This work originated with the Wide Field InfraRed Survey Telescope (WFIRST) slitless spectrometer design (GRISM assembly), which attempted to follow the Hubble Space Telescope (HST) method of employing a singleelement GRISM as a slitless spectroscope. However, WFIRST’s field of view (FOV) is ~100x of HST’s wide field camera, and the spectral resolution is ~5x higher with a relatively faster f/8 system. The design turned out to be extremely difficult using only one diffractive surface. Even with many freeform optical elements, and putting a grating on a toroid surface, the image performance was still not satisfactory.

A compact slitless spectrometer was made that can be inserted into a widefield telescope path just like a spectral filter, but provides high dispersion that covers a large wavelength range. It solved a long-existing optical design problem: to obtain good image quality with a grating in non-collimated space for an optical system with a wide FOV. From the astronomical point of view, it opens the door for the astronomer to simultaneously obtain spectral information of all stars and galaxies in the camera’s FOV. From the space mission point of view, a separate, large-volume imaging spectrometer is replaced by a compact GRISM assembly in the telescope optical path that greatly reduces mass, volume, and cost. From the optical designer’s point of view, it provides a method to use wavelength-dispersing grating in non-collimated space.

This compound GRISM assembly consists of three elements. The first acts as a grating to provide required dispersion, the second acts as a prism to balance zero-deviation, and the third performs two functions: wavefront corrector and wideband filter. Among the three elements, there are two flat surfaces that are diffractive surfaces: the grating and the diffractive lens. In theory, both diffractive surfaces create aberrations that are proportional to the wavelength. The optimized GRISM design solved the matching problem and achieved the goals of parfocal with other wavelength channels, zero deviation to the central wavelength, and diffraction-limited image quality at operation wavelength.

The novel feature of this assembly is the use of two different diffractive surfaces to expand the GRISM application significantly: wider FOV, higher spectral resolution, and operation within a faster beam. The other benefit includes using the well-known and well-characterized optical material, fused silica, for all three elements, which reduces the tolerance significantly. The GRISM can be designed very compactly, and does not need any aspheric or freeform optical surface, which simplifies the optical fabrication.

This work was done by Qian Gong, David Content, Jeffrey Kruk, Bert Pasquale, and Thomas Wallace of Goddard Space Flight Center. GSC-17334-1

Photonics & Imaging Technology Magazine

This article first appeared in the January, 2016 issue of Photonics & Imaging Technology Magazine.

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