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 Ly
.
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.
| Item | FUV Band | NUV Band |
| Bandwidth: | 1344 - 1786 Å | 1771 - 2831 Å |
Effective wavelength
( eff): |
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
( /
): |
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
 ): |
||
| 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 |