Department of Astronomy and Astrophysics
University of Toronto Mississauga
L5L 1C6, Canada
|E-mail:||lester AT departmental mail server|
New Instrument - I have developed a small Fourier Transform Spectrometer (FTS) specifically designed to measure spectral irradiance while avoiding the systematic errors that have plagued spectroradiometry. Although I did not realize it when I began this project, this is a resumption of the approach pioneered by H. L. Johnson in the 1970s. With funding from grants provided by the Research Corporation and by the Natural Sciences and Engineering Research Council of Canada, the FTS has been designed and built by ABB Bomem in Quebec City. The spectrometer is now located at the Lowell Observatory, Flagstaff, Arizona, which is hosting this project.
How Bright is the Sun? Although we know more about the Sun than any other star, its absolute spectral energy distribution is, surprisingly, more uncertain than the bright nighttime stars. Because the Sun is so bright and its angular size is so large, the measurement of its spectral irradiance is dominated by systematic effects, not the number of photons as is usually the case in astronomy. Because of the fundamental importance of the Sun, and because the FTS is able to measure large, bright sources with great accuracy, the Sun is the first star in this program.
Very Tiny Telescope - Because of the Sun's great brightness, it can be observed with a very small telescope. My approach is to mount a small heliostat on top of the stationary spectrometer, as shown in this image of the heliostat. This image also shows Ralph Ney and Jim Darwin of the Lowell Observatory, who created this arrangement. The heliostat tracks the Sun and reflects the light to stationary folding mirrors that direct the light into the "telescope", which is a small lens just above the entrance aperture of the spectrometer. The amount of light is still too much for the detectors of the FTS, so a neutral density filter must be placed in front of the lens, reducing the effective diameter of the telescope to only 50 microns, surely one of the smallest telescopes in the world.
Spectroscopy - To illustrate the spectroscopic performance of the instrument, this observation of H alpha, is compared to the same feature in the Kurucz et al. Solar Flux Atlas, convolved to my resolution.
Photometry - The tests from 2004 June measured the solar spectral irradiance every 30 seconds for several hours, and these data were used to derive the atmospheric extinction. For example, the extinction at 600 nm agrees with the value expected for Lowell observatory in June. Extinction such as this is measured at every wavelength with a spectral resolution of 2 cm-1. For example, the violet extinction shows the rising extinction due to Rayleigh scattering.
Calibration - The next stage of the project is to develop a calibrated light source to remove the instrument's reflections and transmissions.
Former Graduate Students