GMRT Status : June 1999
Frontends and Backends
- Antennas: All 30 reflectors using the SMART technology have been fabricated and installed on cement towers.
- Servo Systems: Servo system for alt-azimuth mount has been installed on all 30 antennas and these can be controlled remotely from the CEB. The servo system has been designed by BARC.
- Feeds : The 610, 325, 210 and 150 MHz feeds have been installed on all the antennas. The 1420 MHz feed has been installed on 28 antennas. The 50 MHz feed design is in progress. All feeds are linearly polarized and these are converted to circular polarizations, RCP and LCP for all frequencies except the L-band. No conversion to circular is made for the L-band to avoid increasing the system temperature. The L-band feeds have been designed and fabricated by the Raman Research Institute, Bangalore.
- Local Oscillators: Frequency synthesizers have been installed on all antennas for all frequency bands except L-band. The L-band LOs (600-1800 MHz) have been installed on 28 antennas.
- Optical Fibre links: All antennas are connected to the CEB through optical fibre links. The faulty Fibre Optic connectors have been replaced by APC connectors. Telemetry, LO and IF signals are relayed between the antennas and CEB through the fibres. A 100 Mbps optic link connecting GMRT, Khodad to NCRA, Pune has become operational.
- Baseband Recievers: The baseband system for both the side-bands, for all the antennas, has been installed and working satisfactorily in the CEB. The available baseband bandwidths are 16, 8, 4, 2, 1, 0.5, 0.25, 0.125, 0.062 MHz.
- Correlator: The 30-station single side-band, dual polarization, ~230,000 channel FX correlator has been installed in the CEB in August 1998. This correlator is designed to generate 128 spectral channels spread over a maximum bandwidth of 16 MHz for each of the polarizations. Fringe stopping and fractional sample time correction is done in the hardware. The design of the correlator for the second side-band is in progress.
- Pulsar Receiver :
- Phased Array :
- Control System : The control software is working satisfactorily.
- DAS : Although not in its final compact form, software for acquiring data, integrating in the memory and writing into data files have been developed. Programmes which maintain the delay and fringe corrections have been found to be working satisfactorily. Work is under progress to integrate the software into one package. The software is also being tested.
- Data Reduction: Software which can display and gain-calibrate the raw data are available. Detailed image processing is done in AIPS.
- Pulsar :
- Antenna Efficiency:
- System temperature:
- Pointing Accuracy :
Observations at 1420, 610, 325, 230 and 150 MHz have been carried out at GMRT using both the 8-station and the 30-station correlators. Presently, at least 20 antennas are available for observations (rest taken away by maintenance).
- Continuum: GMRT correlator always generates 128 channels. Continuum refers to either one channel or collapsing of several channels. Continuum observations of clusters of galaxies ( eg Coma), radio galaxies ( eg ), supernova remnants ( eg Cas A and W4) have given reliable images . Most of these observations have been done in the 325 MHz band. The dynamic range in these images is typically 500:1.
- Pulsar :
- Spectral Line: :Once the continuum is subtracted from the spectral line cube, the image noise characteristics at 325 MHz are seen to improve. The rms noise in the 325 MHz cube differs from the expected noise by only a factor of two which is very encouraging. The spectral dynamic range presently achievable with GMRT in the 325 MHz band is between 1000:1 and 500:1. However, testing the system and removing bugs to improve the dynamic range is in progress. Spectral line observations in the L-band have been satisfactory. Observations of the 21-cm line from galactic sources and the low-redshift damped Lyman alpha absorbers have been fruitful.
- Work in the future: The 50 MHz feeds are presently being designed. The correlator for the second side-band is being designed and is expected to be installed by ?. The DAS software is being integrated. The phased array mode is to become operational. User interface to the control software is being designed.
- Ongoing maintenance: Painting of the antennas is an important maintenance aspect of the telescope. At any given time, at least 1-2 antennas are taken up for painting during the day. Presently, two days in a week are taken up for general maintenance of the system. Rest of the days, the telescope is available to observers for system tests or observations.
Summarizing the GMRT Status:
The 30 antenna single sideband correlator was completed and moved to the telescope site in August 1998 and integrated into the system. The correlator has been used extensively for hardware and software tests and validation. Since January 1999, the correlator has been used fairly routinely for astronomical observations and maps of a variety of sources with nearly the full angular resolution of the GMRT have been produced at 325 MHz. The spectral stability of the correlator has also been extensively tested. While the correlator is functional, a number of low level problems which limit the performance of the system, have been detected and are being studied.
During this year also, the consolidation of the GMRT has continued. The installation of the 1420MHz feed and the Local Oscillator Synthesizers (SYN2) covering 600 to 1800MHz has been completed on all but 2 of the GMRT antennas. Installation of updated Feed Positioning System (FPS) has also been completed on all but 3 antennas. A prototype of an improved FPS unit with nonvolatile RAM that makes the existing differential encoder look like an absolute encoder, has been developed. A prototye of the 50MHz feed for the GMRT antennas has been developed and is being tested. Work has started on a prototype of the battery based Auto stowlock system that can park the GMRT under high wind conditions even in the absence of MSEB power. A 100 Mbps full duplex fibre optic link connecting GMRT, Khodad with NCRA, Pune has been designed and commissioned.
In addition to routine maintenance of the GMRT systems, a project of characterising and optimising the performance of all the subsystems of all the GMRT antennas has started and is continuing. Some of the sub systems that have been giving problems have been improved to increase the relaiblilty of the system. The radio frequency interference environment around the GMRT continues to be a source of concern and is being regularly monitored and studied.