Engineering observation of SWIMS-IFU (Integral field unit for SWIMS, Simultaneous-color Wide-field Infrared Multi-object Spectrograph) on the Subaru Telescope succeeded

Spectroscopic observation is necessary to probe the detailed physical state of stars, galaxies, and other astronomical objects. In spectroscopic observation, dispersing the light from astronomical objects into different wavelengths and measuring the intensity of light at each wavelength allows for detecting various characteristics of the light coming from astronomical objects to determine their temperature, density, elemental abundance, and other properties.

In conventional spectroscopic observation, optical fibers or slits are placed at the position of the object to be observed, and only light passing through them is observed. Therefore, only a portion of the object can be observed when observing a spatially extended object on the celestial sphere, and multiple observations are required to observe entirely. This is inefficient and susceptible to temporal changes in the Earth's atmosphere and the state of the observation instruments.

In recent years, integral field spectroscopy (IFS) has become actively used. IFS is a technique for spectroscopically observing the entire two-dimensional field of view at once by dividing and rearranging the object image obtained by the telescope into a single line. (Fig.1) This method enables spectroscopic observations of the entire objects at once, improving observation efficiency and providing high-quality observation data that is not affected by time variation.

▲Fig. 1. Main methods for IFS: Lenslet array method: an optical element, lenslet array, consisting of a row of small lenses is placed on the telescope focal plane to split the object images. Fiber method: a fiber bundle consisting of optical fibers is placed at the telescope focal plane to divide and rearrange images. Image slicer method (detailed below): a special optical element, image slicer, is used to divide images.

Such IFS observations have been actively conducted in the optical-infrared wavelength in recent years and have produced various results. Especially in optical wavelength, instruments with various features have been developed, such as instruments that can simultaneously perform IFS of a large number of objects and instruments with an extremely wide field of view. On the other hand, in near-infrared wavelength most IFS instruments are optimized for high spatial resolution observations using adaptive optics, resulting in a narrow field of view, and there is a lack of IFS instruments that can observe a wide field of view at once.

So, we are developing SWIMS-IFU which adds an IFS function with a wide field of view to SWIMS (Simultaneous-color Wide-field Infrared Multi-object Spectrograph) for the TAO 6.5m telescope. SWIMS-IFU is an optical unit to switch SWIMS to IFS mode only by inserting SWIMS-IFU into the telescope focal plane. SWIMS-IFU divides an object image obtained by the telescope, rearranges it in a line, and introduces it to the optical system of SWIMS to achieve IFS. We have developed SWIMS-IFU optimized for the optical system of the Subaru Telescope where SWIMS was operated as a PI-type instrument. SWIMS-IFU has a field of view of 13.5" x 10.4", more than twice the area of existing infrared IFS instruments. Combining it with the wide wavelength range of SWIMS spectroscopy (0.9-2.5 μm at once), we can realize much more efficient observations of spatially extended astronomical objects.

SWIMS-IFU uses an optical element called an image slicer. Image slicer consists of multiple strips of mirrors which are stacked and oriented in different directions from each other. When an object image is formed on the image slicer, it is divided into strips and reflected in different directions. The subsequent optical system re-aligns them in a line to achieve integral field spectroscopy of SWIMS. The optical system of SWIMS-IFU is very complicated, it consists of eighty mirrors with a size of a few millimeters and a doublet lens within 235 mm x 170 mm x 55 mm. To realize this complex optical system, we are applying the technique called ultra-precision cutting in cooperation with the Ultrahigh Precision Optics Technology Team, RIKEN. Ultra-precision cutting is a machining method using an ultra-precision machine with nanometer-order operational accuracy and high-precision diamond tools to finish mirrors only by cutting. We can also achieve high accuracy in relative position between mirrors by processing multiple mirrors on a single metal workpiece using this machine. With this method, we have made all the mirrors and completed SWIMS-IFU for the Subaru Telescope in December 2021. (Fig. 2)

▲Fig. 2. Completed SWIMS-IFU (middle) and optical elements: (a)plane mirror and slice mirror array (image slicer)(b)spherical mirror (c)slit mirror array (d)pupil mirror array

Completed SWIMS-IFU was sent to Subaru Telescope, NAOJ from February to March 2022. We installed it into SWIMS at Subaru Telescope in March 2022, and made an engineering observation of SWIMS-IFU successfully (Fig. 3). We observed a standard star with well-known brightness and spectral shape, and a star cluster with multiple stars within the SWIMS-IFU's field of view. Performance evaluation using these data confirmed that image quality and throughput were almost as designed, demonstrating that complicated IFU was made as designed using our ultra-precision cutting technique. A science observation using SWIMS-IFU was scheduled in December 2022, but unfortunately, we couldn't perform it due to the eruption of Mauna Loa.

▲Fig. 3.(left) successful engineering observation at the Subaru Telescope (right) Actual data taken with SWIMS-IFU. Spectra for twelve slices can be seen in each image. The upper one is for the wavelength of 0.9-1.4μm, and the lower one is for 1.4-2.5μm.

We are now preparing for SWIMS-IFU operation on the TAO 6.5m Telescope. We need to develop a new SWIMS-IFU for the TAO Telescope, as the current SWIMS-IFU is optimized for the Subaru Telescope. Based on the technologies we have developed, we are proceeding with optical design and manufacturing studies to achieve better image quality and throughput. Please look forward to SWIMS-IFU on the TAO Telescope.