SOLar STellar Irradiance Comparison Experiment (SOLSTICE)
William McClintock, Gary Rottman, and Tom Woods
Laboratory for Atmospheric and Space Physics, Univ. of Colorado, 1234 Innovation Dr., Boulder, CO 80303, USA, William.McClintock@Colorado.edu
SOLSTICE uses a single optical-detector system to measure ultraviolet (115 - 320 nm) spectral irradiance from the sun and from several bright early-type stars (spectral classes B and A).
It is one of four instruments on the Solar Radiation and Climate Experiment (SORCE), a NASA/EOS satellite mission scheduled for launch in Nov. 2002. SOLSTICE will report four
solar spectral irradiance measurements daily throughout SORCE’s 5-year mission life.
Science Objectives and Measurements
Science Objectives:
SOLSTICE Channel Assembly and Functional Block Diagram
SOLSTICE Channel before Spacecraft
Integration
•Measure solar irradiance from 115 to 320 nm with 0.5 nm spectral resolution and
5% or better accuracy
Grating
Encoder &
Control
•Monitor solar irradiance variation with 0.5% accuracy during the SORCE mission
Diffraction
Grating
Solar/Stellar
Radiation
•Establish the ratio of solar irradiance to the average flux from an ensemble of
bright early-type stars with 0.5% accuracy for future studies of long-term solar
variability.
Solar
Radiation
Measurements:
Sunshade
Vacuum
Door
Solar/Stellar
Entrance Slits
Solar
Position
Monitor
Door
Control
Slit
Control
Fold Mirror I
Fold Mirror II
SPM
Electronics
•Wavelength Coverage: 115 - 320 nm
Elliptical
Mirror
•Solar Spectral Resolution: 0.1 nm
Optical Path
Filter 1
Mechanism
Control
Filter 2
Mechanism
Control
Filter 1 &
Mechanism
Filter 2 &
Mechanism
Mirror
Control
•Stellar Spectral Resolution : 1.1 -2.2 nm
MUV
PMT
MUV
Detector
Electronics
FUV
PMT
FUV
Detector
Electronics
Solar/Stellar
Exit Slits
Control
GCI
Signal Path
Slit
Control
SOLSTICE: Experiment Concept
Solar Observation: Modified Monk-Gillieson Spectrometer
Solar Exit Slit
SOLSTICE: SORCE Experiment Implementation
Photomultiplier Detector
Diffraction
Grating
Camera Mirror
E AU (λ) =
Entrance Slit
Photomultiplier Detector
Camera Mirror
C(λ,τ, Dc, Sl)
⋅T
⋅f
⋅f
RC (λ )⋅ f (θ ,φ )⋅ AEntrance⋅ ∆λ Filter Degradation AU
S( λ) ⋅ N ⋅G(T,V) − Dc − Sl( λ)
C (λ, τ,D,Sl) =
∆t
Stellar Observation: Objective Grating Spectrometer
Stellar Exit Slit
SOLSTICE: Measurement Equation
Diffraction
Grating
Entrance Aperture
• C(λ,τ,DC,Sl)=PMT detector count rate calculated from the observed detector s ignal
counts (S(λ)), detector non-linearity (N), detector sensitivity temperature and voltage
dependence (G(T,V)), detector dark count (DC), spectrometer scattered light (Sl(λ)), and
integration time (∆t)
• RC(λ)=Instrument response function (sensitivity) at its field of view center
•The optical configuration matches illumination areas on the detector
•Interchanging entrance slits and exit slits provides ~ 2x10 5 dynamic range
•Different stellar/solar integration times provide ~ 10 3 dynamic range
•A optical attenuator (a pair of neutral density filters), which can be
measured in flight, provides additional ~ 10 2 dynamic range in the F Mode
for λ>220 nm
• AEntrance=Entrance aperture
• ∆λ? Spectral bandpass
• f(θ,φ)=Correction factor for viewing angle dependence of the instrumen t response
function, RC(λ)
•TFilter=Filter transmission (F Wavelength Mode only)
• fDegradation=Correction factor for time dependent (in-flight) degradation of the instrument
response function
• fAU=Factor used to normalize measured solar irradiance to 1 AU
SOLSTICE Channel Characteristics
Parameter
Configuration
Wavelength Range
Wavelength Resolution
Detector
Absolute Accuracy
Relative Accuracy
Long Term Accuracy
Size (instrument)
Size (electronics)
Mass
Power
Design Lifetime
FOV
Pointing Accuracy/Knowledge
Redundancy
Pre-flight Calibration
In-flight Calibration
Value
Diffraction Grating Spectrometer
115-320 nm
0.1 nm (solar) 1.1-2.2 nm (stellar)
Photon CountingPhotomultiplier Tubes
5%
0.5%
0.5%
88 long x 40 wide x 19 high [cm]
26 long x 21wide x 6 high [cm]
18 kg (instrument)
20 W
5 yrs
0.75Þ calibrated 1.50Þ total
±0.016Þ/±0.008Þ
2 Redundant Channels
NIST SURF-III
UV stars, Redundant Channels
•Two identical instrument channels meet the SORCE Mission
lifetime requirement:
-Channel A primary wavelength range: 170-320 nm
- Channel B primary wavelength range: 115-180 nm
•Each channel covers both wavelength ranges for redundancy and
cross calibration.
•Solar and stellar irradiance are measured with the same opticaldetector chain.
•Accurate pre-flight calibration using the NASA beam line at the
NIST Synchrotron Ultraviolet Radiation Facility (SURF III)
•Precise measurements of solar and stellar irradiance of bright, earlytype stars that, according to stellar theory, vary by <1% in 104 years
•Stellar measurements provide:
- Accurate in-flight instrument calibration tracking
- The basis for comparing SOLSTICE solar irradiance measurementswith
future work
Heritage : UARS SOLSTICE
SORCE SOLSTICE has direct heritage to the very successful
SOLSTICE instrument that is operating on the UARS
Observation Description
Grating
Solar/Stellar
Entrance Slit
Sunshade
Three separate wavelength channels
G: 115-180 nm, CsI Photocathode
F: 160-320 nm, CsTe Photocathode
N: 280-430 nm, Bialkali Photocathode
M e c h a n i s m
F o l dM i r r o r s
OffAxis
Ellipse
Photomultiplier
Solar/Stellar
Entrance
Detector
Slit
M e c h a n i s m
G Optical Configuration
The SOLSTICE instrument consists of two identical scanning grating monochromators that cover the 115 - 320 nm wavelength range. For nominal operations one
channel scans 115 - 190 nm and the other scans 160 - 320 nm, recording a complete ultraviolet solar irradiance spect rum in approximately 1000 seconds. At night
mechanisms within the channels configure channel entrance apertu res and exit slits for stellar observations. Stellar integration times and wavelengths are selected by
ground scheduling and planning software in advance of the observ ations.
References
•Rottman, G., Woods, T., and Sparn, T. “Solar-Stellar Irradiance Comparison Experiment I. Instrument Design and Operation”, JGR. 96, 1993, pp 10667-10677.
•McClintock, W., Rottman, and Woods, T., “Solar Stellar Irradiance Experiment II (SOLSTICE II) for the NASA Earth Observing System’s Solar Radiation and Climate Experiment mission”, SPIE 4135, 2000, pp.
225-234.
•Woods, T., Rottman, G., Harder, G., Lawrence, G., McClintock, B., Kopp, G., and Pankratz, C., “Overview of the EOS SORCE Mission,” SPIE 4135, 2000, pp. 192-203.
•SORCE web page: http://lasp.colorado.edu/sorce/
WEM, 1 2 A u g u s t 2 0 0 2