Development status of a near-infrared integral field unit SWIMS-IFU

SWIMS-IFU is an image slicer type integral field unit of SWIMS (Simultaneous-color Wide-field Infrared Multi-object Spectrograph), the first generation near-infrared spectrograph for the TAO 6.5 m telescope being constructed at the summit of Cerro Chajnantor (5640 m) by Institute of Astronomy, the University of Tokyo. By matching the slice width to a seeing size of similar to 0.4 arcsec, a larger field of view of 13.5 x10.4 arcsec(2) than any existing near-infrared integral field spectrograph is achieved. Combined with the wide simultaneous wavelength coverage of SWIMS of 0.9-2.5 mu m, it enables more efficient observations of spatially extended objects. SWIMS-IFU has to be compact because of the space limitation in SWIMS. This makes an alignment procedure of optical components difficult. Monolithic fabrication of multiple mirrors using an ultra-precision cutting technique reduces the alignment procedure, as well as achieves good surface roughness and an accurate shape. We also used the ultra-precision cutting to make reference surfaces and custom-made shims for assembly with a few pm accuracy. This strategy allowed us to complete the optical alignment with only one adjustment of a lens. Optical tests in the laboratory showed that image quality, exiting ray directions, and vignetting satisfied the requirements. Finally, we carried out an engineering observation at the Subaru telescope. Image qualities were almost as expected and SWIMS-IFU throughput was 55-75%, with some wavelength bands not meeting the specification of 70%. We also observed some thermal stray light. SWIMS-IFU has been accepted for the open-use program at the Subaru telescope on a shared-risk basis in the S22B semester.

Automated summary

SWIMS-IFU is the first generation near-infrared spectrograph for the TAO 6.5 m telescope. It achieves a larger field of view and more efficient observations by matching the slice width and using ultra-precision cutting for alignment.