7.7.1 Low-Dispersion

The low-dispersion SI and VD FITS files (SILO and VDLO, respectively) are the main output data products produced during the image resampling stage. The SILO contains the geometrically resampled intensity data and the resampled $\nu$-flag image corresponding to the intensity data. The intensity data are output as a FITS primary array, and the associated $\nu$ flags are output as the corresponding FITS image extension. The VDLO contains the summation of all the geometric corrections implemented to perform the single image resampling and gives, for each pixel in LI space, the final x and y coordinate in SI space stored in a FITS primary array. As discussed in Chapter 5, the raw cross correlation data are retained in a FITS binary table extension of the VDLO.

The GEOM module writes the following information to the HISTORY portion of the low-dispersion image label:

• four Chebyshev coefficients used to define the wavelength linearization,
• tilt angle (applied only to extended sources),
• wavelength zeropoint and spatial shifts,
• final time/temperature corrected dispersion constants, and
• predicted center line of the large and small apertures.