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The Satellite

The Galaxy Evolution Explorer (GALEX) is a NASA Small Explorer class mission. It consists of a 50 cm-diameter, modified Ritchey-Chrétien telescope with four operating modes: Far-UV (FUV) and Near-UV (NUV) imaging, and FUV and NUV spectroscopy. The telescope has a 3-m focal length and has Al-MgF2 coatings. The field of view is 1.2° circular. An optics wheel can position a CaF2 imaging window, a CaF2 transmission grism, or a fully opaque mask in the beam. Spectroscopic observations are obtained at multiple grism-sky dispersion angles, so as to mitigate spectral overlap effects. The FUV (1528Å: 1344-1786Å) and NUV (2271Å: 1771-2831Å) imagers can be operated one at a time or simultaneously using a dichroic beam splitter. The detector system encorporates sealed-tube microchannel-plate detectors. The FUV detector is preceded by a blue-edge filter that blocks the night-side airglow lines of OI1304, 1356, and Lyalpha. The NUV detector is preceded by a red blocking filter/fold mirror, which produces a sharper long-wavelength cutoff than the detector CsTe photocathode and thereby reduces both zodiacal light background and optical contamination. The peak quantum efficiency of the detector is 12% (FUV) and 8% (NUV). The detectors are linear up to a local (stellar) count-rate of 100 (FUV), 400 (NUV) cps, which corresponds to mAB ~ 14 - 15. The resolution of the system is typically 4.5/6.0 (FUV/NUV) arcseconds (FWHM), and varies by ~20% over the field of view. Further detail about the mission, in general, and the performance of the satellite, in specific, can be found in Martin et al. [6] and Morrissey et al. [7], respectively. The mission is nominally expected to last 38 months; GALEX was launched into a 700 km, 29° inclination, circular orbit on 28 April 2003.

Table 1. Selected Performance Parameters (Morrissey et al. 2005)

Item FUV Band NUV Band

Bandwidth: 1344 - 1786 Å 1771 - 2831 Å
Effective wavelength (lambdaeff): 1528 Å 2271 Å
Field of view: 1.28° 1.24°
Zero point (m0): 18.82 mag 20.08 mag
Image resolution (FWHM): 4.5 arcsec 6.0 arcsec
Spectral resolution (lambda / Delta lambda): 200 90
Detector background (typical):
      Total: 78 c-sec-1 193 c-sec-1
      Diffuse: 0.66 c-sec-1-cm-2 1.82 c-sec-1-cm-2
      Hotspots: 47 c-sec-1 107 c-sec-1
Sky background (typical): 2000 c-sec-1 20000 c-sec-1
Limiting magnitude (5 sigma):
      AIS (100 sec): 19.9 mag 20.8 mag
      MIS (1500 sec): 22.6 mag 22.7 mag
      DIS (30000 sec): 24.8 mag 24.4 mag

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