Results of calculation of characteristics of the RATAN-600 radio telescope in "radio-Shmidt telescope" mode are presented. The flat reflector and the circular mirror are computed as a double-mirror aplanatic system with a planoid mirror which makes "Schmidt" correction of the wave-front of the wave incident at an arbitrary angle with respect to the horizon. Formulae are derived and a package of programmes is created for calculation of configurations of the mirrors of the antenna system and optimization of its basic parameters. Phase distribution of the field over the aperture and the beam pattern (BP) of the telescope for different angles of formation of the antenna and different elevations of a source are computed. A possibility is shown of long-time (up to 1 hour) tracking of sources at waves longer than 4 cm with an aperture of 150 m.

In the "radio-Shmidt telescope" mode the antenna system the South sector of the main mirror with the flat mirror ("South+Flat") of the RATAN-600 radio telescope is used. A correction of the wave-front is made with help fashioning of the flat mirror the necessary curvature, which is calculated for a given elevations of a source.

Fig.1.  The antenna system the South sector with the flat mirror.

The method is evolved from the idea of creation of a prefical aplanatic double-mirror system with a wide field of view, free from aberration and coma. The flat and circular mirrors of the RATAN-600 form the basis of the system. An analogs of this system in optics are "mirror Schmidt" and "mirror Wright" - systems with a planoid mirrors. The surface curvature in planoids at the vertex equals zero, so the focal length of the system is approximately equal to that of the second mirror. In the case of antenna system "South+Flat" the planoid is flat mirror, the circular reflector of RATAN-600 is the second mirror. To follow a source, the secondary mirror moving on the arch rails is used and a change of the tilt angle of the flat mirror within small limits is made. The elements of the flat and circular reflectors is placed according to the necessary curvature. A distinguishing feature of this procedure is that the main mirror shape close to a circular cylinder does not change in the course of source tracking. The "radio-Shmidt telescope" mode allow to solve problem of a long tracking of a cosmic sources at RATAN-600.

Fig.2 Shape variation of the flat reflector with different value of parameter. (F'-М).

The performed computations show, that it is possible to construct an aplanatic system based on the antenna system "South+Flat", which is similar to those of Schmidt and Wright and they demonstrated the necessity of optimization of its parameters to attain the utmost efficiency of the telescope. It is shown in Fig.2 how the shape of the flat mirror change with variation of parameter (F'- M), where F' is the paraxial focal distance of the entire system, M is the distance of the circular reflector vertex from the focus where the feed is placed. x is the variation of the flat mirror longitudinal coordinate. The basic criterion in the computation was the creation of such configurations of the mirrors that could enable a maximum number of panels of the flat and circular reflectors to be set up, i.e. attaining a maximum effective surface and the longest time of following a cosmic source.

Fig.3.  BP of the RATAN-600 at 6 cm in mode "radio-Schmidt telescope" for observations in azimuths:   =0o (1),  3o (2),  5o (3).   (а),(b) - real arch rail track,   (c),(d) - optimal arch rail track.   Horizontal size of the flat mirror L=100 m.   (а), (с) - the flat mirror elements are displaced in longitude and in azimuth. ("exact radio-Schmidt telescope"),   (b),(d) - the flat mirror elements are displaced only in longitude ("approximate radio-Schmidt telescope").

The phase distributions of the field in the aperture and BP of the radio telescope in mode "radio-Schmidt telescope" (Fig.3) have been computed for arbitrary angles of antenna formation and angles of source observation. The computations performed have made is possible to optimize the parameters of the system discussed, in particular, the paraxial focal distance of the telescope, the curvature of the arch rails that carry the feed, the range of angles of antenna formation. The "radio-Schmidt telescope" with the currently existing arch rails has been shown to enable tracking of sources for an hour with no essential distortions of the BP at wavelengths, longer than 4 cm, with the 100 - 150 m aperture. The optimization of the arch rails will permit long-time observations at wavelengths up to 1 cm to be made.

For the tasks that do not need long tracking of a source but only signal accumulation at one declination, one can use special case of the "radio-Schmidt telescope" mode, when the feed is located near the correcting mirror. In this case the largest size of the aberration-free zone and the least curvature of the field in the focal plain can be achieved with an antenna aperture up to 200 m. A wide field of view is achieved through placing a multielement focal array along the focal line of the secondary mirror. Multielement array can be constructed in the form of the line strip array (one-dimensional array), in the form of the plain array (two-dimensional array) and in the form of the "terrace-like" array (three-dimensional array) [1].