Create Smoothed Background Image

 

The smoothed background image is created by cutting out sources found by the local detection from an image and by performing a bicubic spline fit to the so-called cheesed image. The command for that task is

CREATE/BG_IMAGE dbpar edit

  A parameter file dbpar.par is created and is offered to be edited (cf. Table 5.1).

  
Table 5.1: Parameter file for CREATE/BG_IMAGE

• 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 for which a background image is calculated by a spline-fit. The maximum allowed number is 6.
• IMAGE_N is the name of the N-th image. It must be of size image pixels and one image pixel must be half of the size of the on-axis FWHM of the instrument. Such an image will be created with the command CREATE/SOURCE_DETECT_IMAGE.
• MIN_ML determines which sources from the local detection source table are cut out from the image. The criterion is SIGNA MIN_ML, where SIGNA is the existence likelihood of the source and MIN_ML is the minimum of maximum likelihood of existence as given in the parameter file. The likelihood of existence is related to the probability of existence of a source (P) as MIN_ML = -ln(1-P).
• CUT_RADIUS_FWHM is the cut-out radius of the source in units of the FWHM at that position. The default value is 2.0 (determined at calibrated amplitude channel 7) for the PSPC Pointing case. A value of 5.0 should be used for the ROSAT Survey case: There a fixed value of FWHM=60 arcsec is used.
• LDETECT_LIST_N is the name of the N-th source table coming from the local detection (command DETECT/LOCAL). No other table (format) is allowed.
• 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, number of pixels and bin size as the image. It is checked, whether the descriptors START, STEP, and NPIX agree. 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 PSPC default is a circular mask with a radius of 57 arcmin, EXSAS_CAL:mask057.bdf, valid for ROSAT,PSPC and POINTING. Note, that this mask can not be used for survey data.  
• EXPOSURE_FLAG is a flag to use the exposure image (T) or (F).
• EXPOSURE_IMAGE_N is the name of the N-th exposure image. The exposure image is used to obtain a better spline fit to the background. It is not used to correct the resulting background map for exposure.
• BG_IMAGE_N is the name of the N-th background image. The default name is bacmpN.bdf.

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_FITRUN is the number of iterations in the spline-fit. A value of 4 (the default) should be sufficient. In case the spline fit goes wrong (the chi-square becomes zero) after M iterations this parameter should be set to M-1. The reduced chi-square is written to the keyword OUTPUTR(1). So the condition of proper convergence of the spline-fit can be checked by reading this keyword.
• FULL_OUT_FLAG is a flag controlling the output. If activated, some additional output files for debugging issues are produced.
• FWHM_CAL_FILE is the FWHM calibration table (to be found in the calibration area). Allowed values are EXSAS_CAL:fwrops.tbl (PSPC), fwrohr.tbl (HRI), fwrowf.tbl (WFC), and frwos1.tbl, fwros2.tbl, fwrop1.tbl, fwrop2.tbl (WFC filter-specific tables). This parameter has to be set only if a WFC filter-specific table shall be used.
• BIN_RESOLUTION is the resolution of the rebinned image in units of image pixels. To the rebinned and cut-out image a bicubic spline is applied. The default value of 4 should not be modified.
• BG_SCALE is the scaling factor of the background image. The default value of 1 should not be modified.

There is an alternative method to create a smoothed background image   by first smoothing the image with a Gaussian filter, then cut-out the sources, interpolate the cut-out pixels by the mean neighbourhood pixels and smooth the result by applying a bicubic spline rebinning. The command for that task is

CREATE/SMOOTH_BG in_image bg_image LDET_table [cut_rad] - 
                 [val_smoo] [deg_poly] [draw_flag] [fullout]

 

• in_image is the name of the image from which the smoothed background image will be calculated. It must be of size image pixels and one image pixel must be half of the size of the FWHM of the on-axis point spread function of the instrument. Such an image will be created with the command CREATE/SOURCE_DETECT_IMAGE.
• bg_image is the name of the result smoothed background image.
• LDET_table is the name of the source table coming from the local detection (command DETECT/LOCAL). The name of a column can be appended according to which the table will be sorted (e.g. lslst1,:COUNT(-), the default is to sort the table as :SIGNA)
• cut_rad is a radius in units of the FWHM of the source to cut out the photons.
• val_smoo is the filter smooth radius in units of image pixels. The default is 16.
• deg_poly is the degree of the polynomial used to fill the regions cut out at the source positions (0,1, or 2). The default is 1.
• draw_flag is the display/draw flag, if set to N no output is given to the display, if set to Y the modified area is displayed step for step. The default is Y.
• fullout is the full output flag. If set to Y, besides the background image a cheesed image (with filled holes), bldfl1.bdf, and a binned background image binfl1.bdf are created. If set to N the images bldfl1.bdf and binfl1.bdf is not created. The default is fullout=N.

Note that for background images produced with EXSAS versions earlier than 96JAN the background image was scaled with a factor 100. This scaling is not necessary anymore, the default for the scale factor is now set to 1.