Weights for Point Sources

Calnic C uses pre-computed weights for the extraction of spectra of point sources. These weights were computed from artificial PSFs generated by the PSF generation software TinyTim For the weighted extraction, the weight for a given wavelength l and position perpendicular to the dispersion direction i is given by:

$$w_{i,l}= P_{i,l}/\sum_i P_{i,l}^2$$ (6)

where P_i,l is the grism image of a point source with a flat spectrum. This is the optimum extraction for point sources when the noise is independent of the flux, i.e. is background dominated.

A FITS file containing the weight matrix for each grism is used for extracting spectra for point sources.