Measuring the gravitational astigmatism of a radio telescope

Most conventional radio telescopes will show a strong astigmatism when tilted, resulting in surface degradation, which can be corrected with a mechanically deformable subreflector. Astigmatism means different focal lengths ( ) in two perpendicular planes. A most direct way of measuring the astigmatism is to measure with an elongated rotatable feed, illuminating only a central strip of the telescope. Equations are given for the relations between , rim deviation , and rms surface deviation , and for the dependence of these quantities on elevation angle. This method was applied to the 140-ft. telescope. Between zenith and horizon, the measured range is mm, yielding an astigmatic rim deviation of mm, and an astigmatic rms surface deviation of mm. The latter agrees with astronomical efficiency measurements, showing that the total gravitational deformation is mainly of a simple astigmatic shape, to be corrected by relatively simple means.