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HST/ACS Grism: Updating Trace and Wavelength Calibrations

Published onJun 01, 2020
HST/ACS Grism: Updating Trace and Wavelength Calibrations

We present latest results from our ongoing effort to obtain new calibrations of the field dispersion of the trace and wavelength solutions of ±1 and ±2 orders of the HST/ACS G800L grism, an optical slitless spectroscopy mode on ACS. This astrophysically important spectroscopic mode has been very stable since its inception in 2002, but no major effort to improve its calibration has been attempted in nearly 15 years. The accurate calibration of the spectral trace and the dispersion solution are crucial to locate the spectrum in the grism image as well as to precisely identify spectral features in a spectrum. By combining large number of archival datasets that contain stellar sources including a calibration target Wolf-Rayet star (WR96), observed over the full field of view of the ACS detector, we can now derive a more finely sampled field dependence of the grism dispersion solution. These data, combined with a new approach of solving for the polynomial coefficients describing the field dependence, allow us to improve the accuracy of the grism trace and wavelength calibrations.

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