GR5

The GALEX GR5 data release was delivered to MAST is early 2009.  It contains 870 new tiles of sky coverage from the primary mission surveys.  In addition, 266 GR4 tiles were delivered with extended exposure times, comprised of additonal visits and replacement coadds.  See the GR5 Tile List for more details.

GR4

The GALEX GR4 data release was delivered to MAST is early 2008.  It contains 25,000 square degrees of sky coverage from the primary mission surveys and features a number of calibration improvements.  The most notable changes between GR3 and GR4 are as follows:

• A drift in the calibration of both imaging bands has been corrected.  GALEX photometry has been drifting dimmer by 1.5% (0.25%) per year NUV (FUV).

• New flat fields have been implemented.  A 5% systematic error in the FUV photometry has been corrected, making the FUV GR4 release brighter.  Changes to NUV are primarily in the shape of the flat field at the 2% level, based on unsaturated field stars.  See the Photometry Section for more details. 

• New code has been implemented that tracks and removes reflections from bright objects at the edge of the NUV field of view.

• A new mask has been implemented that removes unusable data at the extreme edge of the instrument field of view.

• The spectroscopy calibration has been updated and addresses some issues particularly with the FUV band.  See the Spectroscopy Section for more details.

• Several changes affect the band-merged and spectral catalog data.  See the Catalog Section for more details.

Readers are encouraged to view the recent GALEX calibration and data paper:

Morrissey et al, 2007, “The Calibration and Data Products of the Galaxy Evolution Explorer,” ApJS, 173(2), 682.

GR4 vs GR2/3 Photometry

We have matched sources in fields processed under both the GR2+GR3 and the GR4 pipelines. Histograms of magnitude differences for sources brighter than 20th magnitude are shown for the FUV (left) and NUV (right) in the sense of GR2/3 minus GR4. The magnitude differences should reflect the differences in the flat fields. In the NUV, the mode of the distribution is +0.03 mag. While the standard deviation is 0.028 mag, there is a significant skew towards brighter magnitudes (negative differences) in the GR4 data. In the FUV, there is a slightly larger average offset of  +0.04 mag but the spread is considerably larger with a standard deviation of 0.062 mag. In both bands, the magnitude differences are correlated with position in each field since the magnitude differences are mainly due to differences in the flat field. In the NUV band in particular, the tail to negative magnitude differences (i.e. brighter fluxes in GR4) are primarily sources in the outer portion of the field.

See the Photometry Comparison (pdf) for more details.

GR4 Spectroscopy

The GALEX spectral calibration was updated using a fainter standard star HZ21 to avoid detector saturation issues that have affected the GR3 and earlier releases. The new spectral calibration was cross-checked against the imaging-mode photometric calibration using GALEX grism observations of the imaging-mode standard LDS749B and found to agree to within the errors.  The largest change is in the FUV, having an average Effective Area (EA) that is 79% of the EA from GR3 with excursions to as low as 57%.  In the NUV, the change is much less significant: the GR4 EA curve averages 99% of the GR3 curve with excursions to as low as 86% and as high as 106%.

See the Effective Area Plots (pdf) for more details.

GR4 Catalog Changes

Bandmerged Catalog Changes for GR4:

For GR4, 8 new columns were added to the bandmerged source catalog (-mcat.fits files).  These new columns are related to determining the source density and reliability of merged NUV and FUV detections. In GR4, the threshold for matching detections is now based on the Chi-squared probability.  The threshold (10^-5 given by the 'PROBLIM' header card) allows some matches beyond the previous 3 arcsecond separation limit used in GR3.  In GR4, any match within 3 arcseconds is allowed and matches beyond 3 arcseconds are allowed if their probability value is greater than 0.00001 .  So all sources which would have been matched in GR3, will again be matched in GR4, but there may be some additional matches which have larger positional errors and fall within the new Chi-squared probability limit.  In GR5, we plan to change to 'reliability' limit which takes into account source density of a given field. The new 8 columns include:  n/fuv_s2n, match_density, n/fuv_unmatched_density, reliability (not computed yet), adj_sep, and adj_fuv_det_y.

Spectral Catalog Changes for GR4:

For GR4, there are 22 new columns  in the merged spectral catalog file (-xg-gsp.fits files).  These include detector positions and Q values of each source at the undeviated wavelength position and the middle of the primary order (NUV 1st and FUV 2nd).  The new 22 columns include: 'n/fuv_nc, n/fuv_nr, n/fuv_mag, n/fuv_detx/y,n/fuv_Q, n/fuv_mid_detx/y, n/fuv_mid_Q, n/fuv_mid_ra, and n/fuv_mid_dec.