NIRSpec auto-cals (auto-calibrations) are internal calibration exposures that can be acquired with lamps within the NIRSpec calibration assembly (CAA). These auto-cals can be acquired for flat fields, wavelength reference exposures, and broad line peaks measured for dispersion verification.
NIRSpec auto-cals have been added to the science templates for FS, IFU, and MOS observing modes to make it possible to acquire lamp exposures at the same time as on-sky external exposures. It is expected this capability will primarily be used for instrument commissioning and observatory calibration observations.
NIRSpec auto-cals are not recommended for science observations for 3 reasons:
- Acquiring auto-cals can cause moderate overheads, ~5 minutes or more per exposure.
- Acquiring auto-cals with lamp illumination may cause low levels of persistence in subsequent science that could effect very deep sky exposures of faint objects.
- The NIRSpec grating wheel assembly includes a sensor to calibrate the wavelength shift that results from the grating wheel settling at slightly different positions. Presently, the instrument-model based wavelength calibration (from ground-based argon lamps) gives wavelength solutions with accuracy exceeding the requirement (1/8th of a resolution element). External wavelength calibration observations will be used to update the instrument model during commissioning. Internal lamp exposures are not expected to offer much improvement in the wavelength solution.