Stellar Calibration at ISR
calibration of stellar spectral energy distributions
Calibrating visible and infrared sensors on stars is a common step for any instrument which observes faint point-sources. Stars are scattered conveniently over the sky, are accessible simultaneously to multiple observers, may be revisited at intervals in order to track and characterize any changes in instrument response, and exhibit long term continuity which allows archived observations to share a baseline of comparison with observations made using current and future instruments. The quality of this shared calibration, however, depends in any particular time period on the quality of our knowledge of the spectral energy distributions (SEDs) of the reference stars observed.
Work at ISR has focused on progressively refining and updating knowledge of the visible-through-infrared spectra of commonly observed standard stars using all available strands of information. This effort brings together and cross-correlates theoretical modeling, data from various calibration experiments, exoatmospheric spectrometry (e.g., ESA’s ISO SWS, HST’s STIS & NICMOS, Spitzer's IRS) and relative photometry (e.g., DIRBE, MSX, Hipparcos, Spitzer's IRAC & MIPS, WISE) taken from satellite observatories, as well as the large ground databases of relative magnitude comparisons between types of stars. These data are culled, reanalyzed and reconsidered in light of subsequent information, such as evidence of variability in the traditional primary standard Vega. The results are synthesized into continuous composite spectra (i.e. SEDs stored as computer files) which constitute the best available estimate of each standard star’s absolute spectral calibration.
Such absolutely calibrated “composite” spectra for stars of various temperatures are then used to find a best-fit function of temperature at each catalogued wavelength which generates the mean predicted template spectral shape for a star of any temperature in the range covered by the original standard stars. These can be used to represent stars which have had no direct continuous spectrometry performed on them, greatly expanding the range of stars which can become calibrators in the sky. We have created some 500 calibrators with continuous visible to infrared SEDs in this manner.
The figure above shows some sample composite spectra of specific standard stars (black lines) and our computer generated generic templates (red dotted lines) for temperatures corresponding to those of the standards and for stars of intermediate temperatures. In each case the flux is normalized to a continuum and shown plotted against wavelength (0.3µm - 35µm) on a logarithmic scale.
Engelke, C. W., Price, S. D., & Kraemer, K. E. 2010, “Spectral Irradiance Calibration in the Infrared. XVII. Zero-magnitude Broadband Flux Reference for Visible-to-infrared Photometry,” AJ, 140, 1919-1928. ADS, DOI.
Kennelly, E. J., Price, S. D., Kraemer, K. E., & Aschbrenner, R. 2010, “Calibration against the Moon I: A disk-resolved lunar model for absolute reflectance calibration,” Icarus, 210, 14-36. ADS, DOI.
Engelke, C. W., Price, S. D., & Kraemer, K. E. 2006, “Spectral Irradiance Calibration in the Infrared. XVI. Improved Accuracy in the Infrared Spectra of the Secondary and Tertiary Standard Calibration Stars,” AJ, 132, 1445-1463. ADS, DOI.
Price, S. D., Paxson, C., Engelke, C., & Murdock, T. L. 2004, “Spectral Irradiance Calibration in the Infrared. XV. Absolute Calibration of Standard Stars by Experiments on the Midcourse Space Experiment,” AJ, 128, 889-910. ADS, DOI.
Point of Contact for this project is Charles Engelke.