Radio Astronomy Centre


				It is a tuned amplifier tuned to 30 Mhz and
				bandwidth is limited to 4 Mhz to avoid unwanted
				signals. The receiver gain around 100 db. The
				receiver system is known as Super-heterodyne
				receiver.
Radio Astronomy Centre
				ELECTRICAL SYSTEMS
Tata Institute of Fundamental Research				The main functional units are as follows:

Ooty				1. Power Supply
				2. Rotation
				3. Protection


				Power supply
				the incoming supply for RAC is 22 KVA from
				the sandynalla substation. There are two lines
				which are provided in case of failure from any
CONTENTS				one of the lines. One supply is given to current
				transformer and to the protection circuit. From
				this, it is stepped down to measuring device. Since
				the three phase power lines have to travel a
- Introduction				large distance before reaching the centre. From
- Construction				220 KV supply –stepped down to 440V. The
- Electronic systems				transformer contains oil cooling and silica gel to
- Electrical systems				observe moisture. The 440V is taken to
- Protection systems				distribution board and is given to various loads. If
- Data acquisition system				there is no need of heavy loads, a small
				generator of 40 KVA is used.

INTRODUCTION

The Radio Astronomy

Radio astronomy is the study of universe with
the help of radio waves emitted by celestial
objects. Radio waves are mostly coming from
highly energetic charged particles moving with
velocities close to that of light in presence of
magnetic fields with the help of directional antenna				ROTATION
or telescope the strength of radio waves				The telescope can rotate in East-west direction.
received from different directions from sky can
be determined and a map of the radio signals				Before rotation
from the sky can be made.

Radio Astronomy Centre at Ooty

The radio astronomy is one of the research
institutes under Tata Institute of Fundamental
Research(TIFR) proposed the hail tool of radio
astronomy. A large structure radio telescope was				After rotation
designed in July 1963. A suitable site was
Selected in Nilgiri hills. The work that started in
1965 was completed by 1969. the co-ordinate
latitude:11 22' and co-ordinate longitude:-76 40'.

Ooty Radio Telescope (ORT)

ORT is located on the hill slopes close to muthorai
village in Ooty. The unique feature of telescope is
that it is stepped in the north to south direction				There are two modes of rotation
down a natural hill slope of about 11 equal to the				1. Slew circuit
ideal latitude of the observatory. This				2. Track circuit
arrangement ensures the axis of the telescope
is parallel to the earth axis. This enables the ORT				Slew circuit
to track celestial objects for about 9.5hours by				- It employs 12.5 hp induction motor at 1440 rpm
simply rotating the telescope mechanically in east				located at 4 trusses (N4,S4,N10,S10)
to west direction.				- Induction motor's speed torque is converted to
				75 rpm using gear trails.
CONSTRUCTION				Track circuit
				- Used to track the object continuously.
The following are the building blocks of ORT:				- It employs 3-phase 5 hp motors that rotates
				the shaft at 3 rpm with the help of gear trails.
- Reflector
- Dipole feed
- Receiver

				Usually the motors brought to a stop, the
				telescope strips further away from the required
				position due to momentum. This is a positional
				error.
Reflector
				PROTECTION SYSTEMS
The antenna reflector of ORT is the parabolic				Overspeed tacho circuit(need)
cylinder with its surface formed by 1100 stainless
steel wires. Each fine stainless steel wire is of				The telescope (ORT) is rotated at a speed of 75
.38mm in diameter. These stainless steel wires are				rpm. Due to any fault conditions such as shaft
stretched along the length of cylinder.				breakage or motor failure, the telescope is
				subjected to overspeed. If the telescope is
Dipole Feed				subjected to overspeed then the entire system
				would fail and telescope will be damaged, so to
The dipole feed consists of 1056 half wave				prevent the damage, overspeed tacho circuit is
dipoles hacked by a corner reflector and placed				used. When the overspeed occurs, it is identified
along the focal line on feed tracks. The physical				by the overspeed Tacho circuit that activates a
aperture of antenna is 530 m long and 30 m wide.				relay.

Receiver
				Misalignment loop:
The local oscillator signal at 296.5 Mhz is generated
in the receiver room that is located near adjacent				If any trusses are not rotated or twisted
to the telescopic antenna. The signal is carried in				misalignment may takes place. To avoid this circuit
the transmission line to a central part where it				rings are at the alternate trusses and a wire is
branches into two parts. One branch is for the				connected in between them with a 30-v dc
northern element modules and the other for the				supply. When any misalignment occur to wires will
southern element modules. The RF amplifier is				touch the ring, which will then ac the relay to
followed by a mixer which down converts the RF				switch off supply.
band to an intermediate frequency(IF) band
centred at 30 Mhz. the IF signal from each module				Differential Synchro
is transmitted to the receiver room through a
separate coaxial cable.				The system is used to detect the misalignment in
				the main shaft of the ORT.
ELECTRONIC SYSTEMS				The synchro motor is fixed at both the ends of
				the shaft and the differential error W is fed to an
Low Noise Amplifier(LNA)				electronic circuit, which operates a relay. The relay
				is set to trip the main supply when the
The LNA, which comes immediately after the				misalignment goes beyond 150 in either direction.
dipole, plays major role in determining the system
temperature, and thus the overall sensitivity of				Braking
the telescope. It should have a lowest noise figure				After a track or slew operation, brakes are used
at the frequency of operation with a noise figure				to place the antenna at desired position without
of .06 db at 2 Ghz was chosen for LNA.				failing. The braking is in nature and when given a
				DC supply pushes the brake shoes and stops the
GaAs FET devices are generally used at				rotation of the shaft.
microwave frequencies for optimum gain and low
noise performance using high gain, high frequency				Data Acquisition System For Eas At Grapes-III
gas FET at VHF and UHF frequencies however
possess special problems.				Introduction

A simple technique broadband loading of FET				The Cosmic Ray Laboratory(CRL) is situated jus
drain, was used to ensure stability over the				few meters above the RAC. The CRL
wideband.				constructed with more than 700 detectors
				arranged in hexagonal manner, to detect the
Phase shifters				cosmic showers from the sky by giving very high
				voltage to plastic scintillation squares.
Scanning the beam of a radio telescope consists				Description
of an array of antennas electronically switchable
phase shifters employing PIN diodes are				The very high voltages in range of 2450-volts are
commonly used.				given to detectors,then the output signal will given
				to controller section.
Types of phase shifters:				In this section the total signal is processed in the
- Phase lead-lag type				form of ,
- Micro strip line type				- TDC
				- ADC


The second type is used in ORT. The				Possible Radio Interferences
characteristic impedance is 50 ohm. Here extra				- Spurious effects as a result of the sun
loss of .1 db and .05 db are provided in the direct				appearing through distant side lobes of the
path.				telescope.
				- The radio interference is generated when faulty
Image rejection filters				television booster amplifiers are in function.
				- Also potato insecticides sprayers used by
It is basically a band reject filter. The o/p of 326.5				farmers , will produce radio interference.
Mhz from RF amplifier is fed to this filter. The o/p				- Hence to avoid it the observations in these are
is obtained is in the band region of 15 Mhz.				temporarily suspended.

Mixer				CONCLUSION

The mixer is generally an adder or subtractor				The IPT which we underwent gave us a new
circuit. The o/p's from IRF and local oscillator				great experience at RAC, Ooty. This training
phase shifter to produce the sum and difference				program will surely help us in our future. We
o/p of the two frequencies.				would like to thank everyone at RAC, Ooty for
				their support and encouragement through out the
Intermediate Frequency amplifier				training period.

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Radio Astronomy Centre

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