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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 .

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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

radiometry in space astronomy tying celestial sources to laboratory standards

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 standards1
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 standards2
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]

radiometry in space astronomy tying celestial sources to laboratory standards3
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]

radiometry in space astronomy tying celestial sources to laboratory standards4
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
radiometry in space astronomy tying celestial sources to laboratory standards5
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 standards6
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 standards7
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 standards8
Radiometry in Space Astronomy — tying celestial sources to laboratory standards

VUV Radiometric Calibration of HST

radiometry in space astronomy tying celestial sources to laboratory standards9
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

radiometry in space astronomy tying celestial sources to laboratory standards10
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.

radiometry in space astronomy tying celestial sources to laboratory standards11
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

radiometry in space astronomy tying celestial sources to laboratory standards12
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

radiometry in space astronomy tying celestial sources to laboratory standards13
Radiometry in Space Astronomy — tying celestial sources to laboratory standards

Solar Irradiance: The Solar ‘Constant’

radiometry in space astronomy tying celestial sources to laboratory standards15
Radiometry in Space Astronomy — tying celestial sources to laboratory standards

_____________________

__________________________________

__________________________________

__________________________________

_________________

radiometry in space astronomy tying celestial sources to laboratory standards16
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 standards17
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 standards18
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 standards19
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 standards20
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 !

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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