Radio Astronomy Centre

 

 

 

 

 

 

 

Tata Institute of Fundamental Research

 

Ooty

 

 

 

 

 

 

 

 

CONTENTS

 

 

 

• Introduction
• Construction
• Electronic systems
• Electrical systems
• Protection systems
• Data acquisition system

 

 

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 or telescope the strength of radio waves received from different directions from sky can be determined and a map of the radio signals from the sky can be made.

 

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 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 down a natural hill slope of about 11 equal to the ideal latitude of the observatory. This arrangement  ensures the axis of the telescope is parallel to the earth axis. This enables the ORT to track celestial objects for about 9.5hours by simply rotating the telescope mechanically in east to west direction.

 

CONSTRUCTION

 

The following are the building blocks of ORT:

 

• Reflector
• Dipole feed
• Receiver

 

 

 

 

 

Reflector

 

The antenna reflector of ORT is the parabolic cylinder with its surface formed by 1100 stainless steel wires. Each fine stainless steel wire is of .38mm in diameter. These stainless steel wires are stretched along the length of cylinder.

 

Dipole Feed

 

The dipole feed consists of 1056 half wave dipoles hacked by a corner reflector and placed along the focal line on feed tracks. The physical aperture of antenna is 530 m long and 30 m wide.

 

Receiver

 

The local oscillator signal at 296.5 Mhz is generated in the receiver room that is located near adjacent to the telescopic antenna. The signal is carried in the transmission line to a central part where it branches into two parts. One branch is for the northern element modules and the other for the southern element modules. The RF amplifier is followed by a mixer which down converts the RF band to an intermediate frequency(IF) band centred at 30 Mhz. the IF signal from each module is transmitted to the receiver room through a separate coaxial cable.

 

ELECTRONIC  SYSTEMS

 

Low Noise Amplifier(LNA)

 

The LNA, which comes immediately after the dipole, plays major role in determining the system temperature, and thus the overall sensitivity of the telescope. It should have a lowest noise figure at the frequency of operation with a noise figure of .06 db at 2 Ghz was chosen for LNA.

 

GaAs FET devices are generally used at microwave frequencies for optimum gain and low noise performance using high gain, high frequency gas FET at VHF and UHF frequencies however possess special problems.

 

A simple technique broadband loading of FET drain, was used to ensure stability over the wideband.

 

Phase shifters

 

Scanning the beam of a radio telescope consists of an array of antennas electronically switchable phase shifters employing PIN diodes are commonly used.

 

Types of phase shifters:

• Phase lead-lag type
• Micro strip line type

 

 

 

The second type is used in ORT. The characteristic impedance is 50 ohm. Here extra loss of .1 db and .05 db are provided in the direct path.

 

Image rejection filters

 

It is basically a band reject filter. The o/p of 326.5 Mhz from RF amplifier is fed to this filter. The o/p is obtained is in the band region of 15 Mhz.

 

Mixer

 

The mixer is generally an adder or subtractor circuit. The o/p’s from IRF and local oscillator phase shifter to produce the sum and difference o/p of the two frequencies.

 

Intermediate Frequency amplifier

 

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.

 

ELECTRICAL SYSTEMS

The main functional units are as follows:

 

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 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 large distance before reaching the centre. From 220 KV supply –stepped down to 440V. The transformer contains oil cooling and silica gel to observe moisture. The 440V is taken to distribution board and is given to various loads. If there is no need of heavy loads, a small generator of 40 KVA is used.

 

 

 

 

 

 

 

 

 

 

ROTATION

The telescope can rotate in East-west direction.

 

Before rotation

 

 

 

 

 

 

 

After rotation

 

 

 

 

 

 

 

 

 

There are two modes of rotation

1. Slew circuit
2. Track circuit

 

Slew circuit

• It employs 12.5 hp induction motor at 1440 rpm located at 4 trusses (N4,S4,N10,S10)
• Induction motor’s speed torque is converted to 75 rpm using gear trails.

Track circuit

• Used to track the object continuously.
• It employs 3-phase 5 hp motors that rotates the shaft at 3 rpm with the help of gear trails.

 

 

 

 

Usually the motors brought to a stop, the telescope strips further away from the required position due to momentum. This is a positional error.

 

PROTECTION SYSTEMS

Overspeed tacho circuit(need)

 

The telescope (ORT) is rotated at a speed of 75 rpm. Due to any fault conditions such as shaft breakage or motor failure, the telescope is subjected to overspeed. If the telescope is subjected to overspeed then the entire system would fail and telescope will be damaged, so to prevent the damage, overspeed tacho circuit is used. When the overspeed occurs, it is identified by the overspeed Tacho circuit that activates a relay.

 

 

Misalignment loop:

 

If any trusses are not rotated or twisted misalignment may takes place. To avoid this circuit rings are at the alternate trusses and a wire is connected in between them with a 30-v dc supply. When any misalignment occur to wires will touch the ring, which will then ac the relay to switch off supply.

 

Differential Synchro

 

The system is used to detect the misalignment in the main shaft of the ORT.

The synchro motor is fixed at both the ends of the shaft and the differential error W is fed to an electronic circuit, which operates a relay. The relay is set to trip the main supply when the misalignment goes beyond 150 in either direction.

 

Braking

After a track or slew operation, brakes are used to place the antenna at desired position without failing. The braking is in nature and when given a DC supply pushes the brake shoes and stops the rotation of the shaft.

 

Data Acquisition System For Eas At Grapes-III

 

Introduction

 

The Cosmic Ray Laboratory(CRL) is situated jus few meters above the RAC. The CRL constructed with more than 700 detectors arranged in hexagonal manner, to detect the cosmic showers from the sky by giving very high voltage to plastic scintillation squares.

Description

 

The very high voltages in range of 2450-volts are given to detectors,

then the output signal will given to controller section.

In this section the total signal is processed in the form of ,

• TDC
• ADC

 

 

Possible Radio Interferences

• Spurious effects as a result of the sun appearing through distant side lobes of the telescope.
• The radio interference is generated when faulty television booster amplifiers are in function.
• Also potato insecticides sprayers used by farmers , will produce radio interference.
• Hence to avoid it the observations in these are temporarily suspended.

 

CONCLUSION

 

The IPT which we underwent gave us a new great experience at RAC, Ooty. This training program will surely help us in our future. We would like to thank everyone at RAC, Ooty for their support and encouragement through out the training period.

 

 

 

 

 

 

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