Initial Commit

backend/src/astronomy/coords.rs

@@ -0,0 +1,44 @@
+/// Convert RA/Dec to Altitude/Azimuth.
+/// All inputs and outputs in degrees.
+pub fn radec_to_altaz(
+    ra_deg: f64,
+    dec_deg: f64,
+    lst_deg: f64,
+    lat_deg: f64,
+) -> (f64, f64) {
+    let ha = (lst_deg - ra_deg).rem_euclid(360.0);
+    let ha_rad = ha.to_radians();
+    let dec_rad = dec_deg.to_radians();
+    let lat_rad = lat_deg.to_radians();
+
+    let sin_alt = dec_rad.sin() * lat_rad.sin()
+        + dec_rad.cos() * lat_rad.cos() * ha_rad.cos();
+    let alt_rad = sin_alt.asin();
+
+    let cos_az = (dec_rad.sin() - lat_rad.sin() * sin_alt)
+        / (lat_rad.cos() * alt_rad.cos());
+    let cos_az = cos_az.clamp(-1.0, 1.0);
+    let az_rad = cos_az.acos();
+
+    let az_deg = if ha_rad.sin() < 0.0 {
+        az_rad.to_degrees()
+    } else {
+        360.0 - az_rad.to_degrees()
+    };
+
+    (alt_rad.to_degrees(), az_deg)
+}
+
+/// Rozenberg airmass formula — valid to horizon.
+pub fn airmass(alt_deg: f64) -> f64 {
+    if alt_deg <= 0.0 {
+        return 40.0; // clamp at horizon
+    }
+    let z_rad = (90.0 - alt_deg).to_radians();
+    1.0 / (z_rad.cos() + 0.025 * (-11.0 * z_rad.cos()).exp())
+}
+
+/// Extinction in magnitudes. k = 0.20 mag/airmass (Bortle 5 site).
+pub fn extinction_mag(alt_deg: f64) -> f64 {
+    airmass(alt_deg) * 0.20
+}