Make correlation lengths configurable #183
Description
In aperture synthesis images noise is partially correlated across all pixels of the image.
This effect is incorporated in the error bars on the source parameters in a heuristic manner: any variance further away than the FWHM clean beam is considered uncorrelated, while any image noise at a pixel position nearer than the FWHM clean beam is considered fully correlated.
This is effectuated in calculate_correlation_lengths:
return 2.0 * semimajor, 2.0 * semiminor
This is hard-coded now - and copied from the NVSS paper (equations 25/26) - but we should offer the option of changing the correlation lengths through an ImgConf setting.
For example, for images from observation with very poor uv-coverage, one may want to increase the correlation lengths beyond the FWHM clean beam.