Merge pull request #174 from sot/dyn-bgd-min-anchor-stars

Require minimum number of anchor stars for dyn bgd bonus

sparkles/core.py

@@ -38,6 +38,13 @@
 CACHE = {}
 FILEDIR = Path(__file__).parent
 
+# Minimum number of "anchor stars" that are always evaluated *without* the bonus
+# from dynamic background when dyn_bgd_n_faint > 0. This is mostly to avoid the
+# situation where 4 stars are selected and 2 are faint bonus stars. In this case
+# there would be only 2 anchor stars that ensure good tracking even without
+# dyn bgd.
+MIN_DYN_BGD_ANCHOR_STARS = 3
+
 
 def main(sys_args=None):
     """Command line interface to preview_load()"""
@@ -555,10 +562,13 @@ def __init__(self, *args, **kwargs):
             t_ccds = np.full_like(mags, self.guides.t_ccd)
             if self.dyn_bgd_n_faint > 0:
                 # Apply the dynamic background t_ccd bonus to the
-                # dyn_bgd_n_faint faintest stars. See:
+                # dyn_bgd_n_faint faintest stars, ensuring that at least
+                # MIN_DYN_BGD_ANCHOR_STARS are evaluated without the bonus. See:
                 # https://nbviewer.org/urls/cxc.harvard.edu/mta/ASPECT/ipynb/misc/guide-count-dyn-bgd.ipynb
                 mags = np.sort(mags)
-                t_ccds[-self.dyn_bgd_n_faint:] += self.dyn_bgd_dt_ccd
+                n_faint = min(self.dyn_bgd_n_faint, len(t_ccds))
+                idx_bonus = max(len(t_ccds) - n_faint, MIN_DYN_BGD_ANCHOR_STARS)
+                t_ccds[idx_bonus:] += self.dyn_bgd_dt_ccd
 
             self.guide_count = guide_count(mags, t_ccds)
             if self.is_ER:

sparkles/tests/test_checks.py

@@ -121,25 +121,34 @@ def test_n_guide_mon_check_atypical_request():
          'category': 'caution'}]
 
 
-def test_guide_count_dyn_bgd_bonus():
+vals = [
+    (5, 4.07, 4.66),  # n_guide, legacy_guide_count, dyn_bdg_guide_count
+    (4, 3.25, 3.60),  # 4 stars, legacy and dyn bdg guide counts closer together
+    (3, 2.42, 2.42),  # 3 stars, legacy and dyn bdg equal (no bonus stars)
+]
+
+
+@pytest.mark.parametrize('vals', vals)
+def test_guide_count_dyn_bgd_bonus(vals):
+    n_guide, leg_guide_count, dyn_guide_count = vals
     stars = StarsTable.empty()
 
-    stars.add_fake_constellation(mag=np.linspace(10, 10.2, 5),
-                                 size=2000, n_stars=5)
+    stars.add_fake_constellation(mag=np.linspace(10, 10.2, n_guide),
+                                 size=2000, n_stars=n_guide)
 
     aca_leg = get_aca_catalog(**STD_INFO, dark=DARK40, stars=stars, dyn_bgd_n_faint=0)
     aca_dyn = get_aca_catalog(**STD_INFO, dark=DARK40, stars=stars, dyn_bgd_n_faint=2,
                               dyn_bgd_dt_ccd=-4.0)
     # Same catalog but with different attributes
-    assert len(aca_leg.guides) == 5
-    assert len(aca_dyn.guides) == 5
+    assert len(aca_leg.guides) == n_guide
+    assert len(aca_dyn.guides) == n_guide
     assert np.all(aca_leg.guides['mag'] == aca_dyn.guides['mag'])
 
     acar_leg = ACAReviewTable(aca_leg)
     acar_dyn = ACAReviewTable(aca_dyn)
-    # Computed guide counts are different and more with dyn_bgd_n_faint=2
-    assert np.isclose(acar_leg.guide_count, 4.07, rtol=0, atol=0.1)
-    assert np.isclose(acar_dyn.guide_count, 4.66, rtol=0, atol=0.1)
+    # Computed guide counts without / with dyn_bgd_n_faint=2
+    assert np.isclose(acar_leg.guide_count, leg_guide_count, rtol=0, atol=0.1)
+    assert np.isclose(acar_dyn.guide_count, dyn_guide_count, rtol=0, atol=0.1)
 
 
 def test_n_guide_too_few_guide_or_mon():