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exposure xray


NAME · GENERAL_INTRODUCTION · EXPOSURE_MAPS · EXPOSURE_MASK_VS._EXPOSURE_ARRAY
VIGNETTING · EXPOSURE_CORRECTION_IN_SPECTRA

NAME

exposure -- discussion of exposure files and exposure correction in PROS.

GENERAL INTRODUCTION

The PROS tasks that allow input of an EXPOSURE mask, e.g. IMCNTS, QPSPEC etc, do NOT perform exposure corrections. Rather they allow simple screening out of events from portions of the observation where the exposure drops below a specified percentage.

This screening ability is useful for EINSTEIN observations, where the exposure map is simply a map of exposure times, corrected for instrument features, e.g. ribs but contains no correction factors, e.g. vignetting.

For ROSAT, the exposure files provided with the PSPC are not very useful for screening purposes, due to the INCLUSION of the vignetting factors. However, the *_MEX.IMH file can be used to correct arrays for exposure/vignetting affects, using tasks such as IMCALC.

EXPOSURE MAPS

The exposure map is made by folding the calibration (instrument) map through the aspect history of the telescope so that the exposure at each point in the detector is correct with respect to ribs, wires etc.. To use an exposure map with a task, such as imcnts, it must be the same size as your image. This means either blocking the image down to the size of the exposure map, or magnifying the exposure map up to the size of the image.

The exposure maps provided by SASS are 512 x 512, but they were originally made from 128 x 128 images, and then were expanded to 512 x 512 without interpolation. To make exposure files that are larger than 512x512, you should use IMAGES.BLKAVG to correct for this by blocking the map down to 128 x 128 and then using IMAGES.MAGNIFY with inter=spline3 to get an interpolated exposure map.

When expanding your .pl files, you should specify the .pl extension on the output file explicitly, for the default output is a .imh file. The .pl files do not take up as much space as .imh files, for they are compressed images ( note the difference in size between the *_mex.pl file and the *_mex.imh file).

EXPOSURE MASK vs. EXPOSURE ARRAY

PROS provides the ROSAT/PSPC exposure map in 2 formats.

        a)  *_MEX.IMH
        This is a simple 512x512 array where each pixel value
        represents the exposure time at the location, CORRECTED for
        vignetting at 1 keV.

        b)  *_MEX.PL
        This is a scaled integer version of the _MEX.IMH file with
        a range of values 0-32767.  This is the format expected by
        PROS tasks.  To use it in a PROS task, it must have the
        same dimension as the data file.  'MAGNIFY' will allow
        this file to be expanded to whatever dimension needed.
        (This is a special feature of the FORMAT.)

You should also note that it is possible to make .pl files from .imh files with the execution of the imcopy command, specifying the suffixes explicitly. PL files occupy much less space on disk than do images. Image arrays (like one that is 15360x15360) store images in an inefficient way... entire arrays - even when the majority of the pixels are empty as in the case of xray images. A PL file is a pixel list - it doesn't allocate any memory for empty pixels. It will therefore save you quite a bit of memory; it's dependent upon your image specifically.

VIGNETTING

To apply vignetting, you should use XRAY.XSPATIAL.MAKEVIG and XRAY.XSPATIAL.VIGDATA. MAKEVIG allows you to calculate vignetting at any resolution, but for PSPC it is valid only within the inner-ring, i.e., the first 24 arcmin. Also, it only calculates for a single energy.

VIGDATA allow you to correct for vignetting independent from the exposure correction. These task were developed early in the software development before it was known that the exposure files would be corrected for vignetting.

EXPOSURE CORRECTION IN SPECTRA

The off-axis histogram is used to correct the spectral model for vignetting of the ROSAT mirror. This is done when the fitting task fits the model. The task qpspec can also correct for the difference in vignetting between the background and source regions, properly allowing for charged particle background which is not vignetted. However, if you use a background region close to or surrounding the source this will not be a big affect. See help pspc_fitting for more details.

If you then run xflux, you will get the flux based on the total accepted time (or the time interval you specified when extracting the data). The resulting flux has been corrected for vignetting, but not for rib/edge shadowing or deadtime. If one wanted to estimate a flux from net counts extracted from the image, for example with the task "imcnts", multiplying by the ratio:

  (total accepted time/time in the exposure map at the source location)

will correct the counts for shadowing AND vignetting (but not for deadtime), and the corrected counts could then be converted to a flux by applying an appropriate "counts-to-flux" conversion factor.


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