In this paper we discuss the final mechanical design of the GIRMOS Cryostat. GIRMOS is an adaptive optics fed multi-object Integral-Field Spectrograph (IFS) with a parallel imaging capability and will be installed at the Gemini North Observatory. This instrument includes four identical spectrograph channels arranged symmetrically around a central optical axis which provides it with a multiplexing capability. GIRMOS has an object selection sub-assembly (OSEL) located upstream of the spectrographs which contains four motorized fold mirror assemblies that can scan the incoming light from the telescope to look at four separate sections of the sky simultaneously or can combine their efforts to look at a single larger section of the sky in a tiled mode. Each of the four individual IFS beams from the OSEL assembly is directed into the instrument Cryostat via its dedicated optical entrance window. Within the Cryostat each IFS beam moves through an anamorphic relay, an optical image slicer assembly, and eventually makes it to a spectrograph unit. In this paper, we will provide some details regarding the vacuum chamber design that houses the spectrographs, the design of the internal support structure, the opto-mechanical packaging of the IFS along with some of the thermal load mitigation techniques employed.
Image slicers (SLI) are now standard equipment in astronomical instrumentation, particularly for Integral Field Spectroscopy (IFS). Our team at the Advanced Optical Fabrication Infrastructure (AOFI) has been tasked with the fabrication of the slicer-based IFU for GIRMOS (Gemini InfraRed Multi-Object Spectrograph), which contains 42 250-μm wide slices along with the corresponding pupil and field mirrors. Diamond turning image slicers is complicated as they often include tilted slices smaller than 500-μm wide. The surface finish of diamond-turned optics must be optimal in astronomical applications such as integral field spectroscopy. Here we present a test plan and characterization of the GIRMOS SLI system. One notable challenge is the disparity between room temperature testing and the cryogenic environment of the actual system. This requires the design of a warm model, which has been numerically validated. A comprehensive metrology process is implemented, utilizing equipment such as a Talysurf PGI Freeform profilometer, a ZYGO Verifire HD Fizeau interferometer, a coordinate measuring machine (CMM), a point source microscope (PSM) equipped with a motorized gimbal mount, a scanning electron microscope (SEM), and a custom SLI-M1 test bench. The test bench facilitates verification of intermediate pupil positions, calculation of the slices' tilts and curvatures, measurement of diffraction efficiency, and estimation of surface roughness through total integrated scatter (TIS) analysis. Additionally, the bench is used to characterize the pseudo-slit output by replacing front-end optics and enables the measurement of tilts on all surfaces using a precision motorized gimbal stage. The test plan presented establishes a robust methodology for comprehensive characterization, essential for high precision integral field spectroscopy in astronomical observations.
In this paper we discuss the mechanical design of the GIRMOS Cryostat. GIRMOS is an adaptive optics fed multi-object Integral-Field Spectrograph with a parallel imaging capability and will be installed at the Gemini North Observatory. This instrument includes four separate identical spectrograph channels arranged symmetrically around the central axis of the instrument which provide it its multiplexing capability. Each spectrograph channel starts off at the object selection mechanism. The object selection mechanism contains four motorized fold mirror assemblies which scan the incoming light from the telescope to look at four separate objects simultaneously or combine their efforts to look at a single object in a tiled mode. Each of the four individual beams from the object selection system are then directed into the instrument dewar via separate entrance windows. Within the dewar each IFS beam moves through an anamorphic relay, an optical image slicer assembly and eventually makes it to a Spectrograph unit. All of these assemblies are located on a single cold bench within the dewar. The instrument imager is located along the central axis of the dewar and is housed in the cold bench as well. In this paper we will provide some details regarding the Cryostat design, the mechanical packaging of the IFS and imager along with some of the thermal load mitigation techniques employed. We will also discuss some key performance requirements that were expected from the Cryostat and the design choices we made in order to achieve them.
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