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COBE

DIRBE Data Products

The DIRBE data sets cover the whole sky and provide photometric data in 10 bands ranging in wavelength from 1.25 to 240 microns, plus polarimetric data at 1.25, 2.2, and 3.5 microns. The Project Data Sets give the sky brightness as observed, including zodiacal and Galactic components, as well as the Cosmic Infrared Background radiation which the DIRBE was designed to seek. Some of the higher level Analyzed Science Data Sets include estimates of the zodiacal light intensity, or have the zodiacal light subtracted. Individual data products are comprised of various subsets of the sky survey observations made by the DIRBE over a 10-month period while the COBE cryogen lasted.

Detailed descriptions of the DIRBE, the data processing, and the data products are given in an Explanatory Supplement. A Small Source Spectral Energy Distribution Browser can be used to assess the visibility of an unresolved or small extended source in the DIRBE data and see its spectral energy distribution. As noted in section 5.6.6 of the Explanatory Supplement, the DIRBE Calibrated Individual Observations (or Time-ordered Data) are required to derive definitive point source fluxes. A point source photometry research tool can be used to extract point source photometry from the Calibrated Individual Observations. A catalog of point sources derived from the DIRBE data, has been developed by Beverly Smith and collaborators.

Browsing of the data is not supported here, but please note that DIRBE near-IR images of the Galactic plane may be browsed at the Multiwavelength Milky Way page.

The Project Data Sets are:

The Analyzed Science Data Sets are:

The following ancillary data products are provided to facilitate analysis of the Project Data Sets and Analyzed Science Data Sets:

The maps are distinguished by the time interval over which multiple observations of a single celestial position are coadded. All of the sky maps are quadrilateralized spherical cube projections in ecliptic coordinates presented in FITS binary tables. Each cube face covers 4pi/6 sr in 256 x 256 pixels of approximately equal area; the area is equivalent to a square 0.32 deg on a side. The DIRBE beam (instantaneous field of view) is 0.7 x 0.7 deg.

Software specifically designed to ingest and analyze COBE data is also available.

The names of files that contain data for a single photometric band include the band numbers (e.g., DIRBE_BAND09_AAM_P3B.FITS). The following table gives the effective wavelengths, bandwidths and beam solid angles for each of the DIRBE bands:

Band

Wavelength
(microns)
Effective bandwidth
(THz)
Beam Solid Angle
(10^ - 4 sr)
1*
1.25
59.5
1.198
2*
2.2
22.4
1.420
3*
3.5
22.0
1.285
4
4.9
8.19
1.463
5
12
13.3
1.427
6
25
4.13
1.456
7
60
2.32
1.512
8
100
0.974
1.425
9
140
0.605
1.385
10
240
0.495
1.323

* A => full intensity; B and C => polarimetry

Note: the quoted bandwidths assume a source spectrum nu*I (nu) = constant.


Time-ordered Data (TOD)

Consider using the Calibrated Individual Observations instead. The TOD product includes calibrated sky brightness values and is the most complete public record of the DIRBE observations. It is necessary to understand the operating modes of the instrument and to recognize various mission events in order to select and interpret the data of interest; thorough familiarity with information contained in the DIRBE Explanatory Supplement is recommended. The contents and format of the TOD product are given in an ASCII file called dirbe_tod_record_structure.asc. The TOD product is not intended for heavy use by the research community, although there may be some applications, such as correlative studies of transient events, for which this product is necessary. The DIRBE TOD - in 41 files each covering one week - are available on tape from the NSSDC. For information, contact the Request Coordination Office.

Calibrated Individual Observations (CIO)

The CIO files contain calibrated 1/8th second sampled sky survey data, in pixel number order, for each day of the cryogenic mission. In nearly all applications, the CIO can be thought of as a user-friendly version of the TOD. There are 285 CIO data files. Corresponding to each data file is a CIO Pixel Index file, which is designed to facilitate data retrieval (e.g., selection of time strings). All CIO files are FITS binary tables. The CIO files are on line. A typical data file is about 60 MB in size. To order the CIO data on CD-Rs or tape, contact the Request Coordination Office.

Weekly Sky Maps

These maps provide weekly-averaged intensity values for each pixel and photometric band, plus Stokes Q and U parameters at 1.25, 2.2, and 3.5 microns. Each map covers approximately half of the sky. As a set, the 41 Weekly Sky Maps offer an unprecedented view of the interplanetary dust (IPD) cloud as each celestial direction was observed by DIRBE at a variety of angles through the IPD. To order the Weekly Map data on CDs, contact the Request Coordination Office. The Weekly Sky Maps are on line.

DIRBE Calibrated Annual File (DCAF)

Consider using the DIRBE Sky and Zodi Atlas instead. The DCAF files contain the same data as the Weekly Sky Maps but are organized differently. This product gives a pixel-by-pixel view of the intensity variations seen as a function of time, collecting all the weekly-averaged intensities measured at a given pixel into a single file. Those who wish to study or model time-variable signals, such as the zodiacal light, will find this organization convenient. There are 6 DCAF data files, one for each face of the COBE Sky Cube, and each data file is accompanied by a DCAF Pixel Index file, which is designed to facilitate data retrieval. The files are FITS binary tables. A typical DCAF data file is ~200 MB in size. To order the DCAF data on CDs, contact the Request Coordination Office. The DCAF files are on line.

Annual Average Maps

These maps provide a single, mission-averaged intensity value per pixel for each of the 10 DIRBE bands. The effect of coaddition is to improve sensitivity to faint emission. Since these maps average over the variable zodiacal light signal, they are useful primarily at wavelengths at which the IPD signature is weak, especially 140 and 240 microns. The Annual Average Maps also provide information on the depth of sky coverage over the whole cryogenic mission. These maps (1 FITS file per wavelength band, each 10.6 MB) are network-accessible and will later be provided on CD-ROM.

Solar Elongation = 90 deg Sky Maps

These maps provide an estimate of the infrared intensity at each pixel and wavelength band based on an interpolation of the observations made at various times at solar elongations close to 90 degrees. To first order, this product depicts the sky as if it were observed through a temporally constant IPD foreground, thus enabling straightforward modeling and subtraction of the zodiacal light to a level adequate for many Galactic studies, the obvious drawback being that only a small portion of the available DIRBE data is represented. The Solar Elongation = 90 deg Sky Maps (1 FITS file per wavelength band, each 12.9 MB), are network-accessible and will later be provided on CD-ROM.

Galactic Plane Maps

These maps are subsets of the Solar Elongation = 90 deg Sky Maps designed to facilitate studies of the Galaxy. The maps cover Galactic latitudes |b| < 10 deg at longitudes 30 deg < l < 330 deg and cover |b| < 15 deg elsewhere. The Galactic Plane Maps (all 10 DIRBE bands) are in a single 5.9 MB FITS file. The file is network-accessible and will later be provided on CD-ROM.

DIRBE Sky and Zodi Atlas (DSZA)

In many applications, the DSZA can be used instead of the DCAF; most of the information in the DCAF is repeated in the DSZA. However, the DSZA also includes estimates of the zodiacal light (thermal emission and scattered light from the interplanetary dust) intensity which can be compared directly with, or subtracted from, the DIRBE measurements. For example, using the DSZA, one can readily compare the apparent time variability of the sky, as observed by the DIRBE, with that of the DIRBE interplanetary dust model (Kelsall et al. 1998, ApJ, submitted), or construct sky maps like the ZSMA maps using different data selection criteria (e.g., maps based on data obtained during different weeks in the cryogenic mission). The DSZA is accompanied by a pixel index and organized in the same fashion as the DCAF. Data and model intensities from all 10 full-intensity wavelength bands appear in the DSZA. The DSZA files are on line.

Zodi-Subtracted Mission Average (ZSMA) Maps

The ZL intensities recorded in the DSZA were subtracted week by week and the residual intensity values were averaged to create the ZSMA Maps, which give the best available picture of the Galactic and extragalactic diffuse infrared emission on degree or coarser angular scales. In many applications, the ZSMA Maps will supersede the Annual Average Sky Maps. Like the Annual Average Maps, the ZSMA Maps are provided in ten FITS binary tables, one each at 1.25, 2.2, 3.5, 4.9, 12, 25, 60, 100, 140 and 240 microns. Each row of the FITS table corresponds to a DIRBE pixel. One zodiacal light-subtracted intensity value, representing an average over the cryogenic mission, is given per pixel. The ZSMA files are on line.

Photometric Standard Values Table

This FITS binary table contains photometric data obtained on 92 standard objects which were judged not to vary in brightness at a particular wavelength over the course of the cold mission and were used to stabilize the DIRBE photometric system. The Table is on line, as is an explanatory document (an excerpt from the DIRBE Explanatory Supplement).

Solar System Objects Data

The Solar System Object Dataset provides flux densities and other data pertaining to individual passages of solar system objects through the DIRBE field of view during the period of cryogenic operation, from 11 December 1989 to 21 September 1990. The data are given in FITS tables, one for each of the following objects: Mars, Jupiter, Saturn, Uranus, Ceres, Pallas, and Vesta. The Solar System Object data are on line.

Faint Source Model

The Faint Source Model gives the predicted contribution to the diffuse sky brightness at 1.25, 2.2, 3.5, 4.9, 12 and 25 microns due to (faint) stars and other discrete Galactic sources. It was used in the search for the cosmic infrared background to subtract an important component of the observed near-infrared sky brightness. The model is described by Arendt et al. (1998, ApJ, submitted) and in Appendix F of the DIRBE Explanatory Supplement. The Faint Source Model files are on line.

Beam Profile Maps

The DIRBE beam profiles are provided in a set of 16 FITS (image) files, one file for each of the 10 full intensity bands, plus `B' and `C' polarization channels for the three short-wavelength bands. As explained in the DIRBE Explanatory Supplement, the beam profile in bands 1 - 4 changed slightly with time during the mission; the FITS files for these bands each contain three beam maps corresponding to three time intervals. The Beam Profile Maps are on line and will later be provided on CD-ROM.

Beam offsets and solid angles are given in Table 4.2-1 of the DIRBE Explanatory Supplement, and also, for convenience, in an ASCII table.

System Spectral Response Functions

Normalized system (filter + detector) response functions for each of the 10 DIRBE bands are given in an ASCII table. The table also appears in Appendix A of the DIRBE Explanatory Supplement.

Color Correction Tables

Following the IRAS convention, DIRBE flux densities are derived assuming a nu*I(nu) = constant source spectrum. Color corrections must be applied to the quoted flux densities to derive true flux densities when the spectrum differs in shape from that assumed. The set of Color Correction Tables given in Appendix B of the DIRBE Explanatory Supplement take into account the system spectral response and pertain to a variety of source spectra (power laws and modified Planck functions). The same data are provided in an ASCII table.


The COBE datasets were developed by the NASA Goddard Space Flight Center under the guidance of the COBE Science Working Group .




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