|Figure 1: A large, almost flat primary reflector is supported by the ground and slightly alterable in shape through the adjustment of linear actuators|
|Figure 2: The telescope receiver is carried by an airborne vehicle and driven towards the target position shown here. As a result of corrections to the receiver feed point and reflector adjustments which move the diffraction image, telescope efficiency and correct pointing are maintained with the receiver anywhere within a relatively large volume surrounding the target position|
Emission from a radio source is focussed by the reflector to form a diffraction image. Ideally, this image is at a target position as shown in Fig. 2. This position is at the zenith angle of the source and at an azimuth 180 from the source azimuth. The reflector at all times forms part of a paraboloid and has no appreciable path-length errors or aberrations. To maintain these conditions the reflector is continuously adjusted with the linear actuators so that the particular paraboloidal section being used remains suited to the changing azimuth and zenith angle of the source. Wide sky coverage is thereby achieved, though the collecting area is reduced by foreshortening at large zenith angles (by a factor of two, for example, at a zenith angle of 60). This reduction, and a corresponding change in the primary beam shape with zenith angle, would also be encountered with the phased-array type of telescope being studied for the one-square kilometer telescope (Braun 1993). With both the presently proposed telescope and the phased array, the primary collecting element is fixed to the ground. This ensures that accurate account can be taken of the foreshortening effect in processing a synthesized image.
The position of the airborne vehicle is measured and controlled from the ground to keep the receiver feed in alignment with the diffraction image as closely as possible. Any displacement of the vehicle from this alignment is dealt with in two ways. Large displacements ( m, for example), are compensated by adjusting the primary surface actuators so as to move the focussed diffraction image of the source to follow the vehicle. This greatly eases the problem of controlling the vehicle position by allowing it to be located anywhere within a large volume surrounding the target position as shown in Fig. 2. This may be a feature which makes the telescope scheme practicable. Moving the diffraction image to follow the vehicle does not, however, remove all of the effects of vehicle motion. For a 200 m telescope considered below as an example (Table 1), a 15 m displacement of the vehicle can change the telescope collecting area by up to 1% as a result of changed foreshortening. Illumination efficiency can also change by about 1%. These effects would need to be corrected in the image processing.
At short wavelengths, where accurate pointing is specially important, smaller vehicle displacements of a few metres are compensated by an equal and opposite displacement of the area over which source energy is collected, thereby keeping the pointing constant. This is done essentially instantaneously using a phased receiving array and allows the surface actuator adjustments to be made more slowly and less often. (At mm wavelengths the phased array is replaced with a small mechanically swivelled mirror). At longer wavelengths, where the size of the diffraction image is large, small vehicle displacements are less important and are tolerated. However, large displacements are still corrected by moving the focal point using the surface actuators. The longer wavelength observations are made by reducing the altitude of the vehicle and re-focusing the primary reflector so as to operate with a smaller f/D ratio.
For mm wavelengths, a large f/D ratio gives the opportunity of constructing very accurate flat panels inexpensively from thin stretched sheets of metal. The stable support of each panel by the ground avoids the need for a protective radome.
Copyright The European Southern Observatory (ESO)