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COBE

Cosmic Background Explorer/DMR

Product Name
DMR Calibrated Individual Observations (CIO)
Mission
COBE/DMR

Description

The DMR CIO files contain the DMR 4-year time-ordered data in a format that is user-friendly and platform-independent. These data have the same calibration and systematic error corrections applied as was used in processing the 4-year project sky maps. In addition, the data flags and pointing information are supplied in a transparent form. The data are packaged in files containing 30 days of observations, with the 30 day interval specified in the file name (e.g., dmr_cio_91206-91236.fits contains data from day 206 of 1991 to day 236 of 1991). There are two files for each 30 day interval. The first file, named DMR_CIO*.fits, contains the detector data and pointing information in FITS binary table format. The second file, DMR_ANC*.txt, contains ancillary housekeeping information in a flat ascii format. More detailed information about the contents of each file is provided in the following paragraphs.

These files were produced by DMR team member Tony Banday, with the assistance of Gary Hinshaw. They were used to produce the COBE-DMR maps in HEALPix format , and were validated by reproducing the released 4-year sky maps in their original quad-cube pixel format.

Detector and Pointing data (dmr_cio*.fits files)

The sky signal and pointing information are contained in the first binary table extension of these files. The first six columns tabulate the differential temperature observations, in units of antenna temperature, taken by each of the 6 DMR detectors: 31 GHz channels A & B, 53 GHz channels A & B, and 90 GHz channels A & B (see the DMR Explanatory Supplement for more information about the DMR experiment). No explicit time-stamp is provided for each observation, but it can be roughly reconstructed from the START time given in the main FITS header using an increment of 0.5 seconds per observation. Precise time stamps are provided in the ancillary files described below.

Spacecraft pointing information is provided in the last four columns of the binary table in the form of spacecraft quaternions. The FITS extension header provides references on how to reconstruct sky coordinates from the quaternions.

Ancillary information (dmr_anc*.txt files)

Ancillary information that was used to calibrate and correct the science data are provided in ascii format. The columns are:

columns 1 & 2 : time, recorded as two fields from VAX ADT format, which is the number of 1.e-07 second intervals since 0h UT 17 Nov 1858. Further information on working with ADT time is given in Appendix E of the DMR 4yr Explanatory Supplement.
columns 3,4,5 : Spacecraft XYZ velocity in m/sec, J2000 celestial coordinates.
columns 6,7,8 : Spacecraft XYZ position in m, J2000 celestial coordinates.
column 9 : Moon Phase, in radians.
column 10 : Instrument temperature in data units (DIPDUATH).
column 11 : Instrument bus voltage in data units (DP28VRUA_Orbit_Var).

A more detailed descriptions of these fields is provided in Appendix E of the 4yr DMR Explanatory Supplement. The conversion of columns 10 and 11 to physical units is described in Appendix F of the DMR Explanatory Supplement.

Special data files (dmr_cio*.fits, dmr_anc*.txt files)

As detailed in the DMR 4-year Explanatory Supplement, the 31 GHz channels required special handling due to two distinct effects, 1) an unusual susceptibility to systematic errors during the summer eclipse seasons, and 2) changes in the instrument noise level as a function of mission epoch (channel 31B only). The former effect meant that data for a period of roughly 3 months each year was rejected for use in the final 4-year sky maps. The latter effect led to the production of distinct sky maps from three separate time periods within which the rms noise was approximately constant. The final 4-year 31B map was formed from a weighted average of these three maps.

To accommodate these exceptions, we have generated a set of special files whose start and stop times align with the time boundaries described by these events. These files otherwise contain the same information as the standard cio and anc files described above. In more detail,

31A eclipse season, days 121-216

Use the files:

dmr_cio_*111_*121.fits
dmr_cio_*216_*236.fits

in place of the files:

dmr_cio_*111_*141.fits
dmr_cio_*141_*171.fits
dmr_cio_*171_*206.fits
dmr_cio_*206_*236.fits

and similarly for the anc files.

31B eclipse season, days 141-206; 31B noise changes

The three distinct time intervals within which the 31B rms noise was nearly constant are denoted A, B, and C. Accounting for the eclipse seasons noted above, the time intervals comprising periods A, B, and C are:

Interval A: 1989:328-1990:141 + 1990:206-1991:141 + 1991:206-1991:277
Interval B: 1992:224-1993:085
Interval C: 1993:086-1993:111 + 1993:236-1993:362

This requires the use of the following special files:

dmr_cio_89328_89356.fits ; start of A
dmr_cio_91266_91277.fits ; end of A
dmr_cio_92224_92236.fits ; start of B
dmr_cio_93051_93085.fits ; end of B
dmr_cio_93086_93111.fits ; start of C
dmr_cio_93326_93362.fits ; end of C

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