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
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 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.
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.
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
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.