2.1.

Overview of SWIMS

The internal structure of SWIMS is shown in Fig. 1. Imaging with a large FoV and spectroscopy with a wide simultaneous wavelength coverage of 0.9 to $2.5\mu \mathrm{m}$ are realized by dividing collimated light into two wavelength ranges of 0.9 to $1.45\mu \mathrm{m}$ (blue arm) and 1.45 to $2.5\mu \mathrm{m}$ (red arm) by a dichroic mirror and re-imaging them simultaneously on focal plane array detectors of each arm. Switching between imaging and spectroscopy modes is done by changing the filters and grisms in the filter wheels. Two $2048\times 2048$ format HAWAII-2RG (H2RG) array detectors for each focal plane cover a $6.7\times 3.3{\mathrm{arcsec}}^2$ FoV, and the goal is to increase them to four in a $2\times 2$ matrix to cover the whole FoV delivered by the optics. Spectra are dispersed over the two detectors to cover the wide wavelength range.

Fig. 1

Internal structure of SWIMS. A slit mask or the IFU is placed on the telescope focal plane indicated by the arrow.

For multi-object spectroscopy, a slit mask is placed at a telescope focal plane by a multi-object spectroscopy unit (MOSU).²³ The MOSU consists of a carousel that stores up to 22 slit masks under vacuum and cryogenic conditions and a robotic arm that handles the slit masks.

SWIMS will be installed at the Nasmyth focus of the TAO 6.5 m telescope, which has a focal ratio (F-ratio) of 12.2. This is the same as the F-ratio of the Cassegrain focus of the Subaru telescope. SWIMS was carried to the Subaru Telescope to evaluate and optimize its on-sky performance before the completion of the TAO 6.5 m telescope. Engineering observations were conducted between 2018 and 2020. Thereafter, the instrument was operated as a PI-type instrument from 2021 to 2022.

2.2.

Concept of the SWIMS-IFU

The SWIMS-IFU is an image-slicer type IFU. It should be stored and operated in a cryogenic environment to suppress thermal background radiation from the IFU itself. By placing the IFU at the telescope focal plane as slit masks, SWIMS can be switched to an IFS mode. Thus, the IFU is also stored in a dedicated slot of the carousel and installed at the telescope focal plane by the MOSU. At the telescope focal plane, the IFU is held in place by magnets embedded in both the IFU and focal plane structure, which is the same mechanism as slit masks. This basic concept of the IFU does not require any modification of the optics of SWIMS. On the other hand, the size of the IFU is restricted to within $235\times 170\times 55{\mathrm{mm}}^3$ due to space limitations in the carousel.

4.1.

Mirror Fabrication by Ultra-Precision Cutting

Figure 5 shows a mechanical structure of the SWIMS-IFU. It consists of five optical elements (PO0+slice-mirror array, PO1, PO2, pupil-mirror array, and slit-mirror array) and a base plate. All optical surfaces except for the PO1 and interface surfaces including those of the base plate are fabricated with ultra-precision cutting. Our goal is to achieve sufficient surface quality of the mirrors by only cutting and to guarantee the relative positions and orientations between mirrors within a single optical element as well as between the elements, which will greatly save time and effort for alignment procedures.

Fig. 5

Mechanical structure of the SWIMS-IFU. Vignetting points of pupil images and the FoV described in Sec. 5.1 are also shown.

The ultra-precision cutting of the surfaces was done with ULG-100D(5A) (Shibaura Machine Co., Ltd.), operated by RIKEN, using various machining methods, as summarized in Table 2. All mirrors except for the PO2 are made of a rapidly solidified aluminum alloy RSA6061²⁵ to achieve a good surface roughness of less than 10 nm. The completed optical elements are shown in Fig. 6. Unique points in our fabrication are (1) the PO0+slice-mirror array, which has two types of flat mirrors in a single work piece, and (2) the pupil-mirror array with spherical and ellipsoidal mirrors and the slit-mirror array with various spherical mirrors. In the former fabrication, to complete two types of flat mirrors in a single operation, two types of tools were mounted simultaneously on the ultra-precision machine. One of the tools is a custom-made flat diamond tool for the slice mirrors, which has the same width and cross-sectional shape as the slice mirrors.²⁶ The latter fabrications adopted a ball-end mill tool with a small cutting-edge radius to fabricate the spherical and ellipsoidal mirrors in a single operation. The method can process various spherical and ellipsoidal mirrors with a radius of curvature larger than that of the tool.

Table 2

Summary of fabrication by ultra-precision cutting.

Fig. 6

Completed mirrors for the SWIMS-IFU: (a) PO2, (b) PO0+slice-mirror array, (c) pupil-mirror array, and (d) slit-mirror array.

4.2.

Mirror Surface Quality

4.2.1.

Surface roughness

Surface roughness is an important indicator for assessing the scattering loss at a mirror surface. The scattering loss $L_{\text{scatter}}$ by the five mirrors of the IFU at a wavelength $\lambda$ is estimated by the following equation:⁸

Eq. (1)

$$L_{\text{scatter}}=1-\prod _{i=1-5}(1-{\left(\frac{4\pi {\sigma }_i}{\lambda }\right)}^2),$$

where ${\sigma }_i$ is the RMS surface roughness of each mirror. A requirement for the total scattering loss is less than 10% at $0.9\mu \mathrm{m}$, which is translated to a mean RMS surface roughness of less than 10 nm per surface when surface roughness tolerances are equally assigned to all mirror surfaces.

The surface roughness of each mirror was measured using a white light interferometer (Zygo NewView 7200). Figure 7 shows examples of the measured surface images. For the PO0 and the PO2, five and six points in each mirror surface were measured, respectively, with a measurement area of $140\times 105\mu {\mathrm{m}}^2$. For the three mirror arrays, we used a different objective lens with a long working distance not to interfere with their complex structures, which results in lower spatial sampling but a larger measurement area of $702\times 526\mu {\mathrm{m}}^2$. Three points in each mirror were measured for the slice-mirror array and the slit-mirror array, whereas five points were measured for the pupil-mirror array. Although all mirrors were measured for the pupil-mirror array and the slit-mirror array, only 10 mirrors from CH-1 to CH+9 were measured for the slice-mirror array due to the interference between the work piece and the objective lens. For the pupil mirrors and the slit mirrors, a low-pass filter was applied to remove patterns with a spatial period larger than $20\mu \mathrm{m}$ (twice the finishing pitch), which were considered to be shape errors rather than surface roughness.

Fig. 7

Measured surface images of (a) PO0, (b) PO2, (c) slice mirror of CH-1, (d) pupil mirror of CH-1, and (e) slit mirror of CH-1.

As shown in Table 3, the mean RMS surface roughness, except for the slice mirrors, is 5 to 8 nm, which meets the requirement of less than 10 nm. The slice mirrors have the larger mean RMS surface roughness of 12.74 nm, which is due to many impurities and unsmooth patterns of the base material of RSA6061. The resulting RMS surface roughness is comparable to previous studies using a fly-cut method.^8,27,28 Combining the mean surface roughness of each mirror in Table 3, the total scattering loss of five mirrors is estimated to be 6.4%, which still meets the requirement of a scattering loss of less than 10%.

Table 3

Measured RMS surface roughness of the mirrors.

	PO0	PO2	Slice mirrors	Pupil mirrors	Slit mirrors
Mean (nm)	8.08	4.90	12.74	5.50	7.36
Min (nm)	6.62	4.22	8.09	3.83	4.42
Max (nm)	9.43	5.49	18.84	9.53	12.80

4.2.2.

Shape error

The shape error is an important parameter related to image quality. In particular, the shape error of the pupil mirrors largely affects the image quality because pupil images are formed on the pupil mirrors. Therefore, we simulated degradation of the image quality caused by the shape error of the pupil mirrors using Zemax. In this simulation, we required the degradation of the RMS spot radius at the position of the slit mirror to be less than 10%, and we found that the peak-to-valley (P-V) shape error of each mirror should be less than 300 nm.

The shape errors of the fabricated mirrors were measured with a laser interferometer (Zygo Verifire QPZ). Figure 8 shows examples of the measured shape error profiles. The measurement of the slice mirrors was performed only for six channels, CH±1, $\pm 6$, and $\pm 13$, due to time constraints. Also, only the spherical surfaces of the pupil mirrors were measured because an aspherical surface is difficult to be measured with an ordinary laser interferometer. Note that P-V values of the pupil mirrors were measured within an effective area of 4.5 mm in diameter.

Fig. 8

Measured shape error profile of (a) PO0, (b) PO2, (c) slice mirror of CH-1, (d) pupil mirror of CH-1, and (e) slit mirror of CH-1.

All mirrors except the PO2 meet the requirement for the P-V shape error of less than 300 nm, as summarized in Table 4. The large shape error of the PO2 comes from its thickness of $\sim 3\mathrm{mm}$ due to the limited total size of the IFU; we fixed the work piece to a jig during the process, and slight deformation of the shape may have occurred when it was removed from the jig after finishing. We performed ray-tracing analysis to evaluate the degradation of the image quality caused by the shape error of the PO2, in which the measured shape error of the PO2 was simulated as a vertical astigmatism, and we confirmed that only a few % degradation of the RMS spot radius at the final image plane of the IFU was caused, which is acceptable.

Table 4

Measured P-V shape error of mirrors.

	PO0	PO2	Slice mirrors	Pupil mirrors	Slit mirrors
Mean (nm)	240	507	138	211	169
Min (nm)	—	—	86	155	117
Max (nm)	—	—	228	297	277

4.3.

Assembly

The optical elements of the IFU should be assembled within the tolerances listed in Table 5. They were derived from a tolerance analysis using Zemax, in which the sensitivity of each degree of freedom to throughput, the image quality at the slice-mirror array and at the detector, and the image displacement at the detector were evaluated first. Then, a Monte Carlo simulation with 1000 trials was performed to evaluate the effect of combined errors. Within the tolerances in Table 5, 99.7% of the trials show a throughput degradation of less than 10%; a variations of the RMS spot radius at the slice-mirror array and at the detector of less than 20% and 10%, respectively; and an image misalignment at the detector of a few pixels.

Table 5

Installation tolerances for the optical elements of the IFU. Coordinates are illustrated in Fig. 2, except for the PO1, which has tilted coordinates with the Y axis aligned with the optical axis at the PO1.

The ultra-precision cutting was also used in the assembly process to realize simple and precise assembly. First, mounting surfaces of the base plate were machined by ultra-precision cutting to control their height with a few $\mu \mathrm{m}$ accuracy [Fig. 9(a)]. Fixing flanges of the pupil-mirror array and the slit-mirror array were also machined in the same operation as the mirrors [Fig. 9(b)], so relative positions and orientations between the mirrors and the base plate are guaranteed. For unmachined fixing surfaces of the pupil-mirror array and the slit-mirror array and for the optical elements with fixing flanges that cannot be machined, shims with a thickness that was controlled by ultra-precision cutting were used for accurate positioning. Positioning pins were used to fix the optical elements in the plane of the machined surfaces or shims with an accuracy of 10 to $20\mu \mathrm{m}$. A lens holder of the PO1 was fixed to fixing flanges next to the PO0 mirror.

Fig. 9

Ultra-precision cutting for assembly. Solid boxes show machined surfaces. (a) Mounting surfaces of the base plate. Blue boxes show mounting surfaces for the pupil-mirror array and red boxes for the slit-mirror array. Black boxes show machined surfaces for unmachined surfaces of the pupil-mirror array and the slit-mirror array shown in dashed black boxes in panel (b). (b) Fixing flanges of the pupil-mirror array (top) and the slit-mirror array (bottom).

As a result, the assembly of the IFU was completed with only one adjustment of the PO1 lens holder. The completed IFU is shown in Fig. 10. The assembly of the SWIMS-IFU using ultra-precision cutting is simple and does not require a precise adjustment. The concept is applicable to not only IFUs but also a variety of optical units with complex designs.

Fig. 10

Completed IFU: (a) without a cover and (b) with a cover.

5.1.

Vignetting

There are two vignetting points for the SWIMS-IFU: one is vignetting of the pupil images at the edges of the FoV by an aperture of the PO1 and the other is vignetting of the FoV by a lens holder of the PO1 (refer to Fig. 5).

To measure the vignetting of the pupil images by the aperture of the PO1, we set a light source in the visible wavelength with an input F-ratio of 12.2 at the input side of the IFU to simulate incident light of the Subaru telescope, and we took an illumination pattern at the pupil-mirror array from the output side by a CMOS camera, as shown in Fig. 11(a). Figure 11(b) shows the observed illumination patterns for various channels with different positions in it. The vignetting of the pupil images at the edges of the FoV is larger than at the center, resulting in a lower throughput there. This is caused by an insufficient effective clear aperture diameter of the PO1 lens due to space limitations.

Fig. 11

(a) Configuration to measure vignetting of the pupil images by the aperture of the PO1. (b) Observed pupil images at the center and both edges of CH+13, $+6$, $+1$, $-1$, $-6$, and $-13$. The pupil image of the central part of CH+13 is missing due to the vignetting of the FoV.

To measure the vignetting of the FoV by the lens holder of the PO1, we set a diffused visible light source at the output side of the IFU, shown in Fig. 12(a), and took an illumination pattern at the slice-mirror array from the input side by a CMOS camera. Figure 12(b) shows the observed illumination pattern at the slice-mirror array. It can be seen that the centers of CH+13 and CH+12 are vignetted, which is caused by obstruction of the light between the slice-mirror array and the PO2 by a lens holder of the PO1 lens.

Fig. 12

(a) Configuration to measure vignetting of the FoV by the lens holder of the PO1. (b) Observed image of the slice-mirror array. There is vignetting at the top of the image, affecting the central part of CH+13 and CH+12.

5.2.

Image Quality

To measure the image quality, we again set the light source with F/12.2 and took the pseudo-slit images of all 26 channels by the CMOS camera from the output side of the IFU. The setup for this test was the same as Fig. 11(a), except that the CMOS camera focused on the pseudo-slit image instead of the pupil image.

To evaluate the image quality, cross sections of each pseudo-slit image were fitted with a convolution function of a top-hat function with the designed width and a Gaussian function. The analysis was done for the center and both edges of each pseudo-slit image.

Figure 13 shows double the standard deviations of the Gaussian functions, which we interpret as RMS spot diameters, for all channels, as well as the RMS spot diameters from a ray-tracing simulation at the position of the pseudo-slit image. We should note that these values include degradation caused by various elements. The ray-tracing simulation simulates a spot for a point source at infinity degraded by the telescope and all optical elements of the IFU. On the other hand, the laboratory measurements consider only the degradation by the pupil mirrors. Thus, some measurements at the edge of the outer channels show an RMS spot diameter smaller than the ray-tracing simulation. Still, the overall trend of the measurement is consistent with the ray-tracing simulation, demonstrating the effect of the off-axis ellipsoidal pupil mirrors for the outer channels.

Fig. 13

Measured image quality of the pupil mirrors at the center and both edges of the pseudo-slit image and ray-tracing simulated values in 700 nm. Black circles show the measured value, and white squares show the ray-tracing simulated values. Dashed vertical lines show the boundaries between channels with a spherical pupil mirror and with an off-axis ellipsoidal pupil mirror. Measured values for the center of CH+12 and CH+13 are missing due to vignetting of the FoV, and measured values for edge1 of CH+13 and edge2 of CH-13 are missing due to the configuration of the measurement.

6.1.

Spatial Scales and FoV

The pseudo-slit imaging data of the globular cluster M3 were used for spatial scale measurements. The pseudo-slit images were reconstructed into an image with the FoV of the IFU. Positions of stars in the reconstructed image were compared with those in a $y$-band image of Pan-STARRS²⁹ to measure the slice width and the spatial scale along the slices (Fig. 15). The positions of the stars were measured with the IRAF imexam task, and the slice width and the spatial scales were computed by the IRAF ccmap task.

Table 6 shows the mean and standard deviation of measurements for each filter, as well as the designed values. Both the slice width and the spatial scale for each filter are almost consistent with the designed values. With the measured slice width and the spatial scale, the FoV of the SWIMS-IFU becomes $13.4\times 4.8{\mathrm{arcsec}}^2$.

Table 6

Measured and designed spatial scales of the IFU.

6.2.

Wavelength Scales and Coverage

Wavelength calibration was carried out using the night sky OH emission lines³⁰ in the images of the standard star. Pixel scales in the dispersion direction were measured to be $2.47\AA {\mathrm{pix}}^{-1}$ for the blue arm and $4.67\AA {\mathrm{pix}}^{-1}$ for the red arm. With the measured pixel scales, the observed spectra cover wavelength ranges of 0.85 to $1.48\mu \mathrm{m}$ for the blue arm and 1.42 to $2.56\mu \mathrm{m}$ for the red arm. It should be noted that a gap between the two detectors, as seen in Fig. 14, corresponds to a $\sim 320Å$ width in the range of 9420 to 10070 Å for the blue arm and a $\sim 585Å$ width in 15510 to 16770 Å for the red arm, affecting the observed spectra.

6.3.

Image Quality

We evaluated image quality using the size of the image of the standard star HIP51684 at the slice-mirror array and at the SWIMS detector. Due to time constraints of the engineering observations, the image quality measurements were made for some of the 12 available channels. However, the laboratory performance test in Sec. 5.2 shows the expected results for all channels, and it is expected that the other channels not evaluated in the engineering observations will show similar results.

The image quality at the slice-mirror array was evaluated by reconstructing a stellar profile at the slice-mirror array from signal counts of the spectra of the standard star as follows. For the obtained spectra, several wavelengths that were not affected by atmospheric absorption or night sky emissions were selected, and signal counts of the standard star at the wavelengths were extracted from the 5–6 channels. The extracted signal counts were arranged in the order of the slice mirrors to yield a stellar profile in the direction perpendicular to the slice mirrors. The stellar profile was fitted with a Gaussian function, and an FWHM of the Gaussian function was used to evaluate the size of the standard star image using the slice width of 0.4 arcsec. The evaluations were performed using data in which the standard star was introduced at the nine positions of the FoV of the IFU.

Figure 16 shows the FWHMs at the slice-mirror array as a function of wavelength. The resulting FWHM was 0.49 to 0.60 arcsec, which is consistent with an expected value of 0.50 to 0.53 arcsec calculated as a root sum square of the image quality from a ray-tracing simulation of 0.205 arcsec and the measured seeing sizes of 0.49 arcsec in the $Y$-band and 0.45 arcsec in the $H$-band.

Fig. 16

FWHM of the standard star at the slice mirror as a function of wavelength at the nine positions in the IFU FoV.

The image quality at the detectors was evaluated by fitting the spatial extent of the standard star spectra observed in $\mathrm{CH}\pm 1$ and $\mathrm{CH}\pm 4$ along the slice length direction with a Gaussian function at wavelengths of $1.0\mu \mathrm{m}$ (the central wavelength of the $Y$-band), $1.2\mu \mathrm{m}$ ($J$-band), $1.6\mu \mathrm{m}$ ($H$-band), and 2.1 m ($K_S$-band). The Gaussian fitting was done by the IRAF imexam task, and the pixel scales in Table 6 were used. Table 7 shows the measured FWHMs. They range 0.46 to 0.65 arcsec depending on the wavelength, which agrees with an expected value of 0.53 to 0.56 arcsec calculated as a root sum square of the image quality from a ray-tracing simulation of 0.268 arcsec and the measured seeing sizes of 0.49 arcsec in the $Y$-band and 0.45 arcsec in the $H$-band. There is no significant difference between the channels. It should be noted that a gap in the FWHM between the blue ($Y$- and $J$-band) and the red ($H$- and $K_S$-band) arms is caused by SWIMS optics and is not due to the IFU.

Table 7

FWHM of the standard star in the spatial direction at the detectors.

6.4.

Throughput

The throughput of the SWIMS-IFU was evaluated as a ratio of counts of dome flat data with the IFU to that without the IFU. The dome flat data were obtained in the $Y$-, $J$-, $H$-, and $K_S$-band for the imaging mode and in the spectroscopy mode. For the spectroscopy mode, dome flat data without the IFU were obtained using a slit mask with slits in the same positions and widths as those of the IFU pseudo-slit images.

Figure 17 shows the results, which range from 50% to 75% depending on the wavelength and channel. The results are almost consistent between the imaging and the spectroscopy modes. In Fig. 17, we also show throughput models including scattering losses by the measured surface roughness of manufactured mirrors using Eq. (1), reflectivity of uncoated aluminum mirrors,³¹ internal transmittance and reflectivity of anti-reflection coating of the PO1, and losses by vignetting for $\mathrm{CH}\pm 1$ and $\mathrm{CH}\pm 6$. The throughput models for the other channels should be between the models of $\mathrm{CH}\pm 1$ and $\mathrm{CH}\pm 6$ because the loss by vignetting is between those of $\mathrm{CH}\pm 1$ and $\mathrm{CH}\pm 6$. We can see that the measured values are consistent with the throughput models. The throughput is dominated by the reflectivity of uncoated aluminum mirrors in shorter wavelengths and the reflectivity of the anti-reflection coating of the PO1 in longer wavelengths. The vignetting described in Sec. 5.1 also affects the throughput over the entire wavelength range.

Fig. 17

Throughput of the SWIMS-IFU. Open circles show the average throughput of twelve channels by $Y$-, $J$-, $H$-, and $K_S$-band imaging. Error bars show the wavelength range of each filter and the range from minimum to maximum values for the 12 channels. Colored lines show the throughput from the spectroscopy data. Black lines show the throughput models calculated from the scattering loss by the measured mirror surface roughness, aluminum mirror reflectivity, internal transmittance and reflectivity of anti-reflection coating of the PO1 lens, and losses by vignetting for CH±1 and CH±6.

6.5.

Stray Light

During the first engineering observation, we observed stray light affecting wavelengths longer than $2.1\mu \mathrm{m}$ [Fig. 18(a)]. Ray-tracing analysis revealed that the stray light was caused by thermal emission from a baffle around the Cassegrain hole of the Subaru telescope, which enters the IFU at different incident angles and paths from the designed light, is reflected by a slice mirror, and then enters a pupil mirror in an adjacent channel. Based on the results of the ray-tracing analysis, we narrowed the entrance aperture of the IFU in the slice length direction from 14.5 to 8.5 mm to suppress the stray light, as shown in Fig. 18(b).

Fig. 18

Stray light observed in the pseudo-slit image of CH-1 in the $K_S$-band, (a) before modification of the entrance aperture of the SWIMS-IFU and (b) after modification. White arrows indicate the position of the stray light.

Although we could reduce the thermal stray light, it persists in the current optical design of the SWIMS-IFU, and modification in the optical design is necessary to eliminate the stray light in the future upgrade for the TAO 6.5 m telescope. One solution is to narrow the FoV and thus the shape of the entrance aperture of the IFU to prevent the stray light from entering the IFU. Another is to change the arrangement of the pupil mirrors, especially in the vertical direction, to prevent the stray light from entering the adjacent channel, such as SPIFFI of VLT/SINFONI.³²