ANTENNA CHARACTERISTICS OF RATAN-600 RADIO TELESCOPE IN "RADIO-SCHMIDT" MODE



    In "Radio-Schmidt" mode a flat Periscope mirror is given the necessary curvature to remove spherical aberration caused by the Main mirror in the form of unmovable "flattened" cilinder [1-3]. This is technically possible at RATAN-600 for an effective aperture size of 170 m.
    The main advantages of "Radio-Schmidt" mode at RATAN-600 are:

1.The longest aberrationless focal zone and widest radio telescope field of view. Absence of spherical aberration and coma in radio image in the wide band 1-50 cm for an optimal focal way.

2.Possibility of long time cosmic source tracking or repeated observations   with an unmovable Main mirror.

3.Possibility  to install a multi-beam feed array with the greatest number of   receiver elements along the focal line that may give a significant (one-two orders) growth of integral radio telescope sensitivity in background tasks [4].

4.Possibility to use one, two and three dimensional focal arrays.

 In Table 1 some characteristics of antenna system in "Radio-Schmidt" mode are presented for 150 m aperture size, optimal and real focal way at 7.6 cm.
 Calculations show that less than half an hour Sun tracking is available with the real focal way at more than 8 cm wavelength and one hour Sun tracking at 1 - 50 cm with optimized focal way.

Table 1
 
Relative source  azimuth 
deg. 
 Time of Sun tracking pattern 
hours
  Maximal aperture phase error 
deg.
 Amplitude of the main lobe of beam pattern 
%
 Level of side lobes of beam pattern 
%
 Coma like aberration 
%
 Curvature radius of the focal way
0 
1-4 
0-8 		 <1 0 
< 2.0 
<10.0		 1.0 
1.0 
1.0 
 		 5.0 
5.0 
5.0		 No 
No 
No		 optimal
0 
2.0 
3.0 
3.5		  <0.5 0 
18.4 
41.3 
56.8		 1.0 
0.965 
0.837 
0.720		 5.0 
7.0 
11.0 
16.7		 No 
Yes 
Yes		 real


 The focal aberrationless zone in "Radio-Shcmidt" mode in the range of 1-30 cm for 150 m aperture, elevation angle 45 deg. and Secondary mirror N3 at a real focal way is shown in  Fig.1.

Fig 1. Power aberration curves for  RATAN-600  Secondary mirror N3

So it looks very attractive to apply  multi-beem feed arrays in "Radio-Schmidt" mode[4].

Sun tracking and repeated observations at longest wavelengths (more than 8 cm) are available at RATAN-600 in quazi-Schmidt mode with the unmovable Main mirror and reduced or "shortened" aperture (less than 100 m) even with the flat Periscope mirror. A first trial Sun tracking during an hour in "quazi-Schmidt" mode with an 80 m aperture is shown in Fig.2. RATAN-600 beam patterns for different elevation angles and azimuths in "Radio-Schmidt" mode (a real non optimal focal way) in are shown in Fig.3.

Fig. 2.  The Sun in channels of the panoramic analyser of spectrum of the RATAN-600 observation cabin No 3 during trial  one hour tracking  in quazi-Schmidt mode with 80 m aperture.
 
Fig. 3. RATAN-600 beam patterns for different elevation angles and azimuths in "Radio-Schmidt" mode at 4 cm and 8 cm(a real focal way)
 
References

1.V.Khaikin, E.Majorova, M.Chukhlebov. RATAN-600 as a "Radio-Schmidt" telescope. XXVII Radioastronomical Conference, v.3, 1997.

2.E.K.Majorova, V.B.Khaikin. Characteristics of RATAN-600 radio telescope as two-mirror aplanatic system. Radiophizika, 2000, 4, pp.1-16.

3.V.B.Khaikin,E.K.Majorova,I.G.Efimov, O.A.Victorov. Long time cosmic source tracking at RATAN-600 in "Radio-Schmidt" mode. Astronomy & Astrophysics Transactions, 2000,N3-4, pp.608-615.

4.V.Khaikin,E.Majorova, R.Shifman, M.Parnes, V.Dobrov,V.Volkov,V.Korolkov and S.Uman Multi-element MMIC array technologies for a radio telescope. Astronomy & Astrophysics Transactions, 2000, v.19, N3-4, pp.596-611.