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One of the important demonstrations provided by the Spacewatch Telescope team is that image-recognition algorithms such as their Moving Object Detection Program (MODP) are successful in making near-real-time discoveries of moving objects (asteroids and comets). False detections are almost eliminated by comparing images from three scans obtained one after the other. At present, the Spacewatch system makes detections by virtue of the signal present in individual pixels. With the incorporation of higher-speed computers, near-real time comparison of individual pixels to measure actual image profiles would lead to a great reduction in the most frequent sources of noise, cosmic ray hits and spurious electrical noise events.
In light of the successful performance of Spacewatch, we have rejected a photographic survey. Even though sufficiently deep exposures and rapid areal coverage could be attained to fulfill the survey requirements using a small number of meter-class Schmidt telescopes (similar to the Oschin and U.K. Schmidts), there is no feasible way, either by visual inspection or digitization of the films, to identify and measure the images in step with the search. A photographic survey would fail for lack of adequate data reduction and follow-up. Future developments in electronics and data processing will further enhance the advantages of digital searches over the older analog methods using photography.
In the coming decade, we envisage a trend toward smaller and more numerous CCD pixels covering the same maximum chip area as at present. No great increase in spectral sensitivity can be expected. At the telescope, the pixel scale must be matched to the image scale (the apparent angular size of a stellar image) in good or adequate atmospheric (seeing) conditions. In what follows, we assume a pixel scale of 1 arcsec/pixel (25-micrometer/arcsec, or 40 arcsec/mm), which implies a telescope of 5.2-m focal length. For a telescope of 2 m aperture, the focal ratio is f/2.6; for a 2.5-m, f/2.1; and for a 3-m, f/1.7.
A single 2048x2048 CCD chip simultaneously detects the signals from more than 4 million individual pixels. This is a very powerful data-gathering device, but it still falls short of the requirements for wide-field scanning imposed by the proposed NEO survey. At the prime focus of a telescope of 5.2 m focal length, such a CCD covers a field of view on the sky about one half degree on a side. However, we wish to scan an area at least 2 degrees across. Therefore, we require that several CCD chips be mounted together (mosaicked) in the focal plane. The mosaicking of CCD chips is not a simple process, but it is one that is being vigorously pursued today by astronomers. At Princeton University, for example, a focal plane with 32 CCDs is under development. Mosaicking of 4 to 10 CCD chips into a single focal plane should not be a problem for the proposed survey telescopes by the time they are ready to receive their detector systems.
Studies and planning are underway at the University of Arizona for a modern 1.8-m Spacewatch telescope. The new telescope will be an excellent instrument to test and develop some of the necessary instrumental and strategic considerations outlined in this report. From the Spacewatch design considerations, it is safe to assume that 2- to 3-m-class telescopes can be built having focal lengths near 5 m and usable fields of view between 2 and 3 deg. Refractive-optics field correction is probably required, and it appears advantageous to locate CCD mosaics at the prime focus of such instruments. Here, we indicate telescope functional requirements but do not exactly specify the size or design of the proposed survey telescopes.
The limiting (faintest) stellar magnitude that can be observed by a telescope can be determined as a function of the ratio of the source brightness to that of the sky, the number of pixels occupied by a star image, the pixel area, the light-collecting area of the telescope, and the effective integration time (Rabinowitz 1991). For certain detection, the source brightness must be at least six times that of the sky noise. We have normalized to the performance of the Spacewatch Telescope, which achieves a stellar limit of V = 20.5 using an unfiltered 165-s scan at sidereal rate, and we have allowed for an improvement over the performance of that system arising from improved detector quantum efficiency and improved image-recognition algorithms. We find for the survey telescopes that a single CCD should be able to achieve the survey requirement of V = 22 with the following combinations of telescope aperture and scan speed:
Primary Exposure Scan
Diameter Time Rate
(m) (s) (x sidereal)
2.0 21 6
2.5 14 10
3.0 10 14
Primary Area/month/ Total number
Diameter CCD of CCDs
(m) (sq. deg) required
2.0 260 28
2.5 420 18
3.0 600 13
In computing values for the total number of CCD chips required in the worldwide network of telescopes we assume that no two CCD chips together scan the same region of the sky. These are minimum requirements for the telescopes; in practice more scans may be needed for reliable automatic detection, and probably there will be some overlap of coverage between telescopes.
Searching to +/- 60 deg celestial latitude implies sky coverage, over the course of a year, at almost all declinations. Thus telescopes must be located in both hemispheres. Usable fields of view of between 2 and 3 deg probably limit the number of CCD chips in a telescope's focal plane to about ten at the scales we have been considering. However, real-time image processing is simplified if each chip independently samples the sky. Most likely, four CCDs chips/telescope can be accommodated in a linear array in the focal plane. Thus, it appears that seven 2.0-m telescopes, five 2.5-m telescopes, or four 3-m telescopes suffice to fulfill the search, follow-up, and physical observations requirements of the idealized 6,000-square degree survey. Most likely, there would remain extra observational capability to enhance the detection rates of Atens and LPCs by scanning a few times per month outside the standard region. We note that each telescope must be equipped with a minimum of four 2048x2048 CCD chips or their equivalent in light-collecting ability. If space remains in the focal plane, additional filtered CCD chips could be inserted to undertake colorimetry, which would give a first-order compositional characterization of some of the NEOs discovered while scanning.
If a single-point failure due to weather or other adverse factors is not to hamper effective operation of the survey network, we conclude that three telescopes are required in each hemisphere. With fewer telescopes, orbital, and perhaps parallactic, information on NEOs would be sacrificed. The desirability of searching near the celestial poles calls for at least one telescope at moderate latitude in each hemisphere. In summary, we propose a network of six 2-m or larger telescopes distributed in longitude and at various latitudes between, say, 20 deg and 40 deg north and south of the equator.
At the proposed 1.8-m Spacewatch telescope, it is planned to make three scans of each region of the sky (as is currently done at the 0.9-m Spacewatch telescope). Each scan would cover 10 deg in 26 min, so the interval between the first and third scans is sufficiently long that objects moving as slowly as 1 arcmin/day can be detected. For the proposed NEO survey, we envisage two or three longitudinal scans per sky region, about an hour apart. Thus, at a scan rate of 10 times sidereal, each scan could cover an entire strip of the 60-deg-wide search region, with a second search strip being interposed before the first was repeated. We assume that false positive detections, being somewhat rare, will not survive scrutiny on the second night of observation, and thus will not significantly corrupt the detection database.
There are at least three levels of observational data storage that can be envisaged: (1) preservation of image-parameter or pixel data only for the moving objects detected; (2) preservation of image-parameter or pixel data for all sources detected (mostly stars); (3) storage of all pixel data. The first option is clearly undesirable, because data for slow-moving NEOs mistaken as stars would be lost. The first two options have the disadvantage that there would be no way to search the database, after the event, for sources whose brightnesses are close to the limiting magnitude and that would therefore have been discarded. The third option---the most attractive scientifically---may appear to result in serious problems of data storage and retrieval. However, we anticipate that, using technology shortly to be available, the third option is tractable.
On the order of one thousand NEOs and one million main-belt asteroids could be detected each month---about ten detections per second of observing time. Therefore, only moderate-speed data communication is needed between observing sites and a central-processing facility. Careful observational planning will be required to ensure efficient coverage of pre-programmed scan patterns, to avoid unintentional duplication of observations, to schedule the necessary parallactic and follow-up observations, and to optimize program changes so as to maintain robustness of the survey in response to shutdowns. Successful operation of this survey system will also require the coordination and orbital computation capabilities of a modern central data clearinghouse as described in Chapter 6.
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