(1) Pulkovo Observatory, St.Petersburg, Russia
(2) S.I.Vavilov State Optical Institute, St.Petersburg, Russia
The Struve astrometric satellite which is being developed at Pulkovo Observatory in cooperation with Krasnoyarsk Institute of Applied Mechanics, S.I.Vavilov State Optical Institute and some other space instrumentation institutes, will produce observations of a second epoch for the Hipparcos stars. The project is devoted to maintaining the Hipparcos coordinate system as well as extending it to a density of $\approx 100$ stars per squre degree. New possibilities to increase accuracy and limiting magnitudes of the observationes are discussed. These possibilities are based on using new optical materials and techniques developed at the S.I.Vavilov State Optical Institute that allows to enlarge entrance pupils of the on-board telescopes, improve temperature stability of the optics and reduce scattered light from the mirrors. Requirements to the optical scheme, to the dynamic properties of the spacecraft and to the focal detectors are formulated.