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Appendix A: Data Product Detailed Description
The tables below list various files released to the public. For descriptions of more complete pipeline data products, please refer to pipeline_files.txt.
Direct Imaging Data Products
As an example, the FITS headers for the GR4 data, MISWZS03_18853_0284 (eclipse 23828 data & its single-visit coadd data), are linked to all FITS filenames in the first columns (NUV FITS files are used when filenames contain "[f/n]" option).
Eclipse/Visit Data Products
| Top Directory | |||||
| Filename | File Type | Detail | |||
| -scst.fits | FITS table | Spacecraft state file: Most relevant spacecraft and instrument housekeeping values (e.g. ra_acs, roll_acs, etc.) as a function of time in one second intervals. | |||
| -[f/n]d-cnt.fits | FITS image | Count map (J2000): photons per (1.5 arcsecond) pixel corrected for dither with attitude refinement. | |||
| -[f/n]d-exp.fits | FITS image | Exposure map (J2000): An image of the time (in seconds) spent observing within the GALEX field of view (no response correction). | |||
| -[f/n]d-movie.fits | FITS image cube | Time-slice count maps (J2000): Multiple images of the photons per pixel within selected segments of time in 16 second intervals. | |||
| _premap-[f/n]d-movie.fits | FITS image cube | Premap movie file, created before artifact varpix masking. This is the one that will have all the artifacts still in it and is thus most like the GR2 and GR3 movies. | |||
| -[f/n]d-objmask.fits | FITS image | Image of the masks of objects used in estimating the background; it is NOT a mask of all sources extracted by SExtractor. | |||
| -[f/n]d-int.fits | FITS image | Intensity map (J2000): Photons per pixel per second corrected for the relative response. Note that "-int.fits" is equal to "-cnt.fits" divided by "-rrhr.fits". | |||
| -[f/n]d-skybg.fits | FITS image | Sky background image (J2000): Photons per pixel per second estimate of the background. This background is subtracted from the intensity image which is then used for source detection. | |||
| -[f/n]d-rrhr.fits | FITS image | High resolution relative response (J2000): Effective exposure time per pixel upsampled from the -rr.fits image. | |||
| -[f/n]d-wt.fits | FITS image | Weight/threshold image (J2000): Used by SExtractor for thresholding sources. Pixels > 1 are above the detection threshold. This image is derived by dividing the background-subtracted intensity by a detection threshold image. | |||
| -[f/n]d-intbgsub.fits | FITS image | Background subtracted intensity map (J2000): Intensity map image with the background removed (used for source detections). Note that "-intbgsub.fits" is equal to "-int.fits" minus "-skybg.fits". | |||
| -aspraw.fits | FITS table | Raw spacecraft pointing data. Not used by the pipeline. | |||
| -asp.fits | FITS table | Satellite attitude solution: Raw spacecraft pointing data using gryo rate data. | |||
| -asprta.fits | FITS table | Refined attitude solution: Spacecraft pointing solution after refinement using bright stars in GALEX field of view. | |||
| -[f/n]d-cat.fits | FITS table | SExtractor catalogs for images: Table of sources extracted by GALEX reduction pipeline. The table contains positions, flux, magnitude, and major and minor axes. | |||
| -xd-mcat.fits (Detailed column descriptions: mcat_columns_long.txt) | FITS table | Merged source catalog: Band-merged table of extracted sources. Contains all sources contained in "[n/f]d-cat.fits" matched to the best candidate from the other band. Only sources with a S/N greater than 2 are matched. Sources without matches are also listed. For each source, the table contains a global identifier, merged position, calibrated fluxes and magnitudes, etc. All the data from the "[n/f]d-cat.fits" tables are contained in "-xd-mcat.fits" file, as well as fluxes from the "nd-fcat.fits" and "fd-ncat.fits" files. | |||
| -nd-fcat.fits | FITS table | NUV extractions using FUV positions: A SExtractor catalog of source fluxes and magnitudes from the NUV image, but with source positions taken from the FUV source catalog "fd-cat.fits". | |||
| -fd-ncat.fits | FITS table | FUV extractions using NUV positions: A SExtractor catalog of source fluxes and magnitudes from the FUV image, but with source positions taken from the NUV source catalog "nd-cat.fits". | |||
| -[f/n]d-flags.fits | FITS image | Artifact Flag image (J2000): Identifies predetermined regions which may be introduce systematic errors in data extraction. These artifact regions are caused primarily by reflections of known bright stars. | |||
| -[f/n]d-flag_tbl.fits | FITS table | Table of geometric (static) artifacts. | |||
| -[f/n]d-flagstar.fits | FITS table | Star catalog for flagging: List of known bright stars used to determine artifact regions. | |||
| -rtastar.fits | FITS table | Star catalog used for attitude refinement. | |||
| -nd-cat_mch_rtastar.fits | FITS table | This catalog includes all matches between -rtastar.fits catalog and -n-cat.fits files. | |||
| -[f/n]d-cat_mch_flagstar.fits | FITS table | The matches of our extractions to the bright stars used for artifact flagging. It is used only as a bright subset of stars with which to do plate solutions. | |||
| -[f/n]d-flag_varmask.fits | FITS table | Pixels masked by the varpix movie mask. Usually corresponds fairly well with the varpix flags. | |||
| -[f/n]d-flag_varpix.fits | FITS table | Varpix flag image. | |||
| -[f/n]d-movie_mask.fits | FITS image cube | Movie of varpix masks. | |||
| -[f/n]d-varpix.fits | FITS image | Varpix image. An image showing the relative variability of the pixels over time, scaled roughly as an SNR. | |||
| -[f/n]d-flag_hotmask.fits | FITS table | Image showing where hotspots have been masked. | |||
| _premap-[f/n]d-flag_hotmask.fits | FITS table | Same as above but generated during the premap step, whereas the one above is the official product generated during the map step. | |||
| -[f/n]d-flag_dichroic.ds9reg | ds9 Region file* | Dichroic reflection flag regions: Reflections of stars off of the dichroic optical element. | |||
| -[f/n]d-flag_rim.ds9reg | ds9 Region file | Rim flag regions: Identifies rim artifacts. | |||
| -[f/n]d-flag_brtedge.ds9reg | ds9 Region file | Same as flag_edge except for only bright edge reflections. | |||
| -nd-flag_edge.ds9reg | ds9 Region file | Edge flag regions: Identifies the edge of the detector. | |||
| -nd-flag_window.ds9reg | ds9 Region file | Window reflection regions: Reflections of stars off of the optical window. | |||
| -calinfo.txt | ASCII text file | Calibration parameter values including a directory listing of calibration directory files. | |||
| -[f/n]d-gauss_*.txt | ASCII text file | The gaussian template used by SExtractor for matched filtering. | |||
| -[f/n]d-objmask_out.txt | ASCII text file | Text output of the SExtractor run made to generate the object mask. | |||
| -[f/n]d-sexcols.txt | ASCII text file | List of columns in the SExtactor output. | |||
| -[f/n]d-sexcols_orig.txt | ASCII text file | Local copy of global SExtractor column list. | |||
| -[f/n]d-sexnnw.txt | ASCII text file | SExtractor input to the (uncalibrated and largely useless) object type identification neural net. | |||
| -[f/n]d-sexparams.txt | ASCII text file | List of parameter values used by SExtractor (source extractor). | |||
| -[f/n]d-sexparams_orig.txt | ASCII text file | Local copy of the global, initial SExtractor parameter list. | |||
| -status.txt | ASCII text file | Status of data. | |||
| -summary.txt | ASCII text file | Summary of observational and data values for this eclipse. | |||
| CmdLine.txt | ASCII text file | Command line parameter values. | |||
| Out.txt | ASCII text file | Complete standard output from all pipeline programs. | |||
| OrbPipe.log | ASCII text file | Significant output messages from individual visit pipeline. | |||
| qa Directory | |||||
| Filename | File Type | Detail | |||
| -asp_stats.ps | PostScript | Aspect solution plots. | |||
| -asp_stats.txt | ASCII text file | Aspect solution statistics. | |||
| -aspraw_stats.ps | PostScript | Aspect solution plots (with no rate corrections). | |||
| -aspraw_stats.txt | ASCII text file | Aspect solution statistics (with no rate corrections). | |||
| -[f/n]d-cat.ds9reg | ds9 Region file | Ellipses of sources listed in -[n or f]d-cat.fits. | |||
| -[f/n]d-cat_mch_flagstar_stats.ps | PostScript | Plots of match parameters between extractions and bright stars used for artifact flagging. | |||
| -[f/n]d-cat_mch_flagstar_stats.txt | ASCII text file | Statistics for match parameters between extractions and bright stars used for artifact flagging. | |||
| -nd-cat_mch_rtastar_stats.ps | PostScript | Statistics for bright stars used for aspect solution. | |||
| -nd-cat_mch_rtastar_stats.txt | ASCII text file | Statistics for bright stars used for aspect solution. | |||
| -[f/n]d-cat_stats.txt | ASCII text file | SExtractor catalog statistics. | |||
| -[f/n]d-flag_dichroic_eq.ds9reg | ds9 Region file | Equatorial (as opposed to pixel) DS9 regions for the dichroic flags areas. | |||
| -[f/n]d-flag_rim_eq.ds9reg | ds9 Region file | Regions effected by a rim artifact. | |||
| -[f/n]d-flagstar.ds9reg | ds9 Region file | Regions of bright stars used to determine artifact regions. | |||
| -[f/n]d-map_stats.txt | ASCII text file | Mapping (image creation) statistics. | |||
| -fd-ncat.ds9reg | ds9 Region file | Ellipses of sources listed in the -fd-ncat.fits file. | |||
| -fd-ncat_stats.txt | ASCII text file | Statistics derived from the -fd-ncat.fits file. | |||
| -nd-fcat.ds9reg | ds9 Region file | Ellipses of sources listed in the -nd-fcat.fits file. | |||
| -nd-fcat_stats.txt | ASCII text file | Statistics derived from the -nd-fcat.fits file. | |||
| -[f/n]d-psf_eq.fits | FITS image | Composite PSF images in images oriented with the equatorial. (We no longer produce the ones that were in detector orientation.) | |||
| -[f/n]d-psf_eq_stats.txt | ASCII text file | Statistics generated (FWHMs etc.) when making the PSF FITS file. | |||
| -nd-flag_brtedge_eq.ds9reg | ds9 Region file | Equatorial (as opposed to pixel) DS9 regions for the brtedge flags areas. | |||
| -nd-flag_edge_eq.ds9reg | ds9 Region file | Regions affected by an edge artifact. | |||
| -nd-flag_window_eq.ds9reg | ds9 Region file | Regions affected by a window reflection. | |||
| -rtastar.ds9reg | ds9 Region file | Ellipses of bright stars from the file -rtastar.fits used in the attitude solution (aspect correction). | |||
| -rtastar_used.ds9reg | ds9 Region file | Ellipses of bright stars used. | |||
| -sumstats.fits | FITS table | A collection of statistics from various pipeline programs. | |||
| -sumstats.txt | ASCII text file | A collection of statistics from various pipeline programs. | |||
| -xd-int_2color.jpg -xd-int_2color_large.jpg -xd-int_2color_medium.jpg -xd-int_2color_medium_annot.jpg -xd-int_2color_small.jpg -xd-int_2color_thumb.jpg | JPEG image file | Two color images of the whole field in various sizes with and without annotations of bright stars. NUV flux is yellow and FUV is blue. | |||
| -xd-qa_tabmcat.ps | PostScript | Plots of statistics derived from the -mcat.fits merged catalog file. | |||
| -xd-qa_tabmcat_stats.txt | ASCII text file | Statistics derived from the -mcat.fits merged catalog file. | |||
| _asprefine-* (_asprefine-nd-cat.fits) (_asprefine-nd-cat_mch_rtastar.fits) | various files | Products generated during the aspect static refinement (asprefine) step. | |||
| qa/manual Directory | |||||
| Filename | File Type | Detail | |||
| -qa.txt | ASCII text file | Same as the most recent version of the qafiles, "*-qa*.txt" files in qa/manual subdirectory. | |||
| -qa_v00.txt | ASCII text file | Auto generated QA data, includes PASS/FAIL and Coadd status. | |||
| -qa_v##.txt | ASCII text file | Manually generated QA data (## runs from 01 up to 99); highest number/version is the most recent qafile. | |||
| -fd-flag_blob_eq.ds9reg -nd-flag_asteroid_eq.ds9reg -nd-flag_transit_eq.ds9reg -[f/n]d-flag_hotspot_eq.ds9reg -nd-flag_variable_eq.ds9reg -[f/n]d-flag_miscel_eq.ds9reg | ds9 Region file | Manually identified regions/positions effected by various artifacts such as asteroid, blob, hotspot, transit, variable source, and etc. | |||
Coadd Data Products
| Top Directory | |||||
| Filename | File Type | Detail | |||
| -[f/n]d-cnt.fits | FITS image | Count map (J2000): photons per (1.5 arcsecond) pixel corrected for dither with attitude refinement. | |||
| -[f/n]d-rr.fits | FITS image | Relative response map (J2000): This is the relative sensitivity multiplied by the exposure time image (-exp.fits). | |||
| -[f/n]d-exp.fits | FITS image | Exposure map (J2000): An image of the time (in seconds) spent observing within the GALEX field of view (no response correction). | |||
| -[f/n]d-objmask.fits | FITS image | Image of the masks of objects used in estimating the background; it is NOT a mask of all sources extracted by SExtractor. | |||
| -[f/n]d-int.fits | FITS image | Intensity map (J2000): Photons per pixel per second corrected for the relative response. Note that "-int.fits" is equal to "-cnt.fits" divided by "-rrhr.fits". | |||
| -[f/n]d-skybg.fits | FITS image | Sky background image (J2000): Photons per pixel per second estimate of the background. This background is subtracted from the intensity image which is then used for source detection. | |||
| -[f/n]d-rrhr.fits | FITS image | High resolution relative response (J2000): Effective exposure time per pixel upsampled from the -rr.fits image. | |||
| -[f/n]d-wt.fits | FITS image | Weight/threshold image (J2000): Used by SExtractor for thresholding sources. Pixels > 1 are above the detection threshold. This image is derived by dividing the background-subtracted intensity by a detection threshold image. | |||
| -[f/n]d-intbgsub.fits | FITS image | Background subtracted intensity map (J2000): Intensity map image with the background removed (used for source detections). Note that "-intbgsub.fits" is equal to "-int.fits" minus "-skybg.fits". | |||
| -[f/n]d-cat.fits | FITS table | SExtractor catalogs for images: Table of sources extracted by GALEX reduction pipeline. The table contains positions, flux, magnitude, and major and minor axes. | |||
| -xd-mcat.fits (Detailed column descriptions: mcat_columns_long.txt) | FITS table | Merged source catalog: Band-merged table of extracted sources. Contains all sources contained in "[n/f]d-cat.fits" matched to the best candidate from the other band. Only sources with a S/N greater than 2 are matched. Sources without matches are also listed. For each source, the table contains a global identifier, merged position, calibrated fluxes and magnitudes, etc. All the data from the "[n/f]d-cat.fits" tables are contained in "-xd-mcat.fits" file, as well as fluxes from the "nd-fcat.fits" and "fd-ncat.fits" files. | |||
| -nd-fcat.fits | FITS table | NUV extractions using FUV positions: A SExtractor catalog of source fluxes and magnitudes from the NUV image, but with source positions taken from the FUV source catalog "fd-cat.fits". | |||
| -fd-ncat.fits | FITS table | FUV extractions using NUV positions: A SExtractor catalog of source fluxes and magnitudes from the FUV image, but with source positions taken from the NUV source catalog "nd-cat.fits". | |||
| -[f/n]d-flags.fits | FITS image | Artifact Flag image (J2000): Identifies predetermined regions which may be introduce systematic errors in data extraction. These artifact regions are caused primarily by reflections of known bright stars. | |||
| -[f/n]d-flag_tbl.fits | FITS table | Table of geometric (static) artifacts. | |||
| -[f/n]d-flagstar.fits | FITS table | Star catalog for flagging: List of known bright stars used to determine artifact regions. | |||
| -rtastar.fits | FITS table | Star catalog used for attitude refinement. | |||
| -nd-cat_mch_rtastar.fits | FITS table | This catalog includes all matches between -rtastar.fits catalog and -n-cat.fits files. | |||
| -[f/n]d-gauss_*.txt | ASCII text file | The gaussian template used by SExtractor for matched filtering. | |||
| -[f/n]d-objmask_out.txt | ASCII text file | Text output of the SExtractor run made to generate the object mask. | |||
| -[f/n]d-sexcols.txt | ASCII text file | List of columns in the SExtactor output. | |||
| -[f/n]d-sexcols_orig.txt | ASCII text file | Local copy of global SExtractor column list. | |||
| -[f/n]d-sexnnw.txt | ASCII text file | SExtractor input to the (uncalibrated and largely useless) object type identification neural net. | |||
| -[f/n]d-sexparams.txt | ASCII text file | List of parameter values used by SExtractor (source extractor). | |||
| -[f/n]d-sexparams_orig.txt | ASCII text file | Local copy of the global, initial SExtractor parameter list. | |||
| CmdLine.txt | ASCII text file | Command line parameter values. | |||
| Out.txt | ASCII text file | Complete standard output from all pipeline programs. | |||
| TargPipe.log | ASCII text file | Significant output messages from coadding pipeline. | |||
| qa Directory | |||||
| Filename | File Type | Detail | |||
| -[f/n]d-cat.ds9reg | ds9 Region file | Ellipses of sources listed in -[n or f]d-cat.fits. | |||
| -[f/n]d-cat_stats.txt | ASCII text file | SExtractor catalog statistics. | |||
| -fd-ncat.ds9reg | ds9 Region file | Ellipses of sources listed in the -fd-ncat.fits file. | |||
| -fd-ncat_stats.txt | ASCII text file | Statistics derived from the -fd-ncat.fits file. | |||
| -nd-fcat.ds9reg | ds9 Region file | Ellipses of sources listed in the -nd-fcat.fits file. | |||
| -nd-fcat_stats.txt | ASCII text file | Statistics derived from the -nd-fcat.fits file. | |||
| -[f/n]d-psf_eq.fits | FITS image | Composite PSF images in images oriented with the equatorial. (We no longer produce the ones that were in detector orientation.) | |||
| -[f/n]d-psf_eq_stats.txt | ASCII text file | Statistics generated (FWHMs etc.) when making the PSF FITS file. | |||
| -nd-cat_mch_rtastar_stats.ps | PostScript | Statistics for bright stars used for aspect solution. | |||
| -nd-cat_mch_rtastar_stats.txt | ASCII text file | Statistics for bright stars used for aspect solution. | |||
| -sumstats.fits | FITS table | A collection of statistics from various pipeline programs. | |||
| -sumstats.txt | ASCII text file | A collection of statistics from various pipeline programs. | |||
| -xd-int_2color.jpg -xd-int_2color_large.jpg -xd-int_2color_medium.jpg -xd-int_2color_medium_annot.jpg -xd-int_2color_small.jpg -xd-int_2color_thumb.jpg | JPEG image file | Two color images of the whole field in various sizes with and without annotations of bright stars. NUV flux is yellow and FUV is blue. | |||
| -xd-qa_tabmcat.ps | PostScript | Plots of statistics derived from the -mcat.fits merged catalog file. | |||
| -xd-qa_tabmcat_stats.txt | ASCII text file | Statistics derived from the -mcat.fits merged catalog file. | |||
| qa/manual Directory | |||||
| Filename | File Type | Detail | |||
| -qa.txt | ASCII text file | Same as the most recent version of the qafiles, "*-qa*.txt" files in qa/manual subdirectory. | |||
| -qa_v00.txt | ASCII text file | Auto generated QA data, includes PASS/FAIL and Coadd status. | |||
| -qa_v##.txt | ASCII text file | Manually generated QA data (## runs from 01 up to 99); highest number/version is the most recent qafile. | |||
| -fd-flag_blob_eq.ds9reg -nd-flag_asteroid_eq.ds9reg -nd-flag_transit_eq.ds9reg -[f/n]d-flag_hotspot_eq.ds9reg -nd-flag_variable_eq.ds9reg -[f/n]d-flag_miscel_eq.ds9reg | ds9 Region file | Manually identified regions/positions effected by various artifacts such as asteroid, blob, hotspot, transit, variable source, and etc. | |||
| -visit_qatool_report.txt | ASCII text file | Information on the visits data used in making the coadd. | |||
Spectroscopy Data Products
As an example, the FITS headers for NGPDWS_00 (eclipse 5856; visit 100 data) are linked to all FITS filenames in the first columns (NUV FITS files are used when filenames contain "[f/n]" option).
Eclipse/Visit Data Products
| Top Directory | |||||
| Filename | File Type | Detail | |||
| -scst.fits | FITS table | Spacecraft state file: Most relevant spacecraft and instrument housekeeping values (e.g. ra_acs, roll_acs, etc.) as a function of time in one second intervals. | |||
| -[f/n]g-cnt.fits | FITS image | Count map (J2000): photons per (1.5 arcsecond) pixel corrected for dither with attitude refinement. | |||
| -[f/n]g-rr.fits | FITS image | Relative response map (J2000): This is the relative sensitivity multiplied by the exposure time image (-exp.fits). | |||
| -[f/n]g-exp.fits | FITS image | Exposure map (J2000): An image of the time (in seconds) spent observing within the GALEX field of view (no response correction). | |||
| -[f/n]g-movie.fits | FITS image cube | Time-slice count maps (J2000): Multiple images of the photons per pixel within selected segments of time in 16 second intervals. | |||
| _premap-[f/n]g-movie.fits | FITS image cube | Premap movie file, created before artifact varpix masking. This is the one that will have all the artifacts still in it and is thus most like the GR2 and GR3 movies. | |||
| -[f/n]g-int.fits | FITS image | Intensity map (J2000): Photons per pixel per second corrected for the relative response. Note that "-int.fits" is equal to "-cnt.fits" divided by "-rrhr.fits". | |||
| -[f/n]g-skybg.fits | FITS image | Sky background image (J2000): Photons per pixel per second estimate of the background. This background is subtracted from the intensity image which is then used for source detection. | |||
| -[f/n]g-rrhr.fits | FITS image | High resolution relative response (J2000): Effective exposure time per pixel upsampled from the -rr.fits image. | |||
| -[f/n]g-wt.fits | FITS image | Weight/threshold image (J2000): Used by SExtractor for thresholding sources. Pixels > 1 are above the detection threshold. This image is derived by dividing the background-subtracted intensity by a detection threshold image. | |||
| -[f/n]g-intbgsub.fits | FITS image | Background subtracted intensity map (J2000): Intensity map image with the background removed (used for source detections). Note that "-intbgsub.fits" is equal to "-int.fits" minus "-skybg.fits". | |||
| -aspraw.fits | FITS table | Raw spacecraft pointing data. Not used by the pipeline. | |||
| -asp.fits | FITS table | Satellite attitude solution: Raw spacecraft pointing data using gryo rate data. | |||
| -asprta.fits | FITS table | Refined attitude solution: Spacecraft pointing solution after refinement using bright stars in GALEX field of view. | |||
| -rtastar.fits | FITS table | Star catalog used for attitude refinement. | |||
| -[f/n]g-pri.fits (-[f/n]g-pri_rtastar.fits) | |||||
| -[f/n]g-gsp.fits (-[f/n]g-gsp_rtastar.fits) | |||||
| -xg-gsp.fits (-xg-gsp_rtastar.fits) (Detailed column descriptions: gsp_columns_long.txt) | |||||
| -[f/n]g-xsp.fits (-[f/n]g-xsp_rtastar.fits) | |||||
| -[f/n]g-gsax.fits (-[f/n]g-gsax_rtastar.fits) | |||||
| -[f/n]g-flag_varpix.fits | FITS table | Varpix flag image. | |||
| -[f/n]d-varpix.fits | FITS image | Varpix image. An image showing the relative variability of the pixels over time, scaled roughly as an SNR. | |||
| -[f/n]g-flag_hotmask.fits | FITS table | Image showing where hotspots have been masked. | |||
| _premap-[f/n]g-flag_hotmask.fits | FITS table | Same as above but generated during the premap step, whereas the one above is the official product generated during the map step. | |||
| -calinfo.txt | ASCII text file | Calibration parameter values including a directory listing of calibration directory files. | |||
| -status.txt | ASCII text file | Status of data. | |||
| -summary.txt | ASCII text file | Summary of observational and data values for this eclipse. | |||
| CmdLine.txt | ASCII text file | Command line parameter values. | |||
| Out.txt | ASCII text file | Complete standard output from all pipeline programs. | |||
| OrbPipe.log | ASCII text file | Significant output messages from individual visit pipeline. | |||
Extended Photon Files (X-files)
Introduction
This is a brief description of the contents of the GALEX extended photon files -- often called x files -- with a few notes on their use.
The extended photon file contains a list of detector events (not just photons) which occurred during a given obervation. Some events are generated by the instrument's electronic stimulator (a calibration aid), others within the detector material itself (hotspots or background). Each event has an associated detection time and position on the detector (NUV or FUV). Times and detector positions for events which may be photons are then correlated with knowledge of the spacecraft's position vs. time and assigned equatorial coordinates in equinox J2000. In addition, flags are added which indicate which events should be considered actual photon detections, and which should not.
After construction, the extended photon list is used to make the count map, which is a straight-forward inertial binning of the photon data. All other data products flow from this image and the associated relative response map which is constructed in parallel.
Caveat Emptor
Time sliced photon data is a relatively poorly explored frontier and the mission design is not optimized for such analysis. Be appropriately skeptical of your results.
Creation
Here is how x files are made.
Raw material
- Raw instrument telemetry provides compressed, encoded event times and detector positions in a format called raw4 or raw5 depending on the compression level commanded. This data is expanded slightly to make a FITS format file called a raw6 file.
- Spacecraft telemetry provides an estimate of instrument orientation vs. time. We call this the spacecraft's aspect and it's recorded in a record-oriented binary file called an aspect file. For direct imaging data, this aspect is improved using guide star positions in the photon data itself, producing an RTA aspect file (RTA = Rectification, Transformation, Aspect correction). Grism data does not go through an aspect correction step, so the raw spacecraft aspect estimate is used.
- Extensive ground calibration, subsequently improved by inflight measurements, provides a number of static calibration files used to remove the detector signature from photon positions. See below.
The Process
- Raw6 files are decoded into a time and raw detector position.
- A fiducial detector center and plate scale is applied resulting in centered, uncalibrated detector positions in microns.
- The wiggle correction is applied.
- The walk correction is applied. The partially corrected position that results is recorded as scaled integers in the (xdet,ydet) columns in the x file.
- The linearity correction is applied. This fully corrected position is recorded in the x file in spacecraft coordinates (not detector coordinates) in the (thetax,thetay) columns as scaled arc-seconds.
- Knowledge of the spacecraft's orientation (boresite RA,Dec and roll) vs. time from the aspect file, and the photon's spacecraft position and time tag are used to deproject from spacecraft coordinates to sky coordinates. The results are stored in the (X,Y,Z) columns as scaled integers.
Event Flags
The following flags may be applied to each event:
- skip - This flag will be set for all non-photon events (or, rather, those recognized as such). This flag will be accompanied by another flag giving more info.
- Flags which indicate a non-photon event:
- stim - The event has been positionally associated with the location of one of the four electronic stimulators.
- range - The event was out of range of the calibration tables.
- Flags for events which may or may not be photons:
- masked - The event occurred at a location which is masked in software. Regions of high intrinsic detector emission are declared unusable as part of the calibration process and events from those areas are masked out with this flag.
- badasp - The event occurred at a time without a good aspect correction.
- badwalk - The event has a suspect walk correction. Note that currently this flag DOES NOT lead to the skip flag being set. Photons in this category are deemed acceptable but flagged for information.
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
- 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 * (3600*1.25)/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
Keywords from the main header in the first data unit (HDU 0).
- 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 EXPTIME).
- RA_CENT,DEC_CENT - Map center.
Not the same as the FOV center. The FOV center
can be got 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.
Reading Photon Files
Fairly efficient FITS table input is possible using the CFITSIO library (heasarc.gsfc.nasa.gov/docs/software/fitsio/fitsio.html <http://heasarc.gsfc.nasa.gov/docs/software/fitsio/fitsio.html>) as well as standard IDL library tools such as MRDFITS (idlastro.gsfc.nasa.gov/mrdfits.html <http://idlastro.gsfc.nasa.gov/mrdfits.html>) or FTAB_EXT (using FITS_READ). A simple IDL program using FTAB_EXT is supplied below.
To achieve decent throughput, read the photons in chunks of rows big enough that processing overhead is small compared to I/O time, but all the data can fit in internal buffers. Experimentation may be required to find this value. The sample program below uses a chunk size of 50,000 rows by default. The CFITSIO library supplies a routine to provide the optimal row count.
Photometry of Time-Sliced Photon Data
To get the magnitude of an object for which you have extracted photons from the photon list, you need to know the exposure time, and the detector response. The rr map (relative response map) contains an image in RA and Dec of the relative detector response times the exposure time for whole exposure for that observation. If no great accuracy is required, you can assume the response for all time intervals is approximately the same. If this is adequate, you can derive the average response to use from the pixel value read from the rr map at the object's location:
response = rr/exptime * (1 - fdead)
where rr is the rr map value at the appropriate location, exptime is the value from the EXPTIME keyword in the rr map header, and fdead is a dead time correction. This correction can be read from the AVEFDED rr map header card value.
Then the flux for the object (in whatever aperture you've chosen) will be
flux(photons/sec) = N(photons)/t(exposure)/response
Computing t(exposure) (the interval exposure time) is not as trivial as one might hope. Because of data outages and periods of time when all photons are flagged to be skipped due to poor aspect correction, any given time interval may contain periods of time during which the instrument was effectively not exposing (even though it was). The simplest way to account for this is to add up all the (e.g.) 0.1 second time periods which possess a photon which does not have the badasp or stim flag set. One must consider *all* photons, not just those positionally matching a given object. Thus one must compute the exposure time *before* selecting the desired photons. Here's some pseudo-C code (untested) to illustrate the technique:
#define TICKLEN 0.1
#define MAXTICKS ((int)(2000/TICKLEN))
#define MAXPHOTS (2000*100000)
#define PHSTIM 32u
#define PHBADASP 128u
... blah blah blah ...
int ticks[MAXTICKS];
double t0;
double tph[MAXPHOTS];
unsigned int flags[MAXPHOTS];
int i, nph, tick;
... blah blah blah ...
for(i=0; i<MAXTICKS; ++i) ticks[i] = 0;
... populate tph and flags arrays with nph photons of data ...
t0 = tph[0];
for(i=0; i<nph; ++i) {
if((flags[i]&(PHSTIM|PHBADASP)) != 0) continue;
tick = (tph[i]-t0)/TICKLEN;
ticks[tick] = 1;
... select desired photons ...
... do something with them ...
}
for(i=0; i<MAXTICKS; ++i) exposure += ticks[i]*TICKLEN;
(That's not the real way one will loop over the photon list. There are usually too many photons to fill up arrays with all of an observation's data. The code above is just illustrative.)
Photometry Considerations
There are three main problems with this photometric technique:
- For intervals which are short enough that the source hasn't sampled much detector area, the response derived from the low-resolution flat will be inaccurate. Intervals above 100 seconds or so ought to be more or less OK. Shorter than 50 seconds and you could definitely be affected by small response changes lost in the average value derived from the rr map. To watch out for this, look around your object's position and see if the rr map values are smooth. If they are, you're probably more or less OK. For very short time intervals, you're in uncharted territory. There is no good knowledge of detector response at the spatial scales sampled in intervals less than several seconds.
- Masking. Big problem. If a high-detector-background area (hotspot) dithers over your object in a time interval, the mask dramatically alters the response around your object. But since the rr map is time averaged over the whole exposure, a shorter interval will not have masking properly accounted for. The best you can easily do is watch for masked photons matching your source and if there's too high a fraction, discount that interval. The rr map smoothness test mentioned for problem 1 above will also be useful here.
- Gain sag. Actually, this is a problem for all data, not just time sliced photon data. Gain sag is the loss of responsivity due to depletion of electrons in the detector around a bright object. Sources with fluxes above 100 photons/sec in FUV and 500 photons/sec in NUV will show increasingly non-linear response. It can be so bad, in fact, that extremely bright objects might have a comparatively modest measured flux since the vast majority of photons were never detected. To watch out for this, examine not only the object's measured flux, but also examine the extended emission. If there's lots of flux in the extended PSF, watch out!
