FIRAS Data Products
The Data Products described here are from Pass 4 of the FIRAS Processing Pipeline. They supersede all data products released prior to 01-Jan-1998.
The FIRAS Project Data Sets and Analyzed Science Data Sets (ASDS) cover the whole sky and provide spectrophotometric data at frequencies ranging from 1 to 97 cm-1 (30 to 2910 GHz). The main celestial emission components in this interval are the cosmic microwave background, the spectrum of which FIRAS was designed to measure precisely, thermal emission from interstellar dust, and line emission from various molecules, atoms, and ions, including the CO rotational ladder (from J=1-0 to J=6-5), the C+ 158 micron line, and other important interstellar cooling lines.
The FIRAS instrument, its operating modes, calibration, and the data products are described in detail in the FIRAS Explanatory Supplement. Briefly, the FIRAS is a scanning, four-port (two input; two output) Michelson interferometer that uses polarizing grids as beamsplitters and creates an interferogram (i.e., the Fourier transform of the source spectrum) by scanning a movable mirror platform (the "mirror transport mechanism," or MTM). A dichroic splitter at each output port (arbitrarily designated "left" and "right") further splits each beam into low (2 - 20 cm-1) and high (20 - 100 cm-1) frequency bands. The four detectors are thus referred to as RH, RL, LH, and LL, for "right high," etc. The MTM can be scanned at either of two speeds: "slow" or "fast". And the MTM sweep can be set to one of two scan lengths, "long" or "short," thus affecting the spectral resolution. Files that provide data collected in the right, high, short, slow mode, for example, have "RHSS" in their names (e.g., FIRAS_CALIBRATED_SKY_SPECTRA_RHSS.FITS).
Most research applications will call for one or more of the following high-level products (ASDS)
or one (or more) of the following Spectral Sky Maps
These and the other FIRAS data products are briefly described below.
The FIRAS sky maps are quadrilateralized spherical cube projections in ecliptic coordinates. presented in FITS binary tables. Each cube face covers 4pi/6 sr in 32 x 32 pixels of approximately equal area; the area is equivalent to a square 2.59 deg on a side. The FIRAS horn has a 7 degree circular top hat power pattern, and motional smearing adds 2.3 degrees to the effective beam in a direction perpendicular to the ecliptic plane.
FIRAS data products in HEALPix format are also available, from partial reprocessing of data products from the final FIRAS data release. The products are at HEALPix resolution 4 (Nside=16), which has mean pixel spacing of 2.66 degrees.
Software specifically designed to ingest and analyze COBE data may be obtained via anonymous ftp. You may also wish to take advantage of the FITSIO FORTRAN subroutine library developed by W. Pence.
How to Obtain the Products
The Line Emission Maps cover the entire sky and give the intensity and the associated uncertainty in each of 18 emission lines, of which the following 9 were detected (Bennett et al. 1994; included as Appendix D of the FIRAS Explanatory Supplement): the 158 micron ground state transition of C+, an important neutral interstellar gas coolant; the N+ 122 micron and 205 micron transitions, which trace extended low-density ionized gas; the 370 micron and 609 micron lines of neutral carbon, which arise in photodissociation regions; and the CO J=2-1, 3-2, 4-3, and 5-4 lines, which trace the molecular gas. As one would expect, the line emission is strongest in the Galactic plane and concentrated in the inner Galaxy.
There are three Line Emission Map FITS files. Two of these are at low spectral resolution: FIRAS_LINE_EMISSION_MAP_LOWF.FITS covers frequencies < 630 GHz; FIRAS_LINE_EMISSION_MAP_HIGH.FITS covers higher frequencies and also includes the parameters of an interstellar dust model (see below). FIRAS_LINE_EMISSION_MAP_HRES.FITS contains high spectral resolution data from 30 to 660 GHz.
Synthetic spectral line profiles (FIRAS_LINE_PROFILES_*.FITS) are provided for each of the channel and scan modes or merged modes. The synthetic profiles can be used in conjunction with a FIRAS spectrum to calculate line intensities.
The two dust spectrum maps (FIRAS_DUST_SPECTRUM_HIGH.FITS and FIRAS_DUST_SPECTRUM_LOWF.FITS) cover the whole sky and give the residual sky spectrum after modelled emission from the CMBR, interplanetary dust (zodiacal emission), and interstellar lines have been subtracted. The remaining signal is dominated by thermal continuum emission from Galactic interstellar dust.
Maps of the interstellar dust temperature and the 167 micron (1800 GHz) optical depth are included in the high-frequency Line Emission Map file. A single-temperature dust model was used to make these maps and the far-infrared dust emissivity was assumed to have a power law spectral index of 2.0. Different models can be used and assumptions made, and corresponding temperature and optical depth maps can be derived straightforwardly from the Dust Spectrum Maps. Reach et al. ( 1995, ApJ, 451, 188), for example, report evidence for a ubiquitous cold (~5 K) dust component.
A file called FIRAS_TEMPERATURE_MAP_LOWF.FITS contains maps of the CMB temperature and its uncertainty, parameters of a dipole fit to the CMB temperature map (amplitude, direction, and associated uncertainties), and the average CMB temperature measured by FIRAS (2.728 +/- 0.004 K).
The high-level data products described above derive from FIRAS spectra and ultimately from the original interferograms. Calibrated spectra, interferograms and various ancillary products were released as FIRAS Project Data Sets.
There are three main Spectral Sky Maps: two low-spectral resolution products (FIRAS_DESTRIPED_SKY_SPECTRA_LOWF.FITS, and FIRAS_DESTRIPED_SKY_SPECTRA_HIGH.FITS, which cover the full FIRAS frequency range at ~25 GHz resolution; and a high-spectral resolution product called FIRAS_DESTRIPED_SKY_SPECTRA_HRES.FITS, which covers the frequency range 30 - 660 GHz at 6.9 GHz resolution. These files contain a spectrum at each pixel and were created by coadding spectra obtained from various FIRAS channel and scan mode combinations. Prior to coaddition, the observations made in different modes were calibrated to remove the instrument signature. "Destriping" is a secondary calibration step applied to compensate for unidentified instrumental offsets. The existence of such offsets is suggested by stripes that appear in an untreated sky map at a particular frequency.
In addition to the spectral maps mentioned above, Spectral Sky Maps representing data from individual instrument modes and various intermediate mode combinations are also available for analysis. You may wish to compare corresponding spectra from these files to gauge their consistency. For a description of how the data were processed, a discussion of the random and systematic errors, and advice on how to use the ancillary data products, see the FIRAS Explanatory Supplement. Files of the following types are also available:
* All names have "FIRAS_" as a prefix and ".FITS" as a suffix. [mode] is one of the 4-letter symbols tabulated below; [mode] is either LOWF, HIF2, HIF3 or HIF4. All of the files are FITS binary tables.
Calibration model parameters are given in the Calibration Model Solution files. Errors associated with the calibration model are given in the Calibration Error Terms files.
The Covariance Matrices express all frequency-to-frequency variances in the calibrated spectra, including correlation effects induced by apodization prior to taking the Fourier transform. Covariances with instrument characteristics such as calibration source temperatures are included as well.
Four Model Selection Errors files give estimates of the possible systematic errors caused by the choice of destriper model functions at low spectral resolution. The file designated "LOWF," pertains to low-frequency (30 - 660 GHz) data. The other three files ("HIF2,","HIF3,","HIF4,") are subsets of the high frequency data processed with separate destriper model functions (see the table below for frequency ranges).
The table below explains the symbols used in file names to signify data derived from various instrument modes or mode combinations:
Time-ordered interferograms and engineering data are provided for those interested in the original data and to enable searches for time-dependent phenomena such as the effect of auroral electrons on the FIRAS antenna. Cosmic ray glitch data are included.
The COBE datasets were developed by the NASA Goddard Space Flight Center under the guidance of the COBE Science Working Group .