Space Reconnaissance: Scanning the Sky with an Optical Arecibo

Duncan Steel
Australian Centre for Astrobiology, University of NSW

Harrison Steel
Wesley College, University of Sydney

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     Last modified: July 29, 2011

Abstract
A novel optical design is described for a sky-scanning search/reconnaissance instrument, intended primarily to be used to discover small, fast-moving asteroids comparatively close to Earth (within 0.5 AU) although it might also have applications for identifying other classes of object both in deep space (main-belt asteroids, distant comets and minor planets) and near space (satellites and debris in geocentric orbits).

The concept is based on a large, segmented, spherical primary mirror that has no moving parts and scans across the sky purely due to the Earth’s diurnal rotation. It might therefore be thought of as being an optical counterpart of the Arecibo radio/radar telescope in Puerto Rico.
This system is visualised as being built into a 30-metre wide concavity near ground level which is shaped as the cap of a sphere with half-angle 45 degrees and rotational symmetry about a vertical axis. That concave depression would be largely covered with ~1,000 identical hexagonal spherical mirrors each one-metre across. These mirrors could be of relatively poor quality, for example being produced by slumping centimetre-thick optical blanks into moulds in an oven before their concave reflecting surfaces are aluminised.

In front of the prime focus a cluster of high-quality convex hyperbolic secondary mirrors perform first-order correction of the spherical aberration produced by the primary mirrors. Occluding baffles would be used to restrict the light paths reaching these secondary mirrors to those reflected from the primary mirror segments within ~3 metres of the image positions (i.e. each of the ~100 image positions – located below the primary mirror surface – would be served by only a limited area of the 30-m wide assembly of hexagonal segments). On the other hand, each primary mirror segment would contribute light to several secondaries/images.

Coma, astigmatism and field curvature might be reduced or corrected by auxiliary refractive optics close in front of the image plane. Sky scanning for asteroids often employs a V or R filter, and with a similar narrow band-pass filter in use here the blurring due to the relatively crude primary mirror optics might be ameliorated through the use of holographic correctors placed close in front of the image detectors. Continuous sky-scanning is accomplished by reading out the CCDs column-by-column at the diurnal rotation rate, with image strips from detectors covering identical declinations but different hour angles being compared to identify moving objects. The only moving parts in the whole system would be the slowly-turning CCD detectors, compensating for field rotation.