Symposium in Honour of Philippe Lemaire's Retirement Four Solar Cycles of Space InstrumentationInstitut d’Astronomie Spatiale, Orsay

M.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

Martin C.E. HUBERLaboratory for Astrophysics

Paul Scherrer InstitutCH-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 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 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

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benötigt.

Radiometry in Space Astronomy — tying celestial sources to laboratory standards

VUV Radiometric Calibration of HST

IUE HST HST

vacuum ultraviolet visible/infrared

celestial standard

ground-basedtelescopes

black-bodystandard

terrestrialatmosphere

model

terrestrialatmosphere

model

white-dwarfmodel 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. 145-160

• SUMER Stellar Observations to Monitor Responsivity Variations P. 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 standards

VUV Radiometric Calibration of SOHO

vacuum ultraviolet

storagering

hollow-cathodecalibration source

SUMERCDS

SUMERCDS

contamination ?

IUE

SOLSTICE

Comparison between Primary and Normal-incidence

Transfer Source Standard

hollow-cathodelamp

Transportable Normal-incidence Source Standard

hollow-cathodelamp

HC-Lamp Spectra: Al + Ne

Table 3: Radiant power and photon flux of selected emission lines of the normalincidence source standard – working point: 1 A, 500 V

Wavelength Spectrum Radiant PowerpW

Photon Fluxs-1

Uncertainty (1σ)

53 .70 nm He I 55 .3 1.49 . 10 7 6 %54 .29 / 54.32 nm Ar II 50 .9 1.39 . 10 7 6 %54 .75 Ar II 67 .9 1.87 . 10 7 6 %58 .34 nm Ar II 14 .3 4.20 . 10 6 6 %58 .43 He I 2394 7.04 . 10 8 6 %60 .29 / 60.42 nm Ar II/III 19 .8 6.02 . 10 6 6 %61 .24 nm Ar II 13 .6 4.19 . 10 6 6 %67 .09 – 67 .29 nm Ar II 66 .9 2.26 . 10 7 6 %71 .81 nm Ar II 18 .8 6.80 . 10 6 7 %72 .20 nm Kr III 33 .5 1.22 . 10 7 7 %72 .34 / 72.55 nm Ar II 68 .1 2.48 . 10 7 7 %73 .09 nm Ar II 23 .0 8.47 . 10 6 7 %73 .59 nm Ne I 955 3.54 . 10 8 7 %74 .03 nm Ar II 55 .7 2.08 . 10 7 7 %74 .37 nm Ne I 526 1.97 . 10 8 7 %74 .49 / 74.53 nm Ar II 42 .5 1.59 . 10 7 7 %76 .92 nm Ar III 24 .7 9.56 . 10 6 7 %88 .41 / 88.63 nm Kr II 212 9.45 . 10 7 7 %89 .10 nm Kr II 75 .8 3.40 . 10 7 7 %91 .74 nm Kr II 382 1.77 . 10 8 7 %91 .98 nm Ar II 812 3.76 . 10 8 7 %93 ,21 nm Ar II 486 2.28 . 10 8 7 %96 .50 nm Kr II 455 2.21 . 10 8 7 %104.82 nm Ar I 508 2.68 . 10 8 7 %106.67 nm Ar I 406 2.18 . 10 8 7 %110.04 nm X e II 108 5.98 . 10 7 7 %115.85 nm X e II 47 .6 2.78 . 10 7 7 %116.49 nm Kr I 136 7.98 . 10 7 6 %118.31 nm X e II 95 .0 5.66 . 10 7 6 %123.58 nm Kr I 421 2.62 . 10 8 6 %124.48 nm X e II 22 .5 1.41 . 10 7 6 %146.96 nm X e I 200 1.48 . 10 8 6 %

Table 5: Overall relative uncertainty (1σ) o f thephoto n flu x of th e normalincidenc e sourc e standard

Source of uncertainty a t55 nm a t90 nm

Imaging mirror:

Au SiC Au SiC

Spectra l photo n flu x o f synchrotro n radiation 1.4 % 1.4 % 1.3 % 1.3 %

Hig -h orde r synchrotron radiation -- -- 2.5 % 0.6 %

Inhomogeneity o f optica l component s anddetector

3.0 % 3.5 % 3.0 % 3.5 %

Linearity o f detector 0.6 % 0.6 % 0.6 % 0.6 %

Wavelength calibration 0.4 % 0.9 % 0.1 % 0.1 %

Stability o f hollo w cathod e source 5 % 5 % 5 % 5 %

Total s um in quadratureΔΦSR/ΦSR 6.0 % 6.4 % 6.5 % 6.3 %

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

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 InstrumentationInstitut d’Astronomie Spatiale, Orsay

M.C.E. Huber Radiometry in Space Astronomy — tying celestial sources to laboratory standards19 November 2004, 09 h 00 min - 09 h 20 min