PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
International Mercury Watch Group Observatoire de Paris 5-6 April 2008 2
PHEBUS PLACEMENT AND DESIGN
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy• Signal-to-noise ratio of 3 is achieved in the
radiometric modelling for most important species (depends on integration time)
• EUV = 55 - 155 nm• FUV = 145 - 315 nm • NUV = 404.7 nm (K) and 422.8 nm (Ca)
• EUV: FWHM ~0.48 FW1% ~0.80 • FUV: FWHM ~0.94 FW1% ~1.48
• Ingenious rotating mechanism with attached baffle - impact parameter from 0 to 1500 km
• With current configuration and 3600 s integration time sensitivity to lines as weak as 0.1 Rayleigh is achieved in the radiometric model
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
International Mercury Watch Group Observatoire de Paris 5-6 April 2008 3
PHEBUS DESIGN
• S
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
International Mercury Watch Group Observatoire de Paris 5-6 April 2008 4
PHEBUS DESIGN
Baffle
Mirror
Slith
EUV
FUV
NUV
Grating
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
International Mercury Watch Group Observatoire de Paris 5-6 April 2008 5
PHEBUS MISSION OBJECTIVES
1. To measure the exosphere
(constituents, species densities, vertical structure, seasonal-, local- and temporal variations)
2. To measure the coupling of the surface-exosphere-magnetosphere system
3. Measuring the escape rates of species from the surface and the composition of the eroding regolith
4. Search for surface ice in the polar regions
5. Low priority: Stellar- and interplanetary physics
Magneto- sphereSurface
Exosphere
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
International Mercury Watch Group Observatoire de Paris 5-6 April 2008 6
1. Twilight Mode
2. Vertical Scan Mode
3. Fixed LOS Mode
4. Feature Tracking Mode
5. Nadir Mode
6. Star Mode
7. ???
PHEBUS OBSERVATIONS
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
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PHEBUS MODES
Twilight Mode
Allows search for trace species near surface and studies of the exosphere’s source and dynamics.
Observations when in the shadow of Mercury. The line of sight is as low as possible with no bright emission from the surface is in the field of view
Vertical scan Mode
Give information on the vertical distribution and its variation with solar zenith angle
Fixed LOS Mode
A fixed line of sight respective to the spacecraft. A slow vertical scan is obtained from the altitude variation of the spacecraft.
Gives information about local density of observed species along the orbit
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
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PHEBUS MODES
Line of sight
Line of sight
Feature tracking Mode
The LOS moves slightly as in Twilight Mode to follow a feature (or altitude range) along the orbit
Nadir Mode
The LOS is pointed towards the Nadir on the nightside of the orbit and at high latitude which records the reflectivity of the surface by interplanetry glow (water, sulphur…)
Star Mode
The aim of the star mode is to follow the ageing of the instrument.Observation of some hot stars within the spectral range 100-300 nm. The number of available stars within the Phebus field of view is TBD (by me!).
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
International Mercury Watch Group Observatoire de Paris 5-6 April 2008 9
PHEBUS MODES
?
Tail mode (under construction)
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
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PHEBUS MODELING: Radiometric Model
Complete theoretical model
Calibration model(in progress)
Instrument Response model
Emissions from Mercury(theoretical values)
Collecting: Detectors
Gathering: Entrance Pupil and Baffle
Reflecting: Entrance mirror Gratings
Different reactions due to different photocathodes
Constraints of field of view and stray light
Depends on the separation of the two gratings and the reflectance of coating
Not perfectly reflective
InstrumentResponse
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
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RADIOMETRIC MODEL: Instrument Response and spectra generation
1. Raw list of species with their assumed intensities (preliminary since Mercury’s exosphere is virtually unexplored)
2. List of species subjected to the instrument response
3. ‘Spectrum’ of the species list after addition of noise – 1 s integration time
1.
2. 3.
blue arrow is oxygen at 130,4 nm – ‘easily’ detectablered arrow is clorine at 133,6 nm – ‘difficult’ to detect
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
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RADIOMETRIC MODEL: Instrument Response
10 seconds
60 seconds
600 seconds
3600 seconds
With increasing time the dark current will overtake some lines and change line-ratios
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
International Mercury Watch Group Observatoire de Paris 5-6 April 2008 13
RADIOMETRIC MODEL: Signal-to-Noise Ratio
Species detectable:< 60 s (to make thorough investigations)
EUV: He I, CO, O I, H I, C I, N IFUV: Mg I, Si I, Na I, C I, Fe I, S I, Al I,
CO, Ni I, Mg II, OH, Ca I, Fe II, H2>> 3600 s (to confirm that the species exist)
EUV: Kr I, Ar I, Cl I and Si IIFUV: Al II and Si II
Numbers in the tables are integration time in seconds
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
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RADIOMETRIC MODEL: Detection limit
The lowest (integration time of 3600 s) mean detection limit can thus be estimated to:
EUV range: ~ 0.1 Rayleigh
FUV range: ~ 0.2 Rayleigh
red dotted line marks the limit of 0.1 Rred area marks most common utilised integration time
1-10 s int. time gives lower limit of 10 R
1-10 s int. time gives lowerlimit of 5 R
1-10 s int. time gives lowerlimit of 20 R
PHEBUSProbing of Hermean Exosphere By Ultraviolet Spectroscopy PHEBUS Science Performance
International Mercury Watch Group Observatoire de Paris 5-6 April 2008 15
PHEBUS DESIGN ACHIEVEMENTS
• Signal-to-noise ratio of 3
• Spectral range of 55 - 315 nm and the NUV of 404 nm and 422 nm
• Spectral resolution: FWHM < 1 nm for EUV, < 1.5 nm for FUV. FW1% < 2 nm for EUV and < 3 nm for FUV
• A vertical scanning range equivalent to an impact parameter from 0 to 1500 km
• Vertical resolution of ~ 1/2 scale height on most species(~ 20 km). From apoherm this gives spatial resolution ≥ 0.37°
• Sensitivity to lines as weak as 0.1 Rayleigh
Is achieved in the radiometric modelling for most important species (depends on integration time)
EUV = 55 - 155 nm, FUV = 145 - 315 nm, NUV = 404.7 nm and 422.8 nm
EUV: FWHM ~0.48 FW1% ~0.80 FUV: FWHM ~0.94 FW1% ~1.48
Ingenious rotating mechanism with attached baffle
With margin the spatial resolution is ~0.4° in the current evaluation of the geometric performance
With current configuration and 3600 s integration time this is achieved in the radiometric model for EUV ~0.1 R (FUV ~0.2 R)
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