4.1 Introduction The first Earth-crossing asteroid, Apollo, was discovered photographically in 1932 at Heidelberg and then lost until 1973. In the following decades only a handful of additional ECAs were discovered, and many of these were temporarily lost also. Not until the 1970s were regular searches initiated, using wide-field Schmidt telescopes of modest aperture. Some of these photographic survey programs continue today with steadily increasing discovery rates. In the early 1980s these photographic approaches were supplemented by a new technique of electronic CCD scanning implemented at the University of Arizona, and by the late 1980s this more automated approach was also yielding many new discoveries. Even today, however, the total worldwide effort to search for NEOs amounts to fewer than a dozen full-time-equivalent workers! In this chapter we briefly review the history and current status of both the photographic and CCD searches.
4.2 Photographic Search Programs
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An alternative to photographic search programs was developed at the University of Arizona under the name "Spacewatch" by T. Gehrels in collaboration with R. MacMillan, D. Rabinovich, and J. Scotti. This system makes use of a CCD detector instead of photographic plates. It differs from the wide-field Schmidt searches in scanning smaller areas of sky but doing so to greater depth. In 1981, the Director of the University of Arizona Observatories made the Steward 0.9-m Newtonian reflector on Kitt Peak available, and initial funding for instrument development was obtained from NASA. By 1983 Spacewatch had a 320 x 512 pixel CCD in operation, which was too small for discovery of near-Earth asteroids on that telescope, but was exercised in order to get experience with CCD modes of operation. Later this was upgraded to a 1048x1048 pixel CCD.
The basic construction and operation of the CCD are ideal for scanning. It is referred to as the "scanning mode"; in older literature it is called Time Delay Integration (TDI). The scanning is done by exactly matching the rate of transfer of the charges, from row to row of the CCD chip, with the rate of scanning by the telescope on the sky. A basic advantage of scanning is the smooth continuous operation, reading the CCD out during observing, compared to stop-and-go resetting the telescope for each exposure and waiting for the CCD to be read out before the next exposure can be started. Another advantage of scanning is that the differences in pixel sensitivity are averaged out, and two-dimensional "flat fielding" calibration is therefore not needed.
As each line of the CCD image is clocked into the serial shift register, it is read out by the microcomputer and passed on to the workstation. There the data are displayed, searched for moving objects, and recorded on magnetic tape. As each moving object is discovered, from the three repeated scan regions of about 30 minute length, its image is copied to a separate "gallery" window for verification by the observer. Some five years of computer programming went into this system.
Currently this Spacewatch system is discovering approximately as many NEOs as the photographic surveys. As a consequence of its more sensitive detector, it also tends to discover more smaller objects, including three objects found in 1991 that are only about 10 m in diameter. Substantial increases in capability are proposed with a new telescope of larger aperture (1.8 m) to replace the current Spacewatch telescope in the same dome.
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The following Chapters of this Report describe a survey program based on a new generation of scanning telescopes. However, there is still excellent work to be done with current instruments during the transition to the new survey. The near-term potential of photographic techniques may be considered in the following context. With the provision of about $1 million capital costs and $1 million per year operating expenses it would be possible to boost the current worldwide photographic discovery rate from about 20 per year to 100 per year. Similarly, an upgrade of the Spacewatch CCD scanning system to 1.8-m aperture would more than double the output of this system, and still greater gains are possible utilizing advanced, large-format CCDs. This instrument can also be used as a test-bed for new NEO survey techniques such as use of CCD arrays, optimizing of scanning strategies, and refinement of automated search software.
By the time large search telescopes with CCD detectors become available later in this decade it would be possible to have a sample of at least 1000 NEO's with well determined orbits. From this sample, which should include about 10 percent of the larger bodies, we will gain a much better idea of the physical properties and dynamical distribution of the total population. Such information will be invaluable in optimizing the search strategy of the large new telescopes. In addition, the operation of the large CCD search facilities will require trained personnel and a complex organization to utilize them to the fullest extent, and expansion of current programs can provide the experienced staff that will be required if and when the full survey begins operation.
We assume here that wide-field photography will continue in a substantially productive manner for a number of years. CCD work is expected at the Spacewatch telescope on Kitt Peak in Arizona (with proposed upgrade to 1.8-m aperture) and with the French OCA Schmidt and the Palomar 0.46-m Schmidt, both of which are proposed for conversion to CCD operation.
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