Spacewatch: Beginning in 1984, the 0.9-meter, Newtonian f/5 Steward
Observatory Spacewatch telescope has been used full time for surveying
comets and asteroids under the leadership of Tom Gehrels.
First installed on the University of Arizona campus
in 1923, this telescope was moved to Kitt Peak, Arizona in 1963.
In 1983, this instrument was donated to the Spacewatch team and
in 1984 it then became the first telescope
to detect and discover asteroids and comets with electronic detectors (CCDs, as opposed to photograph=
ic plates or film).
The initial 320 x 512 RCA CCD detector used from 1984 to 1988 was
replaced with a large format 2048x2048 CCD detector used during the
interval 1989-1992. This system had a field width of 38 arc minutes and
limiting magnitude of 20.5. The sensitivity of the CCD (quantum
efficiency) was doubled to 70% in 1992 when a thinned 2048 x 2048
CCD was installed and extended the limiting magnitude down to 21.0.
The 0.9-meter telescope is used about 23 nights per month to search for
near-Earth objects. By locking the right ascension axis in place and
allowing the star fields to drift through its field of view ("drift-scan") while the CCD
detector was constantly read out, this telescope scanned at a rate that
covers about 200 square degrees of sky each month down to magnitude
21. Each region of sky is scanned three times, about thirty minutes apart,
to examine which objects have moved relative to the background stars.
This system was the first to discover NEOs with CCDs, the first to discovera comet with a CCD, and
the first to discover an NEO with automated image processing software. In 2000, large minor planet (20000)
Varuna in the outer solar system was discovered with this system. Spacewatch has discovered
many of the smaller near-Earth objects that pass close to the Earth,
including a ten-meter sized asteroid (1994 XM1) that had a then record close Earth passage
(105,000 km on 1994 Dec 9). Discoveries of Potentially Hazardous Asteroids (PHAs) with the
0.9-m drift scan system total 45, NEOs total 274, and follow-up observations of NEOs many thousands.
In 2001, the Spacewatch group began observing with a newly built 1.8-meter
aperture telescope designed for follow-up of asteroids that get fainter after discovery.
In late 2002, a large-mosaic CCD camera (four 4608 x 2048 CCDs) was added to the 0.9 meter, and the optical
system has been modified to allow a wider field-of-view (2.9 square degrees). The 0.9 meter design now operates in
the "stare" mode rather than in the previous "drift-scan" mode, whereas the 1.8-meter telescope operates in the "drift-scan" mode.
From 2005 through 2008 the Spacewatch group gradually shifted their emphasis to follow-up observations
that are critical for securing accurate orbits. In addition to these activities, the Spacewatch team has
been involved with studies of the Centaur and Trans-Neptunian minor planet populations and the sizes of short period comet nuclei.
Robert S. McMillan: Principal Investigator
Robert Jedicke and Jeff Larsen: Collaborators
Joe Montani and Jim Scotti: Senior Research Specialists
Look here for additional information on the Spacewatch program: