Create Images for Source Detection

 

In the present version of Source Detection images must be of the size 512 512 pixels and have a bin size of 50 to 100 % of the size of the FWHM of the on-axis point spread function of the experiment. Such images can be created with the commands

CREATE/SOURCE_DETECT_IMAGE  pet  ampl [center] [bin] [evrat] 
CREATE/SOURCE_DETECT_IMAGE  parfil edit

 

• pet is the name of the photon event table from which the image(s) will be created.
• ampl is the range of amplitudes in which the images are accumulated. Amplitudes have to be separated by dashes (-) and amplitude ranges have to be separated by commas. Up to 6 images can be created. The names of the images are image1.bdf, image2.bdf, ... image6.bdf. Amplitudes have also to be specified for the ROSAT HRI experiment.  
• center is the x,y sky pixel coordinate of the center of the image to be binned. The default is the value of the descriptors SKY_CEN_X, SKY_CEN_Y. The image center should be in the ROSAT Pointing mode the mean pointing direction of the telescope axis (see note below). ATTENTION, this parameter should not be set in case one does not know how it influences the results.
• bin is the binsize of the image in sky pixels. The binsize must be about 50 to 100 % of the FWHM of the on-axis point spread function in the ROSAT Pointing mode. This is guaranteed in the default mode (30 for ROSAT PSPC, 10 for ROSAT HRI, 7 for ROSAT WFC) ATTENTION, any settings different from the default result in a warning message. Some of the results from the local and map source detection may no longer be correct.
• evrat name of eventrates table to compute the lifetime factor and write it to the descriptor LIF_TIM of image1.bdf, image2.bdf, etc. The default is eventrates.tbl .
• parfil is the name of the parameter file in which the number of images, the names of the images and the amplitude ranges can be specified.
• edit is an edit flag which must be set to edit in case one wants to edit the parameter file, or to noedit.

The center of the binned image should be the sky position mapped to the detector center. As the telescope pointing is not stable, a mean sky position is recommended to be given. This mean position xcen, ycen can be calculated from the attitude table as
 
XCEN = <XOFFSET> + SKY_CEN_X
YCEN = <YOFFSET> + SKY_CEN_Y

where <XOFFSET>, <YOFFSET> are the mean values of the :XOFFSET and :YOFFSET columns respectively and SKY_CEN_X and SKY_CEN_Y are the descriptors giving the pointing direction of the telescope. In case the data are   larger than the size of the image, a patchwork of images has to be   binned. Then the values XCEN, YCEN should be calculated as

XCEN = x sky pixel coordinate of center of image in patchwork
YCEN = y sky pixel coordinate of center of image in patchwork

Now the detection cell size may no longer be adjusted to the image pixel size. Also the range in image pixels over which the detection cell is slided may be different from the default. This means a careful handling of the extended parameters NUM_RUNS, DEAD_X_ROWS and DEAD_Y_ROWS is necessary in the local and map detection commands (see sections 5.2.5 and 5.2.6).   It often is sufficient to use the ROSAT Survey defaults in such a case: NUM_RUNS = 2, DEAD_X_ROWS = 3,3,0,0,0,0, DEAD_Y_ROWS = 3,3,0,0,0,0.
Examples:

CREATE/SOURCE_DETECT_IMAGE events 7-240,7-40,41-240
CREATE/SOURCE_DETECT_IMAGE events1 7-240 ? ? eventrates1
CREATE/SOURCE_DETECT_IMAGE impar edit
CREATE/SOURCE_DETECT_IMAGE events 7-240 ? 15
CREATE/SOURCE_DETECT_IMAGE events 7-240 2000.,2000. 48

In the last two examples the values of the parameters center and bin deviate from the standard settings. These settings may result in wrong results in Source Detection.

The following default bin-sizes are installed for different experiments in the program (see Table 5.2).  

  
Table 5.2: Default image parameters for SOURCE DETECTION

The parameter file impar.par has default values for ROSAT PSPC and pointed observations (see Table 5.3).

  
Table 5.3: Parameter file for CREATE/SOURCE_DETECT_IMAGE parfil