Detect Sources using Local Background

 

This is a sliding window source detection technique. The image must be   of the size 512 512 image pixels. A 3 3 image pixels window is slided over the image. Source counts are determined in the 3 3 pixels box and background counts in the surrounding 16 pixels frame. The probability of existence of a source is calculated from a   chi-square probability table EXSAS_CAL:pchsq.tbl in case the   counts in the source cell exceed the value 100, and from a binomial   probability table EXSAS_CAL:pbin.tbl otherwise. The image used in the local detection can be created with the command CREATE/SOURCE_DETECT_IMAGE, see section 5.2.4.

The command

DETECT/LOCAL dlpar edit

works via a parameter file dlpar.par which is offered to be edited (cf. Table 5.4). or which can be created (for the experiment ROSAT PSPC) by the command

  
Table 5.4: Parameter file for DETECT/LOCAL for ROSAT PSPC

 

 
CREATE/PARFIL EXDL dlpar ROSAT,PSPC

Some parameters of the parameter file dlpar.par are described in more length:

• MISSION,DETECTOR is the experiment identifier. Allowed values are ROSAT,PSPC, ROSAT,HRI, and ROSAT,WFC.
• OBS_MODE is the observation mode. Allowed values are POINTING or SURVEY.  
• NUM_IMAGES is the number of images analysed with the local detection method. Up to 6 images can be analysed. The names of these images have to be given under the parameters IMAGE_1 to IMAGE_N, (N = NUM_IMAGES).
• IMAGE_1 is the name of the first image, broad band image (calibrated amplitude channel 7-256 in the default case, but the new recommended amplitude boundaries are 11-235).
• IMAGE_2 is the name of the second image, soft band image (calibrated amplitude channel 8-40 in the default case, but the new recommended amplitude boundaries are 11-41).
• IMAGE_3 is the name of the third image, hard band image (calibrated amplitude channel 41-256 in the default case, but the new recommended amplitude boundaries are 52-201).
• MIN_ML is the minimum of the maximum likelihood of existence for the acceptance of a source. Only in case the likelihood of existence of the source falls above MIN_ML it will be written to the source table. The likelihood is defined as   MIN_ML = ln (1 - probability of existence). 1,2,3,4,5,10 Gaussian sigma correspond to a likelihood of 1.1, 3.1, 5.9, 9.7, 14, and 53, respectively.  
• MASK_FLAG is a flag to use the mask (T) or not (F).
• MASK is the name of the mask image to screen the image. This image must have the same world coordinates, the same number of pixels and bin-size as the image (the same descriptors START, STEP, and NPIX). The intensity per pixel has to be 1. for the illuminated parts and 0. for the shadowed parts. Such an image can be created with one of the commands CREATE/MASK, CREATE/WOBBLED_MASK (PSPC only), or ADJUST/MASK. The default is a circular mask with a radius of 57 arcmin, EXSAS_CAL:mask057.bdf. For ROSAT PSPC a mask image containing the struts of the window support structure, EXSAS_CAL:mask0thick.bdf, can be used. This image has to be adjusted by the command ADJUST/MASK (see section 5.2.35). Alternatively to this method the command CREATE/WOBBLED_MASK can be used (see section 5.2.35) to obtain a mask including the window support structure and corrected for spacecraft wobble. For survey data either no mask or a user-defined mask has to be used.
• LDETECT_LIST_N is the name of the N-th local detection table which is created by this command. The column labels of this table are described in Table 5.29.

There is an additional set of extended parameters which can be accessed either with WRITE/PARFIL or with CREATE/PARFIL using the extended option. For standard source detection they are not needed.

• NUM_RUNS is the number of compression runs. In each run the image is further rebinned by a factor of 2. The image compression is done to allow to
  • map the point spread function which broadens as a function of off-axis angle (distance to image center) in the Pointing mode due to   the telescope vignetting.
  • detect extended objects.  
• DEAD_X_ROWS are the ranges in image pixels in the x-direction (counted from the outer image boundary) over which the detection cell is slided. Values have to be specified for the NUM_RUNS compression steps and are defined in units of compressed image pixels. The default values are specified in order to map the ROSAT PSPC point spread function growing with increasing distance to the detector center (image center). This means, the image center must be the detector center and the bin-size of the image must be half the on-axis FWHM. Such an image can be binned with the command CREATE/SOURCE_DETECT_IMAGE. Any deviation of the image parameters (bin-size, center) from the defaults must also be considered in the DEAD_X_ROWS values. In the ROSAT Survey case DEAD_X_ROWS should be set to 3,3,0,0,0,0 as the point spread function will stay about constant   across the image. Two values are given to take account for extended sources.
• DEAD_Y_ROWS analogous to parameter DEAD_X_ROWS but for y-coordinate.
• IMAGE_PRIORITY are the priorities assigned to the detections in the NUM_IMAGES bands (images). In the default case the priorities 16, 4 and 1 are assigned to the broad, soft and hard image. This means IMAGE_1 must be the broad band image, IMAGE_2 the soft band image and IMAGE_3 the hard band image.

In the parameter file listed in Table 5.4 three images IMAGE_1, IMAGE_2 and IMAGE_3 were specified and the name of a mask MASK to screen the image. The mask flag MASK_FLAG was set to T. The minimum maximum likelihood MIN_ML for detecting sources was set to 10. The names of the source tables LDETECT_LIST_1, LDETECT_LIST_2 and LDETECT_LIST_3 were chosen as default values. The tables LDETECT_LIST_1, LDETECT_LIST_2 and LDETECT_LIST_3 contain the parameters of the detected sources from image IMAGE_1, IMAGE_2 and IMAGE_3, the source counts COUNT, the background counts per image pixel at the source position BACKG, the source x- and y-position in image pixels X_POS and Y_POS, the distance to the next source in image pixels SOURD and the distance to the next rib or edge in image pixels RIBD (cf. Tables 5.28 and 5.29).

The number of compression steps and the values for the pixel ranges of the image have not to be specified explicitely. The default values for these extended parameters NUM_RUNS, DEAD_X_ROWS, DEAD_Y_ROWS and IMAGE_PRIORITY are set automatically according to the default values for the ROSAT PSPC POINTING and SURVEY, ROSAT HRI, or ROSAT WFC case respectively. In case they really have to be changed, they can be accessed via WRITE/PARFIL or CREATE/PARFIL with the extended option.