VINROUGE : a very compact 2-5 μm high-resolution spectrographwith Germanium immersion grating
Takayuki. Arasakia, Naoto. Kobayahsia,b, Yuji. Ikedaa,c, Sohei. Kondoa, Yuki. Sarugakud, Sayumi.
Kajie, and Hideyo. Kawakitaa,eaLaboratory of Infrared High-resolution spectroscopy (LiH), Koyama Astronomical Observatory,
Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-855, Japan; bInstitute of
Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan; cPhotocoding,460-102 Iwakura-machi, Sakyo-ku, Kyoto 606-0025, Japan; d Kiso Observatory, Institute of
Astronomy, University of Tokyo, 10762-30 Mitaka, Kiso-machi, Kiso-gun, Nagano 397-0101,
Japan; e Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku 603-8555,
Kyoto, Japan
ABSTRACTThe infrared high-resolution and highly-sensitive spectroscopy can provide new and deep insights in many fields ofastronomy. The 2.0-5.5 μm region is a very unique and important wavelength region for astrochemistry and astrobiology,
because the vibrational transitions of C-H, N-H, O-H, C-O, and C-N bonds in many molecules, which are of astrophysicalinterest, concentrate in this wavelength range. To advance the study in this wavelength range, we are developing a newnear-infrared spectrograph: VINROUGE (= Very-compact INfrared high-ResOlUtion Ge-immersion Echelle
spectrograph). The instrumental concepts of VINROUGE are “high-resolution”, “highly-sensitive”, and “very-compact
instrumentation”. With (i) Germanium immersion grating, (ii) white pupil spectrograph design, (iii) reflective optics using
the integrated off-axis mirrors and the optical bench by ceramic (cordierite CO-220), and (iv) highly-sensitive array(HAWAII-2RG 5.3μm cutoff array), we could obtain a solution of optical design with a spectral resolution of 80,000, total
throughput of > 0.28, and a compact volume that is smaller than 600 mm×600 mm×600 mm even for 10-m classtelescope. We have already completed the development of Germanium immersion grating. In this year, we plan to fabricate
a set of integrated off-axis ceramic mirrors together with the ceramic optical bench to demonstrate that the reflective optics
was an athermal performance. The first light of VINROUGE is expected in 2019.
Keywords: Infrared, High-resolution, High-sensitivity, Very compact instrumentation, Immersion grating, Spectrograph

1. INTRODUCTION
Infrared wavelength region from 2-20 μm is so unique that there are rovibrational transitions of various kinds of molecules(including the symmetric molecules without permanent electric moment, which cannot be observed in the radio domain).
Therefore, it is considered as the key wavelength region for astronomy, astrochemistry, and astrobiology. Especially, thevibrational transitions (e.g., for C-H, N-H, O-H, C-O and C-N stretching vibration modes) for the astrophysically interested
molecules concentrate on the near-infrared wavelength region from 2.0-5.5 μm (Figure 1). High-resolution spectroscopy
exhibits a great power for observing minor molecules (e.g., more complicated molecules, deuterated molecules or minor
isotopologures including such as 13CO and C15N, and the molecules including minor but important elements such P) in
this spectral region, because the “high-resolution” can allow us to distinguish the weak lines from the strong ambient
background radiation and other atomic or molecular lines. The high-resolution can also realize highly-sensitive for thelines of which intrinsic widths are narrower compared to the spectra resolution, such as the interstellar absorption lines.
For example, although H3+ and CH3+ are important pivotal ions in the chemical network for astrophysical plasmas (Herbst& Klemperper 19731), CH3+ (which is a starting point of the chemistry for complex organic molecules including carbons)is not yet detected in 3.2 μm region due to their weak transition probabilities while H3+ is well observed in ~ 3.6 μm region
under various environments in the universe (e.g., Geballe et al. 19962, McCall et al. 20023, Oka 20134). Thus, highresolution and highly-sensitive spectroscopy of the molecules with large telescopes will provide the deep insights for
astronomy. Using large telescopes are essential for observing weak lines because not only it is necessary to collect a large
number of photons but also the thermal background noise is extremely reduced by the improvement of the spatial resolution.

Updated 3/20/14

Figure 1. Model spectra of various molecular absorption lines in the wavelength range of 2.0-5.5 μm.

However, there are various technical issues on the development of infrared high-resolution spectrographs attached tolarge telescope. Among them, the instrument size is the biggest one.
The maximum spectral resolution of a diffraction grating for the slit-limited case is given byR=

2nφcol tanθB
sDtel

(1)

where n is the refractive index of which the grooves are immersed (n = 1 for the classical grating), φcol is the collimatedbeam diameter, θB is the blaze angle, s is the slit width, and Dtel is the entrance pupil diameter of the telescope. It is foundfrom Eq. (1) that the collimated beam diameter, that is the size of optics, is proportional to the spectral resolution and theentrance pupil diameter of the telescope equipped with it. Therefore the high-resolution spectrograph in the coming 30-m
telescope ages requires the much larger optics and vacuum chamber enveloping them than those currently attached to 10m class telescopes. To realize these extremely large instruments, some technological innovations are strongly required on
fabrication of the extremely large optics and cryostat, resulting in that it takes a great cost for time and money. Under thesecircumstances, we start the project of a new near-infrared high-resolution spectrograph: VINROUGE. VINROUGE uses
a Germanium immersion grating with which the instrument can be dramatically reduced in size, similar to or smaller than
the spectrographs using classical echelle gratings (see Eq. (1)).
VINROUGE is a PI-type infrared high-resolution spectrograph which is developed by LiH (= Laboratory of Infrared
High resolution spectroscopy) funded by the Koyama Astronomical Observatory of Kyoto Sangyo University, Japan. We
describe the design concepts of VINROUGE in section 2, and we introduce a preliminary optical design for realizing theconcepts in section 3 and the fabrication and alignment strategy on the designed reflective optics in section 4.

2. CONCEPTUAL DESIGN
Table 1 summarizes the specifications of VINROUGE. The instrumental concepts of VINROUGE are to aim “highresolution (Rmax = 80,000)”, “very-compact instrumentation” attachable to the Cassegrain focus, and “highly-sensitive (thetotal throughput > 0.25)”. To achieve these concepts simultaneously, we decided to employ (i) Germanium immersion
grating, (ii) white pupil spectrograph, (iii) reflective optics with the integrated off-axis mirrors and the optical bench byceramic, and (iv) highly-sensitive array (HAWAII-2RG 5.3μm cutoff).
The high-resolution and the very-compact instrumentation are realized with a Germanium immersion grating and thewhite pupil spectrograph design. Immersion grating is a reflection type grating of which the diffraction surface is immersedin the material with a high refractive index (n > 2). It provides an n times higher spectral resolution compared to a classicalreflective grating if using the same collimated beam in diameter, or reduces the size of optics to 1/n if the spectral resolution
is maintained (see Eq. (1)). Since Germanium has the highest refractive index (n = 4) among available infrared optics samematerials for immersion grating, we can obtain an extremely compact high-resolution spectrograph as never before in thiswavelength region. The white pupil spectrograph also contributes to compactification of the size of optics because itreduces the collimated beam diameter on the gratings and camera unit by producing the second pupil image where is aconjugate position to the surface of the first disperser.
Highly-sensitive is achieved with a Germanium immersion grating, a highly-sensitive infrared array, and the reflective
optics. The absorption in the material is very important for immersion grating, because even small absorption coefficientα [cm-1] which is negligible for general applications such as lenses should influence the absolute diffraction efficiency due
to the long traveling path in the immersion grating (α < 0.01 cm-1 is needed for keeping the absorption loss in < 0.1 for a
Germanium immersion grating with R = 100,000 at 8 μm). The single crystal Germanium shows ideally low absorption
coefficients (α < 0.001 cm-1) between 2 and 10 μm (see Kaji et al. 20145 and 20166). With the same ingot, we havesucceeded to fabricate the large Germanium immersion grating with the high diffraction efficiency (> 0.7) even under the
cryogenic condition (see section 3.2). As for the infrared array, we use the HAWAII-2RG 5.3 μm cutoff array for
VINROUGE, which is the most sensitive array in this wavelength region at present. It has the high QE of > 0.8 and thelow readout noise of < 16 e- (from the catalog of Teledyne Inc.). Finally, the reflective optics provides the continuously
high throughput optics in the wide spectral range of the infrared region by using Au or the protected Ag coating as thereflective materials. Recently, the higher (spatial or spectral) resolution instruments has been increasing using the large
format arrays like a 2K×2K array and/or Adaptive Optics (AO) technology. Consequently, the refractive optics tends tobe preferred for these instruments because it is easier to correct the aberrations (although the chromatic aberrations isgenerated), and to align lenses high-precisely by using the axial symmetric lenses and barrels. However, since the white
pupil spectrograph, which is also often used for the modern high-resolution spectrographs, needs a large number of lenses,
the total throughput is considerably degraded due to the reflection loss on the lens surfaces. For instance, if we assume anspectrograph consisting of 15 lenses with the Broad-Band AR coating (BBAR) of R < 3% from 2 to 5.5 um (= 3 lenses fora collimator, 3 lenses for a re-imager, 4 lenses for a relay, and 5 lenses for camera), the throughput of lens systems is
estimated to be 0.40 (= 0.9715×2) in total. On the other hand, the reflective optics can be expected to be the high throughput
of > 0.77 for 13 reflective surfaces with the reflectance of > 0.98 per one surface (= 3 surfaces for a collimator, 3 surfaces
for a re-imager, 3 surfaces for a relay, and 4 surfaces for a camera) with a very slow wavelength dependence. The
difficulties of the aberration correction and alignment for the reflective optics can be overcome with the integrated offaxis mirrors and the optical bench made from the same material to the mirrors, which could provide an athermal optics.
This technology had been often used with aluminum for infrared instrument (e.g., CRIRES+ (Follert et al. 20147), iSHELL(Rayner et al. 20128)), but the aluminum mirrors are not applicable for the high-resolution instruments because they areeasy to be deformed by the inner remaining stress when they are cooled down, and linear scars produced by the machining
process degrade the image qualities. Instead, VINROUGE employs a low CTE ceramic, cordierite CO-220, for thematerials of mirrors. The CO-220, which is provided by Kyocera Corporation, Japan, shows an extremely low CTE (< 2.0
×10-8@23℃)similar as Zeroduar and ULE. It is not only easy to be polished and but also easy to be proceeded to any
forms as obtained with metals. This means that the highly accurate integrated off-axis mirror system could be assembled
with this material, as they had been tried with aluminum. See details of this athermal reflective optics in section 4.
Table 1. Specifications of VINROUGE.
Items
Wavelength coverage
Maximum spectral resolution
Total throughput
Instrumental volume
Slit width×length
Pixel scale
Array

Specifications
2.1–5.3 μm
80,000> 0.25600 mm×600 mm×600 mm
0.13 mm×3.64 mm*1
(0.18”×5.0”)*2
0.07 [arcsec/pixel]*2
HAWAII-2RG 5.3 μm cutoff*1More wider and longer slits will be available as options.
*2 For 10-m with f/15 telescope.

3. OPTICAL DESIGNThe optical layout of VINROUGE is shown in Figure 2. The main optical train of VINROUGE consists of two individual
units: the fore optics and the spectrograph. The fore optics play a role as suppressing of the ambient thermal backgroundradiation by the cold stop located on the pupil image produced in the optics. The focal plane images of telescope arerefocused on the slit of spectrograph by the fore optics. The spectrograph produces the final echellogram spectrum of Rmax
= 80,000. Table 2 summarizes the optical parameters of optical elements used in VINROUGE.

Figure 2. Optical layout of VINROUGE.

3.1 Fore optics
The optical layout of the fore optics is shown in Figure 3. We employ the Offner type optics as the fore optics. It is a reimaging optics consisting two mirror (the primary and secondary mirrors), producing the image with the lateralmagnification of one. The curvatures of two spherical mirrors are 314.7 mm for the primary and 156.7 mm for thesecondary. The f/15 beam from the telescope is reflected by the primary, which produces the pupil of the entrance apertureof the telescope. The secondary mirror, which plays a roll on the cold stop, has a hole at the center and is cut with the sharpedge at the circumference. The former excludes the radiation reflected by the center cone on the secondary mirror, and thelatter also prevents the thermal sky background radiation which directly goes to the optics through the side of the telescope
secondary mirror. The reflected lights from the Offner secondary mirror is re-focused by the primary mirror. The field ofview of the fore optics is 60”×60” (for the 10-m telescope with f/15). The classical Offner optics, in which the centers ofradius of the primary mirror perfectly corresponds to that of the secondary mirror, produces a little astigmatism. In order
to correct this aberration, we slightly shift the positon of the Offner secondary mirror by 0.35 mm away from the Offner
primary mirror.

Table 2. Optical parameters of optical elements in VINROUGE.
Optical unit

Component

Fore optics

Offner optics

Spectrograph

Entrance slit
Collimator and relay optics

Main disperser(immersion grating)

Cross disperser I
Cross disperser II
Cross disperser III
Cross disperser IV
Cross disperser V
CameraDetector

Item

Specification

Material

Cordierite CO-220

Curvatures of primary mirror

314.7 mm

Curvatures of secondary mirror

156.7 mm

Cold stop diameter

φ12 mmMagnification factor

1.0

Physical size

0.13 mm × 3.64 mm

Type

TMA

Material

Cordierite CO-220

Effective focal length348.1 mmMaterial

Single crystal Ge

Entrance/exit aperture

φ28 mmGrating area

31 mm×104 mm

Groove pitch

57.1 μm

Blaze angle

75°

Apex angle

86°

Reflective coating

Protected Ag

Groove density

250 lines/mm

Blaze angle

37.8°

Groove density

625 lines/mm

Blaze angle

50.7°

Groove density

476 lines/mm

Blaze angle

46.3°

Groove density

400 lines/mm

Blaze angle

43.8°

Groove density

323 lines/mm

Blaze angle

40.6°

Configuration

FMA

Material

Cordierite CO-220

Effective focal length

119.7 mm

Format2048×2048 pixels

Pixel size

18 μm

Remarks
CTE < 2.0×10-8

On the secondary
mirrorOffner type optics

CTE < 2.0×10-8

Ks and M -mode
Kl -mode
Ls -mode
Lm -mode
Ll -mode

CTE < 2.0×10-8
HAWAII-2RG 5.3 μm
cutoff (Teledyne Inc.)

QE

> 0.8

From a catalog byTeledyne Inc.,
measured at 2.0, 3.5,
and 4.4 μm

Dark current< 0.05 e-/s

From a catalog byTeledyne Inc.

Readout noise< 15.5 e-

Cut-off Wavelength

5.3 μm3.2 Spectrograph
The optical layout of the spectrograph is shown in Figure 4. The spectrograph is the full reflective optics consisting of
the combination of the three-mirror anastigmat (TMA) and the four-mirror anastigmat (FMA). At the re-focal plane of thefore optics, slit mirrors are located. The size of a slit is 0.13 mm in the width and 3.64 mm in the length (0.18”×5.0” for10-m telescope with f/15). The much wider or longer slits will be prepared for various purpose of observations. The stellar
light passing through the slit is collimated by the TMA collimator unit with the effective focal length of 348.1mm. The
collimated beam size is φ23.2 mm. The collimated light is dispersed by the Germanium immersion grating with the blaze
angle of 75 degrees, the groove pitch of 57.1 μm, and the apex angle of 86 degrees. The entrance/exit clear aperture is
φ28 mm and the grating surface is 31 mm×104 mm, (see Table 2 and section 3.3). The dispersed spectra by theGermanium immersion grating is re-focused near the slit, where the flat mirror is located and reflects back to the TMA.

Figure 3. Optical layout of the fore optics of VINROUGE.

Figure 4. Optical layout of the spectrograph of VINROUGE.

The TMA produces the second pupil image spatially away from the immersion grating to work as the relay optics. The
cross-dispersers are located on the second pupil position (that is the white pupil). VINROUGE employs five kinds ofreflective gratings as the cross-dispersers. Each grating covers the half of K-band (Ks-band and Kl-band) or one third of Lband (Ls-band, Lm-band, and Ll-band), or M-bands.

Figure 5. Simulated echellograms from K- to M-bands. VINROUGE provides six wavelength regions by changing
five reflective grating as the cross-dispersers. Ks-mode and M-mode are obtained by one cross-disperser with thedifferent diffraction order. The other wavelength regions are obtained by the individual cross-dispersers (see Table 2).

One grating among five cross-dispersers is used for both Ks-band and M-modes with the different diffraction orders m = 3
and 1, respectively. The other four gratings are used for Kl-, Ls-, Lm, and Ll-modes with m = 1 (see Table 2). The diffracted
spectra by the cross-dispersers are separated into different orders, which are focused on the infrared array by the camera
optics. The camera optics is the FMA with the effective focal length of 119.7 mm. The final echellograms are shown in
Figure 5. The spectral resolution is achieved to be R = 80,000 with the sampling of 2.5 pixels. The spot diagrams of Lmmode are also shown in Figure 6.

Figure 6. Spot diagrams at various positions on the array for Lm-mode. The alphabets on the left figure show theevaluated positions of the spot diagrams. The spot diagram diameters (RMS) are measured in μm and given in
the right table together with the corresponding wavelengths and the diffraction orders. The sizes of the boxes
surrounding the spots are equivalent with 2.5 pixels×2.5 pixels of the infrared array.

3.3 Germanium immersion grating
Figure 7 shows the Germanium immersion grating for VINROUGE. The designed optical parameters of Germanium
immersion grating are described Table 2. The sharp apex angles of 85 degrees maximizes the diffraction efficiency by
reducing the shadow effect caused by the counter groove surface. It is optimized using the RWCA method with anassumption of Ag coating on the groove surface. We designed the tilted wedge with 5 degrees on the entrance surface toprevent the reflected light on the entrance surface from directly going to the array. We prepare the steps with the 2 mm
width at both sides of grating surface, which are used for holing the grating without any thermal stresses on the groove
surfaces under the cryogenic condition.
We have already completed the Germanium immersion grating, which was fabricated by Canon Inc. We adapt the BBAR(< 3% at 2.0-5.5 μm) on the entrance/exit surface and the protected Ag coating on the grating surface. The carefulevaluations confirm that the completed Germanium immersion grating achieves a high absolute diffractive efficiency of
0.75 at 4.52 μm in both room and cryogenic temperatures (Sarugaku et al. 20169).

Figure 7 3D model (right) and the fabricated Germanium immersion grating (left).

3.4 Total throughput
Table 3 summarizes the estimated total throughput of VINROUGE. We also show the efficiencies of individual
optical elements used for this estimation. The reflectance of Au mirrors are assumed as 0.98 per one surface. The
diffraction efficiency of the immersion grating is a value measured in the cryogenic condition by Sarugaku et al.
(2016)9. This table does not include any slit losses because they are dramatically changeable for the telescopes,
the performance of the AO system, and the weather or site conditions. The final total throughput is estimated as≥ 0.28 which fulfills the requirements in Table 1.

Table 3. Estimated total throughput of VINROUGE.
Optical component

Transmittanceor efficiency

Fore optics

0.904

Collimator

0.868

Relay optics

0.922

3 spherical mirrors and 2 pick-off mirrors
3 aspheric mirrors (including reciprocating path) and1 pick-off mirror
3 aspheric mirrors and 1 pick-off mirror

Immersion grating≥ 0.75

As measured under the cryogenic condition

Cross disperser≥ 0.70Remarks

Camera0.922

4 aspheric mirrors

Detector≥ 0.80From the catalog of Teledyne Inc. at 3.5μm

Total throughput≥ 0.28Not including any slit losses

4. ATHERMAL REFLECTIVE OPTICS USING CORDIERITE
In general, it is a critical issue for infrared instruments including high resolution spectrographs to realize the highly
accurate alignment of the optics under the cryogenic condition. The optical elements such as lenses and glass mirrors often
are held by the metallic holders. However, since there is a large difference of the CTEs between the optical elements andthe holder, the relative positional relations among the optical elements are changed under the cryogenic condition, and theresultant optical performances are degraded. To compensate this degradation, the optical elements are previously alignedwith the displacements, which are predicted by the optical and the mechanical-thermal analyses, in the room temperature.
Since this often used method requires the interactive alignments accompanied with frequent thermal cycles until the optical
performances are satisfied, it could provide the much time and the cost for development. Additionally, because themechanisms which are applied on the holder for releasing the thermal stress onto the optical elements under the cryogenic
condition often have irreversible properties, the alignment becomes more difficult.
In order to solve such problems, athermal reflective optics had been classically employed for the infrared instrument.
This optical system uses only a single material like aluminum alloys for all optical and opt-mechanical parts like themirrors, the mirror holders, and the optical bench. Because of the self-similar expanding or shrinking of the optics for any
temperature variations, it shows no temperature dependence on the aberration in principles. However, this athermal optics
with aluminum alloys has disadvantages that aluminum mirrors produce many ghost or flare images diffracted by theprocessing traces, and they often never shrinks with the self-similarity as expected because of the remaining inner stresses.
Under this situations, we propose alternative athermal reflective optics using ceramic. It consists of the integrated offaxis mirrors and the optical bench, made of only ceramic, cordierite CO-220 produced by Kyocera Co. This material iseasy to be polished like glasses and to be processed to any complicate forms like metals. And, it also has an extremely low
CTE (< 2.0×10-8 @23℃). The easiness of polishing enables the ghost free and high performance mirrors because of noprocessing trace. The free-formability by machining enables to not only the athermal but also the alignment-free reflective
optics even for the cryogenic condition, because we can directly produce holes on the integrated mirror structure for thegauge pins, which are directly stuck through the optical bench made of the same material (see Figure 8). We plan toproduce a testing bench with the off-axis cordierite mirrors and the optical bench, and demonstrate the optical
performances, the athermlity, the effect of hysteresis, and the durability of the metal coating under the cryogenic conditions.

Figure 8. 3D model of the integrate off-axis mirror made by cordierite.

5. SUMMARY
We are developing a new NIR high-resolution, highly-sensitive, and very-compact spectrograph, VINROUGE. By
employing a Germanium immersion grating, the white pupil spectrograph, HAWAII-2RG 5.3 μm cutoff array, and theintegrated reflective optics using a ceramic (cordierite CO-202), we derive a preliminary optical design which achieves
the spectral resolution of R = 80,000, the total throughput of > 0.28, and the instrument size is smaller than 600 mm×600
mm×600 mm even for 10-m class telescope. We have already completed the development of Germanium immersion
grating and confirmed that it shows the highly absolute diffraction efficiency of approximately 0.75 at 4.52 μm in bothroom and cryogenic temperatures. In this year, we plan to fabricate a set of integrated off-axis ceramic mirrors together
with the ceramic optical bench to demonstrate that the reflective optics was an athermal performance. The first light of
VINROUGE is expected in 2019.

ACKNOWLEDGEMENTS
We would like to thank all staff of Koyama Astronomical Observatory of Kyoto-Sangyo University for supports
throughout the production of this work. This study was financially supported by Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of the Japanese government and Kyoto-Sangyo University in Japan (2014-2018).

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