fix bug when moms give a bad size for guesser and using a random number

generator that checks low/high such as (np.random.default_rng)

ngmix/guessers.py

@@ -873,6 +873,9 @@ def _get_T_flux_from_moms(self, obs):
             # deweight assuming true profile is a gaussian
             T = 1.0/(1/Tmeas - 1/Tweight)
             flux = res['flux'] * np.pi * (Tweight + T) / area
+            if T < 0:
+                T = res['T']
+                flux = res['flux']
 
         return T, flux
 

ngmix/tests/_sims.py

@@ -1,6 +1,6 @@
 import numpy as np
 
-from ngmix import DiagonalJacobian, GMix, GMixModel
+from ngmix import DiagonalJacobian, Jacobian, GMix, GMixModel
 from ngmix import Observation, ObsList, MultiBandObsList
 
 PIXEL_SCALE = 0.263
@@ -226,3 +226,31 @@ def get_psf_obs(*, rng, T=TPSF, model="turb", noise=1.0e-6):
         'obs': Observation(im, weight=weight, jacobian=jacob),
         'gmix': gm,
     }
+
+
+def get_noisy_obs(rng):
+    """
+    highlight a single pixel
+    """
+    # import fitsio
+    dim = 25
+    noise = 50.559849582916115
+
+    im = rng.normal(scale=noise, size=(dim, dim))
+
+    dims = im.shape
+    cen = (np.array(dims) - 1.0) / 2.0
+
+    jacob = Jacobian(
+        row=cen[0], col=cen[1],
+        dvdrow=-0.15179598030886227,
+        dvdcol=0.13007044200963258,
+        dudrow=-0.13014613410130665,
+        dudcol=-0.15185634442578344,
+    )
+
+    noise = im.std()
+    weight = im*0 + 1/noise**2
+    return {
+        'obs': Observation(im, weight=weight, jacobian=jacob),
+    }

ngmix/tests/test_guessers.py

@@ -4,7 +4,7 @@
 import ngmix
 from ngmix import guessers
 from ngmix.gmix import get_coellip_npars
-from ._sims import get_model_obs, get_psf_obs
+from ._sims import get_model_obs, get_psf_obs, get_noisy_obs
 from ._priors import get_prior
 
 
@@ -245,3 +245,16 @@ def test_guessers_errors():
 
     with pytest.raises(ValueError):
         ngmix.guessers.CoellipPSFGuesser(rng=rng, ngauss=1000)
+
+
+def test_mom_guesser_badsize():
+    """
+    tests a case where moms used for guess are bad
+    so a fallback guess is used
+    """
+    rng = np.random.default_rng(354365)
+    guesser = ngmix.guessers.GMixPSFGuesser(
+        rng=rng, ngauss=1, guess_from_moms=True,
+    )
+    data = get_noisy_obs(rng=rng)
+    guesser(data['obs'])