Using an Exposure Map in Fitting Image Data
OverviewLast Update: 1 Dec 2006 - reviewed for CIAO 3.4: no changes Synopsis: This thread shows how to use an exposure map when fitting 2-D spatial data. The exposure map file is input to Sherpa through the file-based exposure map model (FEXPMAP). |
Contents
- Getting Started
- Reading and Plotting 2-D FITS Data
- Setting the Exposure Map
- Defining and Fitting the Source
- Saving a Sherpa Session
- Summary
- History
- Images
Getting Started
Please follow the "Sherpa Threads: Getting Started" thread.
Reading and Plotting 2-D FITS Data
We are using 2-D spatial data from the FITS datafile img.fits. This dataset is input into Sherpa with the DATA command:
sherpa> DATA img.fits
The dataset may be viewed as a contour plot, surface plot or an image. Here we show the surface plot method, creating a postscript file of the output as well:
This creates Figure 1 .
Setting the Exposure Map
The following is similar to the process of defining responses for spectral data, where a 1-D instrument model (RSP) is defined and set as the instrument.
We define the exposure map model:
sherpa> FEXPMAP[emap] emap.file parameter value ["none"] expmap.fits emap.norm parameter value [1] sherpa> INSTRUMENT = emap
To display the status of the model emap, use the SHOW command. Note that FEXPMAP is an alternate name for FARF2D; Sherpa identifies it by the latter:
sherpa> SHOW emap farf2d[emap] Param Type Value Min Max Units ----- ---- ----- --- --- ----- 1 file string: "expmap.fits" 2 norm frozen 1 0 1000
The normalization (norm) is frozen at 1, since the exposure map is normalized to the exposure time of the image; this is how all exposure maps are generated in the CIAO exposure map threads.
Defining and Fitting the Source
One can now define a model to be used as a source model. After viewing Figure 1 , the BETA2D model is found to be a promising candidate for the source. Since we want to set the initial values, we leave on the parameter prompting:
sherpa> BETA2D[bm] bm.r0 parameter value [80] 30 bm.xpos parameter value [36.5] 40 bm.ypos parameter value [41.5] 40 bm.ellip parameter value [0] 0.3 bm.theta parameter value [0] 5 bm.ampl parameter value [1.51294] 3.0 bm.alpha parameter value [1] 1.5 sherpa> THAW bm.ellip bm.theta
The BETA2D model is defined for the source, then the data is fit:
sherpa> SOURCE = bm sherpa> FIT LVMQT: V2.0 LVMQT: initial statistic value = 4.88095e+06 LVMQT: final statistic value = 3255.75 at iteration 11 bm.r0 12.4624 bm.xpos 39.5139 bm.ypos 40.8959 bm.ellip 0.0259204 bm.theta 4.72828 bm.ampl 1.31312 bm.alpha 1.66641
To display the fit and residuals of the plot, we again use SPLOT:
sherpa> SPLOT 2 SOURCE RESIDUALS
where RESIDUALS refers to the absolute residuals, calculated as (data - model). This creates Figure 2 .
Alternately, use DELCHI, the sigma residuals of the fit; these are calculated as ((data - model)/error):
sherpa> SPLOT 2 SOURCE DELCHI
as shown in Figure 3 .
Saving a Sherpa Session
To save the Sherpa session:
sherpa> SAVE ALL expmap.shp
where expmap.shp is the output ASCII file. The information is written in the form of a Sherpa script. The USE command will restore the session when desired.
The source (unconvolved model amplitudes), model (convolved model amplitudes), and residuals may all be written out in FITS format with the WRITE command:
sherpa> WRITE SOURCE expmap_source.fits Write X-Axes: (Bin,Bin) Y-Axis: Amplitude (Photons/bin) sherpa> WRITE MODEL expmap_model.fits Write X-Axes: (Bin,Bin) Y-Axis: Counts sherpa> WRITE RESIDUALS expmap_residuals.fits Write X-Axes: (Bin,Bin) Y-Axis: Counts
The output may be examined as any standard FITS file, e.g. with prism, ds9, or dmlist.
Summary
This thread is complete, so we can exit the Sherpa session:
sherpa> EXIT Goodbye.
History
14 Jan 2005 | reviewed for CIAO 3.2: no changes |
21 Dec 2005 | reviewed for CIAO 3.3: no changes |
01 Dec 2006 | reviewed for CIAO 3.4: no changes |