In proceedings of 20th ESTEC Antenna Workshop on Millimetre Wave Antenna Technology and Antenna Measurement, Nordwijk, Netherlands, June 1997 

IMPROVEMENT OF THE RATAN-600 PANEL SURFACE USING A NEW LASER INSTRUMENT
V.Khaikin, S.Golosova *
F.Danilevich, Yu.Kamach, E.Kozlovsky, L.Shapiro, K.Lyozov, Z.Ulman **
e-mail: vkh@ratan. sao. ru
*The Special Astrophysical Observatory RAS, N. Arkhyz, 357147, Russia
** AO LOMO, St.Petersburg, 194044, Russia   ABSTRACT
 
Improvement of the RATAN-600 radio telescope panels using a new laser instrument is described. RMS surface error after panel surface correction with the new instrument falls from 1.2 mm to 0.18 mm giving significant (factor 3-4) growth of surface efficiency in the range of 30-36GHz.
 

l. INTRODUCTION

Geodesic and holographic tests showed that RATAN-600 radio telescope panels have been degrading with the speed of 0.05 mm RMS a year (Ref.2) during the 20 years of the telescope existence due to rotation of adjusting screws at the panel surface. The reason of this process is panel surface thermal distortions in conditions of Sun heating, night cooling, significant overfalls of surrounding temperature. Before we started panel surface restoration special fixators had been applied to prevent adjusting screws from rotation (Ref.5).

 
2. A NEW LASER INSTRUMENT
 

 
Fig. 1. A new laser instrument
 
The main idea of the method of high precision one or two coordinate measurements at a remote object is the fulfillment of comparator (Abbe) principle when the image plane of the control mark projected and the measurement plane of the count device used are brought in line. In this case the precision of coordinate measurements at a remote object primarily depends on the count device error.

The new laser instrument (Fig. 1) gives a clear image of the control mark at the remote object in the form of a cross with the use of a convergent laser beam (so called "laser string"(Ref.1)). The laser (optical) beam is rotating in the plane just in front of the panel surface under test by turning of beam forming optical tube (with pentaprizm) around its horizontal axis. Deviations of a panel surface from this plane are measured by mechanical optical or automatic photo-electrical count device driven along the surface by an operator in the lift. In the last case the surface deviations from the ideal precalculated surface form are recorded by a portable PC. The measurement accuracy of the method is 10-50 microns at a distance of 3-30 m with the ground location of the laser instrument and in working position of the panels.

3. INDEPENDENT HOLOGRAPHY TEST1NG

We also applied microwave holography at 22 GHz for independent panel surface testing (Ref. 3). A holographic map of the panel with test plate is given in Fig.2. Resolution of the holographic method is not enough to correct panel surface but holography is methodically useful to check the absence of large scale systematical errors of the laser (optical) measurement technique we use.
 
 

Fig.2. Reconstructed holographic map of the panel with 3 mm thickness test plate, a-model, b- experiment 
 
 
 
 
   4. PANEL SURFACE TESTING AND  IMPROVEMENT

Using the new  instrument we tested panel surface deformations during daytime and at night. Most of the day time they have less than 0.15 mm RMS value but can reach 0.3-0.5 mm in unfavorable conditions (strong gradients of surrounding temperature: night cooling after Sun heating, for example). We have chosen morning hours from 6 am to 10 am as the best time for panel surface improvement. Inspite of the Main mirror panels are temperature insulated deformations of several neighboring panels are usually correlated due to similarity of their temperature conditions (gradients). This seems to be the main reason of visible small size distortions at holographic surface maps of the Main mirror (Ref.4).

 

 
Fig.3. One of radio telescope panels before and after correction, RMS errors are 1.2 mm and 0.18 mm correspondently

 
We have already started to improve telescope panels using the new  instrument in optical mode at the South sector of RATAN-600. Surface correction of the panel (2x7.5 m size) is achieved by turning 300 adjusting screws at panel surface. The whole procedure of the measurements and correction takes less than 2 hours. As a result RMS surface error of a panel falls from 1.2 mm to 0.18 mm. This improves significantly (factor 3 - 4) surface antenna efficiency in the range of 30-36 GHz and gives us a chance to work with a renewed surface up to 95 6Hz. An example of panel surface correction is given in Fig.3
 
 

5. REFERENCES
 

    1.Danilevich F M, Chernogubovsky M A & Shinkarenko V A. Russian patent, No 2037772, 23.03.1992 priority.
    2.Khaikin V B, Golosova S J, Zverev Yu K, Venger A P & Majorova E K 1995, Improvement of the reflecting surface of RATAN-600 panels, In Proceedings of XVI Radio Astronomical Conference, St.Petersburg 1995.
    3.Khaikin V B 1994, New applications of radio holography for MM observations with RATAN-600 radio telescope.- A.S.P.Conf.Ser.,Vol.59: Astronomy with Millimeter and Submillimeter Wave Interferometry, 104-112.
    4.Khaikin V B 1996, Holographic Testing of the Antenna Surface Thermal Distortions. In Proceedings of XXV URSI General Assembly in Lille, Lille1996.
    5.Khaikin V B & Golosova S J 1996. Increasing of Surface Precision of RATAN-600 Panels without Interrupting Observations. In Proceedings of XXV URSI General Assembly in Lille, Lille 1996.