The COBE mission and FIRAS instrument have been described by
Mather at this
Symposium. I therefore focus on the Diffuse Infrared Background
Experiment (DIRBE).
The primary aim of the DIRBE is to conduct a definitive search for an
isotropic CIBR,
within the constraints imposed by the local astrophysical
foregrounds. The experimental
approach is to obtain absolute brightness maps of the full sky in 10
photometric
bands at 1.25, 2.2, 3.5, 4.9, 12, 25, 60, 100, 140, and 240 µm. In
order to facilitate
discrimination of the bright foreground contribution from interplanetary
dust, linear
polarization is also measured at 1.25, 2.2, and 3.5
µm. Because of the Earth's motion
within the interplanetary dust cloud, the diffuse infrared brightness of
the entire sky
varies over the course of a year. To monitor this variation and use it
to help discriminate
the signal contribution from the interplanetary dust cloud, the DIRBE field of
view was offset from the COBE spacecraft spin axis by
30°. This produces a helical
scan of the sky during each COBE orbit, modulating the signal
from any spherically
symmetric, Sun-centered cloud. Over the course of six months this yields
observations
of all celestial directions hundreds of times at all accessible solar
elongation angles
(depending upon ecliptic latitude) in the range 64° to 124°
. All spectral bands view the
same instantaneous field-of-view, and the helical scan allows the DIRBE
to sample fully
50% of the celestial sphere each day. The instrument is designed to
achieve a sensitivity
at each wavelength for each 0.7° x 0.7° field of view of
I = 1 nW m-2
sr-1 (1 
,
1 year). This is well below the actual sky brightness, and below many of
the predictions for the CIBR brightness, at all wavelengths.
The DIRBE instrument is an absolute radiometer, utilizing an off-axis folded Gregorian telescope with a 19-cm diameter primary mirror. The optical configuration (Magner 1987) is carefully designed for strong rejection of stray light from the Sun, Earth limb, Moon or other off-axis celestial radiation, or parts of the COBE payload (Evans 1983). The instrument, which is maintained at a temperature below 2 K within the COBE superfluid helium dewar, measures absolute brightness by chopping between the sky signal and a zero-flux internal reference at 32 Hz. Instrumental offsets are measured by closing a cold shutter located at the prime focus. A radiative offset signal in the long wavelength detectors arising from JFETs (operating at about 70 K) used to amplify the detector signals was identified and measured in this fashion. Internal radiative reference sources are used to stimulate all detectors when the shutter is closed to monitor the stability and linearity of the instrument response. The highly redundant sky sampling and frequent response checks provide precise photometric closure over the sky and reproducible photometry to ~ 1% or better for the duration of the mission. Calibration of the photometric scale is obtained from observations of isolated bright celestial sources.
The primary DIRBE survey of the sky was carried out from December 11, 1989 until depletion of the liquid helium on September 21, 1990. The interior of the dewar subsequently warmed to about 50 K. Though the detectors at wavelengths longer than 4.9 µm no longer provide useful data, the 1.25 to 4.9 µm detectors continue to provide usable data at about 20% of the original sensitivity. The present plan is to operate the COBE through the end of 1993. A more detailed description of the COBE mission has been given by Boggess et al. (1992), and the DIRBE instrument has been described by Silverberg et al. (1993).
The DIRBE sky maps show the dominant anticipated features of Galactic starlight and zodiacal light at short wavelengths, and emission from the interplanetary and interstellar media at long wavelengths. Four all-sky false color images each containing data from three adjacent DIRBE bands ranging from the near-infrared to the submillimeter are given by Hauser (1993). These images dramatically illustrate the challenge of distinguishing the CIBR from signals arising in our local cosmic environment. The DIRBE data are clearly a valuable new resource for studies of the interplanetary medium and the Galaxy.