Isophote

class photutils.isophote.Isophote(sample, niter, valid, stop_code)

Bases: object

Container class to store the results of single isophote fit.

The extracted data sample at the given isophote (sampled intensities along the elliptical path on the image) is also kept as an attribute of this class. The container concept helps in segregating information directly related to the sample, from information that more closely relates to the fitting process, such as status codes, errors for isophote parameters, and the like.

Parameters:

sampleEllipseSample instance

The sample information.

niterint

The number of iterations used to fit the isophote.

validbool

The status of the fitting operation.

stop_codeint

The fitting stop code:

• 0: Normal.

• 1: Fewer than the pre-specified fraction of the extracted data points are valid.

• 2: Exceeded maximum number of iterations.

• 3: Singular matrix in harmonic fit, results may not be valid. This also signals an insufficient number of data points to fit.

• 4: Small or wrong gradient, or ellipse diverged. Subsequent ellipses at larger or smaller semimajor axis may have the same constant geometric parameters. It’s also used when the user turns off the fitting algorithm via the maxrit fitting parameter (see the Ellipse class).

• 5: Ellipse diverged; not even the minimum number of iterations could be executed. Subsequent ellipses at larger or smaller semimajor axis may have the same constant geometric parameters.

• -1: Internal use.

Attributes:

rmsfloat

The root-mean-square of intensity values along the elliptical path.

int_errfloat

The error of the mean (rms / sqrt(# data points)).

ellip_errfloat

The ellipticity error.

pa_errfloat

The position angle error (radians).

x0_errfloat

The error associated with the center x coordinate.

y0_errfloat

The error associated with the center y coordinate.

pix_stddevfloat

The estimate of pixel standard deviation (rms * sqrt(average sector integration area)).

gradfloat

The local radial intensity gradient.

grad_errorfloat

The measurement error of the local radial intensity gradient.

grad_r_errorfloat

The relative error of local radial intensity gradient.

tflux_efloat

The sum of all pixels inside the ellipse.

npix_eint

The total number of valid pixels inside the ellipse.

tflux_cfloat

The sum of all pixels inside a circle with the same sma as the ellipse.

npix_cint

The total number of valid pixels inside a circle with the same sma as the ellipse.

sareafloat

The average sector area on the isophote (pixel**2).

ndataint

The number of extracted data points.

nflagint

The number of discarded data points. Data points can be discarded either because they are physically outside the image frame boundaries, because they were rejected by sigma-clipping, or they are masked.

a3, b3, a4, b4float

The higher order harmonics that measure the deviations from a perfect ellipse. These values are actually the raw harmonic amplitudes divided by the local radial gradient and the semimajor axis length, so they can directly be compared with each other. The b4 parameter is positive for galaxies with disky (kite-like) isophotes and negative for galaxies with boxy isophotes.

a3_err, b3_err, a4_err, b4_errfloat

The errors associated with the a3, b3, a4, and b4 attributes.

Attributes Summary

eps	The ellipticity of the ellipse.
pa	The position angle (radians) of the ellipse.
sma	The semimajor axis length (pixels).
x0	The center x coordinate (pixel).
y0	The center y coordinate (pixel).

Methods Summary

fix_geometry(isophote)	Fix the geometry of a problematic isophote to be identical to the input isophote.
sampled_coordinates()	Return the (x, y) coordinates where the image was sampled in order to get the intensities associated with this isophote.
to_table()	Return the main isophote parameters as an astropy QTable.

Attributes Documentation

eps

The ellipticity of the ellipse.

pa

The position angle (radians) of the ellipse.

sma

The semimajor axis length (pixels).

x0

The center x coordinate (pixel).

y0

The center y coordinate (pixel).

Methods Documentation

fix_geometry(isophote)

Fix the geometry of a problematic isophote to be identical to the input isophote.

This method should be called when the fitting goes berserk and delivers an isophote with bad geometry, such as ellipticity > 1 or another meaningless situation. This is not a problem in itself when fitting any given isophote, but will create an error when the affected isophote is used as starting guess for the next fit.

Parameters:

isophoteIsophote instance

The isophote from which to take the geometry information.

sampled_coordinates()

Return the (x, y) coordinates where the image was sampled in order to get the intensities associated with this isophote.

Returns:

x, y1D ndarray

The x and y coordinates as 1D arrays.

to_table()

Return the main isophote parameters as an astropy QTable.

Returns:

resultQTable

An astropy QTable containing the main isophote parameters.