Festkolloquium anlässlich des 60. Geburtstags von Herrn Prof. Dr. Michael Kühne, Mitglied des Präsidiums
Astronomical radiometry only complies with the Système International (SI), if the radiance or irradiance of celestial objects is compared with that of a terrestrial laboratory standard. ‘Standard stars’ thus don’t exist and, strictly speaking, celestial objects can be used as model-supported transfer standards at best.
To introduce the topic, we will illustrate some current radiometric practices in astronomy. We will recall how the irradiance of stars in the visible is tied to that of blackbody standards (with an uncertainty of ca. 3 %), and how the calibration of the Hubble Space Telescope was extended from the visible into the ultraviolet by use of white-dwarf model atmospheres. This procedure resulted in a credible accuracy in the ultraviolet of a few percent. However, this result is objectionable from a methodical point of view. Accepting such procedures will eventually result in circular conclusions: the test of stellar models would be based on previous stellar-model assumptions. On the other hand, the measurement in space of the total solar irradiance — the slightly varying ‘solar constant’ — is based on electrically calibrated radiometers, and therefore complies with the SI.
A particularly difficult radiometric problem is the calibration of observations of the solar corona, i.e. the Sun’s outer atmosphere. The corona is best investigated at wavelengths below ca. 120 nm, i.e., in the windowless vacuum ultraviolet (VUV) spectral region, and this requires going into space. Observations of this kind had a radiometric uncertainty of ca. 30 percent before the ESA-NASA Solar and Heliospheric Observatory (SOHO) was launched in 1995. The difficulties in calibrating such observations stemmed from inadequate calibration procedures and laboratory standards, as well as from changes in the reflection of telescopes in orbit, when the solar VUV irradiation broke up residues of initially volatile organic molecules that contaminated the mirror surfaces, and changed the responsivity. Moreover, the VUV radiance of the solar corona varies on time scales from well below a second to the 11-year sunspot cycle (and probably beyond). The Sun itself therefore cannot be used, even as a temporary standard, to reliably evaluate responsivity changes in orbit.
We will discuss the methods, with which we overcame these difficulties for the flight of SOHO. Besides assuring extreme cleanliness, we developed an efficient, much improved calibration scheme for the various VUV instruments on SOHO. A transportable source standard, built at PTB, was used to calibrate SOHO’s VUV instruments in the laboratories of the Principal Investigators. This source, which was based on a high-current hollow-cathode discharge in rare gases, could repeatedly be calibrated on the BESSY-I electron-storage ring. It is a pleasure to acknowledge the contributions of PTB and particularly by Michael Kühne to this project. A much more accurate VUV radiometry of the Sun has been the result.