The first near-infrared IFU spectrograph was the 3D instrument, constructed by the Max Planck Institut für Extraterrestrische Physik. A limitation of this instrument is that each reformatted slitlet is reimaged to only one detector pixel. A mechanism for half-stepping the grating is then required to fully sample the spectrum. NIFS will avoid this complication by mapping each slitlet to two detector pixels in the spectral direction. The 2048 × 2048 pixel detector array then accommodates 29 slitlets, each 0.1" (2 pixels) wide by 3.0" on the sky. The field of view of the IFU is 3.0" × 3.0". An effective slit width of 0.1" was chosen in order to probe to spatial scales approaching the 0.07" diffraction-limited image core size expected from the Gemini adaptive optics system at 2.2 microns while maintaining an acceptable field-of-view.
The IFU is the most critical NIFS component. The reflective "staircase" IFU approach is favored over optical fibres, for example, because it is proven technology requiring the least development investment. Optical fibre solutions are complicated by the need to feed fibres with fast beams. Our approach is similar to that used by the University of Durham group for the GNIRS IFU for Gemini.
A set of image slicer mirrors is located at the f/256 focus (seen at lower-left in the above figure). The f/256 pupil is imaged onto the pupil mirror array (seen at center in the above figure) which reimages the reformatted focal plane at f/16 onto the field mirror array. The field mirror array then feeds the spectrograph collimator mirror (seen at left in the above figure). The pupil mirror array, field mirror array, and spherical collimator mirror are a concentric about the image slicer fanning axis.
The NIFS IFU uses of an 30 mm × 30 mm imager slicer consisting of 29 spherical mirrors each 1.02 mm thick. The displacement angles on the image slicer elements are small so that defocus of the f/256 image plane is not significant relative to the diffraction-limited image size. The image slicer will be manufactured by single point diamond machining a spherical surface on a pinned stack of metal plates. The stack will then be disassembled and re-assembled on a second set of alignment holes so that each plate in the stack is rotated to the required fanning angle.
The pupil mirror and field mirror arrays will be manufactured as monolithic units to improve optical alignment. Each mirror element will be single point diamond machined using a fly-cutting procedure. High quality optical surfaces are required to reduce scattered light, especially for the rejection of terrestrial OH emission.