PSF kernel calibration

Per-iteration PSF kernel fit (gaussian/moffat/drizzle), DAO matched-filter FWHM scan / empirical-FWHM shortcut, and centre-mask helpers.

Per-iteration PSF calibration: sphot’s mainloop hook.

calibrate_psf_step is called per Sersic+iPSF mainloop iteration via sphot.core._maybe_recalibrate_psf when [psf-calib] in_mainloop = true. Each call:

  1. Pulls quality-passing anchors from cutoutdata.psf_table. With no usable photometry the bootstrap blur grid picks a starting PSF from source counts.

  2. Fits a kernel K (Gaussian / Moffat / drizzle) against the anchors via the multi-source NNLS objective.

  3. Replaces cutoutdata.psf with library_psf * K.

  4. Scans dao_fwhm_factor on a 3-point bracket and picks the factor that maximises quality-passing source count under the new PSF.

sphot.calibrate_psf.effective_fwhm_data_px(cutoutdata)[source]

FWHM of cd.psf in data pixels (post-kernel calibration).

Used for sizing the centre-mask radius and any other “where is the PSF actually wide” calculation. Falls back to the library Gaussian equivalent (2.355 × psf_sigma) if photutils.psf.fit_fwhm fails so the caller always gets a finite, sane number.

sphot.calibrate_psf.calibrate_psf_step(cutoutdata, *, family: str = 'gaussian', K: int = 30, fwhm_default_grid: Sequence[float] = (1.0, 1.5, 2.355), bracket_lo: float = 0.5, bracket_hi: float = 3.5, skip_stable_threshold: float = 0.001, progress=None, log=<built-in function print>)[source]

One PSF calibration step. Reuses the just-completed photometry and cd.sersic_residual to fit a kernel and update cd.psf.

Steps:

  1. Pull (x, y, flux) of the K brightest quality-passing sources from cd.psf_table (or run bootstrap blur if none).

  2. Fit kernel parameters via the multi-source NNLS objective.

  3. cd.psf <- library * K.

  4. Bracket-scan dao_fwhm_factor, or reuse the previous one if the kernel L2 distance is below skip_stable_threshold.

  5. Persist cd.kernel_params, cd.dao_fwhm_factor, cd._calibrate_psf_prev_kernel.

family: ‘gaussian’ (default) | ‘moffat’ | ‘drizzle’.