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The high photometric quality and full-sky coverage in the COBE DIRBE datasets make possible detailed studies of the interplanetary medium. This paper presents a preliminary derivation of the infrared scattering phase function of interplanetary dust. The ultimate purpose of these investigations is to use the DIRBE observations to constrain the composition, size and structure of interplanetary dust grains.
The Diffuse Infrared-Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) satellite is a 10-band absolute photometer covering the wavelengths 1–300 microns using photovoltaic, photoconductive, and bolometric detectors. The input is via a 19-cm, off-axis, highly-baffled Gregorian telescope, with the detectors located at a pupil plane so they share the same field of view (0.7 × 0.7 degrees). The whole assembly is mounted inside a 1.4 K; super-fluid, liquid-He dewar, which is shared with the Far Infrared Absolute Spectrometer (FIRAS) instrument. Each day half of the sky is surveyed, as the line-of-sight of the DIRBE is canted 30 degrees to the COBE spin axis. The whole sky is fully observed in 6 months, as the spin axis precesses at about 1 degree per day. At present each sky pixel has been observed at least once. The basic findings on the general brightness of the sky - Zodiacal light and galaxy - are provided, as well as a synopsis of the advantages and disadvantages associated with a space-borne observatory. The relationship of our experience and findings with respect to possible future missions and their scientific goals is presented.
The Cosmic Background Explorer, launched November 18, 1989, has nearly completed its first full mapping of the sky with all three of its instruments: a Far Infrared Absolute Spectrophotometer (FIRAS) covering 0.1 to 10 mm, a set of Differential Microwave Radiometers (DMR) operating at 3.3, 5.7, and 9.6 mm, and a Diffuse Infrared Background Experiment (DIRBE) spanning 1 to 300 µm in ten bands. A preliminary map of the sky derived from DIRBE data is presented. Initial cosmological implications include: a limit on the Comptonization y parameter of 10−3, on the chemical potential μ parameter of 10−2, a strong limit on the existence of a hot smooth intergalactic medium, and a confirmation that the dipole anisotropy has the spectrum expected from a Doppler shift of a blackbody. There are no significant anisotropies in the microwave sky detected, other than from our own galaxy and a cosθ dipole anisotropy whose amplitude and direction agree with previous data. At shorter wavelengths, the sky spectrum and anisotropies are dominated by emission from ‘local’ sources of emission within our Galaxy and Solar System. Preliminary comparison of IRAS and DIRBE sky brightnesses toward the ecliptic poles shows the IRAS values to be significantly higher than found by DIRBE at 100 μm. We suggest the presence of gain and zero-point errors in the IRAS total brightness data. The spacecraft, instrument designs, and data reduction methods are described.
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