Solar-Scatter Reflectors for SNAP Calibration Verification

M.Lampton

UCB SSL

20 March 2002

amended 29 March & 5 April 2002

The Concept

• SNAP requires frequent (monthly?) recalibration to establish wavelength dependent throughput

• The solar irradiance is known to better than 1% in the visible and NIR and could serve as a calibration source if it could be made fainter by a huge factor

• Each month, have SNAP satellite launch a small scattering sphere into space.. Must have known reflectance!

• When it recedes far enough that its scattered sunlight is within the dynamic range of our photometers and spectrometers, use it as a known irradiance source.

We need 18 orders of magnitude

Can it work? Yes.

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Additional Interfering Illumination?

• Scattering sphere views entire sky

• Sun is not alone! Earth, Moon, stars....

• Earthshine: worst at “full Earth”– Closest operating distance = 10Re

– Earth albedo 35% and solid angle = 0.03 ster

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Reflector Materials

• Solid sphere or hollow spherical shell– plastic? aluminum? steel? solid platinum?

• Mechanically stable spherical surface

• Optically stable surface reflectance– specular vs diffuse scattering

– protected silver has >98% reflectance, hence accurate

– gold and platinum have been shown to be unaffected by extended space environment during Long Duration Exposure Facility flight.....

http://setas-www.larc.nasa.gov/LDEF/MATERIALS/mat_metals.html

How to detect a launched reflector• Orbit mechanics based on launcher direction

• Wide field of view in SNAP camera

• Unlike stars, reflector will be *moving*– streak shape unlike stars

– frame subtraction will reveal motion

– streak complicates IFU feed & analysis

– we may prefer to quickly analyze motion, then compensate motion to stabilize image for IFU

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Issues• Sun is variable at a level of 0.2%

– rotates, 28day period

– sunspot groups are cooler than avg photosphere

– sunspot group population varies on all time scales

– spectrum is therefore a complicated varying mixture

• Absolute solar spectrum is imperfectly known

• Albedo of anything is difficult to measure in laboratory– is 1% even possible?

• Albedo of our reflectors may degrade prelaunch or on-orbit– no way to check this; require a robust stable material surface

• Adds parts, cost, complexity to SNAP

• Legal?

Solar Spectrum

Solar Spectra“Solar flux atlas from 296 to 1300nm”, R.L.Kurucz, I.Furenlid, J.Brault National Solar Observatory 1984.

“The solar radiation between 3300 and 12500A”, H.Neckel & D.Labs,Solar Phys. v.90 pp.205-258, 1984: believed errors < 0.5%

“Stellar absolute fluxes and energy distributions from 0.32 to 4.0um”D.S.Hayes, IAU Symposium on Calibration of Fundamental StellarQuantities, v.111, 225, 1985. “A new solar irradiance calibration from 3295 to 8500A derived from absolute spectrophotometry of Vega”, G.Lockwood, H.Tug, N.M.White,Ap.J. 390, 668, 1992; believed errors < 2%

“The 0.12 to 2.5um absolute flux distribution of the sun for comparison withsolar analog stars”, L.Colina, R.C.Bohlin, F.Castelli AJ 112, 307, 1996.

Solar Cycle

Total Solar Irradiance Variationshttp://virgo.so.estec.esa.nl/

Solar Variability

M.Fligge & S.K.Solanki “The solar spectral irradiance since 1700” 2000 Geophys.Res.Letters v27, 2157

Upcoming Missions May Help Us

• SUVIM: ESA, aboard ISS, 2003– www.astro.phys.ethz.ch/papers/fligge/GL10884W01.pdf

– measures total irradiance and coarse solar spectrum

– seven bandpasses: 310, 400, 500, 610, 719, 862nm, and LWIR

– expect 0.1% precision

• SORCE: NASA/GSFC, Pegasus XL, 2003– http://lasp.colorado.edu/sorce/

– Total Irradiance Monitor (TIM): all wavelengths integrated

– Spectral Irradiance Monitor (SIM): 200-2000nm

– Solar-Stellar Irradiance Comparison Expt (SOLSTICE): UV

How good (and how well known) are reflectances?http://www.thermo.com/eThermo/CDA/Technology/Technology_Detail/0,1213,-10334-109,00.html

Comprehensive reflectance models are based on Fresnel equations, for which (n,k) are extensively tabulated

Metal Surfaces in L.E.O.

NASA-95-CR4661pt1.PDF“Space Environmental Effects on Spacecraft: LEO Materials

Selection Guide” p.6-19

Legality: Compare with natural fluxeshttp://lifesci3.arc.nasa.gov/SpaceSettlement/75SummerStudy/2appendA.html

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