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7 Image Resampling (GEOM)

 

Raw IUE images suffer from spatial distortions introduced by the SEC Vidicon cameras. The electrostatically-focused imaging section of the camera produces a pincushion distortion, while the magnetically-focused readout section produces an S-distortion. Furthermore, the dispersion direction lies at an angle of approximately 45 degrees relative to the image axes and the dispersion function is not linear within the spectral orders. The combination of these effects make the task of spectral extraction and subsequent analysis very difficult. The goal of the GEOM module is to create a geometrically-resampled and spatially-rotated image in which the spectral orders are horizontally aligned and the dispersion is linear within each order thus providing an image format that is best suited for scientific analysis. The GEOM step operates on the linearized (i.e., photometrically corrected) image (LI).

The NEWSIPS approach to producing a geometrically resampled image (SI) is to construct a vector field that maps each pixel from its instrumental raw space to a geometrically rectified space. For both low- and high-dispersion images, these vectors include corrections for the following effects:

In addition to the above corrections common to both dispersions, the vectors applied to low-dispersion SI data include: In addition to the corrections common to both dispersions, the vectors applied to high-dispersion SI data include: Both the corrections common to both dispersions and the dispersion-specific corrections are discussed in more detail in the sections which follow.



 
next up previous contents
Next: 7.1 Corrections Common to Up: No Title Previous: 6.5 PHOTOM Output
Karen Levay
12/4/1997