New visions for space telescopes

NASA stumbled into a rare bit of good luck recently when the National Reconnaissance Office did some housecleaning. NRO decided that a pair of space-qualified 2.4 m telescopes dating from the late 1990s were no longer suitable for their original mission in spy satellites. So NRO offered the surplus optics to one of its poorer relations, NASA, for use in new space-based instruments. 

Unexpected hand-me-downs can bring opportunity, like the the piles of Scientific American and Sky & Telescope that came with a house my family rented when I was in high school. Astronomers and NASA scientists are pondering what to do with the windfall. The f/8 Cassegrain telescopes lack instruments, electronics, or spacecraft, but NRO long ago paid for the optics, saving NASA serious money. The Study on Applications for Large Space Optics workshop held February 5 and 6, 2013, in Huntsville, AL heard and discussed 34 proposals for building new instruments around the mirrors. They will be narrowed to six proposals and submitted to NASA management in May.

The range of ideas is impressive. Adaptive optics can do wonders on the ground, but ultraviolet astronomy must remain above the atmosphere, so three proposals call for studying the ultraviolet sky. Other common themes are spectroscopy, planetary science inside the solar system, and attempts to image challenging targets including extrasolar planets.

Bare-bones surplus telescopes inherited by NASA (Government work not subject to copyright)

Some proposals are intriguing. Alfred McEwen of the University of Arizona (Tucson, AZ) envisions the Mars Orbiting Space Telescope, and  Zachary Bailey of the Jet Propulsion Laboratory (Pasadena, CA) proposes "high-resolution surface science at Mars."  Rebecca Farr of the NASA Marshall Space Flight Center (Huntsville, AL) proposes using both mirrors as a deep-space binocular telescope stationed at the Lunar L2 Lagrange point.

Not everything is exactly a telescope. Abhijit Biswas of JPL wants to use a mirror as an optical communications node in space. J. H. Clemmons of the Aerospace Corp. (El Segundo, CA) wants to use one in a lidar to explore the Earth's thermosphere.  Richard Eastes of the University of Central Florida (Orlando, FL) has a plan for "Atmospheric TeleConnections on Earth."

There are plenty more listed on the program, and NASA will be recording the proceedings for later viewing. The ideas are not fully formed, of course, and some seem to duplicate others. But there are enough bright ideas to make one hope that NRO can find more goodies sitting in storage for its needy relatives.

Source:  http://science.nasa.gov/salso/telescope-characteristics-and-capabilities/

Broader view for adaptive optics

Adaptive optics has become standard on large ground-based telescopes because it offers far sharper images than otherwise obtainable. However, standard adaptive optics can compensate atmospheric turbulence only over small areas, so they don't let ground-based telescopes match the celestial panoramas imaged by the Hubble Space Telescope. Now a new generation of adaptive optics has demonstrated high-resolution imaging over a larger field of view with the Gemini South telescope in Chile.

Proposed more than a decade ago by François Rigaut, now at Australian National University (Canberra, Australia), the Gemini Multi-conjugate adaptive optics System (GEMS) uses five laser guide stars and three deformable mirror to measure atmospheric distortion and compensate for its affects. Sampling at 500 to 1000 Hz, GEMS can compensate for turbulence over an area of sky 16 times larger than previously possible.

The picture below tells the story, alternating images of the "Orion Bullets" region in the Orion Nebula taken with GEMS in December 28, 2012 and of the same region taken in 2007 with the previous-generation ALTAIR adaptive-optics system, which uses a single laser guide star. The larger field of view is 85 arcsec across. Without the adaptive optics, the telescope's resolution at the observation time was 0.8 to 1.1 arcsec. Adding GEMS improved resolution by a factor of ten to 0.084 to 0.103 arcsec.  The bright spots are "bullets" of gas ejected from the core of the nebula that are ripping through molecular hydrogen at speeds to 400 km/s, leaving behind wakes of hot hydrogen.

GEMS also benefits from processing enhancements, which use tomographic techniques to map air turbulence in three dimensions, and correct uniformly across the entire field of view. "This is huge when it's time for astronomers to reduce their data," says Adam Ginsburg, a graduate student at the University of Colorado (Boulder, CO), because observers often need to compare objects in the same field.

Field size has long been a crucial limitation on adaptive optics. The 85-arcsec width of the GEMS image still falls well short of the more than nearly 200-arcsec width of the Ultra Deep Field image taken by the Hubble Space Telescope, but it's an important step. With Hubble now well into its third decade in orbit, astronomers need new ways to study the depths of the sky from the ground.

Comparison of images of the same field in the Orion nebula recorded with GEMS and ALTAIR. The white "Orion Bullets" are fast-moving gas clouds leaving hot hydrogen in their wake. Their motion is fast enough to detect in the five years between the 2007 ALTAIR and the 2012 GEMS images.