Notes

• A flat-field spectrum must have been reduced and filed as a calibration. This is essential for locating the positions of the spectra.

• The spectrum from each slice is smoothed with a 1 x 6-pixel box filter before wavelength calibration, primarily to cover bad pixels.

• Variances are propagated, but are not used by Iarc when wavelength calibrating.

• The arc is wavelength calibrated using a list of arc lines from a pre-calibrated arc spectrum to provide an initial fit. These are expected to be constant and are stored in the $ORAC_DATA_CAL/grisms/ directory. The appropriate file (same grism and arc lamp) is provided by the calibration system.

• The recipe checks the r.m.s. value of the wavelength fit returned by Iarc, and will warn the user if this value seems too high. The observer is recommended to always look at the scrunched arc frame. If the wavelength calibration has worked successfully then all the arc lines should be straight, and the frame should be virtually indistinguishable from a long-slit spectrum.