Report at URSI/IEE Convention on Radio Science, Espo, Finland, Oct.2002.

Multi-beam Solar Radio Telescope of Tuorla Observatory : test operations

V.Khaikin*, S.Yakovlev**, A.Kazarinov**, I.Efimov***, A.Volkov***, E.Valtaoja*****

*The Special Astrophysical Observatory of RAS,

357147, N.Arkhyz, Karachai-Cherkessia, Russia

E-mail: vkh@brown.nord.nw.ru

** BUM TECHNO JSC,

Nevsky pr.65, PO BOX 217, 191025, St.Petersburg, Russia

E-mail:bumt@bumt.spb.ru

***The St.Petersburg State Technical University

195251 Polytechnicheskaya 29, St.Petersburg, Russia

**** Tuorla Observatory UTU,

FIN-21500 PIIKKIO, Finland

E-mail:esko.valtaoja@utu.fi

1.Introduction

Multi-beam Solar Radio Telescope (MSRT) of Tuorla Observatory is intended for patrol Solar observations in the band of 3-10 mm. Solar flux and Solar flare monitoring are among main MSRT tasks with the aim of further enhancing the links between ground-based and satellite observations of the Sun.

Low resolution moment images of the Sun at 3 mm (36 pixels) and a differential beam method at 10 mm may be useful for diagnostics of pre-flare and flare process and detection of very weak variations of Solar flux caused by instability of magnetic field structure in active regions before flares [1].

MSRT project was developed in cooperation with NRTT Lab (The Special Astrophysical Observatory/St.Petersburg State Technical University) in 2000, MSRT status at Autumn 2002 is given bellow.

2.MSRT dish

A 2 m precise carbon fiber parabolic dish was developed and fabricated for MSRT at "Luch Co" in Suzran, Russia [2].

The main design features are:

1.A 3 layer honeycomb structure is used in the solid dish design.

2.Additional external metallic strengthening beams of the back structure provide support against expected wind loads in open air conditions without a protecting radome.

3.Four metallic struts connected with beams of the back structure support the secondary mirror and a focal array. The 2 m parabolic dish has the ability to withstand wind speed not less than 40 m/sect.

4.A special anti-corrosion layer was applied to protect the metalized dish surface from the atmospheric effects in open air conditions. A special surface cover also protects MSRT receivers from the heating due to the Sun.

5.Relation of outer and inner foci of the hyperbolic secondary mirror is f3/f2=1.5 that increases an effective radio telescope focal distance, reduces spacing of neighboring beams and off-axis aberrations in a multi-beam case.

Fig.1. The 2 m precise carbon fiber parabolic dish: front(left) and back (right) view.

MSRT characteristics are given in table 1.

Table 1

Location	Latitude: 60° 24'57", Longitude: 22° 26'42"E, Altitude: 53 m a.s.l.
Type/mount	Cassegrain / Equatorial
F/D	0.425
Illumination angle from the Secondary focus, deg	100
Main dish diameter/weight, m/kg	2/74
RMS surface error, μm	85
Drives	Step-motors N1 and N2 with gearboxes, AC motor N3 with gear-box
Control System	PCB with serial PC port, encoders with ISA PC interface
Pointing speed α/δ, arcdeg per sec	1/0.1
Sun tracking time, summer/winter, hr	-+6 / +4
Tracking error (max), arcmin	+-1
Frequency range, MM	10	3*
HPBW, arcmin	17.5’	5’*
Number of beams	2- 6*	25*-36*
MMIC receiver sensitivity in wide channel per beam per sec, mK	50	100*
Polarization	Circular (L/R)	Circular (L/R)
Stocs parameters	I, V	I, V
Time resolution, ms	1	1
Aperture efficiency	0.7	0.6*

* - expected

3.Results of measurements

1.A surface error mapmeasured with the use of a contact 3D coordinate measurement gear has 85 micron RMS deviations of the best fit (Fig.2, left).

Fig 2. MSRT dish surface map, RMS error 85 microns (left), testing of the MSRT dish in Tuorla Observatory, Finland (right)

2.A tuning of adjusting elements at 8 dish supporting beams of a back structure was necessary in working conditions to provide a 100 micron standard deviation at the periphery of a parabolic dish. A carbon fiber material is sufficiently plastic to remove, if necessary, deviations of the best fit in the range +-0.5 mm.

A laser theodolite of AXYZ STM/MTM industrial measurement system (Leica Geosystems) was used for this procedure. A method to adjust up to 10 m precise carbon fiber compound dish in working conditions with RMS accuracy up to 20 microns was developed and tested as well (Fig.3) [3].

Fig. 3. Results of measurements of the parabolic dish under test. Positions of measurement points (top, left), isolines with the step 0.1 mm (top, right), the needle in different projections (bottom, left, right).

3.MSRT polar axis and nulls of encoders were fixed by a laser theodolite in the absolute coordinate system with accuracy 15”. Positions of the secondary mirror and focus were preliminary fixed as well but will be specified later on results of antenna and geodesic measurements.

4.MSRT radiation patterns at 10 mm for an off-axis feed measured on ground-based oscillator in the near (0.1*D**2/λ) and Fresnel zone (0.98*D**2/ λ) are close to expected(Fig.4) but show that some correction of secondary dish position is necessary.

Fig.4. MSRT radiation patterns at 10 mm with (bottom) and without (top) secondary mirror for an off-axis feed measured on ground-based oscillator in the near(left) and Fresnel(right) zone

4.MSRT receivers

The current status:

1.Two array receiver modules at 10 mm of circular polarization(L/R)[4] are presently under test at MSRT(Fig 5 top). Noise road and noise spectrum of a receiver module are given in Fig.6.

2.2x3 or 3x3 element focal receiver arrays may be then assembled of these modules as well (Fig.5 bottom)

3.MMIC technology may be also used to build array receiver modules at 3 mm but if we need 36 or more pixels for Solar flare monitoring with MSRT the mm-wave microbolometer array technology developed by Metorex Oy, Finland [6] seems to be more promising..

Fig.5. Two array receiver modules at 10 mm of circular polarization(L/R) without (top, left) and with thermostatic

Box (top, right), array from receiver modules (bottom)

Fig.6. Noise road of a receiver module with calibration (200 K) (left, top) and corresponding spectrums before and after separation of “white” component.

5. MSRT Data Acqusition System

Data Acqusition System (DAS) for MSRT was realized with Motorola DSP56F807 operating at 80 MGz[5]. DSP provides 3 key functions in DASMA: data acqusiton, digital data filtration with a given filter order and deeper data reduction.

Data reduction includes real time treatment of a “fast calibration” which may be applied in the array to reduce gain variations of array receiver channels in a “total power” operation mode.

Fig.7. The two-channel DAS record of the Sun tracking during 5.5 hours at MSRT.

Fig.7. shows a two channel record of the Sun tracking during 5.5 hours at MSRT (Multibeam Solar Radio Telescope) of Tuorla Observatory [3]. Variations of the signal level reflect mainly variations and the total Sun flux during 5.5 hours. Some atmospheric effects are visible in Fig 8 too. For patrol Solar flare observations with MSRT logarithmic attenuators may be used in DAS channels to provide a necessary dynamic range in a strong flare case.

6. Test operations and first results with MSRT

Periodical Sun observations (Sun tracking up to 10 hours a day) were started in the

Fig.8. One hour Sun tracking with MSRT at 2.7 cm(top), Sun passing the MSRT beam at 10 mm(middle), the first Sun tracking at 10 mm during 10 minutes (bottom)

beginning of 2002(Fig.8 top). Test operations and trial Solar observations in spring and summer 2002 with the testing receiver at 2.5 cm showed insignificant pointing and tracking errors which were studied and may be corrected in a pointing/tracking program. Several Solar flares were registered with MSRT at 2.5 cm during trial observations in spring of 2002. Sun passing the MSRT beam at 10 mm is given in Fig.8 middle. Fig 8 bottom shows the first 10 minute Sun tracking at 10 mm where mainly atmospheric fluctuations are mainly visible during tracking process. The following Sun tracking at 10 mm during 5 hours confirmed that MSRT tracking errors are insignificant for this wavelength too. A strong Solar flare was registered during trial Solar observations at 10 mm on Sept.27, 13:05 UTC.

Fig.8.Solar flare registered at MSRT (12 GHz) and Ondrejov observatory (3 GHz) 10.04.2002, 12:30 UTC (Notes and comparison with GOES data of Silja Pohjolainen, Tuorla Observatory)

7. Conclusion

MSRT test operations are presently in final stage. Results of geodesic/antenna measurements and trial Solar observations during 2001-2002 confirm our intentions of Solar patrol observations with MSRT in the range at least of 3-10 mm. General view of MSRT is given in Fig 9. Achievable field of view with MSRT at 2 mm is shown in Fig.10.

Fig.9. General view of MSRT in Tuorla

Fig.10. Achievable field of view with MSRT at 2 mm

References

1.Pohjolainen S.: Energy release by solar flares: Observations at radio and X-ray wavelengths. Dr. Phil. thesis. Helsinki Univ. of Technology, Metsahovi Radio Research Station, Series A, Report 24, 1996.

2.V.Khaikin, E.Bitkin, A.Matushkin, S.Yakovlev, A.Kazarinov, E.Valtaoja. A carbon fiber parabolic dish for Mulltibeam Solar Radio Telescope. XXV ESA Antenna Workshop, Noordwijk, pp.405-407, Sept.2002.

3.S.Yakovlev, A.Kazarinov, A.Andreev, S.Semin, V.G.Gimmelman and V.Khaikin. Testing of precise carbon fiber parabolic dish in working conditions. XXV ESA Antenna Workshop, Noordwijk, pp.577-579, Sept.2002.

4.V.Khaikin, V.Dobrov, M. Parnes, V.Volkov, A.Golovkov, D.Kalinikos, M.Sugak, Yu.Rubakov .Multi-channel array receiver moduls for solar and atmospheric research. XXVII URSI/IEEE Convention on Radio Sciences, Espoo, Finland, Oct.2002.

5.I.Fadeev, V.Khaikin. Data Acqusition System with DSP Motorola for a multi-channel focal receiver array. XXVII URSI/IEEE Convention on Radio Sciences, Espoo, Finland, pp.170-171. Oct.2002.

6.A.Luukanen, V.-P.Viitanen. Terahertz imaging system based on antenna-coupled microbolometers. Proc.SPIE vol.3378, p.34-44, Passive Millimeter-Wave Imaging Technology II, Roger M.Smith; Ed., 1998.