Abstract

For a relatively small mirror diameter of 250 mm and a wavelength of 10 nm, the theoretical diffraction limit is as low as 0.01 arcsec. On the other hand, a surface figure error and irregularity of ~0.5 nm rms must be reached if the Strehl ratio should be high. We present results of wavefront measurements, where a repeatability of 0.2 nm and an absolute accuracy for the shape of > 1 nm have already been reached for such minors The key to a successful telescope is a two-mirror design with only very slight deviations from ideal spheres. We assume that these mirrors could be manufactured with classical polishing technology, with which a very smooth surface without pronounced gradient errors in any spatial frequency region could be achieved. Experience shows that, in this case, the most dominant wavefront errors are longscale, e.g., of the type of spherical aberration, astigmatism and coma, which could be compensated by proper misalignment of the telescope. For this, a setup is necessary where the wavefront error of the telescope could be tested interferometrically in double pass.

© 1991 Optical Society of America