About me

I was born and brought up in the city of Kolkata, and did my schooling at Calcutta Boys' School. In 2007, I moved to Kanpur for a combined Bachelors and Masters degree in Physics from IIT Kanpur. After 5 years, I moved to USA in 2012 to pursue a PhD in Astronomy at the University of Maryland, College Park. The majority of my thesis work was carried out at NASA Goddard Space Flight Center, which shares a good connection with the university.

My primary interest lies in doing instrumentation towards science. I like having an understanding of the entire process starting from a set of ideas, iterating over the design, building the instrument, collecting data using it and finally analyzing the data to derive the science. Over the last ten years I have been involved in many instrumentation projects that are aimed at explaining various physical phenomena, from molecular to astronomical scales. Many of these projects involve working closely in a team over a long period of time and I love this aspect of learning and building together. My major project at IIT Kanpur -building India's first nano-satellite and my major project during the PhD -building a balloon-borne interferometer to study star formation; -both involved team-work towards developing pioneering technology. They also required assuming responsibilities for major sub-systems of the missions, and helped me nurture my leadership abilities.

The technical skills that I acquired through my research range from optical designing and metrology to astronomy data calibration and analysis. I am eager to apply these skills and develop new ones in future projects towards the devlopment of science and technology.

Contact

Email: arnabdh@gmail.com

Phone:

+1-240-351-6961 in US
+91-94333-44872 in India

Current Address:

1417 Duckens St., #308
Odenton, MD 21113, USA

Permanent Address:

48/3 Rishi Arabinda Sarani, Flat 5
Kolkata 700090, WB, India

Current Research

BETTII Optics

The BETTII (Balloon Experimental Twin Telescope for Infra-red Interferometry) is a double-Fourier interferometer that combines Michelson Interferometery with Fourier Transform Spectrocopy to attain spatially resolved spectroscopy in the far-infrared wavelengths of 30-90 microns. It has a baseline of 8 meters providing a spatial resolution of 0.5 arcseconds at 40 microns. It is designed to study star formation in clusters by disentangling the contribution of individual embedded protostars in a cluster and understanding whether the FIR emission originate only from point sources or are they extended. BETTII had a successful engineering flight in summer 2017, where we tested the control system. However, a mishap after the flight during landing has resulted in us having to rebuild the instrument, thereby delaying its science flight. The team is led Dr. Stephen Rinehart at NASA - Goddard Space Flight Center, and is comprised of many scientists including Dr. Maxime Rizzo, Dr. Todd Veach, Mr. Jordi Vila, Dr. Dale Fixsen, Dr. Roser Juanola-Parramon and Dr. Robert Silverberg amongst many others.

The optics of BETTII involves collecting light from two arms (for spatial interferometry) and changing the delay between them systematically (for spectral interferometry) before combining them and focusing them on to detectors. I modeled the effects of optics misalignments and aberrations on the fringes obtained at the BETTII detector using the FRED optical engineering software. Based on the tolerances obtained from the model, I developed and executed the alignment procedure for the entire optical system and associated it with the BETTII control system using laser trackers and theodolites (Dhabal et al., 2017).

The opto-mechanical parts of BETTII also include a warm and a cold delay line. The warm delay line corrects for path length differences between the left and the right arm due to balloon pendulation, while the cold delay line is aimed at introducing a systematic path length difference, thereby generating our interferograms from where we can derive information about the spectra. My instrumentation effort also included calibrating and characterizing the delay lines of BETTII (Dhabal et al., 2016).

These are some more papers on details of the BETTII design:-

Overview
Sensitivity
Telescope Design

Interferometry Simulations

Since BETTII is a two-element fixed baseline interferometer and since balloon flight time is short, it has a limted u-v plane coverage. Standard inversion techniques cannot be used to image the emission as is typically done for radio interferometers. To have a better understanding of the instrument's performance, preferred observing strategies and how to process the data to perform spatially resolved spectroscopy, I developed an interferometry simulator. I used the Hyperion Monte Carlo Radiative Transfer code to produce high resolution images of YSO systems (both spatial and spectral) at different inclination angles by varying YSO parameters. This grid of models are used as inputs for the BETTII Interferometry Simulator to generate outputs interferograms. Variations in fringe amplitudes and visibilities for different input scenes were used to establish that BETTII will be sensitive to inclination angle, disk mass and envelope mass of protostars. Further, the degeneracies between some parameters such as the disk mass and envelope mass can be broken by combining the amplitude and visibility information from the two BETTII bands.

Link to poster

Morphology and Kinematics of Filaments

Filaments in molecular clouds play a fundamental role in the star formation process by setting the initial conditions for core formation and fragmentation, and defining the morphology of the material available for accretion \citep{Hacar2011}. Herschel FIR images demonstrate the prevalence of filaments over a wide range of scales, tracing them into areas of active star formation. I investigated the structure and kinematics of 5 Herschel filaments using dense gas tracer molecular line observations carried out with the CARMA observatory. We mapped filaments in the Serpens Main, Serpens South and NGC 1333 molecular clouds, using H13CO+ (J=1-0), N2H+ (J=1-0) and other dense tracers at an angular resolution of ~ 7" and a spectral resolution of ~ 0.16 km/s.

The MIRIAD package was used to process the visibility data. Single dish maps were used with the interferometric data to generate the data-cubes using the Maximum Entropy method. The combined deconvolution using the single dish images helps in recovering the large-scale structure filtered out by the interferometer. Although the large-scale structure compared well with the Herschel dust continuum maps, we resolved finer structure within the filaments identified by Herschel. The H13CO+ emission distribution agreed with the N2H+ (J=1-0) maps; so they trace the same morphology and kinematics. Almost all the regions have multiple structures partially overlapping in the line-of-sight. Only the H13CO+ emission having a simple spectrum (no hyperfine components) and sufficient signal-to-noise could disentangle the contribution of each sub-structure. We identified 3 of the 5 filaments to have quasi-parallel sub-filaments that are not resolved by the Herschel maps. We reported full-width at half-maximum widths of 0.03-0.08 pc for the filaments and sub-filaments in these tracers. These are much lesser than the widths in the dust continuum maps. The widths vary by up to a factor of two with the same filamentary structure, thereby questioning the universality of filament widths reported in literature.

One of the chief results from this research is that at least 4 of the 8 filamentary structures have distinct velocity gradients perpendicular to their major axis with average values in the range 4-10 km/s/pc. These findings are in support of the theoretical models of filament formation by 2-D inflow in the shock layer created by colliding turbulent cells. Class 0/I/Flat young stellar objects (YSOs) are identified only along two of the filaments, and are found to be preferably located at filamentary structure intersections or regions of overlap. This suggests that the sub-filaments are physically interacting with each other, which plays a role in star formation.

The research on filaments is complemented by observations of NH3 (1,1) and (2,2) VLA maps. As part of the ongoing analysis, I derive the kinetic temperature of the gas by the intensity ratio of the inversion transitions, and the opacity using the hyperfine structure. These allow us to estimate the physical density from the existing N2H+ CARMA observations. I use the combination of VLA and CARMA observations to analyze the transition from the large-scale clouds to filaments, and further to quiescent cores.

Link to poster. Paper to be uploaded on ArXiV soon.

Past Research

Jugnu Nanosatellite Mission

A 3-kg satellite of dimensions 30mm x 10mm x 10mm, with most advanced features of a normal satellite like a two-way communication system, a sophisticated Attitude Determination and Control System using Torquer Coils and Reaction Wheels, sustained power supply using solar panels, and payloads like Imaging, GPS and IMU, with an able On-Board Computer to control everything. The ejection system for the satellite was also designed indigenously. The team comprised undergraduate and graduate students from IIT Kanpur, led by Shantanu Agarwal and Prof. N. S. Vyas.

I was involved in the GPS subsystem of the satellite along with Siddharth Paliwal for three years. We designed the PCB on which the programmable GPS daughter board could be mounted. The system also consisted of a customized Patch Antenna for acquiring the signals. The board was programmed to take into account the high velocity of the satellite (8 km/s). Thorough ground testing and Simulator testing at ISRO Satellite Centre (ISAC) confirmed its functionability.

On 12th October, 2011, Jugnu was ejected into the Megha-Tropiques Orbit at 800 km altitude by a PSLV C-18 rocket launched from Sriharikota. It was a proud moment for the entire team.

Videos of :

1) The Making
2) The Launch

Negative differential resistance in GaN nanowires

GaN nanowires were grown on Si substrate and were studied using field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) pattern. Both straight and zig-zag nanowires were formed. These were then dispersed between two gold contacts and isolated nanowires were identified for current–voltage (I–V ) studies. Subsequently, the connection between the nanowires and the gold pads were established using focused ion/electron beam-induced chemical vapour deposition of platinum. I-V measurements were carried out, and we repeatedly got a negative differential resistance for isolated zig-zag GaN nanowires with the FEB platinum contacts. The results are published here.

Reversals in Rayleigh Bénard Convection

Rayleigh Benard convection involves heating a fluid sample from below and maintaining it at a lower temperature from above that drives a convective flow. The system is defined by three parameters – Rayleigh number, Prandtl number and the geometry (including the aspect ratio). The original aim of the project was to determine (i) the role of geometry in flow reversals, (ii) the dependence of reversals on Rayleigh numbers and (iii) presence of higher Fourier modes of flow.

We designed and fabricated the apparatus for the experiment taking into account factors like expected time of reversal, uniform heating and cooling requirements, thermocouple placements etc. Thermocouples placed at regular intervals registering temperatures are used to detect the direction of flow, and to study other features of the flow like velocity, turnover time and higher Fourier modes.

Project Report

Dwarf Galaxy Detection

The Dwarf Galaxy problem stands in the way of the established dark matter distribution models. In a quest for more of such galaxies having high non-Baryonic content, I developed codes for processing the astronomical images from Canada-France-Hawaii telescope available at the CADC using IRAF and SExtractor. This was a part of the MITACS summer internship under James Taylor at University of Waterloo.

Technical Write-up

Molecules in Femtosecond Laser Pulses

Molecules in high vacuum ionized by ultra-short laser pulses using pump-probe techniques, are collected by an electron multiplier and transferred to an oscilloscope. From time of flight measurements, the ions produced can be determined, which gives a better insight to nuclear structures.

I was involved in writing a Visual C++ code to digitize the entire process using an Acqiris digitizer for real-time data analysis. The project was carried out in the Max Planck Institute of Nuclear Physics, Heidelberg, under the supervision of Priv. Doz. Dr. Robert Moshammer, and along with Dr. Siva Rama Krishnan.

Quantum States in Ultra Thin Films

Studied how Band gaps of semiconductors and certain metals can be used to set up quantum wells. Carried out Energy band analysis for some potential well configurations (Double finite).

Under the supervision of the same professor, I was also involved in analyzing the diffraction pattern produced when photoelectrons pass through a slit pattern of an angular device used in an Angle-Resolved Photoemission Spectrometer(ARPES). I also assisted MSc student Sanjay Mahatha with operating the ARPES, and its Low energy electron diffraction (LEED) facility to study surface crystalline order of a graphite sample.

IMU-GPS Integration using Kalman Filtering techniques

GPS (Global Positioning System) signals cannot be used to determine position in areas where there is no/poor satellite coverage. In such situations, Inertial Measurement Units (IMU) which give acceleration and angular velocity values can be utilized and its data integrated to get the position at any instant, if the position at the starting instant is known.

My work focussed on combining these two forms of data. I built a common platform in Visual Basic to run multiple IMU and GPS devices simultaneously. It involved sound knowledge of UART communication protocols. Once developed, Masters student Azmir Rabian and I implemented Kalman filtering and run time visualization of roll, pitch and yaw of the body. We tested the same by driving a car around with all these devices in the car.