profoundGetEllipse: Calculate single annulus properties of an iso-photal ellipse

Description

Returns single ellipse properties for a specific set of pixels, assumed to be be a narrow range in flux (i.e. an iso-photal annulus).

Usage

profoundGetEllipse(x, y, z, xcen = NULL, ycen = NULL, scale = sqrt(2), pixscale = 1,
dobox = FALSE, plot=FALSE, ...)

Arguments

Numeric vector; x values of pixels. If this is a 3 column matrix then column 1 is used for x, column 2 is used for y and column 3 is used for val, see Examples.

Numeric vector; y values of pixels.

Numeric vector; z values of pixels. This is effectively the height, and would be the pixel flux for an image.

xcen

Numeric scalar; the desired x centre of the ellipse. If this is not provided it is calculated internally.

ycen

Numeric scalar; the desired y centre of the ellipse. If this is not provided it is calculated internally.

scale

How should the standard ellipse covariance be scaled to create a geometric ellipse. The default of sqrt(2) is appropriate to create an ellipse that represents the location of an iso-photal contour of a galaxy.

pixscale

Numeric scalar; the pixel scale, where pixscale=asec/pix (e.g. 0.4 for SDSS). If set to 1 (default), then the output radhi, radlo and radav is in terms of pixels, otherwise they are in arcseconds.

dobox

Logical; should boxiness be computed? If FALSE then boxiness is fixed to be 0. If TRUE then boxiness is computed (and other parameters are refined) using a maximum likelihood method. This is more expensive to compute, so the default is FALSE.

plot

Logical; should an ellipse be drawn on the current plot? This plot is generated by the profoundDrawEllipse function.

…

Further arguments to be passed to profoundDrawEllipse. Only relevant is plot=TRUE.

Value

A numeric vector with the following named elements:

xcen

The flux weighted x centre of the ellipse.

ycen

The flux weighted y centre of the ellipse.

radhi

The major axis extent of the ellipse (units of pixscale, so if pixscale represents the standard asec/pix this will be asec).

radlo

The minor axis extent of the ellipse (units of pixscale, so if pixscale represents the standard asec/pix this will be asec).

radav

The average radius of the ellipse (units of pixscale, so if pixscale represents the standard asec/pix this will be asec).

axrat

The axial ratio of the ellipse as given by radlo/radhi.

ang

The angle of the ellipse in the usual ProFit sense, see profitMakeModel.

box

The boxiness of the ellipse in the usual ProFit sense, see profitMakeModel.

xsd

The flux weighted standard deviation in x (always in units of pix).

xsd

The flux weighted standard deviation in y (always in units of pix).

covxy

The flux weighted covariance in xy (always in units of pix).

corxy

The flux weighted correlation in xy (always in units of pix).

Details

The assumption is this function will largely be used by the profoundGetEllipses function, but it could be useful for computing the shape of a particular iso-photal contour (see Examples).

Examples

Run this code

# NOT RUN {
# We need the ProFit library to show the profile: library(ProFit)
image = readFITS(system.file("extdata", 'KiDS/G266035fitim.fits',
package="ProFit"))$imDat
tempxy=cbind(which(image>2e-11 & image<3e-11, arr.ind=TRUE)-0.5,
             image[image>2e-11 & image<3e-11])
magimage(image>2e-11 & image<3e-11)
points(tempxy[,1:2], pch='.', col='red')
tempellipse=profoundGetEllipse(tempxy)
profoundDrawEllipse(tempellipse['xcen'], tempellipse['ycen'], tempellipse['radhi'],
                      tempellipse['axrat'], tempellipse['ang'], col='blue')
# }