Thomas L. Murdock

The COBE mission - its design and performance two years after launch

COBE, NASAs first space mission devoted primarily to cosmology, carries three scientific instruments to make precise measurements of the spectrum and anisotropy of the cosmic microwave background radiation on angular scales greater than 7° and to conduct a search for a diffuse cosmic infrared background radiation with 0°.7 angular resolution. The mission goal is to make these measurements to the limit imposed by the local astrophysical foregrounds. The COBE instruments cover the wavelength range from 1.2 μm to 1 cm. The instruments are calibrated periodically in orbit using internal calibrators and celestial standards

COBE differential Microwave Radiometers : preliminary systematic error analysis

The Differential Microwave Radiometers (DMR) instrument aboard the Cosmic Background Explorer (COBE) maps the full microwave sky in order to measure the large-angular-scale anisotropy of the cosmic microwave background radiation. Solar system foreground sources, instrumental effects, as well as data recovery and processing, can combine to create statistically significant artifacts in the analyzed data. We discuss the techniques available for the identification and subtraction of these effects from the DMR data and present preliminary limits on their magnitude in the DMR 1 yr maps (Smoot et al. 1992)

COBE differential microwave radiometers - Calibration techniques

The COBE spacecraft was launched November 18, 1989 UT carrying three scientific instruments into earth orbit for studies of cosmology. One of these instruments, the Differential Microwave Radiometer (DMR), is designed to measure the large-angular-scale temperature anisotropy of the cosmic microwave background radiation at three frequencies (31.5, 53, and 90 GHz). This paper presents three methods used to calibrate the DMR. First, the signal difference between beam-filling hot and cold targets observed on the ground provides a primary calibration that is transferred to space by noise sources internal to the instrument. Second, the moon is used in flight as an external calibration source. Third, the signal arising from the Doppler effect due to the earths motion around the barycenter of the solar system is used as an external calibration source. Preliminary analysis of the external source calibration techniques confirms the accuracy of the currently more precise ground-based calibration. Assuming the noise source behavior did not change from the ground-based calibration to flight, a 0.1-0.4 percent relative and 0.7-2.5 percent absolute calibration uncertainty is derived, depending on radiometer channel.

COBE diffuse infrared background experiment observations of Galactic reddening and stellar populations

This Letter describes the results of an initial study of Galactic extinction and the colors of Galactic stellar populations in the near-IR using the Diffuse Infrared Background Experiment (DIRBE) aboard the Cosmic Background Explorer (COBE) spacecraft. The near-IR reddening observed by DIRBE is consistent with the extinction law tabulated by Rieke & Lebofsky (1985). The distribution of dust and stars in most of the first and fourth quadrants of the Galactic plane (0 deg less than l less than 90 deg, and 270 deg less than l less than 360 deg, respectively) can be modeled as a stellar background source seen through up to approximately 4 mag of extinction at 1.25 micrometers. The unreddened near-IR colors of the Galactic disk are similar to those of late-K and M giants. The Galactic bulge exhibits slightly bluer colors in the 2.2-3.5 micrometers range, as noted by Terndrup et al. (1991). Star-forming regions exhibit colors that indicate the presence of a approximately 900 K continuum produced by hot dust or polycyclic aromatic hydrocarbons (PAHs) contributing at wavelengths as short as 3.5 micrometers.

Design of the diffuse infrared background experiment (DIRBE) on COBE

The Diffuse InfraRed Background Experiment (DIRBE) onboard the cosmic Background Explorer (COBE) was designed to conduct a search for a cosmic infrared background (CIB), which is expected to be the fossil radiation from the first luminous objects in the universe. The instrument, a ten-band cryogenic absolute photometer and three-band polarimeter with a 0.7 degree(s) beam and a wavelength range from 1 - 240 micrometers , scans the sky redundantly and samples half the sky each day. During the ten month lifetime of the cryogen, the instrument achieved a nominal sensitivity on the sky of 10-9 W/m2/sr at most wavelengths, or approximately 1% of the natural background at wavelengths where the sky is very luminous. The short wavelength bands from 1 - 5 micrometers continue to operate after exhaustion of the cryogen, although at reduced sensitivity. In this paper, we review the design, testing, and in-flight performance of the DIRBE.

First results of the COBE satellite measurement of the anisotropy of the cosmic microwave background radiation

Abstract We review the concept and operation of the Differential Microwave Radiometers (DMR) instrument aboard NASAs Cosmic Background Explorer (COBE) satellite, with emphasis on the software identification and subtraction of potential systematic effects. We present preliminary results obtained from the first six months of DMR data and discuss implications for cosmology.

Scientific results from COBE

Abstract NASAs Cosmic Background Explorer ( COBE 1 ) carries three scientific instruments to make precise measurements of the spectrum and anisotropy of the cosmic microwave background (CMB) radiation on angular scales greater than 7° and to conduct a search for a diffuse cosmic infrared background (CIB) radiation with 0.7° angular resolution. Data from the Far-InfraRed Absolute Spectrophotometer (FIRAS) show that the spectrum of the CMB is that of a blackbody of temperature T=2.73±0.06 K, with no deviation from a blackbody spectrum greater than 0.25% of the peak brightness. The first year of data from the Differential Microwave Radiometers (DMR) show statistically significant CMB anisotropy. The anisotropy is consistent with a scale invariant primordial density fluctuation spectrum. Infrared sky brightness measurements from the Diffuse InfraRed Background Experiment (DIRBE) provide new conservative upper limits to the CIB. Extensive modeling of solar system and galactic infrared foregrounds is required for further improvement in the CIB limits.

Preliminary DMR measurements of the CMB isotropy

The COBE Differential Microwave Radiometers (DMR) instrument has produced preliminary full-sky maps at frequencies 31.5, 53, and 90 GHz. The redundant channels and matched beams at three frequencies distinguish the DMR from previous large-scale surveys. Galactic emission is seen unambiguously at all three frequencies. The only large-scale anisotropy detected in the cosmic microwave background is the dipole anisotropy. There is no clear evidence for any other large-angular-scale feature in the maps. Without correcting for any systematic effects, we are able to place limits {Delta}T/T{sub 0}{lt}3{times}10{sup {minus}5} for the rms quadrupole amplitude, {Delta}T/T{sub 0}{lt}4{times}10{sup {minus}5} for monochromatic fluctuations, and {Delta}T/T{sub 0}{lt}4{times}10{sup {minus}5} for Gaussian fluctuations (all limits are 95% C.L. with T{sub 0}=2.735 K). The data limit {Delta}T/T{sub 0}{lt}10{sup {minus}4} for any feature larger than 7{degree}. We briefly review the DMR and discuss some implications of these results for cosmology.

Investigation of the zodiacal light from 1 to 240 μm using COBE DIRBE data

The Cosmic Background Explorer (COBE) satellite was launched on November 18, 1989 from Vandenberg Air Force base on a Delta rocket. It carried two superfluid liquid-helium-cooled (LHe) infrared (IR) instruments in a 600 liter dewar, and three microwave radiometers mounted on the outside of the dewar. One of the LHe-cooled instruments is a ten-band photometer covering the spectral range from 1.2 to 240 micrometers - the Diffuse Infrared Background Experiment (DIRBE). A goal of the DIRBE program is to obtain full-sky infrared observations that can be used to model accurately the IR contributions arising from the interplanetary dust (IPD) and the Galaxy. Using such models, the foreground can be removed to expose and underlying extragalactic IR component produced early in formation of the universe. The nature of the IPD IR foreground detected by the DIRBE is found to be quite complex, but amenable to modelling.

Early results from the Cosmic Background Explorer (COBE)

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 cos θ dipole anisotropy whose amplitude and direction agree with previous data. At shorter wavelengths, the sky spectrum and anisot...