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Surface Photometry of Celestial Sources from a Space Vehicle

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Surface Photometry of Celestial Sources from a Space Vehicle Proc. Nat. Acad. Sci. USA Vol. 69, No. 3, pp. 694-697, March 1972 Surface Photometry of Celestial Sources from a Space Vehicle: Introduction and Observational Procedures* (diffuse light/Milky Way/gegenschein/zodiacal light/contre lumiere/OSO-6) FRANKLIN E. ROACHt, BENJAMIN CARROLL:, LAWRENCE H. ALLER§, AND LEROI SMITHS t'$ Department of Chemistry, Rutgers University, Newark, N.J. 07102; § Department of Astronomy, University of California, Los Angeles, Calif. 90024; and t',1 Battelle Pacific Northwest Laboratories, Richland, Washington 99352 Contributed by Lawrence H. Alter, January 14, 1972 ABSTRACT Diffuse celestial sources of relatively low faint, diffuse celestial sources are often frustrated by this surface brightness such as the Milky Way, zodiacal light, In addition, this extra-terrestrial light is weak- and gegenschein (or contre lumiere) can be studied most phenomenon. reliably from above the earth's atmosphere with equipment ened and scattered in the earth's atmosphere. A point source flown in artificial satellites. We review the techniques used like a star is simply dimmed as a consequence of increase of and some of the difficulties encountered in day-time obh. extinction with airpath, but for any diffuse source the effects servations from satellites by the use of a special photom- are much more complicated. eter and polarimeter flown in the orbiting skylab observa- tory, OSO-6. In the present investigation, for reasons noted below, we report on a narrow cone of observations (of the order of i 15°) In addition to the familiar discrete sources-stars, nebulae, about the antisolar point. As the earth moves about the sun, galaxies, planets, and comets-there exist diffuse celestial this cone sweeps along the ecliptic, crossing the Milky Way sources of light: the zodiacal cloud including the gegenschein in the summer and again in the winter. Fig. 1 shows the sur- (counter-glow or contre lumi~re), which is of solar system origin, face brightness for space observations of the antisolar point, the diffuse light of the Milky Way, and possibly also a faint as a function of time, as reported by ground-based observers. glow of "cosmic light" (1). The unit of surface brightness is the number of tenth-magni- The zodiacal light is produced by solar light scattered by tude stars per degree2. The gegenschein or "contre lumire" small particles encircling the sun. The contre lumitre is usually amounts to about 200 such units. Superposed on this bright- intrepreted as the back scattering by particles behind the ness is the contribution of starlight plus diffuse galactic light. earth with respect to the sun. The arrows in the upper-lefthand corner suggest the uncer- The galactic and extra-galactic light arises from discrete tainties involved in ground-based observations because of stars, nebulae, and galaxies. By counting stars and galaxies airglow and scattered light. The variability of this correction in each region of the skA to different limiting brightnesses, we introduces an annoying, variable noise in all ground-based can eliminate the contribution of these denumerable sources. data. What is left, the diffuse light, is comprised of two contribu- tions: (a) starlight scattered by small particles (dust grains) THE INSTRUMENT in interstellar space, and (b) a "cosmic light" of very low Observations from a space vehicle offer the opportunity to intensity that may arise partly from an inverse Compton escape the limitations mentioned above and to secure data effect involving very fast electrons and the 30K cosmic back- of superior quality. One of the experiments on the wheel of ground radiation. This cosmic light must have a very low orbiting skylab observatory (OSO)-6 was devised to measure intensity level, which lies below our present detection limits. the brightness and polarization of the sky, using equipment Each of these diffuse sources is of interest in its own right:- designed by A. L. Rouy, B. Carroll, and L. H. Aller. The the zodiacal light and gegenschein in the context of small late A. Rouy directed the fabrication of this instrument by particles in the solar system and scattered starlight as probes Ball Bros. Corp., Boulder, Colo. of the properties of interstellar grains. The separation of these The instrument includes a main afocal telescope, collimator, components is a problem of analytical skill that is difficult quartz rotators, a quarter-wave plate, a double-cut prism technologically, but that is challenging because of the in- that serves as analyzer, and a field lens, all bringing the light trinsic astrophysical importance of these components. beam through a 20-diameter field stop to an EMR 641 E-type When observations are made from the surface of the earth, photomultiplier. This device is intended to measure the bright- further difficulties are introduced by the presence of upper ness, polarization, and ellipticity of the zodiacal light and atmospheric airglow and light scattered in the terrestrial at- other sources of diffuse cosmic light. The zodiacal light com- mosphere. Airglow brightness increases with decreasing alti- ponent shows pronounced polarization, which varies with tude in the sky, but its behavior is capricious: its intensity angular distance from the sun, dropping to zero at the sun and spectral characteristics depend on time and azimuth in and antisolar points. an unpredictable fashion (2). Ground-based observations of The position of the analyzer is kept invariant with respect to the photomultiplier. To perform the equivalent of a specific rotation of the optic axis of the analyzer, the angular position * This is paper no. 1 in a series: "Surface Photometry of Celestial of the plane of the partially-polarized zodiacal component Sources from a Space Vehicle." of the radiation is shifted by means of appropriate quartz 694 Downloaded by guest on October 1, 2021 Proc. Nat. Acad. Sci. USA 69 (1972) Surface Photometry of Celestial Sources 695 ORBIT NUMBER (1970) 3000 4000 5000 6000 7000 JAN FEB MAR APRIL MAY JUNE JULY AUG SEPT OCT NOV DEC FIG. 1. The absolute brightness throughout the year in the gegenschein direction based on ground observations. OSO-6 orbit numbers are shown for the calendar year of 1970. The arrows in the upper left suggest the uncertainties in ground-based data due to (1) airglow and (2) scattered light. 3000 -' - a.-170 +190 ' 1 < a'. o~wesca-7 +185 t17 _ Ck*,°'(-m) ........LEs ) 2000| h1 .3 ........}i ° Auoo0,7o°;°.....IgO596X4IA -EJ1- Cl) I-- f COUNTS~~~~(SAELIT EVENING 0 ~~~~~~~10002000 3000 COUNTS (SATELLITE EVENING) FIG. 2. Correlation plots of observations-pairs of the same region in the celestial sky. Traverses are B (morning) versus B (evening) and C (morning) versus C (evening) in the upper plot; and D (morning) versus D (evening) and E (morning) versus E (evening) in the lower plot. Downloaded by guest on October 1, 2021 696 Astronomy: Roach et al. Proc. Nat. Acad. Sci. USA 69 (1972) 5. 2-. 4 0 JUNE (1970) JULY AUGUST FIG. 5. Comparison of some OS0-6 observations (open circles) of the celestial sky in traverse D (elongation equals -180°) with GALACTIC LONGITUDE the corresponding ground-based observations (closed circles and FIG. 3. The loci in galactic coordinates of six traverses of the solid curve). The calibration of Fig. 4 is based on these same ob- Milky Way (A,B,C,D,E,F) made by the OSO-6 polarimeter- servations, and the least-squares solution obtained from the plot photometer for orbits 4672 to 4856. of Fig. 4 was used to put the observations of OS0-6 on the absolute scale of Fig. 5. rotators. These rotators are mounted on two wheels so that various combinations of rotators, with or without the quarter- brightnesses 10-9-10-1' that of the sun. When observations wave plate, may be introduced into the optical path. Rouy, have to be made during satellite daytime, difficulties arise, Carroll, and Quigley (3) have described the use of "imposed not only from light contamination by the sun shining on parts rotations" in polarimetry. A wheel is placed between the of the instrument, but also from light reflected by the sunlit analyzer and photomultiplier that contains three interference earth, which fills a large solid angle. It was immediately filters, centered at 4000 A, 5000 A, and 6100 A, so that colors evident that at line-of-sight elongations within 900 from the and polarization may be compared in three separate spectral sun, the data were seriously diluted by instrumental scattered regions. light. However, scrutiny of the data indicated that measure- The line of sight of the instrument is fixed at right angles ments in the neighborhood of the antisolar point were not to the spin axis of the orbiting vehicle. Hence, in each rotation so affected, and the accumulated tapes provided a scan with of the wheel, the viewing telescope passes near the sun and the a diameter of about 300 i 50 along the ecliptic. antisolar point. A small sampling telescope, which is pro- The instrument was programmed to start measurements at grammed to look directly into the sun, serves for calibration an elongation of -195° (i e., beyond the antisolar point) as purposes. It has a secondary optical path that attenuates the satellite emerged from the night (labeled "morning" sunlight by a factor of about 1012, and then introduces it into observations). Then, as it approached the earth's shadow the main optical system at the analyzing prism. again, the instrument measured to + 1950, i.e., 150 on the THE OBSERVATIONS opposite side of the antisolar point ("evening" observations). Pairs of elongation such as [-1700(morn.), +1900(eve.)], We are concerned here with the photometric data and their [-175° (morn.), + 1850 (eve.)], [-1850 (morn.), + 175' reliability, reserving polarization studies for later discussions. (eve.)], and, of course, [-180'(morn.), +1800(eve.)] cor- The diffuse sources with which we are concerned have surface respond to the same point in the sky.
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