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:

• the displacements between the raw science image and the Intensity Transfer Function (ITF),
• the displacement of the reseau pattern (camera fiducials) in the ITF from its original square grid,
• the rotation of the spectral format to lie along image rows, and
• the small change of scale needed to linearize the dispersion.

In addition to the above corrections common to both dispersions, the vectors applied to low-dispersion SI data include:

• the shift to align the two spectrograph apertures in wavelength space,
• corrections for the spatial deviations (cross-dispersion ``wiggles'') in the long-wavelength cameras,
• a correction (detilting) for extended sources to account for the fact that the major axis of the large aperture is not exactly perpendicular to the dispersion direction, and
• shifts in both the wavelength and spatial directions to maintain a fixed starting wavelength and spatial position in the image,
• an adjustment to the LWP data to put the large-aperture data at the top, and
• an adjustment so that both long wavelength cameras provide coverage of the same spectral range.

In addition to the corrections common to both dispersions, the vectors applied to high-dispersion SI data include:

• corrections for the echelle order splaying,
• spatial shifts for proper alignment of the orders to a fiducial location (order registration), and
• corrections for the cross-dispersion wiggles.

Both the corrections common to both dispersions and the dispersion-specific corrections are discussed in more detail in the sections which follow.

 

• 7.1 Corrections Common to Both Dispersions
  • 7.1.1 Measurement of Distortions
    • 7.1.1.1 Mapping of Raw Science Image to ITF Space
    • 7.1.1.2 Mapping of ITF Space to Geometrically Rectified Space
  • 7.1.2 Image Rotation
  • 7.1.3 Wavelength Linearization
• 7.2 Additional Corrections for Low Dispersion
  • 7.2.1 Aperture Alignment
  • 7.2.2 Wiggle Corrections
  • 7.2.3 Large-Aperture Tilt Correction
  • 7.2.4 Wavelength and Spatial Normalization
• 7.3 Additional Corrections for High Dispersion
  • 7.3.1 Order De-splaying
  • 7.3.2 Wavelength and Spatial Normalization
  • 7.3.3 Wiggle Corrections
• 7.4 Flux Resampling Algorithm
• 7.5 Data Quality ($\nu$) Flag Resampling
• 7.6 High-Dispersion Cosmic Ray Detection Algorithm ( COSMIC_RAY)
• 7.7 GEOM Output
  • 7.7.1 Low-Dispersion
  • 7.7.2 High-Dispersion
• 7.8 COSMIC_RAY Output