adding a simple orbit parameterization

exoplanet/orbits/__init__.py

@@ -2,8 +2,9 @@
 
 __all__ = [
     "KeplerianOrbit", "get_true_anomaly",
-    "TTVOrbit",
+    "TTVOrbit", "SimpleTransitOrbit",
 ]
 
 from .keplerian import KeplerianOrbit, get_true_anomaly
 from .ttv import TTVOrbit
+from .simple import SimpleTransitOrbit

exoplanet/orbits/keplerian.py

@@ -309,17 +309,17 @@ def get_radial_velocity(self, t, output_units=None):
         v = self.get_star_velocity(t)
         return conv * v[2]
 
-    def approx_in_transit(self, t, r=0.0, duration_factor=3, texp=None):
+    def approx_in_transit(self, t, r=0.0, texp=None, duration_factor=3):
         """Get a list of timestamps that are expected to be in transit
 
         Args:
             t (vector): A vector of timestamps to be evaluated.
             r (Optional): The radii of the planets.
+            texp (Optional[float]): The exposure time.
             duration_factor (Optional[float]): The factor by which to multiply
                 the approximate duration when computing the in transit points.
                 Larger values will be more conservative and might be needed for
                 large planets or very eccentric orbits.
-            texp (Optional[float]): The exposure time.
 
         Returns:
             inds (vector): The indices of the timestamps that are expected to

exoplanet/orbits/simple.py

@@ -0,0 +1,83 @@
+# -*- coding: utf-8 -*-
+
+from __future__ import division, print_function
+
+__all__ = ["SimpleTransitOrbit"]
+
+import theano.tensor as tt
+
+
+class SimpleTransitOrbit(object):
+
+    def __init__(self, period=None, t0=0.0, b=0.0, duration=None,
+                 r=None, r_star=1.0):
+        self.period = tt.as_tensor_variable(period)
+        self.t0 = tt.as_tensor_variable(t0)
+        self.b = tt.as_tensor_variable(b)
+        self.duration = tt.as_tensor_variable(duration)
+        self.r = tt.as_tensor_variable(r)
+        self.r_star = tt.as_tensor_variable(r_star)
+
+        self._b_norm = self.b * self.r_star
+        x2 = (self.r + self.r_star)**2 - self._b_norm**2
+        self.speed = 2 * tt.sqrt(x2) / self.duration
+        self._half_period = 0.5 * self.period
+        self._ref_time = self.t0 - self._half_period
+
+    def get_star_position(self, t):
+        nothing = tt.zeros_like(t)
+        return nothing, nothing, nothing
+
+    def get_planet_position(self, t):
+        return self.get_relative_position(t)
+
+    def get_relative_position(self, t):
+        """The planets' positions relative to the star
+
+        Args:
+            t: The times where the position should be evaluated.
+
+        Returns:
+            x, y, z: The components of the position vector at ``t`` in units
+                of ``R_sun``.
+
+        """
+        dt = tt.mod(tt.shape_padright(t) - self._ref_time, self.period)
+        dt -= self._half_period
+        x = tt.squeeze(self.speed * dt)
+        y = tt.squeeze(self._b_norm + tt.zeros_like(dt))
+        z = -tt.ones_like(x)
+        return x, y, z
+
+    def get_planet_velocity(self, t):
+        raise NotImplementedError("a SimpleTransitOrbit has no velocity")
+
+    def get_star_velocity(self, t):
+        raise NotImplementedError("a SimpleTransitOrbit has no velocity")
+
+    def get_radial_velocity(self, t, output_units=None):
+        raise NotImplementedError("a SimpleTransitOrbit has no velocity")
+
+    def approx_in_transit(self, t, r=None, texp=None, duration_factor=None):
+        """Get a list of timestamps that are in transit
+
+        For this orbit, this function is exact.
+
+        Args:
+            t (vector): A vector of timestamps to be evaluated.
+            r (Optional): The radii of the planets.
+            texp (Optional[float]): The exposure time.
+
+        Returns:
+            inds (vector): The indices of the timestamps that are in transit.
+
+        """
+        if r is None:
+            r = self.r
+        dt = tt.mod(tt.shape_padright(t) - self._ref_time, self.period)
+        dt -= self._half_period
+        tol = self.duration
+        if texp is not None:
+            tol += 0.5 * texp
+        mask = tt.any(tt.abs_(dt) < tol, axis=-1)
+        return tt.arange(t.size)[mask]