Experimental investigations of the beam
E.K.Majorova, S.A.Trushkin, SAO RAS
Results are presented of experimental investigations of the
beam pattern (BP) of the RATAN-600 radio telescope observing the bright discrete
sources. The measurements were carried out in the range from 1.4 to 49 cm
wavelength at the elevations of sources from 10 to 90 degrees. The main beam
of the BP was measured to a level of 0.5 - 3% of the BP maximum.
The aberration curves were measured that describe the fall-down of signal
in observations with transverse shifts of the horn from the antenna focus.
The measurements of the BP made it possible to check the accuracy of calculation
and reveal important effects which influence on the structure of BP far out of
its main axes. A comparison was made of the drift scans of the Moon with the
convolutions of the two-dimensional computed BP with the uniform disk of
angular dimensions of the Moon, the root-mean-square error of setting the
main mirror elements is estimated. It is equal to 0.55 mm.
It is show that the new computation of the BP , with takes account of the
diffraction effects and the finite size of the RATAN-600 main mirror ring have
a better fit to the data of measurements than the early calculations .
The BP of the North sector of the RATAN-600 radio telescope was measured in
three sets of observations (2001 February-March, 2001 Oktober, 2002 April).
Sources were observed in the mode of transit of a source across the immovable
BP of the telescope. The flux densities of the sources were above 3 Jy.
The higt sensitivity radiometric complex of the feed-cabin No.1 was used for
the measurements. The sensitivity of its radiometers is from 2.5 mK to 20 mK.
With the effective area of about 1000 m2 a high signal-to-noise
ratio was realized in one observation even in the cross-sections far from
the central one. In the process of observations the drift scans of point sourse
( PKS0521-36, PKS1830-21, 3C161, 3C454.3, 3C121,
2005+403, 3C84) with an elevation H were registered.
Transits of sources were observed across the horizontal cross-sections of the
BP which differed in elevation from the central horizontal cross-section by
The relations between the maximum value of the
in different horizontal cross-sections and
the shift value of the section (
in elevation with respect to the central cross-section at the
wavelengt 13 cm.
The squares show the data of measurements made in 2001 Oktober,
solid lines are the computations of the BP, with takes account of the
diffraction effects and the finite size of the RATAN-600 main mirror ring .
A comparison of the computed and experimental relations
as well as comparison of drift scans of point sources across different
horizontal cross-sections of the BP with the corresponding computed cross-sections
were made. Under a good state of the antenna, the concidence of the experimental
and computed curves is reached at the level 2-3% of the BP maximum
Fmax(0) in the central cross-section.
In a good state of the antenna the root-mean-square error of setting the elements
of the main mirror in radius and elevation is not worse than 2-2.5 precise scale
graduations of the synchro.
Using the observational data, experimental two-dimensional BPs at the waves
13 and 7.6 cm. In Fig.3 these BPs
are presented as isophotes. The BPs are normalized to the maximum in the central
cross-section. When constructing experimental two-dimensional BPs, the drift
scans of sources across equally spaced cross-sections were used.
The two-dimensional experimental BP of the RATAN-600 radio telescope.
The observations were carried out in Oktober, 2001.
Individual cross-sections of the BP were simulated in the presence of errors
in setting the panels in elevation and radial coordinates. It is show how the
structure of the BP changes at distant cross-sections with changes of the value
and character of distribution of these errors over the aperture of the main mirror.
As can be seen from the curves shown in Fig.4, precence of panels with large
elevation errors lead to a considerable change in the BP structure. These
changes are the more pronounced the shorter is the wavelength. On condition
that the number of panels with great errors is rather large, a general shift
of the BP in elevation may occur if errors of one sign predominate.
The radial errors also have an effect on the BP structure, but considerably
weaker than errors in positions of the panels in elevation. Any errors in
setting of the reflecting elements of the calculed position cause not only
distortion of the BP structure but also a fall-down in gain of the antenna.
The drift scans of the point source 3ó84(0316+41)
at wavelength 7.6 cm across the
horizontal cross-section of the BP in Oktober observations (black lines) and
the computed horizontal cross-sections of the BP corresponding to them without
allowance for errors (red lines) and with allowance for errors (blue lines)
in settings of the main mirror reflecting elements in elevation and radius.
As the latter, the differences of "zero positions" in elevation and radius
between auto-collimation adjustments in 2002 March and 2001 April were used.
A modeling of transit of an extended source across the computed two-dimensional
BP of RATAN-600 and a comparison of the results of modeling with actual
records of transit of the Moon across the BP were made. Convolutions of the
computed BP with the uniform disk of angular dimensions of the Moon.
The background scattering brought by occasional errors of setting the panels
was simulated at short wavelengths
( 1.38, 2.7, 3.9 cm). It was
specified by the two-dimensional Gaussian, the halfwidths of which are equal
in the horizontal and vertical plane the width of panel and the effective
vertical size of the panel correspondingly. Occacional errors do not affect
the shape of the drift scan at wavelengths
> 7.6 cm. At these wavelengths the
experimental drift scans of the Moon and the convolutions of the two-dimensional
BP were compared with the disk of homogeneous brightness of the Moon's angular
dimensions. In all the records, but for those with dominating receiver's
noises, the experimental and calculated curves are almost identical.
The drift scans of the Moon at wavelength
1.38 cm (black lines) and
the computed curves corresponding to the values of errors in radius
= 0.8, 0.55, 0.31 mm (colour lines).
The drift scans of the Moon across the BP of RATAN-600 at wavelengths
1.38 - 47.6 cm (black lines)
and the convolutions of two-dimensional calculated BP with the uniform disk
of sizes of the Moon (red lines).
Relations between the maximum values of the BP in the central cross-section
Fmax and the transverse displacement
of the receiving horn from the
antenna focus (the so called aberration curves) were measured using the
reference point sources. It can be seen that at elevations < 80o
computed curves are in good agreement with the results of measurements.
The discrepancy in plotting the computed and experimental curves at elevation
88o with very large displacements of the horn from the focus is
likely to be associated with incorrect setting of the horn at wavelength
1.38 cm along the focal line of the secondary mirror.
Probably, additional measurements are needed to refine the results obtained.
The aberration curves Fmax(),
derived from observations of reference sources (squares)
and calculated with allowance for the diffraction effects (red lines).
 Esepkina N.A., 1972, Astrophyz.Issled. (Izv.SAO), No.4, p.157.
 Korzhavin A.N., 1979, Astrophyz.Issled. (Izv.SAO), No.11, p.170.