BEFS Data Products

The Berkeley Extreme and Far-UV Spectrometer (BEFS) observes the spectral range from about 400 to 1200 Å. We divide the spectrum into four chunks, labeled A, B, C, and D. Spectra A and B are recorded by Detector 0, while spectra C and D fall onto Detector 1. The approximate wavelength regions spanned by each spectrum are given as follows:

The BEFS could record light simultaneously in all these segments, but most observers planned their observations to optimize the FUV.

The general design of the BEFS is discussed by Hurwitz & Bowyer (1986), while its calibration and performance on the ORFEUS-I and II missions are described by Hurwitz & Bowyer (1996) and Hurwitz et al. (1998), respectively. The BEFS team is currently at work on a final calibration paper. In the meantime, Hurwitz et al. (1998) provide explanations of the calibration process and a few tricky points (the bright corner, for example) that users should understand before working with these data. Users are advised to read this paper. The rest of this overview will address the format of the files in this distribution.

All BEFS data files are provided in FITS format, with spectra stored as binary table extensions, and are accessible on the catalog page. General information about each observation may be found in the primary file header; detailed information about data formats, etc., are in the headers of the individual extensions. The background spectrum is smoothed by 13 pixels (see caveats.html) for more details).

For each observation, the database contains:

• Two photon-list files (X, Y, T), one for each detector, labeled LST0 and LST1.
• Four calibrated spectral files, labeled SPA1, SPB1, SPC1, and SPD1. Note that EUV spectra (SPA1 and SPB1) are available for only about a third of the observations. Additional files for BEFS II only include: SPD2 which are airglow spectra and the tin-filter (EUV) spectra, SPB3.
• Software to extract time-tagged spectral-photon lists from the LST0 and LST1 files. This code is described in the file time_tagged.html.

Please see the caveats page for information on data problems and subtleties.

Data file types:

• Photon-list files (LST0 and LST1) contain a single binary table extension, three columns wide, which lists the X and Y coordinates and arrival time for each photon event recorded by the detector during a given observation. Only events arriving during designated "good times" are included. The files are typically 10 Mbytes in length, but can be as large as 50 Mbytes. Most researchers will have no use for these files.

• Extracted spectral files (SPA1, SPB1, etc.) each contain one or two binary table extensions. The first is the extracted, flux-calibrated target spectrum. It is identified by the extension name 'SPECTRUM' and consists of twelve columns:

  WAVELENGTH      (float) wavelength in Angstroms
  FLUX_PHOT       (float) flux in photons/cm2/s/A
  FLUX_ERGS       (float) flux in ergs/cm2/s/A
  FRAC_ERR        (float) (sigma of flux) / flux
  X               (int)   pixel number
  SPEC            (int)   raw counts in spectrum
  BG0             (int)   raw counts in background window 0
  BG1             (int)   raw counts in background window 1
  BACKGROUND      (float) normalized background, smoothed by 13 pixels
  BG_ERR          (float) sigma of background
  DELTAW          (float) wavelength increment of this channel
  EFF_AREA        (float) effective area at this wavelength
  

• "Air glow" files were produced in the ORFEUS-1 mission to signify a few accidental pointings and exposures of a sky region near a target. In these cases the RA and Dec values pointed to were retained in the FITS headers. Prior to coaddition, "air glow" files should be culled out of data subsets containing multiple exposures of a target with nearly the same coordinates.
  The ORFEUS-II data set includes spectra of deliberate airglow exposures. [Also look for other warnings (e.g., "Warning: source variable" or "Near edge of aperture") in the primary file headers.]

• Second-order (EUV) spectra, referred to above, are written as a second binary-table extension (EXTNAME = '2NDORDER') to the corresponding SPC1 and SPD1 files. This table has four columns:

  WAVELENGTH      (float) wavelength in Angstroms
  2ND_COUNTS      (float) estimated second-order spectrum in RAW COUNTS
  2ND_SIGMA       (float) sigma of second-order counts
  2ND_AEFF        (float) second-order effective area
  

  Note that the second-order spectrum is written in raw counts, rather than in flux units. The user may flux calibrate the second-order spectrum using the prescription found in the first-order table header and subtract it from the flux-calibrated first-order spectrum.

The files were written, and can be read, using the FITSIO software library, available at  http://heasarc.gsfc.nasa.gov/docs/software/fitsio/fitsio.html  .

These FITS files may be read into IDL using the FITS binary table i/o routines of the IDL Astronomy User's Library, available at http://idlastro.gsfc.nasa.gov/homepage.html  , or the IDL FITS reader (ifitsrd) available from the IUEDAC.

One may consult the quick IDL-tutorial or IRAF-tutorial for the means to read the FITS BEFS data.

Most of the above information was provided by Dr. Van Dixon of the Space Sciences Laboratory, The University of California, Berkeley.