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28 July 2014The spectrograph units for the HARMONI integral field spectrograph
The spectrograph sub-system is responsible for dispersing the light from the slicer with the required spectral resolving
power and imaging the spectra on to a detector. Each image slicer creates a single exit slit feeding a single spectrograph
unit containing visible (VIS) and infrared (IR) cameras. The four HARMONI slicers in total create four exit slits, feeding
four spectrograph units comprising of collimators, dispersers, and cameras. The focal plane of each camera contains a
mosaic of two 4Kx4K detectors, leading to 8K pixels along the length of the slit. The HARMONI wavelength range
(0.43 μm to 2.45 μm) splits into a visible and a near-infrared wavelength range with a transition wavelength at 0.8 μm. The optical design of HARMONI up to the dispersers is fully reflective and therefore the pre-optics and IFU subsystems,
as well the spectrograph collimator, can be used for both the visible and near-infrared wavelength range. Only
the dispersers and the spectrograph cameras are different for the visible and near infrared spectral ranges. To not
duplicate sub-systems unnecessarily the wavelength split in the spectrograph is realised by inserting a dichroic in the
collimated beam before the disperser to either direct the light towards the visible disperser and camera, or let it pass
toward the near-infrared disperser and camera. In contrast to the Phase A study all of HARMONI spectrograph unit will
have both visible and near infrared disperser and cameras.
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Kieran O'Brien, Jamie R. Allen, James D. Lynn, Niranjan A. Thatte, Ian Bryson, Fraser Clarke, Hermine Schnetler, Matthias Tecza, "The spectrograph units for the HARMONI integral field spectrograph," Proc. SPIE 9147, Ground-based and Airborne Instrumentation for Astronomy V, 914797 (28 July 2014); https://doi.org/10.1117/12.2055870