Radiometry in Space Astronomy — tying celestial sources to laboratory standards

1. Symposium in Honour of Philippe Lemaire's Retirement Four Solar Cycles of Space InstrumentationInstitut d’Astronomie Spatiale, OrsayM.C.E. Huber Radiometry in Space Astronomy — tying celestial sources to laboratory standards19 November 2004, 09 h 00 min - 09 h 20 min

3. 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

4. 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

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

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

7. Radiometry in Space Astronomy — tying celestial sources to laboratory standards Nomenclature forradiative measurements: • in Astronomy • photometrymeasured in magnitudes (also within filter bands) • in Physics • radiometrymeasured in units of the Système International, • physics reserves the term photometryfor radiative measurements with units related to human vision • the candela — i.e., luminous intensity of radiation at 540 x 1012 Hz (corresponding to  = 555.016 nm in standard air) — is 1/683 W sr-1 in SI units

8. 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)

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

10. 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

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

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

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

14. 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. 145-160 • SUMER Stellar Observations to Monitor Responsivity VariationsP. Lemaire pp. 265-270 • New UV Detector Concepts J.-F. Hochedez, U. Schühle and P. Lemaire pp. 371-378 Philippe’s Contributions to the SOHO Calibration Volume

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

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

17. Transportable Normal-incidence Source Standard hollow-cathode lamp

18. HC-Lamp Spectra: Al + Ne

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

22. Radiometry in Space Astronomy — tying celestial sources to laboratory standards

23. Radiometry in Space Astronomy — tying celestial sources to laboratory standards _____________________ __________________________________ __________________________________ __________________________________ _________________

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

25. 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

26. 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.

27. 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.

28. 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 !

30. Symposium in Honour of Philippe Lemaire's Retirement Four Solar Cycles of Space InstrumentationInstitut d’Astronomie Spatiale, OrsayM.C.E. Huber Radiometry in Space Astronomy — tying celestial sources to laboratory standards19 November 2004, 09 h 00 min - 09 h 20 min