NIST Stars is an Innovations in Measurement Science project to create a catalog of stars whose top-of-the-atmosphere spectral flux has an SI traceable radiometric calibration of better than 0.5%.
A wide range of scientific and technical fields will benefit from having SI traceable stellar radiometric flux measurements (SI Stars). Weather and climate research satellites could use SI Stars for on orbit calibration of their spectroradiometers and astronomers could use them to calibrate measurements of dark energy that will determine the ultimate fate of our universe.
Currently, all SI traceable astronomical measurements are based on the calibration of a single star (Vega) performed through a series of experiments in the 1970’s. More recent observations of Vega have led to questions about its suitability as a radiometric standard. Additionally, improvements in fundamental radiometry and the ability to measure and model the interfering effects of the Earth’s atmosphere should allow for significant reduction in the radiometric uncertainties of stellar flux calibrations.
To calibrate the radiometric flux from a star, we use a redundant system of calibrated detectors and sources. We start by calibrating a telescope-spectrometer system in the Telescope Calibration Facility (TCF). This reference telescope is then placed next to a large astronomical telescope and a calibrated source is placed in a distant location that both telescopes can observe. Both telescopes measure the source, allowing the calibration to be transferred to the astronomical telescope. If both telescopes then measure the flux from a star, the two should agree. However, this gives only the ground-level radiometric flux, which varies with atmospheric conditions and is not a useful calibration standard. Instead, we need to be able to reliably measure the top-of-the-atmosphere flux. We can do this if we know the atmospheric transmittance to correct for the light lost in the atmosphere. This will be accomplished by making measurements of the atmosphere using the remote sensing technique, LIDAR, and using atmospheric models to convert those measurements to atmospheric transmittance. Because the top-of-the-atmosphere flux does not vary for a stable star, repeated measurements under varying atmospheric conditions will provide a measure of the uncertainty involved in this atmospheric correction.
Image of the NIST calibration source on the summit of Mt. Hopkins with the handle of the Big Dipper above. (Photo courtesy of Peter Zimmer)
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