Symposium in Honour of Philippe Lemaire's Retirement Four Solar Cycles of Space Instrumentation Institut d’Astronomie Spatiale, Orsay M.C.E. Huber Radiometry in Space Astronomy — tying celestial sources to laboratory standards 19 November 2004, 09 h 00 min - 09 h 20 min

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Martin C.E. HUBER Laboratory for Astrophysics Paul Scherrer Institut CH-5232 Villigen PSI Switzerland

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Content –astronomical radiometry today (and hopefully in future) –three examples VUV radiometric calibration of HST VUV calibration of SOHO precision calibration of Solar Irradiance –summary and conclusions

Radiometry in Space Astronomy — tying celestial sources to laboratory standards  [W m -2 ] – Spectral Irradiance I (W m -2 nm -1 ): per wavelength interval at wavelength Spectroradiometric observations involve determining Spectral Irradiance or Spectral Radiance: –Irradiance I (W m -2 ): power per unit area (often loosely called ‘flux’)

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Spectroradiometric observations involve determining Spectral Irradiance or Spectral Radiance: –Radiance R (W m -2 sr -1 ): power per unit area per unit solid angle  [W m -2 sr -1 ] – Spectral Radiance R (W m -2 sr -1 nm -1 ) per wavelength interval at wavelength

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Nomenclature for radiative measurements: in Astronomy –photometry measured in magnitudes (also within filter bands) in Physics –radiometry measured in units of the Système International, –physics reserves the term photometry for radiative measurements with units related to human vision  the candela — i.e., luminous intensity of radiation at 540 x Hz (corresponding to = nm in standard air) — is 1/683 W sr -1 in SI units

Radiometry in Space Astronomy — tying celestial sources to laboratory standards the Astronomer says –I define celestial standards –... and then will (somehow) work with ergs, cm etc. the Physicist says – I want to tie the celestial sources to the laboratory standards that define the units – then I do work with the units of the Système International (SI)

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Laboratory Source Standards: Storage Ring and Black Body Radiator

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Content –astronomical radiometry today (and hopefully in future) –three examples VUV radiometric calibration of HST VUV calibration of SOHO precision calibration of Solar Irradiance –summary and conclusions

Radiometry in Space Astronomy — tying celestial sources to laboratory standards VUV Radiometric Calibration of HST

Radiometry in Space Astronomy — tying celestial sources to laboratory standards VUV Radiometric Calibration of HST IUE HST vacuum ultraviolet visible/infrared celestial standard  ground-based telescopes black-body standard terrestrial atmosphere model terrestrial atmosphere model white-dwarf model atmosphere 

Radiometry in Space Astronomy — tying celestial sources to laboratory standards The Radiometric Calibration of SOHO A. Pauluhn, M.C.E. Huber & R. von Steiger, eds. Published in August 2002 ISSI Sci. Report SR-002 (Noordwijk: ESA Publ. Div.) 387 pp.

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Solar Vacuum-ultraviolet Radiometry with SUMER K. Wilhelm, U. Schühle, W. Curdt, I.E. Dammasch, J. Hollandt, P. Lemaire and M.C.E. Huber pp SUMER Stellar Observations to Monitor Responsivity Variations P. Lemaire pp New UV Detector Concepts J.-F. Hochedez, U. Schühle and P. Lemaire pp Philippe’s Contributions to the SOHO Calibration Volume

Radiometry in Space Astronomy — tying celestial sources to laboratory standards VUV Radiometric Calibration of SOHO vacuum ultraviolet storage ring hollow-cathode calibration source SUMER CDS SUMER CDS contamination ? IUE SOLSTICE

Comparison between Primary and Normal-incidence Transfer Source Standard hollow-cathode lamp

Transportable Normal-incidence Source Standard hollow-cathode lamp

HC-Lamp Spectra: Al + Ne

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Solar Irradiance: The Solar ‘Constant’

Radiometry in Space Astronomy — tying celestial sources to laboratory standards

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Radiometry in Space Astronomy — tying celestial sources to laboratory standards (Future) Solar Irradiance Calibration Solar Irradiance Cryogenic Radiometer (primary standard) in orbit

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Content –astronomical radiometry today (and hopefully in future) –three examples VUV radiometric calibration of HST VUV calibration of SOHO precision calibration of Solar Irradiance –summary and conclusions

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Summary Following an introduction on radiometric calibration and the relevant rules and units of the Système International (SI), examples were given of calibrations of three space instruments, namely –the VUV radiometric calibration of STIS on HST, which is related to a laboratory standard (black-body) via observations that require modelling of both the terrestrial atmosphere and atmospheres of white dwarfs; moreover the resulting calibration is given in cgs-units and ignores the radiometric definitions of SI, –the VUV calibration of SOHO, where a primary standard (synchrotron radiation) was used in the laboratory to calibrate a secondary, transportable standard that enabled a convenient overall radiometric calibration of the flight instruments, and –a proposed precision calibration of Solar Irradiance, where a primary standard (cryogenic radiometer) will be flown in the spacecraft itself.

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Summary (cont.) A proper radiometric calibration should be traceable as directly as possible to primary standards, so that it enables an objective test of models of astronomical objects, based on their radiative characteristics. –The philosophy of the procedure employed for STIS on HST is questionable, since it will result in models being tested based on other models that had been used as input to the calibration. –The drawback of the calibration of the SUMER and CDS instruments on SOHO is the remaining possibility of an unnoticed change in radiometric instrument response between the calibration in the laboratory and the start of observations in orbit. –Thus, the proposal to integrate a primary standard into a spacecraft, looks like the ideal approach to radiometric calibration of space intruments.

Radiometry in Space Astronomy — tying celestial sources to laboratory standards Hopefully the radiometric calibration of SOHO was just a step in approaching an astronomical radiometry that is directly based on the Système International This, I am sure, would also be in the sense of today’s Guest of Honour Philippe Lemaire !

Symposium in Honour of Philippe Lemaire's Retirement Four Solar Cycles of Space Instrumentation Institut d’Astronomie Spatiale, Orsay M.C.E. Huber Radiometry in Space Astronomy — tying celestial sources to laboratory standards 19 November 2004, 09 h 00 min - 09 h 20 min