The InfraRed Imaging Spectrograph (IRIS) is one of three first light science instruments for the Thirty Meter Telescope (TMT). It will provide dedicated function of imaging and integral field spectroscopic observations in parallel with the assistance of a Narrow Field InfraRed Adaptive Optics System (NFIRAOS). The IRIS imager delivers celestial light to a dual-channel Integral Field Spectrograph (IFS) through a pair of pick-off mirrors in the central field. The IFS creates multi-functional ability to explore the universe in IR (0.84 – 2.4um) with moderate spectral resolution of R=4,000/8,000 and four spaxel scales of 4, 9, 25, 50 milli-arc-seconds (mas). An image slicer serves one of the two spectral channels as its Integral Field Unit (IFU) in two coarse spaxel scales of 25 and 50mas over the continuous science fields of 2.2x1.125 arc-seconds (arcsec) and 4.4x2.25 arcsec respectively. It splits the field to 88 unit systems, and then re-images at two parallel slits in order to take full advantage of the detector (4Kx4K @ 15um). This paper describes a novel all-reflective design of image slicer, which uses a new ‘brick stage’ layout to stagger the adjacent mirrors and deliver image quality close to diffraction limit. The quasi-telecentric optical design gives more friendly interfaces with pre-optics and spectrograph than the conceptual design. Here, more technical issues are discussed to guide the further study on optical performance and fabrication feasibility.
The Maunakea Spectroscopic Explorer (MSE) project will transform the CFHT 3.6m optical telescope to a 10m class dedicated multi-object spectroscopic facility, with an ability to measure thousands of objects with three spectral resolution modes respectively low resolution of R≈3,000, moderate resolution of R≈6,000 and high resolution of R≈ 40,000. Two identical multi-object high resolution spectrographs are expected to simultaneously produce 1084 spectra with high resolution of 40,000 at Blue (401-416nm) and Green (472-489nm) channels, and 20,000 at Red (626-674nm) channel. At the Conceptual Design Phase (CoDP), different optical schemes were proposed to meet the challenging requirements, especially a unique design with a novel transmission image slicer array, and another conventional design with oversize Volume Phase Holographic (VPH) gratings. It became clear during the CoDP that both designs presented problems of complexity or feasibility of manufacture, especially high line density disperser (general name for all kinds of grating, grism, prism). At the present, a new design scheme is proposed for investigating the optimal way to reduce technical risk and get more reliable estimation of cost and timescale. It contains new dispersers, F/2 fast collimator and so on. Therein, the disperser takes advantage of a special grism and a prism to reduce line density on grating surface, keep wide opening angle of optical path, and get the similar spectrum layout in all three spectral channels. For the fast collimator, it carefully compares on-axis and off-axis designs in throughput, interface to fiber assembly and technical risks. The current progress is more competitive and credible than the previous design, but it also indicates more challenging work will be done to improve its accessibility in engineering.