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COBE

DMR Data Products

This table summarizes the available DMR data products.

The data products are categorized as Project Data Sets (PDS) and Analyzed Science Data Sets (ASDS). The DMR Project Data Sets are Sky Maps and Pixelized Differential Data from each of the six instrument channels, and Time-Ordered Data. The Analyzed Science Data Sets are maps of various Galactic emission components and maps of the cosmic microwave background anisotropy. Maps equivalent to each of the PDS and ASDS Sky Maps but pixelized in Galactic instead of ecliptic (the default) coordinates are also provided. All of the data products except the Time-Ordered Data are in FITS binary tables. Detailed descriptions of the DMR, the data processing, and the data products are given in an Explanatory Supplement.

Observations from the entire four-year period of DMR operation are covered by the products released on 10 January 1996. The four-year Sky Maps are the most sensitive achievable with DMR data and are recommended for new analyses. However, since numerous publications have been based on the one- and two-year maps released as intermediate data products, those maps have been kept on line.

The following DMR Data Products are available on line from the LAMBDA.

In order to reduce the brightness gradient and keep map-making errors to a minimum, a dipole was subtracted prior to pixelizing the data to generate the PDS Sky Maps. (This improvement in the data reduction was implemented after the 1-year maps were produced and released as an "Initial Product" in 1993; the 1-year maps contain the dipole signature, which is due to our motion with respect to the cosmic microwave background.) Note that the dipole parameters, which are given in the FITS headers, are not intended to be definitive, best-fit values.


Sky Maps and Analyzed Science Data Sets

There are six sky maps, corresponding to the two independent channels (designated A and B) and three frequencies (31.5, 53, and 90 GHz) of the COBE Differential Microwave Radiometers (DMR). The maps are quadrilateralized spherical projections in either ecliptic or Galactic coordinates. A sparse matrix least squares minimization technique was used to derive, from highly redundant sky sampling, a differential temperature for each of 6144 map pixels while simultaneously allowing for instrumental and systematic effects. The observations were made with a 7 degree beam and are presented in 2.8 degree pixels. The maps are in FITS files formatted as described in the DMR Explanatory Supplement. File names indicate the frequency, channel, and world coordinates of the map (ecliptic by default; Galactic if specified). For example, the 53 GHz, channel A map in ecliptic coordinates is named dmr_skymap_53ghz_a.fits and its Galactic coordinate counterpart is named dmr_skymap_galactic_53a.fits.

Due to the differential nature of the observations, the maps have unconstrained antenna temperature zero points. While the dipole anisotropy has been removed to first order (see note above), emission from the Galaxy remains. The DMR Analyzed Science Data Sets include Galactic Emission Maps and Cosmic Emission Maps. The latter result when the Galactic emission is removed. The modeled Galactic components are synchrotron emission (dmr_synch.fits), free-free emission (dmr_free.fits), and thermal dust emission (dmr_dust.fits). Two techniques were used to produce the Cosmic Emission Maps, a "subtraction technique" in which the Galactic components are subtracted from the DMR maps (dmr_dsmb.fits), and a "combination technique" in which a linear combination of the DMR maps is used to cancel the Galactic emission (dmr_dcmb.fits). Corresponding to each of these maps is an equivalent map pixelized in Galactic coordinates (e.g., dmr_dust_galactic.fits). For a more detailed description of the Analyzed Science Data Sets and the corresponding Galactic Coordinate Data Sets, see the DMR Explanatory Supplement and references therein.


Pixelized Differential Data

The calibrated differential antenna temperature (DT) and the number of observations are given for each combination of sky positions observed simultaneously by a DMR horn pair. The data (temperature in "horn 1" minus temperature in "horn 2") are sorted according to the COBE sky map pixel number seen by horn 1, and sub-sorted by the horn 2 pixel number. The observations are made with a 7 degree beam and presented in 2.8 degree pixels. The pixelization scheme is described in a FITS file called dmr_firas_skymap_info.fits.

All differential temperatures for a given pixel pair are combined, after correcting for known systematic errors and subtracting a nominal dipole to minimize pixel gradient effects, into four moment sums, specifically the sums of DT, (DT)^2, (DT)^3, and (DT)^4. The Pixelized Differential Data are presented in FITS files formatted as described in the DMR Explanatory Supplement. There are six Pixelized Differential Data files corresponding to two independent DMR channels at each of three frequencies: 31.5, 53, and 90 GHz. The Pixelized Differential Data Sets describe observations made during the same four-year period as the Sky Maps (see above).

The 4-year Pixelized Differential Data files are online, but be aware that each file is about 40 MB in size. If you would prefer not to download these files over the Web, they may be ordered either on CD-ROM or tape through the NSSDC's Coordinated Request and User Support Office.


Time-Ordered Data

Time-Ordered Data (TOD) from the DMR covering the period 19 November 1989 to 29 December 1993 (i.e., the entire period during which observations were made) are available in native format. The contents and format of the TOD are described in the DMR Explanatory Supplement. The data are in daily files, each 5 MB in size. The TOD (approximately 7.5 GB) may be obtained from the NSSDC on tape. To request a set of tapes, contact the Coordinated Request and User Support Office.


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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