Creation of Exposure Maps

              After producing an image, it is sometimes necessary to remove artificial features, like the rib structure of the PSPC or the vignetting effects. In particular, this is important for the study of extended sources. The goal of this analysis is to convert a raw image into a count rate ``clean'' image. This can be accomplished by means of the production of an ``exposure map'', that is an image which contains the corrected exposure as a function of the sky position. A division of the uncorrected count image by the exposure map will then give the desired output. Since the response of the detector is different in different channel ranges, the appropriate instrument map has to be selected.

The command CREATE/EXPOSURE_MAP is available for the exposure map production. Since this task is computationally expensive, it is a rather slow procedure: if you have a long dataset it can sometimes take hours to be completed. The program reads in an attitude table and produces a histogram of the distribution of pointing direction and roll angle during the observation. This histogram can be saved for subsequent use. Then the chosen instrument map is projected to the sky according to this distribution. The syntax of the command is:

CREATE/EXPOSURE_MAP parfil

where parfil is the name of the parameter file (default extension is .par).

The parameter file entries are shown in Table 4.7, where:

  
Table 4.7: Parameter file entries for CREATE/EXPOSURE_MAP

• ATTITUDE_TABLE is the name of the attitude table to be used for the determination of the pointing direction. It is assumed that the attitude times include the photon arrival times. Default is attitude.tbl.
• REFERENCE_IMAGE Name of the reference data image to be used. The dimension and size of the exposure map will be extracted from it. The image is also used to extract the accepted times. This means that the exposure map will be constructed according to the time selections contained in this image. Do not use an image coming from the SASS (it would not work): accumulate it yourself using the EXSAS BIN/IMAGE command. There is no default.
• INSTRUMENT_MAP Name of the instrument map to be used. This will fix the channel selection. Use one of the instrument maps in the calibration area or any combination of them.

   
                  exsas_cal:moimp_8_19_b.bdf    (or _c)
                  exsas_cal:moimp_8_41_b.bdf    (or _c)
                  exsas_cal:moimp_20_41_b.bdf   (or _c)
                  exsas_cal:moimp_42_51_b.bdf   (or _c)
                  exsas_cal:moimp_52_69_b.bdf   (or _c)
                  exsas_cal:moimp_52_90_b.bdf   (or _c)
                  exsas_cal:moimp_70_90_b.bdf   (or _c)
                  exsas_cal:moimp_91_131_b.bdf  (or _c)
                  exsas_cal:moimp_91_201_b.bdf  (or _c)
                  exsas_cal:moimp_132_201_b.bdf (or _c)

  There is no default. true cm

  Let's say you want to produce an instrument map in the channel range 70-131 (because you want to correct an image in that range). It is not enough just to average the two instrument maps moimp_70_90_b.bdf and moimp_91_131_b.bdf, since this would assume a flat spectrum (which is most likely not the case). Also a weighted average of the two instrument maps on your spectrum would be a mistake, because the spectrum is generally far from being constant along your field of view. The best thing is, surprisingly, NOT to combine the two maps anyway. It is better to produce two data images in the two energy ranges 70--90 and 91--131, to correct them separately with the two exposure maps derived from the corresponding instrument maps moimp_70_90_b.bdf and moimp_91_131_b.bdf, and finally to sum the two corrected count rate images into one. Only in this way the result can be considered ``perfect'', as even the energy dependency of the vignetting correction is kept into account.

  The generalization to more than two bands is trivial.

  It is also possible to mask the instrument map before running the exposure map program, for instance to use only the central part of the PSPC detector. The command CREATE/MASK can be used (see chapter 5).

• EXPOSURE_MAP Name of the output exposure map. There is no default.
• ATTITUDE_ROWS_STEP The attitude table has a time step of one second. To speed up the procedure it is possible to read in a row every n. This parameter specifies the value of n. Defaults is 1.
• ATTITUDE_LIST_REBIN The program will produce an attitude histogram. This parameter specifies the bin size for XOFFSET and YOFFSET (in sky pixels). Default is 30.0 (15 arcsec).
• ROLL_ANGLE_REBIN The program will produce an attitude histogram. This parameter specifies the bin size for the roll angle (in degree). Default is 0.25.
• DEADTIME_CORRECTION_FLAG Flag for applying the deadtime correction.
  • 0: no correction
  • 1: correction
  Default is 0.
• EVENTRATES_TABLE Housekeeping table for the deadtime correction. Necessary if the DT correction has to be performed. Default is eventrates.tbl.
• ATTITUDE_LIST_IO_FLAG Flag for attitude list I/O. The program is divided into two main sections. First, an attitude list is produced, and then the instrument map is cast on the sky according to the entries in the attitude list. Since the attitude list depends only on the time selections, if you want to produce more than one exposure map (e.g., different energy selections, or different parts of the detector) with the same timing selections, you can run the first part only once. This parameter can have the following values:
  • W: in this case the attitude list will be produced (and used), but it will also be stored into a table (whose name is specified by the next parameter).
  • R: in this case the attitude list will NOT be produced, but rather read in from a table (whose name is specified by the next parameter).
  • N: (default) no I/O to table will be performed.
• ATTITUDE_LIST_FILE Table name for the attitude list I/O (see above). There is no default.
• INSTRUMENT_MAP_REBIN Rebinning factor for the instrument map. Default is 1.