Optimizing end-to-end system performance for millimeter and submillimeter spectroscopy of protostars : wideband heterodyne receivers and sideband-deconvolution techniques for rapid molecular-line surveys
AbstractThis thesis describes the construction, integration, and use of a new 230-GHz ultra-wideband heterodyne receiver, as well as the development and testing of a new sideband-deconvolution algorithm, both designed to enable rapid, sensitive molecular-line surveys. The 230-GHz receiver, known as Z-Rex, is the first of a new generation of wideband receivers to be installed at the Caltech Submillimeter Observatory (CSO). Intended as a proof-of-concept device, it boasts an ultra-wide IF output range of \sim 6 - 18 GHz, offering as much as a twelvefold increase in the spectral coverage that can be achieved with a single LO setting. A similarly wideband IF system has been designed to couple this receiver to an array of WASP2 spectrometers, allowing the full bandwidth of the receiver to be observed at low resolution, ideal for extra-galactic redshift surveys. A separate IF system feeds a high-resolution 4-GHz AOS array frequently used for performing unbiased line surveys of galactic objects, particularly star-forming regions. The design and construction of the wideband IF system are presented, as is the work done to integrate the receiver and the high-resolution spectrometers into a working system. The receiver is currently installed at the CSO where it is available for astronomers' use. In addition to demonstrating wideband design principles, the receiver also serves as a testbed for a synthesizer-driven, active LO chain that is under consideration for future receiver designs. Several lessons have been learned, including the importance of driving the final amplifier of the LO chain into saturation and the absolute necessity of including a high-Q filter to remove spurious signals from the synthesizer output. The on-telescope performance of the synthesizer-driven LO chain is compared to that of the Gunn-oscillator units currently in use at the CSO. Although the frequency agility of the synthesized LO chain gives it a significant advantage for unbiased line surveys, the cleaner signal and broader tuning range of the Gunn continue to make it the preferred choice. The receiver and high-resolution spectrometer system were brought into a fully operational state late in 2007, when they were used to perform unbiased molecular-line surveys of several galactic sources, including the Orion KL hot core and a position in the L1157 outflow. In order to analyze these data, a new data pipeline was needed to deconvolve the double-sideband signals from the receiver and to model the molecular spectra. A highly automated sideband-deconvolution system has been created, and spectral-analysis tools are currently being developed. The sideband deconvolution relies on chi-square minimization to determine the optimal single-sideband spectrum in the presence of unknown sideband-gain imbalances and spectral baselines. Analytic results are presented for several different methods of approaching the problem, including direct optimization, nonlinear root finding, and a hybrid approach that utilizes a two-stage process to separate out the relatively weak nonlinearities so that the majority of the parameters can be found with a fast linear solver. Analytic derivations of the Jacobian matrices for all three cases are presented, along with a new Mathematica utility that enables the calculation of arbitrary gradients. The direct-optimization method has been incorporated into software, along with a spectral simulation engine that allows different deconvolution scenarios to be tested. The software has been validated through the deconvolution of simulated data sets, and initial results from L1157 and Orion are presented. Both surveys demonstrate the power of the wideband receivers and improved data pipeline to enable exciting scientific studies. The L1157 survey was completed in only 20 hours of telescope time and offers moderate sensitivity over a > 50-GHz range, from 220 GHz to approximately 270 or 280 GHz. The speed with which this survey was completed implies that the new systems will permit unbiased line surveys to become a standard observational tool. The Orion survey is expected to offer \sim 30 mK sensitivity over a similar frequency range, improving previous results by an order of magnitude. The new receiver's ability to cover such broad bandwidths permits very deep surveys to be completed in a reasonable time, and the sideband-deconvolution algorithm is capable of preserving these low noise levels. Combined, these tools can provide line spectra with the sensitivity required for constraining astrochemical models and investigating prebiotic molecules.
Sumner, Matthew Casey (2011) Optimizing end-to-end system performance for millimeter and submillimeter spectroscopy of protostars : wideband heterodyne receivers and sideband-deconvolution techniques for rapid molecular-line surveys. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:05302011-143941777 <http://resolver.caltech.edu/CaltechTHESIS:05302011-143941777>