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Formulae
Equatorial Coordinate
R = tan(π/2 - δ/2)
θ = α
X = R cos(θ)
Y = R sin(θ)
where (α, δ) = (Right ascension, Declination)
Horizontal Coordinate
R = tan(π/2 - alt/2)
θ = az
X = R cos(θ)
Y = R sin(θ)
where (az, alt) = (Azimuth, Altitude)
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Stereographic projection
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