[Paper Review] The Calibration and Data Products of the Galaxy Evolution Explorer

Patrick Morrissey, T. Conrow+21 more|arXiv (Cornell University)|Jun 6, 2007

Photocathodes and Microchannel Plates1 references5 citations

TL;DR

This paper presents the calibration and data products for the Galaxy Evolution Explorer (GALEX) mission's GR2 and GR3 data releases, detailing improved photometric precision (0.05 mAB in FUV, 0.03 mAB in NUV), astrometric accuracy of 0.5′′ RMS, and enhanced pipeline processing for photon-counting detectors. It identifies long-term photometric drifts (1% in FUV, 6% in NUV) and calibration uncertainties tied to non-standard FUV recovery data and saturated white dwarf calibrators, while confirming stable instrument performance and ongoing calibration refinement potential.

ABSTRACT

We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX). These releases have identical pipeline calibrations that are significantly improved over the GR1 data release. GALEX continues to survey the sky in the Far Ultraviolet (FUV, ~154 nm) and Near Ultraviolet (NUV, ~232 nm) bands, providing simultaneous imaging with a pair of photon counting, microchannel plate, delay line readout detectors. These 1.25 degree field-of-view detectors are well-suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 mAB in the FUV and NUV, respectively. We have detected a long term drift of order 1% FUV and 6% NUV over the mission. Astrometric precision is of order 0.5" RMS in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.

Motivation & Objective

• To provide a comprehensive overview of the calibration status and data products for the GALEX GR2 and GR3 data releases.
• To address photometric and astrometric performance issues affecting data quality and reliability in the FUV and NUV bands.
• To identify and quantify long-term calibration drifts and systematic errors in the FUV and NUV channels.
• To support astronomers in using GALEX data by detailing pipeline processing, data products, and known data peculiarities.
• To lay the foundation for future calibration improvements as the mission extends into an extended phase with ongoing instrument health and new guest observer opportunities.

Proposed method

• Utilization of a dithered observing mode with 1′ pattern to mitigate detector non-uniformities and MCP fatigue.
• Implementation of a complex data pipeline integrating detector calibration, aspect reconstruction, background subtraction, and source extraction.
• Application of wavelength-dependent response corrections and PSF-based optimal extraction for spectroscopic data.
• Use of a single, wavelength-dependent calibration per band despite observed spatial and grism-angle variations.
• Employment of a white dwarf calibrator for NUV zero point, with ongoing efforts to validate it using dimmer CALSPEC standards.
• Use of a pulser system to detect and correct for detector 'breathing' effects impacting astrometry.

Experimental results

Research questions

• RQ1 What is the current photometric precision of the GALEX GR2 and GR3 data releases in the FUV and NUV bands?
• RQ2 How do long-term photometric drifts affect FUV and NUV flux measurements over the mission's first four years?
• RQ3 What are the primary sources of systematic error in the FUV and NUV photometric calibrations, and how do they impact data reliability?
• RQ4 To what extent does detector non-uniformity, saturation, and grism position angle affect spectroscopic calibration and extraction?
• RQ5 Can astrometric precision be further improved by modeling detector 'breathing' effects using in-flight pulser data?

Key findings

• The photometric precision of the GR2 and GR3 data releases is 0.05 mAB in the FUV and 0.03 mAB in the NUV, representing a significant improvement over the GR1 release.
• A long-term photometric drift of approximately 1% in FUV and 6% in NUV has been detected over the first four years of the mission, with sources appearing to grow fainter with time.
• The FUV calibration relies on non-standard mode data from the 2005 recovery, which appears 5% brighter than standard mode measurements for the same objects.
• The NUV zero point is defined by a white dwarf that is at least partially saturated, introducing a potential calibration uncertainty of up to 3% in the spectral order body and 10% near the edges.
• Astrometric precision has been improved to 0.5′′ RMS, with evidence that further improvements are possible by modeling detector 'breathing' using in-flight pulser data.
• Spectroscopic data products include primary, secondary, and optimal extraction arrays, with usable wavelength ranges of 1300–1820 Å (FUV, 2nd order) and 1820–3000 Å (NUV, 1st order), and average resolutions of ~200 (8 Å) and ~118 (20 Å), respectively.