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.

Date Published: 28 July 2014
PDF: 8 pages
Proc. SPIE 9147, Ground-based and Airborne Instrumentation for Astronomy V, 914797 (28 July 2014); doi: 10.1117/12.2055870

Published in SPIE Proceedings Vol. 9147:
Ground-based and Airborne Instrumentation for Astronomy V
Suzanne K. Ramsay; Ian S. McLean; Hideki Takami, Editor(s)