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.
It is not recommended for users to acquire NIRSpec auto-cals with their science for two reasons:
- Acquiring auto-cals can cause moderate overheads, ~5 minutes or more per exposure.
- Acquiring auto-cals with lamp illumination may cause low levels persistence in subsequent science that could effect very deep sky exposures on 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.
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.