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Summary
NIST Stars is a project to improve the accuracy of the catalog of standard stars used by astronomers for spectral flux calibration of observatories so that they are SI traceable with uncertainty better than 0.5 %.
Description
Image of the NIST calibration source on the summit of Mt. Hopkins with the handle of the Big Dipper above.
Several current astrophysical research programs require SI traceable flux measurements (also called spectral irradiance) at accuracies higher than currently available. For example, exoplanet research requires improved host star absolute brightness knowledge to better determine habitability. Astrophysicists studying dark energy through supernova cosmology measurements require improved absolute spectral flux ratios over wide spectral ranges. These modern research areas and others are limited by the relatively large uncertainty currently available.
Currently, all SI traceable astronomical spectral flux measurements in the visible-near infrared are based on the calibration of a single star (Vega) performed through a series of experiments in the 1970's, and there are problems with extrapolating this visible-wavelength calibration into the infrared. Also, more recent observations of Vega have led to questions about its suitability as a radiometric standard. Additionally, improvements since the 1970s in fundamental radiometry at NIST, and the ability to measure, model, and correct for 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 NIST 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 a LIDAR for aerosols and a microwave radiometer for water vapor, 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.