Ahelp: plot_eprof

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AHELP for CIAO 3.4

plot_eprof

Context: sherpa

Synopsis

Display a radial profile (elliptical annuli) of 2D data, fit, and residuals.

Syntax

plot_eprofr( [dnum,] min, max, step )
plot_eprofr( [dnum,] modelname, min, max, step )

plot_eprofd( [dnum,] min, max, step )
plot_eprofd( [dnum,] modelname, min, max, step )

plot_eprof() is a synonym for plot_eprofr()

The 'image fit' command shows the data, best-fit model, and residuals for your 2D dataset. Sometimes this can be hard to interpret - in particular when dealing with low-count data as is often the case with Chandra images - and a different view of the fit may be useful. The plot_eprofr() and plot_eprofd() functions provide such a view, as they display two plots; the first is an elliptical profile of the data and best-fit model, while the second is an elliptical profile of the residual image. The residual plot created by plot_eprofr() has the y axis in units of counts, whilst the residual plot from plot_eprofd() uses counts / error.

These plots are similar to the output of 'lplot fit resid' - for plot_eprofr() - and 'lplot fit delchi' - for plot_eprofd() - for 1D data.

The functions provide a usage message if called with either no, or an incorrect set of, arguments. The plot_rprofr() and plot_rprofd() functions produce the same plots but using circular, rather than elliptical, annuli: see "ahelp plot_rprof" for more information.

How are the annuli chosen?

The min, max, and step arguments determine the the minimum and maximum radii of the annuli, and the width of each annnulus repectively. The units of these arguments match the current setting of Sherpa's coordinate command. Unlike the plot_rprofr() and plot_rprofd() functions, the other values that control the annuli - here the center, ellipticity, and position angle - can only be taken from a model component. If the source expression contains only one model with xpos, ypos, ellip, and theta parameters then no other argument is required (other than the optional dataset number, dnum). Otherwise, the name of the model component must be given using the modelname argument.

The following examples assume that a source model has been fit to a 2D dataset.

Example 1

sherpa> plot_eprofr(0,200,20)

This will plot up an elliptical profile of the data and model (top plot) and an elliptical profile of the residual image (bottom plot) for dataset 1. Assuming the values in sherpa.fitplot and sherpa.resplot have not been changed then the data is plotted using squares, the best-fit as the red line, and the residuals as filled circles.

The three arguments in the call give the minimum and maximum radii - here 0 and 200 respectively - and the width of each annulus (20). The units are taken to match the current coordinate settings for the dataset (see 'ahelp coord'). The source expression is assumed to contain only one component with "xpos", "ypos", "ellip", and "theta" parameters; these are used to define the center of the profile, its ellipticity, and angle.

As with the plot_rprofr() and plot_rprofd() functions, a bin will only be displayed if there are any pixels that contribute to that bin.

Example 2

sherpa> plot_eprofd(0,200,20)

This creates the same plot as in the previous example except that the residual plot has units of "sigma" - i.e. counts divided by the error estimate - rather than counts.

Example 3

sherpa> plot_eprofr(2,0,200,20)

This creates elliptical profiles for dataset 2.

Example 4

sherpa> plot_eprof("core",0,200,20)

Here we explicitly list which model should be used to define the center, ellipticity, and angle of the annuli; we assume in this example that "core" is a model component such as BETA2D (i.e. that it has xpos, ypos, ellip, and theta parameters).

This is only necessary when the source expression contains more than one component with "xpos", "ypos", "ellip", and "theta" parameters.