GALEX Scan Mode Data

• Description of Data
• Considerations
• Data Access: HTML/FTP
• Data Access: gPhoton

Description of Data

In the post-NASA (or "CAUSE") phase of the GALEX mission, the spacecraft began observing frequently in a mode known as "scan mode". In scan mode, the spacecraft boresight would traverse and observe many degrees of sky, rapidly, in a long swath. This was in contrast to the traditional boresight dither, "petal pattern," and AIS modes, in which the spacecraft hovered at single sky pointings and was reduced to a non-observing (low voltage) state when orienting between pointings. The benefit of scan mode was that it allowed the team to more rapidly complete the All Sky Survey in the ultraviolet. But scan mode was performed at a time when the mission was operating on both minimal staff and budget, so while nominal data products were produced, the calibration pipeline was not optimized to handle these data, and no additional scientific support was available. MAST provides access to the RAW6 and visit-level data products, but we do not mix these data with other GALEX services (e.g., GALEXView), since the data are so different from the rest of the mission. Because these data cover new areas of the sky never explored by the rest of the GALEX mission, we feel these data are important to release to the community. We'd like to stress, however, that the quality of most of these data products is quite different from the other GALEX data, and in many cases somewhat worse. We expect users will need to perform additional analyses, and perhaps write their own data calibration or reduction scripts, to extract the science from these data files.

The nature and peculiarities of scan mode data are still being investigated. Not many people have looked closely at it, and no careful analysis of how it differs from data observed in the traditional modes has been performed. We summarize below some considerations, but encourage users to provide feedback and input to us as you explore this data. The best way is to send an email to us at archive@stsci.edu.

Considerations

Scan mode has a high "FAIL" rate, as set by both automated flagging in the calibration pipeline and the manual QA. Some FAIL-graded visits may still contain useful information, but, in general, care and caution should be used when performing any investigation with scan mode data, regardless of its PASS/FAIL status. Some users have found that visits with a PASS grade have serious issues, while others with a FAIL grade are usable. We advise that users examine each visit and make their own assessments.

The GALEX calibration pipeline assumed integrated images of a size and shape approximately the same as the detector field-of-view (circular, with a 1.25 degree diameter). To accomodate scan mode, which would produce single observations covering many degrees, the calibration pipeline was hacked to subdivide each scan into multiple observations akin to AIS subvisits. This is the reason that the data is formatted this way in the products (FITS files) released by the mission.

A common artifact in the count and intensity maps is a doubling or ghosting of individual sources or images. We believe that this is caused by jumps in the refined aspect solution, where the aspect correction stage of the calibration pipeline got confused. It might be possible to correct this by building a new and improved aspect refinement routine, but we have no plans to do so at this time.

Photometry with GALEX assumes that the source of interest remains in the same general region of the detector over an observation (i.e. that the source is on the same part of the flat), and users are (in general) advised not to trust flux measurements near the edge of the detector. Both of these conditions are violated by scan mode, where every source samples a cord across the entire detector, including the edges. The method that the GALEX calibration pipeline used for trimming regions of questionable response--masking out portions of the image with an integrated response below a threshhold fraction of the maximum response--would not have worked at all for scan mode data in the downtrack direction.

Coadds were not created for scan mode data.

Data Access: HTML/FTP

Users can download the RAW6 and/or pipeline-producted FITS files via HTML or FTP. For convenience, we have separate links and directories for scan mode data taken by various PI-led teams. MAST will release the scan-mode data from the various projects over time. Users may browse these data through the HTML link, or through anonymous ftp.

Within each directory, we also include "QA" reports from the original GALEX pipeline processing. As previously mentioned, users should use these notes and flags cautiously; we generally advise users to examine the data products of interest and judge for themselves whether the data are useful for their purposes or not.

Data Access: gPhoton

Another way to use the scan-mode data from GALEX is with the gPhoton tool. gPhoton is a database of time-tagged photon events collected by GALEX over its entire 10-year lifetime. In addition to the database, a python-based software package allows users to construct lightcurves, animated movies, and calibrated intensity images with specified coordinates, apertures, and time samplings. Data from any eclipse or visit can be used, since gPhoton does not artificially separate data based on survey type (AIS, MIS, etc.) or eclipse.

The scan mode data will be included in the database. Early tests have shown that gPhoton may handle the scan mode data better than the default GALEX pipeline in some respects. The gPhoton software was in an open beta period when the scan mode data was first released, and the database was still being populated. Check the page above for current status, to request beta access (if relevant), and instructions on how to download and use the gPhoton software tools.