arXiv subjects

A. G. Sreejith

Publications and source records attributed to A. G. Sreejith.

34 records · Page 2Linked to original sources

arXiv2018

Characterisation of the HD219134 multi-planet system II. Stellar-wind sputtered exospheres in rocky planets b & c

We present a 3D study of the formation of refractory-rich exospheres around the rocky planets HD219134b and c. These exospheres are formed by surface particles that have been sputtered by the wind of the host star. The stellar wind properties are derived from magnetohydrodynamic simulations, which are driven by observationally-derived stellar magnetic field maps, and constrained by Ly-alpha observations of wind mass-loss rates, making this one of the most well constrained model of winds of low-mass stars. The proximity of the planets to their host star implies a high flux of incident stellar wind particles, thus the sputtering process is sufficiently effective to build up relatively dense, refractory-rich exospheres. The sputtering releases refractory elements from the entire dayside surfaces of the planets, with elements such as O and Mg creating an extended neutral exosphere with densities larger than 10/cm3, extending to several planetary radii. For planet b, the column density of OI along the line of sight reaches 10^{13}/cm2, with the highest values found ahead of its orbital motion. This asymmetry would create asymmetric transit profiles. To assess its observability, we use a ray tracing technique to compute the expected transit depth of the OI exosphere of planet b. We find that the transit depth in the OI 1302.2A line is 0.042%, which is a small increase relative to the continuum transit (0.036%). This implies that the sputtered exosphere of HD219134b is unlikely to be detectable with our current UV instruments.

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arXiv2018

CUTE Data Simulator and Reduction Pipeline

The Colorado Ultraviolet Transit Experiment (CUTE) is a 6U NASA CubeSat carrying a low-resolution (R ~3000), near-ultraviolet (255 - 330nm) spectrograph fed by a rectangular primary Cassegrain. CUTE, is planned for launch in spring 2020 and it will monitor transiting extra-solar planets to study atmospheric escape. We present here the CUTE data simulator, which is a versatile tool easily adaptable to any other mission performing single-slit spectroscopy and carrying on-board a CCD detector. We complemented the data simulator with a data reduction pipeline capable of performing a rough reduction of the simulated data. This pipeline will then be updated once the final CUTE data reduction pipeline will be fully developed. We further briefly discuss our plans for the development of a CUTE data reduction pipeline. The data simulator will be used to inform the target selection, improve the preliminary signal-to-noise calculator, test the impact on the data of deviations from the nominal instrument characteristics, identify the best spacecraft orientation for the observation of each target and construct synthetic data to train the science team in the data analysis prior to launch.

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arXiv2018

Wide-field Ultraviolet Imager for Astronomical Transient Studies

Though the ultraviolet (UV) domain plays a vital role in the studies of astronomical transient events, the UV time-domain sky remains largely unexplored. We have designed a wide-field UV imager that can be flown on a range of available platforms, such as high-altitude balloons, CubeSats, and larger space missions. The major scientific goals are the variability of astronomical sources, detection of transients such as supernovae, novae, tidal disruption events, and characterizing AGN variability. The instrument has an 80 mm aperture with a circular field of view of 10.8 degrees, an angular resolution of around 22 arcsec, and a 240-390 nm spectral observation window. The detector for the instrument is a Microchannel Plate (MCP)-based image intensifier with both photon counting and integration capabilities. An FPGA-based detector readout mechanism and real-time data processing have been implemented. The imager is designed in such a way that its lightweight and compact nature are well fitted for the CubeSat dimensions. Here we present various design and developmental aspects of this UV wide-field transient explorer.

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arXiv2017

Near UV Imager with an MCP Based Photon Counting Detector

We are developing a compact UV Imager using light weight components, that can be flown on a small CubeSat or a balloon platform. The system has a lens-based optics that can provide an aberration-free image over a wide field of view. The backend instrument is a photon counting detector with off-the-shelf MCP, CMOS sensor and electronics. We are using a Z-stack MCP with a compact high voltage power supply and a phosphor screen anode, which is read out by a CMOS sensor and the associated electronics. The instrument can be used to observe solar system objects and detect bright transients from the upper atmosphere with the help of CubeSats or high altitude balloons. We have designed the imager to be capable of working in direct frame transfer mode as well in the photon-counting mode for single photon event detection. The identification and centroiding of each photon event are done using an FPGA-based data acquisition and real-time processing system.

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arXiv2017

Development of Data Acquisition Methods for an FPGA-Based Photon Counting Detector

MCP-based detectors are widely used in the ultraviolet (UV) region due to their low noise levels, high sensitivity and good spatial and temporal resolution. We have developed a compact near-UV (NUV) detector for high-altitude balloon and space flights, using off-the-shelf MCP, CMOS sensor, and optics. The detector is designed to be capable of working in the direct frame transfer mode as well in the photon-counting mode for single photon event detection. The identification and centroiding of each photon event are done using an FPGA-based data acquisition and real-time processing system. In this paper, we discuss various algorithms and methods used in both operating modes, as well as their implementation on the hardware.

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arXiv2017

Prospect for UV observations from the Moon. II. Instrumental Design of an Ultraviolet Imager LUCI

We present a design for a near-ultraviolet (NUV) imaging instrument which may be flown on a range of available platforms, including high-altitude balloons, nanosatellites, or space missions. Although all current UV space missions adopt a Ritchey-Chretain telescope design, this requires aspheric optics, making the optical system complex, expensive and challenging for manufacturing and alignment. An all-spherical configuration is a cost-effective and simple solution. We have aimed for a small payload which may be launched by different platforms and we have designed a compact, light-weight payload which will withstand all launch loads. No other UV payloads have been previously reported with an all-spherical optical design for imaging in the NUV domain and a weight below 2 kg. Our main science goal is focussed on bright UV sources not accessible by the more sensitive large space UV missions. Here we discuss various aspects of design and development of the complete instrument, the structural and finite-element analysis of the system performed to ensure that the payload withstands launch-load stresses and vibrations. We expect to fly this telescope -- Lunar Ultraviolet Cosmic Imager (LUCI) -- on a spacecraft to the Moon as part of the Indian entry into Google X-Prize competition. Observations from the Moon provide a unique opportunity to observe the sky from a stable platform far above the Earth's atmosphere. However, we will explore other opportunities as well, and will fly this telescope on a high-altitude balloon later this year.

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arXiv2016

A software package for evaluating the performance of a star sensor operation

We have developed a low-cost off-the-shelf component star sensor (StarSense) for use in minisatellites and CubeSats to determine the attitude of a satellite in orbit. StarSense is an imaging camera with a limiting magnitude of 6.5, which extracts information from star patterns it records in the images. The star sensor implements a centroiding algorithm to find centroids of the stars in the image, a Geometric Voting algorithm for star pattern identification, and a QUEST algorithm for attitude quaternion calculation. Here, we describe the software package to evaluate the performance of these algorithms as a star sensor single operating system. We simulate the ideal case where sky background and instrument errors are omitted, and a more realistic case where noise and camera parameters are added to the simulated images. We evaluate such performance parameters of the algorithms as attitude accuracy, calculation time, required memory, star catalog size, sky coverage, etc., and estimate the errors introduced by each algorithm. This software package is written for use in MATLAB. The testing is parametrized for different hardware parameters, such as the focal length of the imaging setup, the field of view (FOV) of the camera, angle measurement accuracy, distortion effects, etc., and therefore, can be applied to evaluate the performance of such algorithms in any star sensor. For its hardware implementation on our StarSense, we are currently porting the codes in form of functions written in C. This is done keeping in view its easy implementation on any star sensor electronics hardware.

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arXiv2016

Pointing System for the Balloon-Borne Astronomical Payloads

We describe the development and implementation of a light-weight, fully autonomous 2-axis pointing and stabilization system designed for balloon-borne astronomical payloads. The system is developed using off-the-shelf components such as Arduino Uno controller, HMC 5883L magnetometer, MPU-9150 Inertial Measurement Unit (IMU) and iWave GPS receiver unit. It is a compact and rugged system which can also be used to take images/video in a moving vehicle, or in areal photography. The system performance is evaluated from the ground, as well as in conditions simulated to imitate the actual flight by using a tethered launch.

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arXiv2016

Noise modeling and analysis of an IMU-based attitude sensor: improvement of performance by filtering and sensor fusion

We describe the characterization and removal of noise present in the Inertial Measurement Unit (IMU) MPU-6050. This IMU was initially used in an attitude sensor (AS) developed in-house, and subsequently implemented in a pointing and stabilization platform developed for small balloon-borne astronomical payloads. We found that the performance of the IMU degrades with time due to the accumulation of different errors. Using the Allan variance analysis method, we identified the different components of noise present in the IMU and verified the results using a power spectral density analysis (PSD). We tried to remove the high-frequency noise using smoothing filters, such as moving average filter and Savitzky-Golay filter. Although we did manage to filter some of the high-frequency noise, the performance of these filters was not satisfactory for our application. We found the distribution of the random noise present in the IMU using a probability density analysis, and identified the noise to be white Gaussian in nature which we successfully removed by a Kalman filter in real time.

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arXiv2016

Balloon UV Experiments for Astronomical and Atmospheric Observations

The ultraviolet (UV) window has been largely unexplored through balloons for astronomy. We discuss here the development of a compact near-UV spectrograph with fiber optics input for balloon ights. It is a modified Czerny-Turner system built using off-the-shelf components. The system is portable and scalable to different telescopes. The use of re ecting optics reduces the transmission loss in the UV. It employs an image-intensified CMOS sensor, operating in photon counting mode, as the detector of choice. A lightweight pointing system developed for stable pointing to observe astronomical sources is also discussed, together with the methods to improve its accuracy, e.g. using the in-house build star sensor and others. Our primary scientific objectives include the observation of bright Solar System objects such as visible to eye comets, Moon and planets. Studies of planets can give us valuable information about the planetary aurorae, helping to model and compare atmospheres of other planets and the Earth. The other major objective is to look at the diffuse UV atmospheric emission features (airglow lines), and at column densities of trace gases. This UV window includes several lines important to atmospheric chemistry, e.g. SO2, O3, HCHO, BrO. The spectrograph enables simultaneous measurement of various trace gases, as well as provides better accuracy at higher altitudes compared to electromechanical trace gas measurement sondes. These lines contaminate most astronomical observations but are poorly characterized. Other objectives may include sprites in the atmosphere and meteor ashes from high altitude burn-outs. Our recent experiments and observations with high-altitude balloons are discussed.

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arXiv2016

An Ultraviolet imager to study bright UV sources

We have designed and developed a compact ultraviolet imaging payload to fly on a range of possible platforms such as high altitude balloon experiments, cubesats, space missions, etc. The primary science goals are to study the bright UV sources (mag < 10) and also to look for transients in the Near UV (200 - 300 nm) domain. Our first choice is to place this instrument on a spacecraft going to the Moon as part of the Indian entry into Google lunar X-Prize competition. The major constraints for the instrument are, it should be lightweight (< 2Kg), compact (length < 50cm) and cost effective. The instrument is an 80 mm diameter Cassegrain telescope with a field of view of around half a degree designated for UV imaging. In this paper we will discuss about the various science cases that can be performed by having observations with the instrument on different platforms. We will also describe the design, development and the current state of implementation of the instrument. This includes opto-mechanical and electrical design of the instrument. We have adopted an all spherical optical design which would make the system less complex to realize and a cost effective solution compared to other telescope configuration. The structural design has been chosen in such a way that it will ensure that the instrument could withstand all the launch load vibrations. An FPGA based electronics board is used for the data acquisition, processing and CCD control. We will also briefly discuss about the hardware implementation of the detector interface and algorithms for the detector readout and data processing.

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arXiv2016

Measurements of gondola motion on a stratospheric balloon flight

Balloon experiments are an economically feasible method of conducting observations in astronomy that are not possible from the ground. The astronomical payload may include a telescope, a detector, and a pointing/stabilization system. Determining the attitude of the payload is of primary importance in such applications, to accurately point the detector/telescope to the desired direction. This is especially important in generally unstable lightweight balloon flights. However, the conditions at float altitudes, which can be reached by zero pressure balloons, could be more stable, enabling accurate pointings. We have used the Inertial Measurement Unit (IMU), placed on a stratospheric zero pressure balloon, to observe 3-axis motion of a balloon payload over a fight time of 4.5 hours, from launch to the float altitude of 31.2 km. The balloon was launched under nominal atmospheric conditions on May 8th 2016, from a Tata Institute of Fundamental Research Balloon Facility, Hyderabad.

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arXiv2015

An Overview of High-Altitude Balloon Experiments at the Indian Institute of Astrophysics

We have initiated the High-Altitude Ballooning programme at Indian Institute of Astrophysics, Bangalore, in the year 2011 with the primary purpose of developing and flying low-cost scientific payloads on a balloon-borne platform. The main aim is the observations of extended nearby objects (e.g. comets) and of diffuse sources (e.g. zodiacal light or airglow) with wide field of view (FOV) UV instruments from near space (20 to 30 km). A brief summary and the results of the tethered flights carried out at IIA CREST campus are given in Ref.~1. Here we present an overview of the nine free-flying balloon experiments conducted from March 2013 to November 2014. We describe the launch procedures, payloads, methods of tracking and recovery. Since we fall in the light balloon category --- payload weight is limited to less than 6 kg --- we use the 3-D printer to fabricate lightweight boxes and structures for our experiments. We discuss the flight and scientific data obtained from different launches and the development of the in-house lightweight sensors and controllers, as well as a fully-fledged 2-axis pointing and stabilization system, for the flights.

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arXiv2014

A Raspberry Pi-Based Attitude Sensor

We have developed a lightweight low-cost attitude sensor, based on a Raspberry Pi, built with readily available commercial components. It can be used in experiments where weight and power are constrained, such as in high- altitude lightweight balloon flights. This attitude sensor will be used as a major building block in a closed-loop control system with driver motors to stabilize and point cameras and telescopes for astronomical observations from a balloon-borne payload.

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arXiv2014

Prospect for UV observations from the Moon

Space astronomy in the last 40 years has largely been done from spacecraft in low Earth orbit (LEO) for which the technology is proven and delivery mechanisms are readily available. However, new opportunities are arising with the surge in commercial aerospace missions. We describe here one such possibility: deploying a small instrument on the Moon. This can be accomplished by flying onboard the Indian entry to the Google Lunar X PRIZE competition, Team Indus mission, which is expected to deliver a nearly 30 kgs of payloads to the Moon, with a rover as its primary payload. We propose to mount a wide-field far-UV (130--180 nm) imaging telescope as a payload on the Team Indus lander. Our baseline operation is a fixed zenith pointing but with the option of a mechanism to allow observations of different attitudes. Pointing towards intermediate ecliptic latitude (50 deg or above) ensures that the Sun is at least 40 deg off the line of sight at all times. In this position, the telescope can cover higher galactic latitudes as well as parts of Galactic plane. The scientific objectives of such a prospective are delineated and discussed.

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arXiv2013

High-Altitude Ballooning Program at the Indian Institute of Astrophysics

We have begun a program of high altitude ballooning at the Indian Institute of Astrophysics, Bangalore. Recent advances in balloons as well as in electronics have made possible scientific payloads at costs accessible to university departments. The primary purpose of this activity is to test low-cost ultraviolet (UV) payloads for eventual space flight, but to also explore phenomena occurring in the upper atmosphere, including sprites and meteorite impacts, using balloon-borne payloads. This paper discusses the results of three tethered balloon experiments carried out at the CREST campus of IIA, Hosakote and our plans for the future. We also describe the stages of payload development for these experiments.

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