The Deep Lamp Project: An Investigation of the Precision and Accuracy of the Echelle Wavelength Scales of Space Telescope Imaging Spectrograph

Abstract

The precision and absolute accuracy of the echelle mode wavelength scales of Space Telescope Imaging Spectrograph (STIS) are investigated. The method is to measure deep exposures of the onboard Pt/Cr-Ne hollow cathode calibration lamp. The standard deviation of emission spots from their laboratory wavelengths in a single image is a measure of the internal precision of the pipeline-assigned scales. The average shift of the image as a whole is a measure of the absolute accuracy. While systematic patterns can be identified in all four echelle modes (E140M, E140M, E230M, and E230H), the overall precision (even without compensating for long-range trends with λ) is excellent: of order one-tenth of the resolution element (σ ~ 600 and 300 m s−1, for medium- [M] and high- [H] resolution modes, respectively). Furthermore, the absolute accuracy and its repeatability (assessed in a time series of WAVECAL images) is of order a remarkable 100 m s−1, aside from one of the E230M modes (secondary tilt λ2269) that shows a systematic offset 10 times larger. The excellent precision of the STIS echelle wavelengths could be improved by adding higher order terms to the biquadratic polynomial currently implemented in the CALSTIS pipeline. On the other hand, the existing small distortions might be resolved more naturally by a physical instrument model, currently under development by the Space Telescope European Coordinating Facility's STIS Calibration Enhancement Project.