add near meridian constraint

astroplan/constraints.py

@@ -35,7 +35,7 @@
            "LocalTimeConstraint", "PrimaryEclipseConstraint",
            "SecondaryEclipseConstraint", "Constraint", "TimeConstraint",
            "observability_table", "months_observable", "max_best_rescale",
-           "min_best_rescale", "PhaseConstraint", "is_event_observable"]
+           "min_best_rescale", "PhaseConstraint", "is_event_observable", "NearMeridianConstraint"]
 
 _current_year = time.localtime().tm_year  # needed for backward compatibility
 _current_year_time_range = Time(  # needed for backward compatibility
@@ -938,6 +938,46 @@ def compute_constraint(self, times, observer=None, targets=None):
         return mask
 
 
+class NearMeridianConstraint(Constraint):
+    """
+    Constraint near the Meridian.
+    """
+    def __init__(self, min=None, boolean_constraint=True):
+        """
+        Parameters
+        ----------
+        min : `~astropy.units.Quantity` or `None`, optional
+            Minimum acceptable distance to meridian.
+            `None` indicates no limit.
+
+        boolean_constraint : bool
+
+        Examples
+        --------
+        Constrain observations to targets that are 3 degrees away from the meridian.
+        >>> import astropy.units as u
+        >>> constraint = NearMeridianConstraint(min=3*u.deg)
+
+        This can be useful for observations using German-Equatorial Mounts, to avoid
+        flipping the side of the pier during exposures.
+        """
+        self.min = min if min is not None else 0*u.hourangle
+        self.boolean_constraint = boolean_constraint
+
+    def compute_constraint(self, times, observer, targets):
+        lst = observer.local_sidereal_time(times)
+        meridian = SkyCoord(ra=lst, dec=targets.dec)
+
+        meridian_separation = meridian.separation(targets)
+
+        if self.boolean_constraint:
+            mask = (self.min < meridian_separation)
+            return mask
+        else:
+            rescale = min_best_rescale(meridian_separation, self.min, less_than_min=0)
+            return rescale
+
+
 def is_always_observable(constraints, observer, targets, times=None,
                          time_range=None, time_grid_resolution=0.5*u.hour):
     """

astroplan/tests/test_constraints.py

@@ -5,7 +5,7 @@
 import numpy as np
 import astropy.units as u
 from astropy.time import Time
-from astropy.coordinates import Galactic, SkyCoord, get_sun, get_body
+from astropy.coordinates import Galactic, SkyCoord, EarthLocation, get_sun, get_body
 from astropy.utils import minversion
 import pytest
 
@@ -20,7 +20,7 @@
                            TimeConstraint, LocalTimeConstraint, months_observable,
                            max_best_rescale, min_best_rescale, PhaseConstraint,
                            PrimaryEclipseConstraint, SecondaryEclipseConstraint,
-                           is_event_observable)
+                           is_event_observable, NearMeridianConstraint)
 from ..periodic import EclipsingSystem
 from ..exceptions import MissingConstraintWarning
 
@@ -200,6 +200,18 @@ def test_sun_separation():
     assert np.all(is_constraint_met == [False, True, True])
 
 
+def test_near_meridian():
+    time_range = Time(["2024-10-08 20:28", "2024-10-08 22:30"])
+    target = FixedTarget(coord=SkyCoord(ra=19.75*u.hour, dec=-22.05*u.deg), name="name")
+
+    # Pico dos Dias Observatory (Brazil)
+    opd = Observer(location=EarthLocation(lat=-22.53, lon=-45.58, height=1864))
+    constraint = NearMeridianConstraint(min=3*u.deg)
+
+    results = constraint(opd, target, times=time_grid_from_range(time_range))
+    assert np.all(results == [True, True, False, True, True])
+
+
 def test_moon_separation():
     time = Time('2003-04-05 06:07:08')
     apo = Observer.at_site("APO")
@@ -419,6 +431,7 @@ def test_rescale_minmax():
     AtNightConstraint(),
     SunSeparationConstraint(min=90*u.deg),
     MoonSeparationConstraint(min=20*u.deg),
+    NearMeridianConstraint(min=3*u.deg),
     LocalTimeConstraint(min=dt.time(23, 50), max=dt.time(4, 8)),
     TimeConstraint(*Time(["2015-08-28 03:30", "2015-09-05 10:30"]))
 ]