, the focus of the screen's orbit will lie at a point displaced toward the Sun, from the Earth's center, by the screen/telescope separation distance. It will be shown that by passing a small photoelectric current through a few turns of wire surrounding the screen, a Lorentz force can be generated that is sufficient to maintain the position of the screen's shifted orbital focal point.
A simple method is presented for measuring boththe real and imaginary parts of low resistivity semiconductors. The method depends on making two reflection measurements from a bulk sample of material. One measurement is made at normal incidence and the other at an arbitrary angle of incidence greater than zero, but less than 90°. Only a simple reflectometer is required. The method offers an alterantive to traditional, and more complex Brewster angle measurements or ellipsometry.
The use of a high velocity stream of carbon dioxide snowflakes to clean large optics is well known, and has gained widespread acceptance in the astronomical community as a telescope maintenance technique. Ultimately, however, the success of carbon dioxide snow cleaning depends on the availability of high purity carbon dioxide. The higher the purity of the carbon dioxide, the longer will be the time interval between required mirror washings. The highest grades of commercially produced liquid carbon dioxide are often not available in the more remote regions of the world - such as where major astronomical observatories are often located. Furthermore, the purity of even the highest grades of carbon dioxide are only nominal, and wide variations are known to occur from tank to tank. Occasionally, visible deposits of organic impurities are left behind during cleaning with carbon dioxide that is believed to be 99.999% pure. A zeolite molecular sieve based filtration system has proven to be very effective in removing these organic impurities. A zeolite is a complex alumino-silicate. One example has an empirical formula of (see paper for formula). The zeolites have an open crystal structure and are capable of trapping impurities like 8-methylheptadecane (an oil) and 2,6-octadine-1-ol,3,7- dimethyl-,(E)- (a fatty acid). In fact, a zeolite can trap 29.5% of its own weight in SAE 20 lubricant at 25 degree(s)C. After filtration of liquid CO2 through zeolites, the concentration of measured impurities was below the detection limit for state-of-the-art gas chromatography systems.
The development of ultra-light fibrous substrate mirrors allows serious contemplation of large multi-mirror space telescopes using rigid segments. Mirrors made of silica and alumina fibers have a small coefficient of thermal expansion and a density competitive with inflatable structures. Furthermore, they are without the imagery problems caused by non parabolic figures, gaseous expansion and contraction, tidal distortion of large gas filled structures, leaks, and long lived transient mirror perturbations caused by intentional pointing and tracking movements, micrometeor and space debris impacts, and mechanical vibrations. Fibrous substrate primary mirrors also have logistical advantages, since segments can be fabricated in orbit from small amounts of dense raw materials. One space shuttle flight, lifting about half its payload capacity, is adequate to transport all the material necessary to fabricate substrates for a one hundred meter telescope whose primary mirror consists of 12,086 hexagonal segments, each having a diameter of 1 meter and an area of 0.6495 square meters.
Cleaning large optics with carbon dioxide snowflakes is a process that has achieved general acceptance as a maintenance technique at astronomical observatories over the past decade. The technique is slowly spreading into the general optics community where smaller optics need to be cleaned, but removal and washing of precisely positioned, or deeply embedded, optics is time consuming and impractical. One obstacle preventing the more widespread use of CO2 snow cleaning has been the fact that the more inexpensive lower grades of liquid CO2, the source material used to produce the snowflakes, are often contaminated with oil. The origin of this oil is believe to be the lubricant used in the compressor that liquefies the CO2 gas. Since liquid CO2 is an excellent solvent for oil, the contaminants eventually turn up in the liquid. Recent development of an inexpensive zeolite based filtering system, allows removal of oil contaminants with nearly 100% efficiency. The data presented in this report show the result of decontamination experiments for a variety of oil components. The snowflakes made from the purified liquid CO2 are capable of producing surfaces that are nearly atomically clean. This level of cleanliness can be important for the successful operation of some ultraviolet and infrared optical systems.
Characterization of the transmission and scattering of ultraviolet light through thin carbon foils is central to the operation of high-energy neutral atom (HENA) images. When unwanted external UV enters a HENA imager through its carbon foil window, some of this radiation is scattered toward the internal side mounted detectors. These detectors may then produce a 'false count' which is unrelated to any neutral particle inspection. It is against the background of ultraviolet produced false counts that the true counts of high-energy neutral particles must be made. This report describes the UV transmission and scattering through thin carbon foils at the Lyman-(alpha) wavelength of 1216 angstroms; the dominant ultraviolet wavelength in the solar system. The films tested were flight hardware from the HENA imager of the IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) spacecraft. The imager ultimately produces a two dimensional map by measuring the neutral atom flux in different directions. The IMAGE spacecraft is scheduled for launch on Jan 1, 2000. A twin HENA was also placed aboard the Cassini mission to Saturn spacecraft, which was successfully launched on Dec. 5, 1997.
Image intensifiers on board UVSTAR, the Ultraviolet Spectrograph Telescope for Astronomical Research, did not seem to be functioning as expected. Therefore, a simplified version of the UVSTAR electronics was used to test the image intensifiers in the laboratory. All photosensitive locations of he intensifier array were exposed at once, and then each line read each sequentially. The images which were produced had an annoying intensity gradient superimposed on them. This gradient is due to the fact that line n of the intensifier array was corrupted by thermal signal n times longer than line 1. The imagery was further degraded by the presence of so-called 'hot-pixels'. These pixels correspond to locations on the intensifier's charge coupled device array which over react to the photon flux and produce abnormally high intensity readings. This paper descries image processing techniques which removed the gradient as well as hot pixels from the intensifier imagery so that an accurate performance assessment could be made. The methods describe can also be used to diagnose flaws in medical and industrial image intensifiers in general.
The deep space one (DS-1) probe is a mini-tour photo reconnaissance mission of Earth-Mars space. Target bodies include an asteroid, a comet, and the planet Mars. Central to this mission is an accurate measurement of the reflectance of the primary mirror of the imaging system. Knowledge of this reflectance will allow calculation of the absolute albedos of the target bodies encountered. Scattering measurements were also made on the so-called 'diffuser plate' of the optical train. This component was intended to be used only for the solar occultation experiment which measures the solar intensity as a function of atmospheric depth as Mars comes between the Sun and the probes optical detectors. The diffuser plate was designed to reduce the solar intensity by scattering light in an isotropic fashion. However, it has been found that the diffuser plate is not an isotropic scatterer. This report describes the equipment used to make the reflectance and scattering measurements, as well as the results obtained from the measurement program.
During the outgassing of orbiting astronomical observatories, the condensation of molecular species on optical surfaces can create difficulties for astronomers. The problem is particularly severe in ultraviolet astronomy where the adsorption of only a few atomic layers of some substances can be very damaging. In this paper the removal of adsorbed atomic layers using carbon dioxide snow is discussed. The rate of removal of adsorbed layers of isopropyl alcohol, Freon TF, and deionized distilled water on Teflon substrates was experimentally determined. The removal of fingerprints (containing fatty acids such as stearic acid) from optical surfaces is also demonstrated. The presence and rate of removal of the multilayers was monitored by detecting the molecular dipole field of adsorbed molecular species. For isopropyl alcohol, Freon TF (trichlorotrifluoroethane), and water adsorbed multilayers were removed in under 1.5 seconds. Fingerprint removal was much more difficult and required 20 seconds of spraying with a mixture of carbon dioxide snow flakes and atomized microdroplets of isopropyl alcohol.
In this paper the fabrication of ultralight mirror segments is described. The mirrors are made from HTP (high thermal performance material), better known as a third generation derivative of the Space Shuttle heat shield tile. The HTP material has a density of only about 0.14 gms/cc and is made from tangled fibers of aluminum oxide and silicon dioxide sintered together as a non-isotropic material. HTP material is also stiff, and undergoes very small shrinkage and expansion due to thermal effects. When one side of an HTP surface is coated with a suitable glassy substance, such as fused silica, fine mirrors can be made. HTP material also outgasses quickly, thereby preventing annoying virtual leaks which can result in condensation of gaseous species onto optical surfaces. Experiments have shown that a one-centimeter-thick specimen of HTP material will outgas from one atmosphere pressure down to the ambient pressure at low earth orbit in eight minutes when the pumping rate is 200 liters/sec.
A scintillation calculation is presented in which the ground intensity is calculated for a satellite transmission through a model atmospheric irregularity of well-defined geometry. Artificial intelligence, in the form of extensive computer generated algebra, has been used to produce a closed form solution whose properties provide new information about the scintillation problem.
The theory, methods, requirements, limitations, effectiveness and economics of cleaning large telescope mirrors
with carbon dioxide (COJ snowflakes are discussed. The method holds promise as a rapid, easy-to-use technique for
routinely maintaining the cleanliness of mirrors between washings.