FittableImageModel¶

class
photutils.psf.
FittableImageModel
(data, flux=1.0, x_0=0.0, y_0=0.0, normalize=False, normalization_correction=1.0, origin=None, oversampling=1, fill_value=0.0, **kwargs)[source]¶ Bases:
astropy.modeling.Fittable2DModel
A fittable 2D model of an image allowing for image intensity scaling and image translations.
This class takes 2D image data and computes the values of the model at arbitrary locations (including at intrapixel, fractional positions) within this image using spline interpolation provided by
RectBivariateSpline
.The fittable model provided by this class has three model parameters: an image intensity scaling factor (
flux
) which is applied to (normalized) image, and two positional parameters (x_0
andy_0
) indicating the location of a feature in the coordinate grid on which the model is to be evaluated.If this class is initialized with
flux
(intensity scaling factor) set toNone
, thenflux
is going to be estimated assum(data)
. Parameters
 datanumpy.ndarray
Array containing 2D image.
 origintuple, None, optional
A reference point in the input image
data
array. When origin isNone
, origin will be set at the middle of the image array.If
origin
represents the location of a feature (e.g., the position of an intensity peak) in the inputdata
, then model parametersx_0
andy_0
show the location of this peak in an another target image to which this model was fitted. Fundamentally, it is the coordinate in the model’s image data that should map to coordinate (x_0
,y_0
) of the output coordinate system on which the model is evaluated.Alternatively, when
origin
is set to(0,0)
, then model parametersx_0
andy_0
are shifts by which model’s image should be translated in order to match a target image. normalizebool, optional
Indicates whether or not the model should be build on normalized input image data. If true, then the normalization constant (N) is computed so that
\[N \cdot C \cdot \sum\limits_{i,j} D_{i,j} = 1,\]where N is the normalization constant, C is correction factor given by the parameter
normalization_correction
, and \(D_{i,j}\) are the elements of the input imagedata
array. normalization_correctionfloat, optional
A strictly positive number that represents correction that needs to be applied to model’s data normalization (see C in the equation in the comments to
normalize
for more details). A possible application for this parameter is to account for aperture correction. Assuming model’s data represent a PSF to be fitted to some target star, we setnormalization_correction
to the aperture correction that needs to be applied to the model. That is,normalization_correction
in this case should be set to the ratio between the total flux of the PSF (including flux outside model’s data) to the flux of model’s data. Then, best fitted value of theflux
model parameter will represent an aperturecorrected flux of the target star. In the case of aperture correction,normalization_correction
should be a value larger than one, as the total flux, including regions outside of the aperture, should be larger than the flux inside the aperture, and thus the correction is applied as an inversely multiplied factor. fill_valuefloat, optional
The value to be returned by the
evaluate
orastropy.modeling.Model.__call__
methods when evaluation is performed outside the definition domain of the model. kwargsdict, optional
Additional optional keyword arguments to be passed directly to the
compute_interpolator
method. Seecompute_interpolator
for more details. oversamplingint or tuple of two int, optional
The oversampling factor(s) of the model in the
x
andy
directions. Ifoversampling
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)
.
Attributes Summary
Get original image data.
Fill value to be returned for coordinates outside of the domain of definition of the interpolator.
Get current interpolator’s arguments used when interpolator was created.
Get normalization constant.
Set/Get flux correction factor.
Get normalization status.
Get normalized and/or intensitycorrected image data.
Number of columns in the data array.
Number of rows in the data array.
A tuple of
x
andy
coordinates of the origin of the coordinate system in terms of pixels of model’s image.The factor by which the stored image is oversampled.
Names of the parameters that describe models of this type.
A tuple of dimensions of the data array in numpy style (ny, nx).
Xcoordinate of the origin of the coordinate system.
Ycoordinate of the origin of the coordinate system.
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.
Attributes Documentation

data
¶ Get original image data.

fill_value
¶ Fill value to be returned for coordinates outside of the domain of definition of the interpolator. If
fill_value
isNone
, then values outside of the domain of definition are the ones returned by the interpolator.

flux
= Parameter('flux', value=1.0)¶

interpolator_kwargs
¶ Get current interpolator’s arguments used when interpolator was created.

normalization_constant
¶ Get normalization constant.

normalization_correction
¶ Set/Get flux correction factor.
Note
When setting correction factor, model’s flux will be adjusted accordingly such that if this model was a good fit to some target image before, then it will remain a good fit after correction factor change.

normalization_status
¶ Get normalization status. Possible status values are:
 0: Performed. Model has been successfuly normalized at
user’s request.
1: Failed. Attempt to normalize has failed.
2: NotRequested. User did not request model to be normalized.

normalized_data
¶ Get normalized and/or intensitycorrected image data.

nx
¶ Number of columns in the data array.

ny
¶ Number of rows in the data array.

origin
¶ A tuple of
x
andy
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
orfloat
may be used. If origin is set toNone
, the origin of the coordinate system will be set to the middle of the data array ((npix1)/2.0
).Warning
Modifying
origin
will not adjust (modify) model’s parametersx_0
andy_0
.

oversampling
¶ The factor by which the stored image is oversampled.
An input to this model is multipled by this factor to yield the index into the stored image.

param_names
= ('flux', 'x_0', 'y_0')¶ Names of the parameters that describe models of this type.
The parameters in this tuple are in the same order they should be passed in when initializing a model of a specific type. Some types of models, such as polynomial models, have a different number of parameters depending on some other property of the model, such as the degree.
When defining a custom model class the value of this attribute is automatically set by the
Parameter
attributes defined in the class body.

shape
¶ A tuple of dimensions of the data array in numpy style (ny, nx).

x_0
= Parameter('x_0', value=0.0)¶

x_origin
¶ Xcoordinate of the origin of the coordinate system.

y_0
= Parameter('y_0', value=0.0)¶

y_origin
¶ Ycoordinate of the origin of the coordinate system.
Methods Documentation

compute_interpolator
(**kwargs)[source]¶ Compute/define the interpolating spline.
This function can be overriden 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 Yaxes. Default value is degree=3.
 sfloat, optional
Nonnegative smoothing factor. Default value s=0 corresponds to interpolation. See
RectBivariateSpline
for more details.
Notes
When subclassing
FittableImageModel
for the purpose of overridingcompute_interpolator()
, theevaluate()
may need to overriden as well depending on the behavior of the new interpolator. In addition, for improved future compatibility, make sure that the overriding method stores keyword argumentskwargs
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, use_oversampling=True)[source]¶ Evaluate the model on some input variables and provided model parameters.
 Parameters
 use_oversamplingbool, optional
Whether to use the oversampling factor to calculate the model pixel indices. The default is
True
, which means the input indices will be multipled by this factor.