Spectral and imaging performance of WFC3 interference filters

Abstract

The Wide Field Camera 3 (WFC3) is a panchromatic imager that will be deployed in the Hubble Space Telescope (HST). The mission of the WFC3 is to enhance the imaging capability of HST in the ultraviolet, visible and near-infrared spectral regions. Together with a wavelength coverage spanning 2000 Angstrom to 1.7 μm, the WFC3 high sensitivity, high spatial resolution, and large field-of-view provide the astronomer with an unprecedented set of tools for exploring all types of exciting astrophysical terrain and for addressing many key questions in astronomy today. The filter complement, which includes broad, medium, and narrow band filters, naturally reflects the diversity of astronomical programs to be targeted with WFC3. The WFC3 holds 61 UVIS filter elements, 15 IR filters, and 3 dispersive elements. During ground testing, the majority of the UVIS filters were found to exhibit excellent performance consistent with or exceeding expectations; however, a subset of filters showed considerable ghost images; some with relative intensity as high as 10-15%. Replacement filters with band-defining coatings that substantially reduce these ghost images were designed and procured. A state-of-the-art characterization setup was developed to measure the intensity of ghost images, focal shift, wedge direction, transmitted uniformity and surface features of filters that could affect uniformity in flat-field images. We will report on these filter characterization methods, as well as the spectral performance measurements of the in-band transmittance and out-of-band blocking.