The magnetic field strengths (B) in the solar photosphere are routinely measured using the Zeeman effect. However, such measurements are not available in the solar chromosphere and corona. The Gauribidanur RAdioheliograPH (GRAPH), near Bangalore in India, has been successfully carrying out total intensity (Stokes I) observations of the solar corona at low frequencies (≲ 100 MHz) for several years now. Radio emission from the Sun in the above frequency range originates typically in the heliocentric distance r ≳ 1.2 R_⊙ . We recently augmented GRAPH with the aim to observe circularly polarized radio emission (Stokes V) from solar corona, in particular, from the 'quiet' corona. Combining the low frequency Stokes I V radio images, and modeling techniques, it is possible to calculate the magnetic field strengths in the solar corona on regular basis. This work details the procedures undertaken to augment the GRAPH for simultaneous observations of both Stokes I V radio emission from the solar corona, and the calibration procedure. Preliminary results indicate that the augmented GRAPH can effectively detect Stokes V emission associated with thermal emission from the 'quiet' Sun at frequencies ≲ 100 MHz, in addition to Stokes V emission associated with the non-thermal radio bursts from the 'active' Sun.

The presence of the magnetic field makes the solar coronal medium birefringent. Research indicates that an appreciable degree of circular polarization (DCP) can be observed in the thermal radio emission at frequencies <100 MHz from the solar corona, due to the difference in the absorption coefficients of the ordinary ("o") and extraordinary ("e") modes of propagation in the magnetized coronal medium. Measurements of this DCP from observations are, however, rare. In this study, we report Stokes-I and Stokes-V observations of thermal radio emission from the "quiet" solar corona in the frequency range 50-80 MHz, using a one-dimensional radio interferometric polarimeter. The estimated DCP in the above frequency range is approximate to 2.5%-1.2%. The results indicate the potential in ground-based low-frequency radio observations to estimate the coronal magnetic field in the "quiet" corona in the heliocentric distance range (r < 2.0 R-circle dot), where solar radio emission in the above frequency range typically originates.

We are exploring the possibility of carrying out radio interferometric observations of the solar chromosphere at ≈ 11.2 GHz ( λ=2.68 cm), in both total intensity (Stokes-I) and circularly polarized intensity (Stokes-V), using low-cost commercial dish TV antennas. Here, we present our initial results on the magnetic field strength (B) estimated using data obtained with a prototype set-up, and compare them with similar observations.

Recently we had reported commissioning of a prototype for pulsar observations at low radio frequencies (<100 MHz) using log-periodic dipole antennas in the Gauribidanur Radio Observatory (approximate to 77 degrees E, 14 degrees N) near Bangalore in India (https://www.iiap.res.in/?q=centers/radio). The aforementioned system (the Gauribidanur Pulsar System) is currently being augmented to directly digitize the radio-frequency signals from the individual antennas in the array. Our initial results using a 1-bit raw voltage-recording system indicate that such a back-end receiver offers distinct advantages like (i) simultaneous observations of any set of desired directions in the sky with multiple offline beams and smaller data rate/volume, and (ii) archival of the observed data with minimal resources for reanalysis in the future, either in the same or a different set of directions in the sky.

The Gauribidanur Pulsar System (GAPS) is a new Radio telescope at the Gauribidanur Radio Observatory (Lon:77.4° E; Lat: 13.6° N)) in India. Its purpose is to observe Pulsars and other astrophysical transients at low radio frequencies (<100 MHz). The system was recently upgraded to a “Digital Radio telescope” where raw voltages from each antenna are recorded separately using 1-bit digitization, and the beamforming is done in the software. Such a software-based binary system offers distinct advantages, like simultaneously probing different directions in the sky with reduced data rate/volume, making it suitable for transient surveys. We present a summary of the 1 -bit raw voltage recoding receiver of GAPS with its initial results.

Recently a prototype for pulsar observations at low radio frequencies (RFs) (<100MHz) using log-periodic dipole antennas (LPDAs) in the Gauribidanur Radio Observatory (approximate to 77 degrees E14 degrees N) near Bangalore, India, was commissioned. The aforementioned system is currently being augmented (i) to directly digitize the RF signals from the individual antennas and (ii) with a digital beamformer to simultaneously observe different regions of the sky present within the primary "beam" of the LPDA used in the array. Our initial results indicate that co-temporal observations of a known pulsar along with the Sun using two different beams could be used to calibrate the dynamic spectrum of the solar radio transients. This is important because the calibration of the latter in observations with the conventional solar radio spectrographs is difficult. (c) 2023 Society of Photo-Optical Instrumentation Engineers (SPIE)

The Zeeman effect has been routinely used to image and quantify the solar photospheric magnetic field (B). Such a direct measuring technique is not yet available for the corona (Lin et al. 2004). Since almost all transient nonthermal radio emissions from the corona are either partially or fully circularly polarized, observing their polarization signatures over broad frequency ranges would be of help to estimate B as a function of heliocentric height. This article aims to describe the design and development of a Cross-polarized Log-Periodic Dipole Antenna (CLPDA), an integral part of a radio spectro-polarimeter, which works in the 50–500 MHz frequency-range and to explain the tests that were carried out to characterize it. The above frequency range corresponds to a heliocentric height range ≈1.03 < r < 2.5 R _⊙ ( R _⊙ = photospheric radius ), wherein the numerous coronal nonthermal transients associated with space-weather effects are observed to originate. The CLPDA is used to determine the strength and sense of polarization of the received radio signal. The uncertainty involved in the determination depends on the polarization-isolation (PI) between the two orthogonal components of a CLPDA. Some of the recent advancements made in the antenna design concepts at high frequencies (∼GHz) were adopted to reduce the PI at low frequencies (∼MHz). Throughout the above frequency range, the CLPDA has a gain, return loss, and PI of ≈6.6 dBi, ≲−10 dB, and ≲−27 dB, respectively. The average PI of the CLPDA varies from −30 to −24 dB over an azimuthal angle range 0° to ±45° within which the observations are performed regularly.

This paper presents high-performance computing efforts with FPGA for the accelerated pulsar/transient search for the square kilometre array (SKA). Case studies are presented from within SKA and pathfinder telescopes highlighting future opportunities. It reviews the scenario that has shifted from offline processing of the radio telescope data to digitizing several hundreds/thousands of antenna outputs over huge bandwidths, forming several hundreds of beams, and processing the data in the SKA real-time pulsar search pipelines. A brief account of the different architectures of the accelerators, primarily, the new generation field programmable gate array-based accelerators, showing their critical roles to achieve high-performance computing and in handling the enormous data volume problems of the SKA is presented here. It also presents power-performance efficiency of this emerging technology and presents potential future scenarios.

A dedicated system for observations of Pulsars and other astrophysical transients in the frequency range of 50–80 MHz has been recently commissioned at the Gauribidanur Radio Observatory near Bangalore, India. One of the salient features of the system is digital delay beamforming to simultaneously phase the antenna response pattern (“beam”) to different directions in the sky as compared to the generic cable phase/delay shifters which help to observe only a particular direction of the sky at a given epoch. Mechanical steering is difficult for the large antenna arrays used at low frequencies. The work reported is expected to provide useful inputs for similar work with SKA-Low.

The objective of this work is to develop a prototype broadband radio antenna that can be used for observing the Sun and other astronomical sources in the 200–600 MHz band; the latter corresponds to a heliocentric height range of ~ 1.01-1.30 $\mathrm{R}_{\odot} ( >\mathrm{R}_{\odot}$ = photospheric radius). In the case of non-solar targets, the above frequency range is well suited for the observations of Fast Radio Bursts (FRBs) and other transients. The Log Periodic Dipole Antenna (LPDA) is chosen as the feed because it has broadband, directional and uniform characteristics over its operating bandwidth as compared to other broadband antennas. Additionally, the antenna is fitted to a rotor system having minimal radio frequency interference $(\lesssim-100\ \text{dBm})$ in order to track the Sun both in hour angle and declination. It also helps to achieve uniform antenna gain as a function of frequency throughout the observing time period as compared to a stationary zenith pointing system. Although a dish antenna with a broadband feed is preferred due to a larger collecting area and better sensitivity, its gain varies appreciably over the operating bandwidth and its return loss is greater than the nominal value $(\approx-9.5\ \text{dB})$ . The performance of this new system (in the standalone receiving element mode at present) is better than a dish antenna feed in terms of uniform gain and return loss over the designed operating bandwidth.

The radio astronomy group in the Indian Institute of Astrophysics (IIA) has been carrying out routine observations of radio emission from the solar corona at low frequencies (≈40 - 440 MHz) at the Gauribidanur observatory, about 100 km north of Bangalore. Since IIA has been performing regular observations of the solar photosphere and chromosphere using different optical telescopes in its Kodaikanal Solar Observatory (KSO) also a a https://www.iiap.res.in/kodai.htm, the possibilities of carrying out Stokes-I and Stokes-V observations of the solar chromosphere using low-cost radio instrumentation to supplement the optical observations are being explored. Note that measurements of Stokes-V help to infer the magnetic field strength of the sunspots. As a part of the exercise, recently the group has developed prototype instrumentation for interferometric observations of radio emission from the solar chromosphere at high frequencies (≈ 11.2 GHz) using three commercial dish TV antennas. The hardware set-up and initial observations are presented.

Abstract. A prototype for dedicated observations of pulsars and other astrophysical transients in the frequency range of 50 to 80 MHz has been recently commissioned in the Gauribidanur Radio Observatory near Bangalore in India. The antenna setup, the analog and digital receiver systems, and the initial observations are presented.

The Indian Institute of Astrophysics (IIA) has commissioned a low frequency (<100 MHz) spectrograph at its Kodaikanal Solar Observatory (KSO) for coordinated observations of transients in the solar atmosphere with other existing optical observing facilities there. The hardware set-up and initial observations are presented. The availability of different instruments in the same observatory helps to quickly plan and jointly observe the energetic phenomena and their signatures in all the three major atmospheric regions of the Sun, i.e. photosphere, chromosphere and corona.

The radio astronomy group in the Indian Institute of Astrophysics (IIA) has been carrying out routine observations of radio emission from the solar corona at low frequencies (≈40 – 440 MHz) at the Gauribidanur observatory, about 100 km north of Bangalore. Since IIA has been performing regular observations of the solar photosphere and chromosphere using different optical telescopes in its Kodaikanal Solar Observatory (KSO) also (see  https://www.iiap.res.in/kodai.htm ), the possibilities of obtaining two-dimensional radio images of the solar chromosphere using low-cost instrumentation to supplement the optical observations are being explored. As a part of the exercise, recently the group had developed prototype instrumentation for interferometric observations of radio emission from the solar chromosphere at high frequencies (≈11.2 GHz) using two commercial dish TV antennas. The hardware set-up and initial observations are presented.

Over the last 5 years, UVIT has completed observations of more than 500 proposals with ~ 800 unique pointings. In addition, regular planned monitoring observations have been made and from their analysis various key parameters related to in orbit performance of UVIT have been quantified. The sensitivities of the UV channels have remained steady indicating no effect of potential molecular contamination confirming the adequacy of all the protocols implemented for avoiding contamination. The quality of the PSF through the years confirms adequacy of thermal control measures. The early calibrations obtained during the Performance Verification (PV) phase have been further revised for more subtle effects. These include flat fields and detector distortions with greater precision. The operations of UVIT have also evolved through in orbit experience, e.g. tweaking of operational sequencing, protocol for recovery from bright object detection (BOD) shutdowns, parameters for BOD thresholds, etc. Finally, some effects of charged particle hits on electronics led to opimised strategy for regular resetting. The Near-UV channel was lost in one of such operations. All the above in-orbit experiences are presented here.

Results of the initial calibration of the Ultra-Violet Imaging Telescope (UVIT) were reported earlier by Tandon et al. (2017). The results reported earlier were based on the ground calibration as well as the first observations in orbit. Some additional data from the ground calibration and data from more in-orbit observations have been used to improve the results. In particular, extensive new data from in-orbit observations have been used to obtain (a) new photometric calibration which includes (i) zero-points (ii) flat fields (iii) saturation, (b) sensitivity variations (c) spectral calibration for the near Ultra Violet (NUV; 2000 - 3000 Angstroms) and far Ultra-Violet (FUV; 1300 - 1800 Angstroms) gratings, (d) point spread function and (e) astrometric calibration which included distortion. Data acquired over the last three years show continued good performance of UVIT with no reduction in sensitivity in both the UV channels.

We have designed a new ROACH-based digital back-end for the existing Gauribidanur RAdio Spectro-Polarimeter (GRASP) for observation of the solar corona in the frequency range 400 -50 MHz, which corresponds to $1.05-2.0 \mathrm { R } _ { \odot }$ on the solar surface. It is well established that the solar radio bursts at our interest of frequencies are found to be circularly polarized. This system consists of a Cross Polarised Log-Periodic Dipole Antenna (CLPDA), which is a combination of two orthogonally oriented LPDAs. The sum of auto-correlation of the two orthogonal signals corresponds to Stokes-I, and the sum of imaginary components of the cross-correlations corresponds to Stokes -V. The RF signals from CLPDA, after passing through a bandpass filter are directly digitized using iADC at its Nyquist rate (800 MHz). The Fourier analysis of this digitized signal is done by a four tap Poly-phase Filter Bank (PFB) to suppress the side-lobes and a 8192 point FFT is computed. In order to correct the polarisation leakage the real component of the cross-correlation is also computed. The performance of the new digital back-end will be compared with the existing spectrum analyser based back-end. The detailed characterization of the digital back-end, calibration of the system using lab test, observations of Galactic Center (GC) and preliminary results will be discussed.