EllipseGeometry¶

class
photutils.isophote.
EllipseGeometry
(x0, y0, sma, eps, pa, astep=0.1, linear_growth=False, fix_center=False, fix_pa=False, fix_eps=False)[source]¶ Bases:
object
Container class to store parameters for the geometry of an ellipse.
Parameters that describe the relationship of a given ellipse with other associated ellipses are also encapsulated in this container. These associated ellipses may include, e.g., the two (inner and outer) bounding ellipses that are used to build sectors along the elliptical path. These sectors are used as areas for integrating pixel values, when the area integration mode (mean or median) is used.
This class also keeps track of where in the ellipse we are when performing an ‘extract’ operation. This is mostly relevant when using an area integration mode (as opposed to a pixel integration mode)
 Parameters
 x0, y0float
The center pixel coordinate of the ellipse.
 smafloat
The semimajor axis of the ellipse in pixels.
 epsellipticity
The ellipticity of the ellipse.
 pafloat
The position angle (in radians) of the semimajor axis in relation to the postive x axis of the image array (rotating towards the positive y axis). Position angles are defined in the range \(0 < PA <= \pi\). Avoid using as starting position angle of 0., since the fit algorithm may not work properly. When the ellipses are such that position angles are near either extreme of the range, noise can make the solution jump back and forth between successive isophotes, by amounts close to 180 degrees.
 astepfloat, optional
The step value for growing/shrinking the semimajor axis. It can be expressed either in pixels (when
linear_growth=True
) or as a relative value (whenlinear_growth=False
). The default is 0.1. linear_growthbool, optional
The semimajor axis growing/shrinking mode. The default is
False
. fix_centerbool, optional
Keep center of ellipse fixed during fit? The default is False.
 fix_pabool, optional
Keep position angle of semimajor axis of ellipse fixed during fit? The default is False.
 fix_epsbool, optional
Keep ellipticity of ellipse fixed during fit? The default is False.
Methods Summary
Compute the semimajor axis of the two ellipses that bound the annulus where integrations take place.
find_center
(image[, threshold, verbose])Find the center of a galaxy.
Initialize geometry attributes associated with an elliptical sector at the given polar angle
phi
.Return the two polar angles that bound the sector.
radius
(angle)Calculate the polar radius for a given polar angle.
reset_sma
(step)Change the direction of semimajor axis growth, from outwards to inwards.
to_polar
(x, y)Return the radius and polar angle in the ellipse coordinate system given (x, y) pixel image coordinates.
update_sma
(step)Calculate an updated value for the semimajor axis, given the current value and the step value.
Methods Documentation

bounding_ellipses
()[source]¶ Compute the semimajor axis of the two ellipses that bound the annulus where integrations take place.
 Returns
 sma1, sma2float
The smaller and larger values of semimajor axis length that define the annulus bounding ellipses.

find_center
(image, threshold=0.1, verbose=True)[source]¶ Find the center of a galaxy.
If the algorithm is successful the (x, y) coordinates in this
EllipseGeometry
(i.e., thex0
andy0
attributes) instance will be modified.The isophote fit algorithm requires an initial guess for the galaxy center (x, y) coordinates and these coordinates must be close to the actual galaxy center for the isophote fit to work. This method provides can provide an initial guess for the galaxy center coordinates. See the Notes section below for more details.
 Parameters
 image2D
ndarray
The image array. Masked arrays are not recognized here. This assumes that centering should always be done on valid pixels.
 thresholdfloat, optional
The centerer threshold. To turn off the centerer, set this to a large value (i.e., >> 1). The default is 0.1.
 verbosebool, optional
Whether to print object centering information. The default is
True
.
 image2D
Notes
The centerer function scans a 10x10 window centered on the (x, y) coordinates in the
EllipseGeometry
instance passed to the constructor of theEllipse
class. If any of theEllipseGeometry
(x, y) coordinates areNone
, the center of the input image frame is used. If the center acquisition is successful, theEllipseGeometry
instance is modified in place to reflect the solution of the object centerer algorithm.In some cases the object centerer algorithm may fail even though there is enough signaltonoise to start a fit (e.g., objects with very high ellipticity). In those cases the sensitivity of the algorithm can be decreased by decreasing the value of the object centerer threshold parameter. The centerer works by looking where a quantity akin to a signaltonoise ratio is maximized within the 10x10 window. The centerer can thus be shut off entirely by setting the threshold to a large value (i.e., >> 1; meaning no location inside the search window will achieve that signaltonoise ratio).

initialize_sector_geometry
(phi)[source]¶ Initialize geometry attributes associated with an elliptical sector at the given polar angle
phi
.This function computes:
the four vertices that define the elliptical sector on the pixel array.
the sector area (saved in the
sector_area
attribute)the sector angular width (saved in
sector_angular_width
attribute)
 Parameters
 phifloat
The polar angle (radians) where the sector is located.
 Returns
 x, y1D
ndarray
The x and y coordinates of each vertex as 1D arrays.
 x, y1D

polar_angle_sector_limits
()[source]¶ Return the two polar angles that bound the sector.
The two bounding polar angles become available only after calling the
initialize_sector_geometry()
method. Returns
 phi1, phi2float
The smaller and larger values of polar angle that bound the current sector.

radius
(angle)[source]¶ Calculate the polar radius for a given polar angle.
 Parameters
 anglefloat
The polar angle (radians).
 Returns
 radiusfloat
The polar radius (pixels).

reset_sma
(step)[source]¶ Change the direction of semimajor axis growth, from outwards to inwards.
 Parameters
 stepfloat
The current step value.
 Returns
 sma, new_stepfloat
The new semimajor axis length and the new step value to initiate the shrinking of the semimajor axis length. This is the step value that should be used when calling the
update_sma()
method.

to_polar
(x, y)[source]¶ Return the radius and polar angle in the ellipse coordinate system given (x, y) pixel image coordinates.
This function takes care of the different definitions for position angle (PA) and polar angle (phi):
\[ \begin{align}\begin{aligned}\pi < PA < \pi\\0 < phi < 2 \pi\end{aligned}\end{align} \]Note that radius can be anything. The solution is not tied to the semimajor axis length, but to the center position and tilt angle.
 Parameters
 x, yfloat
The (x, y) image coordinates.
 Returns
 radius, anglefloat
The ellipse radius and polar angle.

update_sma
(step)[source]¶ Calculate an updated value for the semimajor axis, given the current value and the step value.
The step value must be managed by the caller to support both modes: grow outwards and shrink inwards.
 Parameters
 stepfloat
The step value.
 Returns
 smafloat
The new semimajor axis length.