Revealing the Distant Universe with Space Observatories
Dr. Henry C. Ferguson
Space Telescope Science Institute
James Webb Space Telescope Instrument Team Lead
Abstract
In the past two decades, observations from the Hubble, Spitzer, and Chandra observatories have transformed our understanding of the universe. This talk will explore the process of converting raw data into scientific knowledge for images of very faint and distant galaxies.
The observatories transmit raw telemetry to the ground, which is turned into images. These raw images are affected by imperfections of the optics, detectors, and pointing systems, as well as the immediate and lingering effects of charged-particle radiation. Some of these issues are addressed by strategies for obtaining the observations, and others are addressed by strategies for reducing the data. The goal is to produce images where each pixel value is linearly proportional to the number of photons received from the sky. It is impossible to achieve this perfectly, so the challenge is to come close, and to quantify the uncertainties.
After constructing calibrated images, the next step is to identify the astronomical sources and make measurements of their fluxes and shapes. This is challenging because the observations span a very wide range of wavelength, and have very different spatial resolutions. The sources are not always detectable in all of the images, and their appearance may be changing as a function of wavelength. The spatial resolution of some of the images is low enough that the sources tend to overlap each other. Information from less crowded images taken at other wavelengths can be used to overcome some of this confusion. Once again, this is an imperfect process, and a significant part of the challenge is to quantify the uncertainties.
The image analysis generally results in catalogs of measurements which can be used to test hypotheses and make inferences about cosmology. These range from refining measurements of the acceleration of the universe, to testing whether the energy output from galaxies observed one billion years after the Big Bang was sufficient ionize most of the hydrogen in intergalactic space between the galaxies. Depending on the nature of the question, it is often necessary to simulate some part of the detection or measurement process, which can be done at varying levels of sophistication, ranging from simulating the path of each photon through the telescope and electron through the electronics, to simulating the last few steps in generating the catalogs.
The talk will discuss the current state of the art in various steps of this whole process, and outline areas where improvements are most important for the next generation of telescopes such as the James Webb Space Telescope and the Large Synoptic Survey Telescope.
Biographical Sketch
Dr. Ferguson is an astronomer at the Space Telescope Science Institute (STScI), which operates the Hubble Space Telescope, and is preparing to operate the James Webb Space Telescope, slated for launch in 2018. He has been a leader in deep-field observations with Hubble since 1995, His current project - CANDELS (http://candels.ucolick.org/) - uses Hubble's optical and near-infrared cameras to improve our understanding of the geometry of the universe and the evolution of galaxies within a few billion years of the big bang.
Dr. Ferguson obtained his A.B degree from Harvard University in 1981, and PhD. from the Johns Hopkins University in 1990. He held a postdoctoral fellowship at Cambridge University, and a Hubble Fellowship at STScI, before joining the STScI permanent staff in 1995. Since joining the staff, he has provicded scientific support for several of the focal plane instruments, served as head of the Science-Instruments Support Division, and as Head of Science. He is currently serving as the James Webb Space Telescope Instrument Team lead, preparing to operate instruments on this 6.5-meter cryogenic telescope that is slated for launch in 2018.
Dr. Ferguson is a fellow of the American Association for the advancement of Science, and currently serves as the head of the Galaxies collaboration for the Large Synoptic Survey Telescope, planning for future observations with this major ground-based facility.