What Are We Imaging?• Lunar Atmosphere – interaction with S.W.
– What criteria should be applied?
• Magnetosheath – interaction with S.W.
– Primary driver for aspect selection criteria
• Heliosphere–interaction of neutral ISM with S.W.
– Sample region small
• X-ray Background
– A bonus, does not drive aspect selection criteria
Magnetosheath Model• Bowshock/Magnetosheath model of Spreiter,
Summers, & Alksne (1966)
• Scaled by nominal Solar Wind– ρ=2.0 cm-3
– v=400 km/s
• Exospheric Neutral Model– N(r)=25.0(10.0 RE/R)3 cm-3
• X-ray Emission proportional to– N(r)ρ(r,θ)[v(r,θ)2+3kBT(r,θ)/m]½
Magnetosheath Model• Distances/sizes to scale
• X-ray emission strength unnormalized
• MP~11.95RE
• BS~15.09RE
Baseline Instrument• Zenith telescope at selenographic (λ,µ)
=tilted telescope at some other (λ,µ)
• Incapable of pointing
• Field of view diameter of 10°
• Detector on at all times
Model FOV as f(t)Given (λ,µ)
Lunar Model• Lunar Orbit
– e=0.05490
– i=5.15° from the ecliptic
– a~384,400 km ~60RE
– dΩ/dt=19.35°/year (18.6 year precession period)
• Moon– Obliquity=6.87° from orbit, 1.54° from ecliptic
– ω=13.17635815 deg/day
Lunar ModelConsequences:
• Cis-lunar space is not scanned evenly
• LOS from moon can pass above/below the Earth-sun line
• LOS from moon has nearly constant ecliptic latitude
Lunar Model• Lunar Orbit
– e=0.05490
– i=5.15° from the ecliptic
– a~384,400 km ~60RE
– dΩ/dt=19.35°/year (18.6 year precession period)
• Moon– Obliquity=6.87° from orbit, 1.54° from ecliptic
– ω=13.17635815 deg/day
JPL DE405 ephemeris
IDL implementation by Markwardt
P.K.Seidelmann, IAU Working Group (2000)
LCaMD (JPL D-32296)
Lunar Model• Assumed telescope at lunar barycenter
• Width of MP-BS at subsolar point:3.14RE~3°
• Radius of bright subsolar “spot”: 8RE~7.5°
Model for (λ,µ)=(0°,0°)
Galactic Coordinates, FOV centers only
Subsolar point appears uniformly along band
“Conclusions”|µ|≥10° removes the
paths of both the sun and earth from the FOV
“Spot” remains in the FOV
MP-BS line on the edge of field
Tolerances are fairly tight!
Need sun/earth constraints.