# Fitting Image Data with Convolved and Unconvolved Model Components

Sherpa Threads (CIAO 4.14 Sherpa)

## Overview

#### Synopsis:

This thread demonstrates the use of the set_full_model functionality for manually defining a complete source
model expression for fitting spatial 2-D data, where one or
more components are convolved by a PSF
model, and others are not. An exposure map file is input to *Sherpa*
as a file-based exposure map model via
the load_table_model function, and used
in the fitting of all model components, both unconvolved extended
emission and convolved point-source emission. A
Point-Spread-Function (PSF) image is loaded via load_psf to be used as the convolution
kernel for the point-source emission, only.

**Last Update:** 10 Dec 2015 -
reviewed for CIAO 4.8, no content change.

## Contents

**Getting Started****Reading and Plotting 2D FITS Data**-
**Defining a Source Model** **Fitting the model****Saving a Sherpa Session****Scripting It****Summary****History**-
**Images**

## Reading and Plotting 2D FITS Data

In this thread we use 2D spatial data from the FITS data file
src_image.fits, which was created by binning a *Chandra*
events table in sky coordinates and filtering it to contain
only the source region of interest—a square
region of extended source emission containing a bright central
point source. This data set is input to
*Sherpa* with the load_image command,
and can be visualized with the image_data or
contour_data commands.

sherpa> load_image("src_image.fits") sherpa> image_data()

The image_data command opens the source data image in DS9, as shown in Figure 1, where in this example, each pixel subtends 0.492 arcseconds (a sky coordinate binning factor of 1 was used).

### Figure 1: Source data image

The show_data command lists information available for this data set, such as the binning of the image, physical dimensions, and coordinate system.

sherpa> show_data() Data Set: 1 Filter: name = src_image.fits x0 = Float64[263169] x1 = Float64[263169] y = Float64[263169] shape = (513, 513) staterror = None syserror = None sky = physical crval = [ 3815.5 3688.5] crpix = [ 0.5 0.5] cdelt = [ 1. 1.] eqpos = world crval = [ 116.0688 37.9087] crpix = [ 4096.5 4096.5] cdelt = [-0.0001 0.0001] crota = 0 epoch = 2000 equinox = 2000 coord = logical

## Defining a Source Model

In order to define a multi-component source model expression
in *Sherpa* containing
both convolved and unconvolved components with which to fit
a single data set, the model must be manually defined using the
set_full_model function (as opposed
to the set_source function,
which automatically convolves an entire source model
expression with a PSF or response model).
After examining Figure 1, it is apparent that a
2D Beta model is appropriate for describing the extended
component of the source
emission, while a PSF-convolved 2D Gaussian model
can be used to describe the central point-source component.
We choose to apply an (optional) exposure map model to the full source
model expression to account for a variable response across
the data image, and convolve only the point-source model
component by a PSF model. First, we set up the exposure map and PSF
model components, and then define the full source model
expression with set_full_model.

### Setting the exposure map

We define a file-based exposure map model by loading an exposure map file with the load_table_model command:

sherpa> load_table_model("emap", "expmap.fits")

To display the status of the model emap, use the
print() command; notice that *Sherpa* identifies the
exposure map model as "tablemodel.emap":

sherpa> print(emap) tablemodel.emap Param Type Value Min Max Units ----- ---- ----- --- --- ----- emap.ampl thawed 1 -3.40282e+38 3.40282e+38

### Setting the PSF convolution model

A PSF instrument model may be established by loading an
image of a PSF into *Sherpa*, e.g. one created with
the *Chandra* Ray
Tracer (ChaRT), or by defining a *Sherpa* model
expression which describes the shape of the PSF. Both
types of PSF models are added to the current list of models
in the session using the load_psf
command. In this example, we load an image containing
the *Chandra* PSF at 1 keV, at the position of our
point source on the ACIS-S3 chip.

sherpa> load_psf("psf0", "psf_image.fits") sherpa> print(psf0) psfmodel.psf0 Param Type Value Min Max Units ----- ---- ----- --- --- ----- psf0.kernel frozen psf_image.fits psf0.radial frozen 0 0 1 psf0.norm frozen 1 0 1

Normally—i.e., when set_source is
used to define a source model expression—the model to be
used as the PSF convolution model must be defined as such
with the set_psf command; however, it
is not necessary in this case since we are manually defining
the full source model expression using the set_full_model functionality, as shown in the
next section of this thread. To learn how to fully establish a PSF
model when defining a source model with set_source, see the
*Sherpa* thread Accounting for PSF
Effects in 2D Image Fitting.

While it is not strictly necessary to run the set_psf
command in this context, it must be run in order to utilize
most of the PSF-specific *Sherpa* commands, e.g. the image_psf command for viewing the PSF image in
DS9. Below, we go ahead and run set_psf in order to
visualize the PSF model:

sherpa> set_psf(psf0) sherpa> image_psf()

The PSF image is shown in Figure 2.

### Figure 2: PSF image

Running the set_psf command also allows us to take advantage of the show_psf and show_kernel commands, which offer more information than "print(psf0)":

sherpa> show_kernel() PSF Kernel: 1 psfmodel.psf0 Param Type Value Min Max Units ----- ---- ----- --- --- ----- psf0.kernel frozen psf_image.fits psf0.size frozen (256, 256) (256, 256) (256, 256) psf0.center frozen (128, 128) (128, 128) (128, 128) psf0.radial frozen 0 0 1 psf0.norm frozen 1 0 1 sherpa> show_psf() PSF Model: 1 name = psf_image.fits x0 = Float64[65536] x1 = Float64[65536] y = Float64[65536] shape = (256, 256) staterror = None syserror = None sky = physical crval = [ 3944. 3817.] crpix = [ 0.5 0.5] cdelt = [ 1. 1.] eqpos = world crval = [ 116.0688 37.9087] crpix = [ 4096.5 4096.5] cdelt = [-0.0001 0.0001] crota = 0 epoch = 2000 equinox = 2000 coord = logical

### Defining a manual source model with set_full_model

We define the complete source model expression using set_full_model as follows, where we
also include a *Sherpa* const2d model component to describe the
constant background level contributing to the total emission:

sherpa> set_full_model(emap*(const2d.bgnd + beta2d.ext + psf0(gauss2d.ps)))

In a set_full_model expression, any response or PSF convolution models which should be applied to one or more individual model components must be explicitly defined—in this example, the PSF convolution model named "psf0" is applied only to the 2D Gaussian model component used to describe the point-source emission.

We can print the default model parameter values for the defined model using show_model:

```
sherpa> show_model()
PSF Model: 1
name = psf_image.fits
x0 = Float64[65536]
x1 = Float64[65536]
y = Float64[65536]
shape = (256, 256)
staterror = None
syserror = None
sky = physical
crval = [ 3944. 3817.]
crpix = [ 0.5 0.5]
cdelt = [ 1. 1.]
eqpos = world
crval = [ 116.0688 37.9087]
crpix = [ 4096.5 4096.5]
cdelt = [-0.0001 0.0001]
crota = 0
epoch = 2000
equinox = 2000
coord = logical
Model: 1
(tablemodel.emap * ((const2d.bgnd + beta2d.ext) + psfmodel.psf0(gauss2d.ps)))
Param Type Value Min Max Units
----- ---- ----- --- --- -----
emap.ampl thawed 1 -3.40282e+38 3.40282e+38
bgnd.c0 thawed 1 0 3.40282e+38
ext.r0 thawed 10 1.17549e-38 3.40282e+38
ext.xpos thawed 0 -3.40282e+38 3.40282e+38
ext.ypos thawed 0 -3.40282e+38 3.40282e+38
ext.ellip frozen 0 0 0.999
ext.theta frozen 0 -6.28319 6.28319 radians
ext.ampl thawed 1 -3.40282e+38 3.40282e+38
ext.alpha thawed 1 -10 10
ps.fwhm thawed 10 1.17549e-38 3.40282e+38
ps.xpos thawed 0 -3.40282e+38 3.40282e+38
ps.ypos thawed 0 -3.40282e+38 3.40282e+38
ps.ellip frozen 0 0 0.999
ps.theta frozen 0 -6.28319 6.28319 radians
ps.ampl thawed 1 -3.40282e+38 3.40282e+38
```

Before fitting the model to the source, we can set some initial model parameter values:

sherpa> ext.r0 = 7.6 sherpa> ext.xpos = 4072 sherpa> ext.ypos = 3945 sherpa> ext.ellip = 0.3 sherpa> ext.theta = 2.4 sherpa> ext.ampl = 0.42 sherpa> ext.alpha = 0.94 sherpa> ps.xpos = 4072 sherpa> ps.ypos = 3945 sherpa> ps.fwhm = 1. sherpa> ps.ampl = 1497 sherpa> bgnd.c0 = 0.002 sherpa> thaw(ext.ellip, ext.theta, ps.ellip, ps.theta) sherpa> freeze(emap.ampl)

The updated model parameters can be viewed again with show_model:

```
sherpa> show_model()
{...psf model information shown above...}
Model: 1
(tablemodel.emap * ((const2d.bgnd + beta2d.ext) + psfmodel.psf0(gauss2d.ps)))
Param Type Value Min Max Units
----- ---- ----- --- --- -----
emap.ampl frozen 1 -3.40282e+38 3.40282e+38
bgnd.c0 thawed 0.002 0 3.40282e+38
ext.r0 thawed 7.6 1.17549e-38 3.40282e+38
ext.xpos thawed 4072 -3.40282e+38 3.40282e+38
ext.ypos thawed 3945 -3.40282e+38 3.40282e+38
ext.ellip thawed 0.3 0 0.999
ext.theta thawed 2.4 -6.28319 6.28319 radians
ext.ampl thawed 0.42 -3.40282e+38 3.40282e+38
ext.alpha thawed 0.94 -10 10
ps.fwhm thawed 1 1.17549e-38 3.40282e+38
ps.xpos thawed 4072 -3.40282e+38 3.40282e+38
ps.ypos thawed 3945 -3.40282e+38 3.40282e+38
ps.ellip thawed 0 0 0.999
ps.theta thawed 0 -6.28319 6.28319 radians
ps.ampl thawed 2018.5 -3.40282e+38 3.40282e+38
```

## Fitting the model

We can now fit the manually defined model to the
data in the usual way using the fit
command. In this example, we decide to fit with the *Sherpa* C-Stat statistic
and the Nelder-Mead optimization method, which we
specify using the set_stat and set_method commands:

sherpa> show_stat() Statistic: Chi2Gehrels Chi Squared with Gehrels variance sherpa> show_method() Optimization Method: LevMar name = levmar ftol = 1.19209289551e-07 xtol = 1.19209289551e-07 gtol = 1.19209289551e-07 maxfev = None epsfcn = 1.19209289551e-07 factor = 100.0 verbose = 0 sherpa> set_stat("cstat") sherpa> set_method("neldermead") sherpa> fit() Dataset = 1 Method = neldermead Statistic = cstat Initial fit statistic = 3.47915e+06 Final fit statistic = 168747 at function evaluation 715 Data points = 263169 Degrees of freedom = 263155 Probability [Q-value] = 1 Reduced statistic = 0.641246 Change in statistic = 3.3104e+06 bgnd.c0 0.000434043 ext.r0 8.60185 ext.xpos 4072.55 ext.ypos 3944.9 ext.ellip 0.488371 ext.theta 3.16468 ext.ampl 0.973904 ext.alpha 3.85123 ps.fwhm 1.62618 ps.xpos 4072.58 ps.ypos 3945.54 ps.ellip 0.649299 ps.theta 1.82501 ps.ampl 1497.42

Normally, the image_model, image_fit, and image_resid commands can be used to display the model and fit residuals images, but these commands are not compatible with the set_full_model functionality. However, we can use the image_source_component command to separately plot each of the unconvolved model components contributing to the fit of the source emission:

sherpa> ext.xpos = 257 sherpa> ps.xpos = 257 sherpa> ext.ypos = 235 sherpa> ps.ypos = 235 sherpa> image_source_component(ext) sherpa> image_source_component(ps, tile=True, newframe=True) sherpa> image_source_component(psf0(ps), tile=True, newframe=True) sherpa> image_data(tile=True, newframe=True)

These commands create the images displayed in Figure 3.

### Figure 3: Images of individual, fitted model components

There are several things to note about the series of commands
issued above for creating the display in Figure 3. First is that, normally, the image_model_component
command is used to plot convolved model components, where
the PSF or response model associated with a given model
component is automatically applied. Since we have defined
our source model manually with set_full_model, we must also ensure that we manually convolve
source model components within the
image_source_model expression, as shown above for the
PSF-convolved point-source component of the model. Second, before running the image_source_component commands,
the (fitted) (x,y) center position of the individual source
model components in physical coordinates had to be adjusted
to coincide with the center of the data image in
*image* coordinates, for proper display in the DS9
frame—so approximately (4072, 3945) → (257,253).

## Saving a Sherpa Session

To save this *Sherpa* session in order to return to the
analysis at a later point, the following commands may be used:

sherpa> save("manual_fit.save") sherpa> save_all("manual_fit.ascii")

The save function records all the information about the current session to the binary file manual_fit.save, and the save_all function records the session settings to an editable ASCII file.

To restore the session that was saved to the binary file manual_fit.save or ASCII file manual_fit.ascii:

```
sherpa> restore("manual_fit.save")
sherpa> execfile("manual_fit.ascii")
```

## Scripting It

The file fit.py
is a Python script which performs the primary commands used
above; it can be executed by typing execfile("fit.py")
on the *Sherpa* command line.

The *Sherpa* script command may be used
to save *everything* typed on the command line in a
*Sherpa* session:

sherpa> script(filename="sherpa.log", clobber=False)

(Note that restoring a *Sherpa* session from such a file could
be problematic since it may include syntax errors, unwanted
fitting trials, etcetera.)

The CXC is committed to helping *Sherpa* users transition to
new syntax as smoothly as possible. If you have existing
*Sherpa* scripts or save files, submit them to us via the
CXC Helpdesk and we will provide the
CIAO/*Sherpa* 4.8 syntax to you.

## Summary

This thread is complete, so we can exit the *Sherpa* session:

sherpa> quit

## History

10 Jan 2010 | original version |

06 Jan 2012 | reviewed for CIAO 4.4 (no changes) |

13 Dec 2012 | reviewed for CIAO 4.5 (no changes) |

06 Dec 2013 | reviewed for CIAO 4.6: typos fixed |

07 Apr 2015 | reviewed for CIAO 4.7, no content change. |

10 Dec 2015 | reviewed for CIAO 4.8, no content change. |