Merge pull request #413 from lpsinger/galactic-plane-separation-const…

…raint

Add GalacticLatitudeConstraint

CHANGES.rst

@@ -1,6 +1,9 @@
 0.6 (unreleased)
 ----------------
 
+- Add ``GalacticLatitudeConstraint`` to constrain the galactic latitudes of
+  targets. This can be useful for planning surveys for which crowding due to
+  Galactic point sources is an issue. [#413]
 
 0.5 (2019-07-08)
 ----------------

astroplan/constraints.py

@@ -15,7 +15,7 @@
 # Third-party
 from astropy.time import Time
 import astropy.units as u
-from astropy.coordinates import get_body, get_sun, get_moon, SkyCoord
+from astropy.coordinates import get_body, get_sun, get_moon, Galactic, SkyCoord
 from astropy import table
 
 import numpy as np
@@ -28,12 +28,12 @@
 
 __all__ = ["AltitudeConstraint", "AirmassConstraint", "AtNightConstraint",
            "is_observable", "is_always_observable", "time_grid_from_range",
-           "SunSeparationConstraint", "MoonSeparationConstraint",
-           "MoonIlluminationConstraint", "LocalTimeConstraint",
-           "PrimaryEclipseConstraint", "SecondaryEclipseConstraint",
-           "Constraint", "TimeConstraint", "observability_table",
-           "months_observable", "max_best_rescale", "min_best_rescale",
-           "PhaseConstraint", "is_event_observable"]
+           "GalacticLatitudeConstraint", "SunSeparationConstraint",
+           "MoonSeparationConstraint", "MoonIlluminationConstraint",
+           "LocalTimeConstraint", "PrimaryEclipseConstraint",
+           "SecondaryEclipseConstraint", "Constraint", "TimeConstraint",
+           "observability_table", "months_observable", "max_best_rescale",
+           "min_best_rescale", "PhaseConstraint", "is_event_observable"]
 
 
 def _make_cache_key(times, targets):
@@ -482,6 +482,40 @@ def compute_constraint(self, times, observer, targets):
         return mask
 
 
+class GalacticLatitudeConstraint(Constraint):
+    """
+    Constrain the distance between the Galactic plane and some targets.
+    """
+
+    def __init__(self, min=None, max=None):
+        """
+        Parameters
+        ----------
+        min : `~astropy.units.Quantity` or `None` (optional)
+            Minimum acceptable Galactic latitude of target (inclusive).
+            `None` indicates no limit.
+        max : `~astropy.units.Quantity` or `None` (optional)
+            Minimum acceptable Galactic latitude of target (inclusive).
+            `None` indicates no limit.
+        """
+        self.min = min
+        self.max = max
+
+    def compute_constraint(self, times, observer, targets):
+        separation = abs(targets.transform_to(Galactic).b)
+
+        if self.min is None and self.max is not None:
+            mask = self.max >= separation
+        elif self.max is None and self.min is not None:
+            mask = self.min <= separation
+        elif self.min is not None and self.max is not None:
+            mask = ((self.min <= separation) & (separation <= self.max))
+        else:
+            raise ValueError("No max and/or min specified in "
+                             "GalacticLatitudeConstraint.")
+        return mask
+
+
 class SunSeparationConstraint(Constraint):
     """
     Constrain the distance between the Sun and some targets.

astroplan/tests/test_constraints.py

@@ -5,15 +5,17 @@
 import numpy as np
 import astropy.units as u
 from astropy.time import Time
-from astropy.coordinates import SkyCoord, get_sun, get_moon
+from astropy.coordinates import Galactic, SkyCoord, get_sun, get_moon
 from astropy.utils import minversion
 import pytest
 
 from ..observer import Observer
 from ..target import FixedTarget, get_skycoord
 from ..constraints import (AltitudeConstraint, AirmassConstraint, AtNightConstraint,
                            is_observable, is_always_observable, observability_table,
-                           time_grid_from_range, SunSeparationConstraint,
+                           time_grid_from_range,
+                           GalacticLatitudeConstraint,
+                           SunSeparationConstraint,
                            MoonSeparationConstraint, MoonIlluminationConstraint,
                            TimeConstraint, LocalTimeConstraint, months_observable,
                            max_best_rescale, min_best_rescale, PhaseConstraint,
@@ -136,6 +138,28 @@ def test_compare_airmass_constraint_and_observer():
         assert all(always_from_observer == always_from_constraint)
 
 
+def test_galactic_plane_separation():
+    time = Time('2003-04-05 06:07:08')
+    apo = Observer.at_site("APO")
+    one_deg_away = SkyCoord(0*u.deg, 1*u.deg, frame=Galactic)
+    five_deg_away = SkyCoord(0*u.deg, 5*u.deg, frame=Galactic)
+    twenty_deg_away = SkyCoord(0*u.deg, 20*u.deg, frame=Galactic)
+
+    constraint = GalacticLatitudeConstraint(min=2*u.deg, max=10*u.deg)
+    is_constraint_met = constraint(apo, [one_deg_away, five_deg_away,
+                                         twenty_deg_away], times=time)
+    assert np.all(is_constraint_met == [False, True, False])
+
+    constraint = GalacticLatitudeConstraint(max=10*u.deg)
+    is_constraint_met = constraint(apo, [one_deg_away, five_deg_away,
+                                         twenty_deg_away], times=time)
+    assert np.all(is_constraint_met == [True, True, False])
+
+    constraint = GalacticLatitudeConstraint(min=2*u.deg)
+    is_constraint_met = constraint(apo, [one_deg_away, five_deg_away,
+                                         twenty_deg_away], times=time)
+    assert np.all(is_constraint_met == [False, True, True])
+
 # in astropy before v1.0.4, a recursion error is triggered by this test
 @pytest.mark.skipif('APY_LT104')
 def test_sun_separation():