The design and fabrication of multiple dichroic beamsplitters for the MIRI spectrometer (4.8-29µm)
M J Wells, G J Hawkins, G Olofsson
Proc. SPIE Vol. 5487, pp 794-803 (2004)
The spectrometer sub-system of the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) contains four channels which spectrally multiplex the incoming radiation. This incident radiation, spanning a wavelength range from 4.8 to 28.8µm is spectrally divided between the channels using sets of three dichroics combined in series along the optical trains. The four channels, with overlapping wavelengths 4.8-7.8, 7.4-11.9, 11.4-18.3 and 18.3-28.8µm, are in-turn each split into three wavelength ranges to provide the required resolving power with the available detector pixels. This splitting of the wavelengths within each channel is achieved using three separate sets of dichroics and diffraction gratings, mounted on two wheels.
This paper describes the design of the dichroics together with a spectral performance model developed to simulate the system spectral throughput for each of the four channels of the MIRI instrument. Details of the spectral design, manufacture, testing and mounting of the dichroics are presented together with the optomechanical layout of the instrument.