We evaluate the exposure during nadir observations with JEM-EUSO, the Extreme Universe Space Obser-\ud vatory, on-board the Japanese Experiment Module of the International Space Station. Designed as a mis-\ud sion to explore the extreme energy Universe from space, JEM-EUSO will monitor the Earth’s nighttime\ud atmosphere to record the ultraviolet light from tracks generated by extensive air showers initiated by\ud ultra-high energy cosmic rays. In the present work, we discuss the particularities of space-based obser-\ud vation and we compute the annual exposure in nadir observation. The results are based on studies of the\ud expected trigger aperture and observational duty cycle, as well as, on the investigations of the effects of\ud clouds and different types of background light. We show that the annual exposure is about one order of\ud magnitude higher than those of the presently operating ground-based observatories

The Extreme Universe Space Observatory on board the Japanese Experiment\ud Module of the International Space Station, JEM-EUSO, is being designed\ud to search from space ultra-high energy cosmic rays. These are charged\ud particles with energies from a few 10(19) eV to beyond 10(20) eV, at the\ud very end of the known cosmic ray energy spectrum. JEM-EUSO will also\ud search for extreme energy neutrinos, photons, and exotic particles,\ud providing a unique opportunity to explore largely unknown phenomena in\ud our Universe. The mission, principally based on a wide field of view (60\ud degrees) near-UV telescope with a diameter of similar to 2.5 m, will\ud monitor the earth's atmosphere at night, pioneering the observation from\ud space of the ultraviolet tracks (290-430 nm) associated with giant\ud extensive air showers produced by ultra-high energy primaries\ud propagating in the earth's atmosphere. Observing from an orbital\ud altitude of similar to 400 km, the mission is expected to reach an\ud instantaneous geometrical aperture of A (g e o) a parts per thousand yen\ud 2 x 10(5) km(2) sr with an estimated duty cycle of similar to 20 \%.\ud Such a geometrical aperture allows unprecedented exposures,\ud significantly larger than can be obtained with ground-based experiments.\ud In this paper we briefly review the history of space-based search for\ud ultra-high energy cosmic rays. We then introduce the special issue of\ud Experimental Astronomy devoted to the various aspects of such a\ud challenging enterprise. We also summarise the activities of the on-going\ud JEM-EUSO program

The Galaxy Evolution Explorer (GALEX), a NASA Small Explorer Mission planned for launch in Fall 2002, will perform the first Space Ultraviolet sky survey. Five imaging surveys in each of two bands (1350-1750Å and 1750-2800Å) will range from an all-sky survey (limit m AB~2 0-21) to an ultra-deep survey of 4 square degrees (limit m AB~2 6). Three spectroscopic grism surveys (R=100-300) will be performed with various depths (m AB~2 0-25) and sky coverage (100 to 2 square degrees) over the 1350-2800Å band. The instrument includes a 50 cm modified Ritchey-Chrétien telescope, a dichroic beam splitter and astigmatism corrector, two large sealed tube microchannel plate detectors to simultaneously cover the two bands and the 1.2 degree field of view. A rotating wheel provides either imaging or grism spectroscopy with transmitting optics. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the UV-global star formation rate relationship in galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the red shift range zero to two. The GALEX mission will include an Associate Investigator program for additional observations and supporting data analysis. This will support a wide variety of investigations made possible by the first UV sky survey. SCIENCE OVERVIEWThe primary goals of GALEX are to address these questions:Local Universe Investigation: What are the UV properties of local galaxies, and how do rest UV properties, measured at high redshift by HST and NGST in their search for galaxy origins relate to star formation rate (SFR), extinction, metallicity, and burst history? Star Formation History Investigation:What is the star formation and metal production history of galaxies over the redshift range 0

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We report the design and performance analysis of an off-axis three-mirror telescope as the fore optics for a new hyperspectral sensor aboard a small unmanned aerial vehicle (UAV), for low-altitude coastal remote sensing. The sensor needs to have at least 4 cm of spatial resolution at an operating altitude of 500 m, 4° field of view (FOV), and a signal to noise ratio (SNR) of 100 at 660 nm. For these performance requirements, the sensor's optical design has an entrance pupil diameter of 70 mm and an F-ratio of 5.0. The fore optics is a three-mirror system, including aspheric primary and secondary mirrors. The optical performance is expected to reach 1/15λ in RMS wavefront error and 0.75 in MTF value at 660 nm. Considering the manufacturing and assembling phase, we determined the alignment compensation due to the tertiary mirror from the sensitivity, and derived the tilt-tolerance range to be 0.17 mrad. The off-axis three-mirror telescope, which has better performance than the fore optics of other hyperspectral sensors and is fitted for a small UAV, will contribute to ocean remote-sensing research.