Demonstration and design of a compact diffraction limited spectrograph.

PIMMS IR is a prototype high resolution diffraction limited spectrograph operating in the near infrared. Its current configuration has a bandwidth of 8nm centred on 1550nm with a resolving power, lambda/Delta lambda, of 31000 with the option to increase this to similar to 60000 using a dual grating system. Remarkably, this is 85% of the theoretical limit for Gaussian illumination of a diffraction grating. It is based upon the PIMMS#0 (photonic integrated multi-mode micro-spectrograph), a design that utilises the multi-mode to single-mode conversion of the photonic lantern. By feeding the spectrograph with the single-mode fibres we are able to design and build a spectrograph whose performance is diffraction limited and independent of the input source (i.e. a telescope) it is attached to. The spectrograph has with a throughput of similar to 70% (that is the light from the single-mode entrance slit that lands on the detector). The spectrograph is also extremely compact with a footprint of just 450mm x 190mm. Here we present the design of PIMMS IR and its performance characteristics determined from ray tracing, physical optics simulations and experimental measurements.