refactor: route size conversions through cs_util.size

The Gaussian size conversions are now sourced from cs_util.size, the
single source of truth shared with sp_validation, instead of being
re-derived locally:

- ngmix.py get_noise window: np.sqrt(guess[4]/2) -> cs_size.T_to_sigma
  (guess[4] is the ngmix area parameter T = 2 sigma^2; bit-exact).
- make_cat.py PSF_FWHM: galaxy.sigma_to_fwhm -> cs_size.sigma_to_fwhm
  (bare conversion, no pixel_scale).
- utilities/galaxy.sigma_to_fwhm becomes a thin wrapper that keeps the
  pixel_scale rescaling and ShapePipe input validation but delegates
  the bare 2*sqrt(2 ln 2) factor to cs_util.size; this preserves its
  second caller (spread_model.py, which uses pixel_scale) and the
  test_utilities contract. Matches the old hard-coded 2.35482004503
  constant to <1e-11 relative.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: cailmdaley

src/shapepipe/modules/make_cat_package/make_cat.py

@@ -13,10 +13,10 @@
 from astropy import coordinates as coords
 from astropy import units as u
 from astropy.wcs import WCS
+from cs_util import size as cs_size
 from sqlitedict import SqliteDict
 
 from shapepipe.pipeline import file_io
-from shapepipe.utilities import galaxy
 
 
 def get_output_name(output_dir, file_number_string):
@@ -725,7 +725,7 @@ def _save_psf_data(self, galaxy_psf_path):
                 )
                 self._add2dict(f"PSF_ELL_{epoch + 1}", e_psf, idx)
 
-                psf_fwhm = galaxy.sigma_to_fwhm(
+                psf_fwhm = cs_size.sigma_to_fwhm(
                     gpc_data["SHAPES"]["SIGMA_PSF_HSM"]
                 )
                 self._add2dict(f"PSF_FWHM_{epoch + 1}", psf_fwhm, idx)

src/shapepipe/modules/ngmix_package/ngmix.py

@@ -14,6 +14,7 @@
 import galsim
 import numpy as np
 from astropy.io import fits
+from cs_util import size as cs_size
 from modopt.math.stats import sigma_mad
 from ngmix.observation import Observation, ObsList
 from sqlitedict import SqliteDict
@@ -970,7 +971,7 @@ def get_noise(gal, weight, guess, pixel_scale, thresh=1.2):
 
     sig_tmp = sigma_mad(gal[m_weight])
 
-    gauss_win = galsim.Gaussian(sigma=np.sqrt(guess[4] / 2), flux=guess[5])
+    gauss_win = galsim.Gaussian(sigma=cs_size.T_to_sigma(guess[4]), flux=guess[5])
     gauss_win = gauss_win.shear(g1=guess[2], g2=guess[3])
     gauss_win = gauss_win.drawImage(
         nx=img_shape[0], ny=img_shape[1], scale=pixel_scale

src/shapepipe/utilities/galaxy.py

@@ -7,6 +7,7 @@
 """
 
 import numpy as np
+from cs_util import size as cs_size
 
 
 def sigma_to_fwhm(sigma, pixel_scale=1.0):
@@ -15,6 +16,11 @@ def sigma_to_fwhm(sigma, pixel_scale=1.0):
     Transform standard deviation of a 1D Gaussian, sigma, to FWHM
     (Full Width Half Maximum).
 
+    Thin wrapper over :func:`cs_util.size.sigma_to_fwhm`, the single
+    source of truth for the bare ``FWHM = 2 sqrt(2 ln 2) sigma``
+    conversion; this layer adds the optional ``pixel_scale`` rescaling
+    and ShapePipe's input validation.
+
     Parameters
     ----------
     sigma : numpy.ndarray
@@ -89,6 +95,4 @@ def sigma_to_fwhm(sigma, pixel_scale=1.0):
             f"Invalid pixel scale {pixel_scale}, needs to be greater than 0.0."
         )
 
-    cst = 2.35482004503
-
-    return sigma * cst * pixel_scale
+    return cs_size.sigma_to_fwhm(sigma) * pixel_scale