PIMMS échelle: the next generation of compact diffraction limited spectrographs for arbitrary input beams

PIMMS échelle is an extension of previous PIMMS (photonic integrated multimode spectrograph) designs, enhanced by using an échelle diffraction grating as the primary dispersing element for increased spectral band- width. The spectrograph operates at visible wavelengths (550 to 780nm), and is capable of capturing ~100 nm of R > 60, 000 (λ/(triangle)λ) spectra in a single exposure. PIMMS échelle uses a photonic lantern to convert an arbitrary (e.g. incoherent) input beam into N diffraction-limited outputs (i.e. N single-mode fibres). This allows a truly diffraction limited spectral resolution, while also decoupling the spectrograph design from the input source. Here both the photonic lantern and the spectrograph slit are formed using a single length of multi-core fibre. A 1x19 (1 multi-mode fiber to 19 single-mode fibres) photonic lantern is formed by tapering one end of the multi-core fibre, while the other end is used to form a TIGER mode slit (i.e. for a hexagonal grid with sufficient spacing and the correct orientations, the cores of the multi-core fibre can be dispersed such that they do not overlap without additional reformatting). The result is an exceptionally compact, shoebox sized, spectrograph that is constructed primarily from commercial off the shelf components. Here we present a brief overview of the échelle spectrograph design, followed by results from on-sky testing of the breadboard mounted version of the spectrograph at the ‘UK Schmidt Telescope’.