GALEX Data Questions

GALEX Cycle 1 Data DATA Issues Header Body-ED

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GALEX Project at Caltech

Extended Photon Record Files – x.fits files

GALEX observations are made using photon-counting detectors. The direct photon data may be useful in analysis of time-variable objects. GALEX proposers may request “Photon Files” (“Photon Lists”, “Photon Record Files”,“Extended Photon Record Files”, GALEX x files”…). However, proposers should be aware that these files are used as input to the GALEX standard data pipeline, and are not generally supported as science analysis data products. GALEX does not provide any software or analysis tools for use with photon files; proposers should be prepared to develop their own tools for use with this data.

The '-x.fits' files are the extended photon record with photon times and calibrated (aspect corrected) positions. The photon files include time-tagged lists of detector positions and pulse heights corresponding to unique photon events. This file is not generated or delivered unless specifically requested by a GI proposal. By default, photon files use 5 millisecond sampling. Users may request 20 microsecond sampling if required by the science. These requests should be explained in the Science Justification.

It is important for people using the file to know that certain photons should be ignored (see the 3rd bit of the 'flags' column).

FILE FORMAT

The extended photon file is a standard binary FITS file with (as is usual) the binary table in the second header data unit (HDU). The first HDU contains only informatory header cards (see below).

TABLE COLUMN DESCRIPTIONS
COLUMN	DESCRIPTIONS
t	Time of the event. IEEE 8-byte float. Seconds since 0UTC 1980-01-06 ("GPS time"). Convert to UNIX time (seconds since 0UTC 1970-01-01): t(unix) = t + 315964800 Standard UNIX/POSIX library tools can be used to manipulate UNIX times. E.g. perl -le 'print scalar gmtime(315964800)' Sun Jan 6 00:00:00 1980
xdet ydet	Partially corrected detector coordinates. Scaled, signed, 2-byte integers. To get microns from the fiducial detector center: x(um) = x(FITS)/32767 * (36001.25)/2 / .068754932 = x(FITS) 0.998716 y(um) = same Note that the NUV and FUV detectors are oriented differently, so photons from the same object will not have the same detector coordinates in the two bands.
Val	Obscure quantity related to decoding raw6 files. Unsigned 1-byte integer.
Q	Event pulse height. Unsigned 1-byte integer.
Flags	Bitwise flags. See above for detailed description. Unsigned 2-byte integer. Least significant bit = 0: 0 - N/A (don't assume it's zero!) 1 - N/A (don't assume it's zero!) 2 - Skip 3 - 0 4 - 0 5 - Stim 6 - Masked 7 - BadAsp 8 - Range 9 - Badwalk 10 - N/A (don't assume it's zero!) 11 - N/A (don't assume it's zero!) 12 - N/A (don't assume it's zero!) 13 - N/A (don't assume it's zero!) 14 - N/A (don't assume it's zero!) 15 - N/A (don't assume it's zero!)
X,Y,Z	Equatorial position in rectangular coordinates. J2000. Scaled, signed 4-byte integer. X = X(FITS)/2147483647 Y,Z = same Dec = asin(Z) RA = atan2(Y,X)
thetax thetay	Fully-corrected spacecraft coordinates (axes not aligned with detector coordinates). Angle from detector fiducial axis. Scaled, signed 2-byte integers. thetax(asecs) = thetax(FITS)/10. thetay(asecs) = same

USEFUL HEADER CARDS
EXPTIME	Exposure time in seconds for the whole observation period. Note that due to data outages and periods of poor aspect correction, this is often not the same as the interval between the first and last photon.
AVEFDED	Average fractional dead time (see above). (no longer used?)
RA_CENT DEC_CENT	Map center. Not the same as the FOV center. The FOV center can be obtained from the AVASPRA and AVASPDEC header cards in the rr map header.
ROLL	Spacecraft roll during the observation. This is needed to derive the detector orientation, in case that ever matters.
ECLIPSE	The eclipse number. This is useful as a reference when talking to the GALEX team.
GRELEASE	Pipeline version tag. This is useful as a reference when talking to the GALEX team.

Responsible NASA Official: Susan G. Neff
Curator: J.D. Myers

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