GALEX Release 1 (GR1) Column descriptions for
-xg-gsp.fits
____________________________________________________________________
-xg-gsp.fits: -tab Jan'04 / Dec'04
Binary FITS table column descriptions for *-xg-gsp.fits files:
(Total number of fields: 58. Total number of bytes: 17280.)
ggoid : TTYPE1 (2J) : Global ID of spectral source
Joining these two integers yields a 64 bit integer uniquely
identifying this source including the tile number, band, product number,
reduction try (path), etc. The bits from most significant to least are:
3 bits: vsn : Version number (of the pipeline)
16 bits: tile : Tile or field or target number
2 bits: type : Observ. type (00=single,01=AIS(multi))
2 bits: ow : optics wheel(01=drct,10=grsm,11=opaq,00=undef)
4 bits: prod : product number(_visits,_best,etc.)
12 bits: img : visit/sub-visit number
{when type=01: 5 bit subvis + 7 bit visit}
3 bits: try : try number (processing number)
2 bits: band : Band (01=nuv,10=fuv,11=both,00=undefined)
20 bits: xid : Extraction ID
The extraction ID is the same as the extraction ID in the -xd-mcat.fits
file used for this spectral reduction.
ggoid_D : TTYPE2 (2J) : Global ID of direct image source
This is the GGOID for the corresponding source in the direct image
catalog (the -xd-mcat.fits file).
band : TTYPE3 (1J) : 1=nuv only, 2=fuv only, 3=both
From the GGOID.
id : TTYPE4 (1J) : Spectral source ID
Extraction ID from the GGOID.
alpha_j2000 : TTYPE5 (1D) : Right Ascension J2000 (deg)
delta_j2000 : TTYPE6 (1D) : Declination J2000 (deg)
Merged position from the merged catalog file (-xd-mcat.fits).
numpt : TTYPE7 (1J) : Number of points in spectra
The actual number of real data points in the spectral arrays
obj,objerr,etc. Note that this may be smaller than the number
of allocated values.
objwidth : TTYPE8 (1E) : 'arcsec ' : Object extraction width (arcsec)
The object extraction window width in arcseconds. This is the
region of the spatial profile centered on the expected center
of the source used to accumulate the flux at each point along
the spectrum. Note that this is the total size of extraction
aperture (i.e a diameter, not a radius).
bckwidth : TTYPE9 (1E) : Background extraction width (arcsec)
This is the width in the spatial direction used for the background
estimate. The background excludes the object window and also
regions immediately adjacent to the object (see objbcksep).
The background width is equal to the number of rows in the spectral
image strip, minus one row used to accumulate the spectral response
deviations (currently not used).
objbcksep : TTYPE10 (1E) : Object-Background separation (arcsec)
The separation between the object window and the background windows
which bracket each side of the object window.
bckcolrad : TTYPE11 (1J) : Radius for averaging background columns (pix)
Number of columns to average on either side of a given column
to compute the background level at each point along the spectrum.
A value of 0 means only use the column at each point to determine
the background. A value of 1 means include 1 column on either
side of the primary column to compute the background at each
spectrum point.
truthid : TTYPE12 (1J) : Truth catalog ID (matched)
Not used.
objtype : TTYPE13 (1J) : Object Type(0=galaxy,1=star)
A guess at the object type based on the direct image colors.
temperature : TTYPE14 (1E) : Estimated effective temper.(K)
From a blackbody fit to the spectrum.
redshift : TTYPE15 (1E) : Estimated redshift
Not used.
slope : TTYPE16 (1E) : Estimated spectral slope
A power-law fit to the spectrum, this is alpha in equation:
f_nu (proportional to) nu^alpha. Or, in our case, photons per
sec. per cm^2 per Ang. proportional to lambda^(-(1+alpha)).
qa : TTYPE17 (1E) : Quality Analysis value
Not currently used.
flag : TTYPE18 (1J) : Miscellaneous flag (not used)
Not currently used.
weight : TTYPE19 (2E) : Average weight (each band)
Average response in effective seconds for the unmasked pixels
in the spectral image strip. (NUV is the first element.)
This includes flat field response and neighbor masking.
background : TTYPE20 (2E) : Average background (each band)
Average background in photons per (effective) second per pixel
for the unmasked pixels within the background limits. The pixel
size in the image strips is nominally 1 square arcsecond.
(NUV is the first element (value), FUV is the second value.)
median_s2n : TTYPE21 (2E) : Median Signal/Noise (each band)
Median S/N per spectral pixel (wavelength point) in the flux
calibrated spectra. Each spectral pixel is nominally 3.5
Angstroms in size. For NUV the range used for this computation
is 1755 to 3000 Angstroms. For FUV the range is 1300 to 1745 Angstroms.
(NUV is the first element (value), FUV is the second value.)
centroid : TTYPE22 (2E) : Relative position(spatial)(each)
Centroid of the profile relative to the expected object position.
The expected object position is the given by the number of rows
in the image strip (-pri.fits or -prc.fits):
(object center) = ((number of rows in image strip) - 2) / 2 .
This assumes the center of a pixel in the first row is 0.0 .
Note that the last row of an image strip is for the spectral
response variation (currently 1.0). The centroid value is
relative to the expected object center, so a perfectly aligned
object would have a centroid value of 0.0.
(NUV is the first element (value), FUV is the second value.)
specfwhm : TTYPE23 (2E) : Estim. Spatial FWHM (each band)
This is an approximate estimate of the FWHM (in arcseconds) of
the spatial profile. Assuming a gaussian, it uses the formula:
FWHM = 0.93944 x (area of profile) / (peak pixel value)
(NUV is the first element (value), FUV is the second value.)
effexp : TTYPE24 (2E) : Effective exposure time in spectral image strip
For each band (NUV and FUV, respectively), the effective exposure time
in seconds for the entire spectral image strip (object and background
regions) for a given source. This value takes into account the varying
response of the flat field and corrections for dead spots. It does not
include masking of neighboring sources by spectral extraction programs.
effmsk : TTYPE25 (2E) : Effective masking fraction in spectral image
For each band, the effective masking fraction over the entire spectral
image strip. This includes masking of neighboring sources. No masking
would be 0., masking of 10% of the image strip would be 0.10. Multiplying
"effexp" and "1.0 - effmsk" gives you approximately the "weight" value.
nuv_numpro : TTYPE26 (1J) : Number of points in NUV profile
Number of points in the NUV profile array. This is one less than the
number of rows in the spectral image strip, since the last row in
the image strip is the spectral response variation.
fuv_numpro : TTYPE27 (1J) : Number of points in FUV profile
Number of points in the FUV profile array.
nuv_profile : TTYPE28 (198E) : NUV profile (spatial)
NUV profile array. Values are in photons/second/pixel (a pixel is
nominally 1 square arcsecond). The column range used to compute the
average profile is offsets +50 to +275 arcseconds for NUV (offset=0 is
the undeviated wavelength position on the image strip). This is
roughly the NUV first order portion of the spectrum. Masked pixels
are ignored. See the description of the "centroid" column for the
definition of the "object center row".
fuv_profile : TTYPE29 (198E) : FUV profile (spatial)
FUV profile array. Values are in photons/second/pixel (a pixel is
nominally 1 square arcsecond). The column range used to compute the
average profile is offsets +175 to +375 arcseconds for FUV. This is
roughly the FUV second order portion of the spectrum. Masked pixels
are ignored.
nuv : TTYPE30 (1E) : Direct image flux rate (NUV)
NUV flux in photons per (effective) second from direct image source
extraction. This is the "nuv_flux" column in the bandmerged direct
image source catalog (-xd-mcat.fits), but converted to photons per
second rather than micro Janskys.
nuve : TTYPE31 (1E) : Error in direct image flux rate
One sigma error in "nuv" column value.
fuv : TTYPE32 (1E) : Direct image flux rate (FUV)
FUV flux in photons per (effective) second from direct image source
extraction.
fuve : TTYPE33 (1E) : Error in direct image flux rate
One sigma error in "fuv" column value.
nuvs : TTYPE34 (1E) : Spectral flux scaled to direct
NUV flux derived from the extracted spectrum. This is computed by
summing the products of the flux in each pixel of the flux calibrated
spectrum by the direct image effective area and the dispersion. The
wavelength range used in the NUV is 1800 to 2800 Angstroms. Since this
does not include the edges of the bandpass, a correction factor (guess) of
1.0254 is applied to the sum. Some discrepancy will remain between the
direct and grism results, but this value is useful for quality analysis.
nuvse : TTYPE35 (1E) : Error in nuvs
One sigma error in "nuvs" column value.
fuvs : TTYPE36 (1E) : Spectral flux scaled to direct
FUV flux derived from the extracted spectrum. This is computed by
summing the products of the flux in each pixel of the flux calibrated
spectrum by the direct image effective area and the dispersion. The
wavelength range used in the FUV is 1350 to 1700 Angstroms. Since this
does not include the edges of the bandpass, a correction factor (guess) of
1.1071 is applied to the sum. Some discrepancy will remain between the
direct and grism results, but this value is useful for quality analysis.
fuvse : TTYPE37 (1E) : Error in fuvs
One sigma error in "fuvs" column value.
nuv0s : TTYPE38 (1E) : NUV Zeroth order flux rate
NUV flux derived from only the 0th order spectrum. The wavelength range
is 2000 to 2300 Angstroms and no correction is applied. Although smaller
than the value "nuvs", each of the values of nuv0s,nuv1s,nuv2s should be
the same and can be used for quality analysis.
nuv0se : TTYPE39 (1E) : Error in NUV Zeroth order flux rate
One sigma error in "nuv0s" column value.
nuv1s : TTYPE40 (1E) : NUV First order flux rate
NUV flux derived from only the 1st order spectrum. The wavelength range
is 2000 to 2300 Angstroms and no correction is applied.
nuv1se : TTYPE41 (1E) : Error in NUV First order flux rate
One sigma error in "nuv1s" column value.
nuv2s : TTYPE42 (1E) : NUV Second order flux rate
NUV flux derived from only the 2nd order spectrum. The wavelength range
is 2000 to 2300 Angstroms and no correction is applied.
nuv2se : TTYPE43 (1E) : Error in NUV Second order flux rate
One sigma error in "nuv2s" column value.
fuv1s : TTYPE44 (1E) : FUV First order flux rate
FUV flux derived from only the 1st order spectrum. The wavelength range
is 1350 to 1600 Angstroms and no correction is applied. Although smaller
than the value "fuvs", each of the values of fuv1s,fuv2s,fuv3s should be
the same and can be used for quality analysis.
fuv1se : TTYPE45 (1E) : Error in FUV First order flux rate
One sigma error in "fuv1s" column value.
fuv2s : TTYPE46 (1E) : FUV Second order flux rate
FUV flux derived from only the 2nd order spectrum. The wavelength range
is 1350 to 1600 Angstroms and no correction is applied.
fuv2se : TTYPE47 (1E) : Error in FUV Second order flux rate
One sigma error in "fuv2s" column value.
fuv3s : TTYPE48 (1E) : FUV Third order flux rate
FUV flux derived from only the 3rd order spectrum. The wavelength range
is 1350 to 1600 Angstroms and no correction is applied.
fuv3se : TTYPE49 (1E) : Error in FUV Third order flux rate
One sigma error in "fuv3s" column value.
zero : TTYPE50 (1E) : Wavelength of first pixel (Ang.)
Zero point for the wavelength scale in Angstroms. The true wavelength is
given by: wavelength = zero + ((pixel#) * disp), where the first pixel is
pixel# 0.
disp : TTYPE51 (1E) : Angstroms per pixel
Dispersion for the wavelength scale in Angstroms per pixel. The true
wavelength is given by: wavelength = zero + ((pixel#) * disp), where the
first pixel is pixel# 0.
obj : TTYPE52 (552E): Flux (pho/sec/cm^2/Ang) (standard)
Spectrum array in photons per second per cm^2 per Angstrom. This is the
standard reduction which means a simple sum of the pixels in the object
region (given by "objwidth", see -gsax.fits file) after dividing by the
response image and subtracting an average background level at each
wavelength point (or column). Only NUV 1st order and FUV 2nd order are
used to compute this spectrum.
objerr : TTYPE53 (552E): Error on obj
One sigma error for obj[] array.
opx : TTYPE54 (552E): Flux (pho/sec/cm^2/Ang) (optimal)
Intended for optimal extraction spectrum, currently not used.
opxerr : TTYPE55 (552E): Error on opx
Not used.
objs : TTYPE56 (552E): Flux (pho/sec/cm^2/Ang) (FUV 3rd + NUV 2nd)
Secondary spectrum-- this is similar to "obj" but only the NUV 2nd order
and FUV 3rd order are used to extract the spectrum (instead of only
NUV 1st and FUV 2nd). This is expected to have a much lower S/N but
will have a higher spectral resolution than "obj".
objserr : TTYPE57 (552E): Error on objs
One sigma error for objs[] array.
objmdn : TTYPE58 (552E): Flux spectrum using median image strip.
Spectrum extracted from the median image strip files (-prm.fits).
Only used for co-added or visit-combined data. This array is not
used for individual visits.