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.