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:
380 - 510 Å
390 - 520 Å
514 - 687 Å
500 - 670 Å
700 - 900 Å
680 - 910 Å
865 - 1175 Å
910 - 1220 Å
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
All BEFS data files are provided in FITS format, with spectra stored as
binary table extensions, and are accessible on the
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
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
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
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
[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