STIS Calibration Enhancement (STIS-CE) : Dispersion Solutions Based on a Physical Instrument Model

The Space Telescope European Co-ordinating Facility (ST-ECF) embarked on the STIS calibration enhancement (STIS-CE) effort as part of the extension of the Memorandum of Understanding between NASA and ESA for the Hubble Space Telescope (HST). The work was done by the ST-ECF’s Instrument Physical Modeling Group in collaboration with the STScI’s Spectrographs group. Traditionally, calibration of spectrographs is done by empirical methods, e.g. a wavelength dispersion solution is derived by applying a polynomial fit to the emission line spectrum of a calibration exposure. In the model based approach we make full use of the engineering information used to build the instrument. Details of the concept are described in Rosa (1995). Our approach comprises a ray trace model (Ballester & Rosa 1997) describing the geometry of the optical elements and the software needed to compare the performance of the instrument as determined by the model with actual calibration data. We then use an optimizer tool which finds, for any given observation, the actual configuration of the instrument. With this method we can close the loop between model description and actual observations resulting in a high fidelity calibration of the instrument. For STIS we have completed the dispersion solution of the high resolution echelle modes and have demonstrated that all central wavelength settings of E140H can be properly described by changing only two tilt angles of the cross-disperser. An integral part of our approach is the provision of high quality input data: in the case of STIS our work in collaboration with the Atomic Spectroscopy Group at the US National Institute of Standards and Technology (NIST) on the output of the calibration lamps has remedied a longstanding shortcoming of the calibration. The standard wavelength calibration of all HST spectrographs has been based on a line list produced by Reader et al. (1990) for a Pt-Ne lamp. STIS uses a Pt/Cr-Ne lamp; the addition of Cr is especially significant in the near UV where up to 90 % of the observed lines are Cr. Without established wavelength standards these could not be used. Our new dispersion solution based on an instrument model and using the new Pt/Cr-Ne line list (Sansonetti et al. 2004) has been successfully applied to scientific data. In order to investigate the improvement achieved in a quantitative manner we have chosen a science case looking at interstellar absorption lines. For the echelle mode E140H we have compared the wavelengths of the same lines in the overlap regions of adjacent orders. Using the CALSTIS pipeline we find a systematic offset of order 0.5 pixel which is within the expected accuracy. The model based solution delivers a factor of five improvement bringing the difference in wavelength to within order 0.1 pixel. Given the fact that STIS is a very well calibrated instrument this nicely illustrates the potential of the physical modeling approach. affiliated to the Space Telescope Operations Division of Research and Scientific Support Department of the European Space Agency