Spectral and wavefront error performance of WFIRST/AFTA bandpass filter prototypes

The Cycle 5 design baseline for the Wide-Field Infrared Survey Telescope Astrophysics Focused Telescope Assets (WFIRST/AFTA) instrument includes a single wide-field channel (WFC) instrument for both imaging and spectroscopy. The only routinely moving part during scientific observations for this wide-field channel is the element wheel (EW) assembly. This filter-wheel assembly will have 8 positions that will be populated with 6 bandpass filters, a blank position, and a Grism assembly that will consist of a three-element assembly to disperse the central wavelength undeviated for galaxy redshift surveys. All elements in the EW assembly will be made out of fused silica substrates (110 mm diameter) that will have the appropriate bandpass coatings according to the filter designations (Z087, Y106, J129, H158, F184, W149 and Grism). This paper presents and discusses filter performance, including spectral transmission and reflected/transmitted wavefront errors measurements, for a subset of the bandpass filter complement of the WFC instrument. The filters that are tested in this effort are the Z087, W149, and Grism. These filter coatings have been procured from three different vendors to assess the most challenging aspects in terms of the in-band throughput, out of band rejection (including the cut-on and cut-off slopes), and the impact the wavefront error distortions of these filters will have on the imaging performance of the wide-field channel in the WFIRST/AFTA observatory.