Infrared irradiance calibration

Abstract

Infrared astronomical measurements are calibrated against reference sources, usually primary standard stars that are, in turn, calibrated either by direct or indirect means. A direct calibration compares the star with a certified source, typically a blackbody. Indirect methods extrapolate a direct measurement of the flux at one wavelength to the flux at another. Historically, α Lyr (Vega) has been used as the primary standard as it is bright, easily accessible from the northern hemisphere, and is well calibrated in the visual. Until recently, the direct absolute infrared calibrations of α Lyr and those derived from the absolute solar flux scaled to the observed spectral energy distributions of solar type stars increasingly diverged with wavelength from those obtained using a model atmosphere to extrapolate the absolute visual flux of Vega into the infrared. The exception is the direct calibration by the 1996/97 Midcourse Space Experiment of the absolute fluxes for a number of the commonly used infrared standard stars, including Vega.In the mid-1980s, the Air Force Geophysics Laboratory began a program that led to the establishment of a network of stars with which to calibrate infrared space-based sensors. α Lyr and a CMa were adopted as the fundamental references and the absolute 1.2 to 35 µm infrared spectral energy distributions for the 616 secondary standard stars in the network were derived through spectral and photometric comparisons with the primary standards. The stars are also used for calibration at ground-based infrared observatories. For applications in which the network stars may not be bright enough, particularly at the longer infrared wavelengths, planets and the larger asteroids are used. Planets and asteroids move and rather sophisticated thermal modeling of the bodies is required to predict the disk-integrated brightness at a specific time with reasonable accuracy. The Infrared Space Observatory applied such a sophisticated 'thermo-physical' model to the largest asteroids to support calibration of the sensors to a claimed accuracy of within 5%. The AFRL program also created a spectral atlas of the brightest stars in the sky that, although they are variable, may be used for calibration if the large(r) attendant uncertainties are acceptable.

Absolute Physical Calibration in the Infrared

“…Since the high-weight calibrations are at 2.20 and 2.25 µm Selby et al 1983;Booth et al 1989), we correct all of them to 2.22 µm; this wavelength has the further advantage that it is well removed from strong spectral absorptions in both A and G stars. Figure 5 of Price (2004) demonstrates that the predictions of A-star models are very similar in spectral shape between 2 and 24 µm, and we obtain a similar result comparing the 1993 and 2003 Kurucz models for Vega. At shorter wavelengths, there can be slight deviations of models.…”

“…Our most important conclusion is that there is very consistent behavior of solar-type and A-type stars, and that they in turn are closely consistent with virtually all direct calibration measurements and with models of their spectra. Concerns of significant inconsistencies (Price 2004;Bohlin 2007) can therefore be put aside, and we can proceed to develop a procedure for calibration of IR measurements with assurance that there are unlikely to be serious undetected systematic errors.…”

“…We do so at a wavelength of 10.6 µm, using the "Vega" SED as the means to interpolate or extrapolate to the same wavelength and thus to relate the measurements to each other. That is, the calibration is relative to the normalization in Price (2004), Figure 5. However, this figure plots the calibration of incorrectly.…”

“…Bessell (2005) has reviewed photometry in general, but with only a few comments on the IR. Price (2004) has reviewed IR calibrations. His review concentrates on a huge body of work by Cohen, Walker, Witteborn, Price, and other co-workers on this topic.…”

Techniques and Review of Absolute Flux Calibration from the Ultraviolet to the Mid-Infrared

“…Excellent reviews of the direct measurement of the physical fluxes of stars in the infrared have been published (Rieke et al 1985, Price 2004, Rieke et al 2008 and are summarized here.…”