EPSFModel

class photutils.psf.EPSFModel(data, flux=1.0, x_0=0.0, y_0=0.0, normalize=True, normalization_correction=1.0, origin=None, oversampling=1, fill_value=0.0, norm_radius=5.5, shift_val=0.5, **kwargs)

Bases: photutils.psf.FittableImageModel

A class that models an effective PSF (ePSF).

The EPSFModel is normalized such that the sum of the PSF over the (undersampled) pixels within the the input norm_radius is 1.0. This means that when the EPSF is fit to stars, the resulting flux corresponds to aperture photometry within a circular aperture of radius norm_radius.

While this class is a subclass of FittableImageModel, it is very similar. The primary differences/motivation are a few additional parameters necessary specifically for ePSFs.

Parameters

oversamplingint or tuple of two int, optional

The oversampling factor(s) of the model in the x and y directions. If oversampling is a scalar it will be treated as being the same in both x and y; otherwise a tuple of two floats will be treated as (x_oversamp, y_oversamp).

norm_radiusfloat, optional

The radius inside which the ePSF is normalized by the sum over undersampled integer pixel values inside a circular aperture.

shift_valfloat, optional

The fractional undersampled pixel amount (equivalent to an integer oversampled pixel value) at which to evaluate the asymmetric ePSF centroid corrections.

Attributes Summary

origin

A tuple of x and y coordinates of the origin of the coordinate system in terms of pixels of model’s image.

Methods Summary

compute_interpolator(**kwargs)	Compute/define the interpolating spline.
evaluate(x, y, flux, x_0, y_0)	Evaluate the model on some input variables and provided model parameters.
normalized_data()	Overloaded dummy function that also returns self._data, as the normalization occurs within _compute_normalization in EPSFModel, and as such self._data will sum, accounting for under/oversampled pixels, to 1/self._normalization_correction.

Attributes Documentation

origin

A tuple of x and y coordinates of the origin of the coordinate system in terms of pixels of model’s image.

When setting the coordinate system origin, a tuple of two int or float may be used. If origin is set to None, the origin of the coordinate system will be set to the middle of the data array ((npix-1)/2.0).

Warning

Modifying origin will not adjust (modify) model’s parameters x_0 and y_0.

Methods Documentation

compute_interpolator(**kwargs)

Compute/define the interpolating spline.

This function can be overridden in a subclass to define custom interpolators.

Parameters

**kwargsdict, optional

Additional optional keyword arguments:

• degreeint, tuple, optional

  Degree of the interpolating spline. A tuple can be used to provide different degrees for the X- and Y-axes. Default value is degree=3.

• sfloat, optional

  Non-negative smoothing factor. Default value s=0 corresponds to interpolation. See RectBivariateSpline for more details.

Notes

• When subclassing FittableImageModel for the purpose of overriding compute_interpolator(), the evaluate() may need to overridden as well depending on the behavior of the new interpolator. In addition, for improved future compatibility, make sure that the overriding method stores keyword arguments kwargs by calling _store_interpolator_kwargs method.

• Use caution when modifying interpolator’s degree or smoothness in a computationally intensive part of the code as it may decrease code performance due to the need to recompute interpolator.

evaluate(x, y, flux, x_0, y_0)

Evaluate the model on some input variables and provided model parameters.

normalized_data()

Overloaded dummy function that also returns self._data, as the normalization occurs within _compute_normalization in EPSFModel, and as such self._data will sum, accounting for under/oversampled pixels, to 1/self._normalization_correction.