Image Analysis

Making an Image from the Events file

You may wish to start your analysis by looking at the ROSAT image, and while 'standard' images are provided with the archival dataset you will probably wish to make your own spatial binning and energy band selections.

xselect> extract image

xselect> saoimage

you will now have extracted a full-band image with the default spatial binning and displayed it in an saoimage window.

You can save this image by :

xselect> save image sourceimage.fits

Image subselections

Image can be extracted under pha, time, spatial or housekeeping filters, pha is loosely used to mean pha or pi and so

xselect> extract image phalcut=12 phahcut=200

extracts an image using PI channels 12 to 200 etc.

xselect> filter time file xsel_cursor_gti.xsl

xselect> extract image

extracts an image using only the good times previously saved in the file xsel_cursor_gti.xsl (see Chapter 4 for details of how to make such a time filter file).

As the raw sky image has 0.5 arcsec pixels, and the binning factor is 15, for example the default image is created as 1024 x 1024 for the PSPC.

To change the resolution of the image you must reset the spatial binning

xselect> set XYBINSIZE 8 (4 arcsec bins)

Care is necessary when switching the xybinsize and using regions defined in saoimage. xselect will assume that the regions that it is using for any extraction were defined on an image binned up by the factor given by the current xybinsize so if you create a region then change the xybinsize you will not get correct results.

To extract a sub-image at high resolution one can reset xysize. Images are constrained to be square and so xysize requires the dimension, in output pixels, of the side of the output image. Each output pixel is of the size determined by xybinsize. For example, if you are working with the sky image (i.e. Coordinates for the image are X Y) xybinsize is set to a factor of 8, so each output pixel will be 8x0.5 = 4 arcsec. Setting xysize to 1000 will yield an image 1000x1000 4 arcsec pixels, or 66.7 arcmin on each side.

As the limit of xysize is 1200, you may wish to recenter your output image, in order to get high resolution information about sources at the edge of the field-of-view. To do this, set xycenter in raw pixels. For the ROSAT PSPC the center pixel in raw 0.5 arcsec sky pixels is 7680 7680.

Example:

xsel:ROSAT-PSPC > set xycenter 6530 6240

Remember to SET XYSIZE, or the extractor will ignore the centering

xsel:ROSAT-PSPC > set xysize 800
xsel:ROSAT-PSPC > extract image
 Extractor 1.0q
 XPI 2.1t
 Doing file: /testdirectory/rp700105.fits
 Getting FITS WCS Keywords

show st100% completed
    Total      Good    Bad: Region      Time     Phase       PHA
   124810    109163          14930         0         0       717
===========================================================================
   Grand Total      Good   Bad:  Region      Time     Phase       PHA
        124810    109163          14930         0         0       717
 Total Counts for image :   109163
 Total Time for image :     9625.0000000000
 Total Counts/Time for image :     11.341610389610

To make an image in detector coordinates

xselect> set image detector

and then proceed as before. Both PSPC and HRI datasets are oversampled to 0.5 arcsec pixels in the sky image (raw images sizes are thus 15360 and 8192 respectively). In detector cordinates they are 0.934 and 0.600 arcsec per pixel, for the PSPC and HRI, respectively.

Region selection

Once the desired image is displayed in the saoimage window you can select a subregion of interest. Click on the ``Cursor'' button. There are two sets of cursor buttons available; toggle between them using the ``Region'' button at the righthand end. To define a region click on the appropriate geometric shape and then manipulate the region using the cursor. The left button changes the center of the region, the middle button changes the size of the region, and the right button changes the orientation. To save the region first type ``s'' to save as an included region then go to the region ``write'' button and at the prompt give the filename for the region specifier. The current version of xselect only supports circles, boxes and triangles, please do not use more complex regions. If you want to specify a region to be excluded then type ``e'' rather than ``s''. To tell xselect to use your region file :

xselect> filter region mysource.reg

You can add other region files simply by repeating this command, but remember to use the reset button to clear old saved cursors. To remove a region file that you don't want to use :

xselect> filter region "-mysource.reg"

and to clear all the regions in filter :

xselect> clear region all

Assuming that you have set the region filter to be mysource.reg you can now remake the image using photons only from the mysource.reg area :

xselect> extract image

xselect> saoimage

Region files are simple ascii format files and can be edited (with care). An example of a region file from saoimage is

# /test_data/xsel_image.xsl
# Tue Sep  30 17:18:03 1994
# shape x, y, [x dimension, y dimension], [angle]
 CIRCLE(502.50,569.00,6.80)

For detailed image analysis the extracted image, or the events file, must be read into ximage, which is described briefly in Chapter 6.