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Дата изменения: Wed Feb 24 16:41:28 2010
Дата индексирования: Mon Oct 1 21:05:32 2012
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The beam pattern
and the polarization characteristics of the radio telescope RATAN-600
E.K. Majorova, SAO RAS
Results ara presented of calculation of the Mueller matrix elements
of the radio telescope RATAN-600 that connect the the Stokes parameters
at the input and output of the antenna setup of the radio telescope.
The computation is performed with allowing for the diffraction effects in the
space between the primary and secontary mirrors and for limited width of the
primary mirrow ring. The proportion of spurious linear and circular
polarization from the antenna is estimated when modeling passage of extended
across the beem pattern.
The Mueller matrix elements was computed via formulas [1]:
where are the beam patterns (BP)
for main polarization and cross-polarization, when the field of the feed is vertically
polarized and
are the beam patterns for main polarization and cross-polarization, when
the field of the feed is horyzontally polarized.
The matrix element
characterize the power BP
of the radio telescope for an unpolarized source of radio emission,
element determine the BP of
spurios circular polarization, element
characterize variations of the position angle of linearly polarized radiation.
The BP were computed by the aperture method, proceeding from the fild distribution
in the aperture of the primary mirror.
The components of the main and cross-polarization components of the electric
field in the vertical aperture of the primary mirror was computed in Frenel
diffraction approximation. The field distribution in vertical
apertute secontary mirrors was computed in the approximation of geometrical
optics, proceeding from the BPs of the primary feeds. [2].
Fig.1.
The BP of RATAN-600 ()and the
two-dimensional matrix elements
() и ()
at wavelength 32 cm,
are computed in approximation of geometrical optics - (a) and in the Frenel
diffraction approximation - (b) for different heights of observed sources h.
For the vertification of computations accuracy a comparison was made of
calculated and experimental relations between the maximum values of the BP
()
in different horizontal cross-sections and the shift value of the section in
elevation with respect to the central cross-sections. The experimental relations
was got from observations of brigth point cosmic sourses in the mode of transit
of a sourses across the immovable BP of the telescope. The observations conducted
for different heights of the sources with the focused antenna and in the presence
of aberrations. In the process of the sourse 1830-211 (h=25o) observation
the primary feed at wavelength 7.6 см
was at the focus of the antenna, the primary feed at wavelength
3.9 см it was displaced from the
focus by 7.8
Рис.2.
Maxsimum of values of the BP in different
horizontal sections for the wavelength 7.6
cm - (a) and for the wavelength
3.9 cm -
(b). Curves 1 is computed in the Frenel diffraction approximation,
Curves 2 - in approximation of geometrical optics, the triangles are the
experimental values derived from observations of hte point source
1830-211 (h=25o).
The calculations carried out with provision for the diffraction effects and also
more correct allowance for thr vertical size of the reflected elements of the
primary mirror made possible revision of the shape of the BP of the radio telescope
RATAN-600 both in intensity and polarization. The power beem patterns of the
radio telescope and the Mueller matrix elements computed with allowance made for
these effects have a smaller extent in the vertical plane than those calculated in
approximation of geometrical optics. The reduction of the solid angle of the BP
proved to be the more considerable the lower elevation of the source and the
longer of the wavelength. From this it follows that at long wavelengths the effect
of confusion will be far weaker than it was assumed earlier.
Allowance for the diffraction effects causes narrowing of the aberration-free zone
of the radio telescope near zenith and disturbance of antisymmetry of the element
with respect to the central horizontal
section ( the maximum (minimum) values of the element in the upper half-plane
differ from the minimum (maximum) values of the lower half-plane in magnitude).
Fig.3. The aberration curves at the wavelength
1 cm for h=90o.
Solid lines - the diffraction approximation,
dashed lines - approximation of geometrical optics.
The portion of spurious
polarization from antenna
as an expended sourse passes across the BP of the radio telescope was computed
as the ratio of the maximum value of convolution of the element
wiht a Gaussian to the maximum
value of convolution of the element
with the same Gaussian. The computations was carried for the case symmetrical
Е and H planes of the BP of the primary feed and for case assymmetrical of
the BP in the range of wavelengths
=132 cm. The calculation have show,
that, when the BPs of the primary feed are symmetrical about the polarization,
the portion of spurious circular polarization does not exceed
35%, the portion of spurious linearly
polarization does not exceed 4%.
The proportion of spurious polarization related to non-symmetry of the horn
BPs in E and H planes is estimated. The calcucation have shown that the portion
of spurious linearly polarization from the antenna at the wavelength 1 cm
in the investigation of the relic background fluctuation with RATAN-600
(experiment "Cosmological gene") will not exceed 1% even at great displacements
of primary feeds from the focus if the asymmetry of the horn BPs in E and H
planes is not higher than 10%.
Fig.4.
Maximum values of elements
и
at the wavelength 32 cm as
dependent on the elevation of the source.
The solid line refers to in the upper
half-plane, the dashed line to in the
lower half-plane.
of the radio telescope RATAN-600 that connect the the Stokes parameters
at the input and output of the antenna setup of the radio telescope.
The computation is performed with allowing for the diffraction effects in the
space between the primary and secontary mirrors and for limited width of the
primary mirrow ring. The proportion of spurious linear and circular
polarization from the antenna is estimated when modeling passage of extended
across the beem pattern.
are the beam patterns (BP)
for main polarization and cross-polarization, when the field of the feed is vertically
polarized and 
are the beam patterns for main polarization and cross-polarization, when
the field of the feed is horyzontally polarized.
characterize the power BP
of the radio telescope for an unpolarized source of radio emission,
element 
determine the BP of
spurios circular polarization, element 
characterize variations of the position angle of linearly polarized radiation.
The BP were computed by the aperture method, proceeding from the fild distribution
in the aperture of the primary mirror.
The components of the main and cross-polarization components of the electric
field in the vertical aperture of the primary mirror was computed in Frenel
diffraction approximation. The field distribution in vertical
apertute secontary mirrors was computed in the approximation of geometrical
optics, proceeding from the BPs of the primary feeds. [2].
32 cm,
are computed in approximation of geometrical optics - (a) and in the Frenel
diffraction approximation - (b) for different heights of observed sources h.
)
in different horizontal cross-sections and the shift value of the section in
elevation with respect to the central cross-sections. The experimental relations
was got from observations of brigth point cosmic sourses in the mode of transit
of a sourses across the immovable BP of the telescope. The observations conducted
for different heights of the sources with the focused antenna and in the presence
of aberrations. In the process of the sourse 1830-211 (h=25o) observation
the primary feed at wavelength 
7.6 см
was at the focus of the antenna, the primary feed at wavelength
as an expended sourse passes across the BP of the radio telescope was computed
as the ratio of the maximum value of convolution of the element
wiht a Gaussian to the maximum
value of convolution of the element 
32 cm. The calculation have show,
that, when the BPs of the primary feed are symmetrical about the polarization,
the portion of spurious circular polarization does not exceed
3