Integral field spectroscopy (IFS) is an observational method for obtaining spatially resolved spectra over a specific field of view (FoV) in a single exposure. In recent years, near-infrared IFS has gained importance in observing objects with strong dust attenuation or at a high redshift. One limitation of existing near-infrared IFS instruments is their relatively small FoV, less than 100 arcsec2, compared with optical instruments. Therefore, we developed a near-infrared (0.9 to 2.5 μm) image-slicer type integral field unit (IFU) with a larger FoV of 13.5×10.4 arcsec2 by matching a slice width to a typical seeing size of 0.4 arcsec. The IFU has a compact optical design utilizing off-axis ellipsoidal mirrors to reduce aberrations. Complex optical elements were fabricated using an ultra-precision cutting machine to achieve root mean square surface roughness of less than 10 nm and a P-V shape error of less than 300 nm. The ultra-precision machining can also simplify the alignment procedures. The on-sky performance evaluation confirmed that the image quality and the throughput of the IFU were as designed. In conclusion, we successfully developed a compact IFU utilizing an ultra-precision cutting technique, almost fulfilling the requirements.
We are developing an image-slicer type integral field unit (IFU), SWIMS-IFU, for SWIMS (Simultaneous-color Wide-field Infrared Multi-object Spectrograph), a near-infrared instrument for TAO 6.5 m telescope. SWIMS- IFU divides a field-of-view of 16:006 12:008 into 26 slices with a width of 0:005, which is the largest FoV among near-infrared IFUs on 8 m class telescopes. It is also capable of obtaining entire near-infrared spectra from 0.9 to 2.5 m with R1000 with a single exposure. Because of limitations of space in SWIMS, SWIMS-IFU should fit in a volume of 170 x 220 x 60 mm3, which results in small and complicatedly aligned mirror facets. To reduce alignment procedures, we adopt an ultra-precision cutting technique to fabricate mirror arrays monolithically. We have completed one of the mirror arrays, the slit-mirror array which consists of 26 spherical mirror facets, and confirmed both their surface roughness and shape errors satisfy the requirements. We also have fabricated a prototype of the pupil-mirror array including some elliptical mirror facets and confirmed that the elliptical mirrors have enough surface qualities and produce better image quality than spherical ones by a pinhole imaging test.
We are developing an optical adaptive optics (AO) system for small telescopes. An AO instrument in optical wavelength mounted on a 1-2 m class telescope located at a good seeing site will make it possible to achieve high angular resolution of 0.1-0.2 arcsec. Such capability will enable us to perform unique astronomical programs, as well as to provide good opportunity in education for both astronomy and engineering. In order to examine the AO capability on small telescopes, we developed an experimental AO instrument, in which inexpensive commercial devices are extensively used to reduce cost for development. We designed the weight and the physical size so small that it is portable and easy to be mounted on a small telescope, which is a unique feature of our AO instrument. After the engineering observations performed in Japan, we mounted it on the 1-m telescope of the European Southern Observatory of La Silla in Chile in March 2018 to examine the performance. We found that there were approximately 4 times and 5 times improvements in the full-width-halfmaximum (FWHM) and Strehl ratio of the PSF from the natural seeing, respectively. The best AO-corrected PSF obtained during the observation achieved FWHM=0.18 arcsec and the Strehl ratio = 0.18. Based on the detailed analysis of the timeseries wavefront and deformable-mirror-operation data, further improvement in AO performance is expected by adjustment of the system parameters. We succeeded in demonstrating the feasibility of an inexpensive optical AO system for small telescopes.
The Simultaneous-color Wide-field Infrared Multi-object Spectrograph (SWIMS) is one of the 1st generation facility instruments for the University of Tokyo Atacama Observatory (TAO) 6.5 m telescope currently being constructed at the summit of Cerro Chajnantor (5,640 m altitude) in northern Chile. SWIMS has two optical arms, the blue arm covering 0.9–1.4 µm and the red 1.4–2.5 µm, by inserting a dichroic mirror into the collimated beam, and thus is capable of taking images in two filter-bands simultaneously in imaging mode, or whole nearinfrared (0.9–2.5 µm) low-to-medium resolution multi-object spectra in spectroscopy (MOS) mode, both with a single exposure. SWIMS was carried into Subaru Telescope in 2017 for performance evaluation prior to completion of the construction of the 6.5 m telescope, and successfully saw the imaging first light in May 2018 and MOS first light in Jan 2019. After three engineering runs including the first light observations, SWIMS has been accepted as a new PI instrument for Subaru Telescope from the semester S21A until S22B. In this paper, we report on details of on-sky performance of the instrument evaluated during the engineering observations for a total of 7.5 nights.
SWIMS-IFU is an image-slicer integral field unit designed for Simultaneous-color Wide-field Infrared Multi-object Spectrograph (SWIMS) of the University of Tokyo Atacama observatory 6.5m telescope. Its field-of-view, slice width and slice number are 17.2 ′′ × 12.8 ′′, 0.4 ′′ and 26, respectively. Due to the space limitation inside SWIMS, the IFU should fit in the dimension of 60mm×170mm×220mm. After finishing development of optical design, we have conducted tolerance analysis. The results show that the probability of vignetting of less than 5% is ∼90%, although at a slice of one side it drops to 50%. We plan to fabricate the mirror arrays monolithically by a ball-end milling with an ultra-high precision machine tool, and have conducted a demonstration process to prove its feasibility. Our requirement for shape error is less than 100 nm P-V and that for surface roughness is less than 10 nm r.m.s. Results of the latest demonstration satisfies the requirement. We will fabricate the mirror arrays and the support structures in 2018, and the IFU will be installed into SWIMS in 2019.
The Mid-Infrared Multi-field Imager for gaZing at the UnKnown Universe (MIMIZUKU) is a mid-infrared camera and spectrograph developed as a first-generation instrument on the University of Tokyo Atacama Observatory (TAO) 6.5-m telescope. MIMIZUKU covers a wide wavelength range from 2 to 38 μm and has a unique optical device called Field Stacker which realizes accurate calibration of variable atmospheric transmittance with a few percent accuracy. By utilizing these capabilities, MIMIZUKU realizes mid-infrared long-term monitoring, which has not been challenged well. MIMIZUKU has three optical channels, called NIR, MIR-S, and MIR-L, to realize the wide wavelength coverage. The MIR-S channel, which covers 6.8–26 μm, has been completed by now. We are planning to perform engineering observations with this channel at the Subaru telescope before the completion of the TAO 6.5-m telescope. In this paper, we report the results of the laboratory tests to evaluate the optical and detector performances of the MIR-S channel. As a result, we confirmed a pixel scale of 0.12 arcsec/pix and a vignetting- free field of view of 2./0 1./8. The instrument throughputs for imaging modes are measured to be 20–30%. Those for N - and Q -band spectroscopy modes are 17 and 5%, respectively. As for the detector performance, we derived the quantum efficiency to be 40–50% in the mid-infrared wavelength region and measured the readout noise to be 3000–6000 electrons, which are larger than the spec value. It was found that this large readout noise degrades the sensitivity of MIMIZUKU by a factor of two.
The Simultaneous-color Wide-field Infrared Multi-object Spectrograph, SWIMS, is a first-generation near-infrared instrument for the University of Tokyo Atacama Observatory (TAO) 6.5m Telescope now being constructed in northern Chile. To utilize the advantage of the site that almost continuous atmospheric window appears from
0.9 to 2.5 μm, the instrument is capable of simultaneous two-color imaging with a field-of-view of 9.′6 in diameter or λ/▵λ 1000 multi-object spectroscopy at 0.9–2.5 μm in a single exposure. The instrument has been trans- ported in 2017 to the Subaru Telescope as a PI-type instrument for carrying out commissioning observations before starting science operation on the 6.5m telescope. In this paper, we report the latest updates on the instrument and present preliminary results from the on-sky performance verification observations.
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