PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 1

JEM-EUSO optics design and its performanceYoshiyuki Takizawa∗, Alessandro Zuccaro Marchi†, Roy Young‡ and Yoshiyuki Takahashi§∗

for the JEM-EUSO collaboration

∗RIKEN, Japan†CNR-INOA Firenze, Italy

‡NASA/Marshall Space Flight Center, USA§University of Alabama in Huntsville, USA

Abstract. JEM-EUSO uses a wide-angle refractivetelescope operating in the near-ultraviolet wavelengthregion to observe the time-and-space-resolved atmo-spheric fluorescence images of extensive air show-ers. The JEM-EUSO Optics Module (OM) has twocurved doublet fresnel lenses and a precision fresnellens for chromatic aberration correction. The OMhas 2 types of designs: the Baseline optics and theAdvanced optics. The former is characterized by 2curved double-sided fresnel lenses and one curvedprecision lens, all made of PMMA-000. The latterhas 2 curved double-sided fresnel lenses, made ofCYTOP, and a flat precision lens, made of PMMA-000. In this paper, we describe these optimized opticsdesigns that maximize the sensitivity of JEM-EUSO.

Keywords: UHECR, fresnel lens optics, Interna-tional space station.

I. INTRODUCTION

The definition of the JEM-EUSO Optics Module(OM) follows and improves what was done during theESA-EUSO Phase-A study[1][2], where the OM con-sisted of two curved double-sided fresnel lenses of Poly-methyl-methacrylate PMMA-000 (Mitsubishi Rayon Co.product). After that, the study for an improved OMhas been continued[3][4], reaching a new baseline forJEM-EUSO as well as the option for a more advanceddesign. Both designs have been verified during theJEM-EUSO Phase-A study. All the details about thebaseline and advanced OM designs will be describedin the following sections. The optics basic requirementparameters and design parameters of both OM are shownin Table I. To take advantage of all the available roomon the Japanese HII Transfer Vehicle (HTV) to transferJEM-EUSO to the ISS, the size of the lenses can beextended in one dimension and a further geometricaldesign has been studied, which will be discussed in theperformance section. Briefly, this design considers lenseswith a maximum extension of 2.65 m in diameter, andminor segments of two parallel edges of the lenses areremoved. A width between the two parallel edges is 1.9m.

II. LENS MATERIALS

The OM has two candidates for lens materials,PMMA-000 and CYTOP. PMMA-000 is a special-grade UV transmittance Poly-methyl methacrylate (Mit-subishi Rayon Corp. product), which was previouslystudied[2]. Another candidate is CYTOP. CYTOP isan amorphous, soluble perfluoropolymer (AGC Corp.product). CYTOP combines the excellent properties ofhighly fluorinated polymers with solubility in selectedperfluorinated solvents to provide outstanding coatingsfor optical, electronic and other applications. CYTOPhas a 95% high transmittance between UV and near-IR. The characteristics of CYTOP and PMMA-000 areshown in Table II.

A. Refractive indexes and transmittance

The refractive indexes of the two materials, PMMA-000 and CYTOP, in the near-UV region are shown inFig. 1a. The refractive index dispersion of CYTOP issmaller than PMMA-000, therefore, CYTOP reduces thecolor aberration effect as compared with PMMA-000.The transmittance curves of 15 mm thickness of CYTOPand PMMA-000 are shown in Fig. 1b.

B. Temperature dependence of refractive index.

JEM-EUSO, attached to the ISS, orbits the Earth in∼90 minutes. Therefore, each lens has a thermal cyclesynchronizing with the orbit. Refractive index is shiftedby temperature changes, which cause defocusing effect.Thermal analyses predicted that each lens shifts ±10◦Cfrom the equilibrium temperature. On the other hand,optics analysis by numerical ray-tracing method requiresthat refractive index shift are below 0.0013/10◦C. Themeasured results of temperature shift is 0.0007/10◦C(CYTOP) and 0.0009/10◦C (PMMA-000); we confirmedthat each value is below the requirement of 0.0013/10◦C.

III. OPTICS DESIGN

A. Baseline optics

The Baseline optics uses the PMMA-000 lenses ver-ified and selected in ESA-EUSO Phase-A study[2],and adds one intermediate curved precision fresnel lensbetween the two curved double-sided fresnel lens, tocorrect for chromatic aberration. The Baseline design

2 Y. TAKIZAWA et al. JEM-EUSO OPTICS DESIGN

TABLE IOPTICS REQUIREMENTS AND BASELINE AND ADVANCED DESIGNS.

Requirements Baseline Advancedf/# (F number) < 1.25 1.0 1.0Lens diameter > 2.5 m 2.65 m 2.65 mField of view > 60◦ 60◦ 60◦

Spot size (RMS) < 5 mm 5 mm (2.5 mm) 5 mm (2.5 mm)Throughput 50% @ 0◦ ÷ 10◦

40% @ 10◦ ÷ 20◦30% @ 20◦ ÷ 30◦

59% @ 0◦ ÷ 10◦52% @ 10◦ ÷ 20◦39% @ 20◦ ÷ 30◦

62% @ 0◦ ÷ 10◦58% @ 10◦ ÷ 20◦42% @ 20◦ ÷ 30◦

Filter transmittance > 90% > 90% > 90%

TABLE IICHARACTERISTICS OF CYTOP AND PMMA-000 MATERIALS.

CYTOP PMMA-000Density (25 ◦C) 2.03 g/cm3 1.19 ∼ 1.20 g/cm3

Glass transition temperature 108 ◦C 105 ∼ 120 ◦CWater absorption < 0.01 0.3

Coefficient of linear expansion 7.4×10−5 cm/cm/◦C 8.0×10−5 cm/cm/◦CRupture strength 40 MPa 65 ∼ 73 MPaBreak elongation 150% 3 ∼ 5%

Yield strength 40 MPa (65) MPaTensile strength 1200 MPa 3000 MPa

(a) Refractive indexes of PMMA-000 and CYTOP in thenear-UV region.

(b) Transmittances of PMMA-000 and CYTOP (15 mmthickness).

Fig. 1. Refractive indexes and transmittance.

(a) Baseline design, which has two double-sided fresnellenses and an intermediate curved precision fresnel lens.

(b) Advanced design, which has two double-sided fresnellenses and an intermediate flat precision fresnel lens.

Fig. 2. Cross-section figures of the optical designs.

is described in Table III and Fig. 2a. B. Advanced opticsThe Advanced optics changes both the material and

the geometrical design from the Baseline optics.The Ad-

PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 3

TABLE IIISPECIFICATIONS OF BASELINE DESIGN

1st lens 2nd lens 3rd lens Focal surfaceMaterial PMMA-000 PMMA-000 PMMA-000 See [6]

Lens type Curved double-sided fresnel Curved fresnel (precision lens) Curved double-sided fresnel N/ADiameter [mm] 2650. 2509 2650. See [6]

Radius of curvature [mm] 2415.085 5486.148 4295.865 See [6]Thickness [mm]. 15. 10. 15. N/A

Weight (BEE) [kg] 89. 55. 84 See [6]

vanced design has the two curved double-sided fresnellenses of CYTOP and one intermediate flat precisionfresnel lens of PMMA-000 between the two curveddouble-sided fresnel lens, to correct for chromatic aber-ration. From the point of view of the manufacturing ofa fine grating structure, we choose PMMA-000 materialfor the precision lens. The Advanced design is describedin Table IV and Fig. 2b.

IV. TOLERANCE ANALYSIS

The JEM-EUSO optics does not need diffraction limitresolution like astronomical telescopes. The JEM-EUSOangular resolution requirement is roughly 300 thousandtimes larger than the diffraction limit. The JEM-EUSOoptics tolerances were developed to produce an error ofless than the spot size. We verified tolerance under twospot sizes, 5 mm and 2.5 mm, by using a ray-tracingcode. The result for the 5 mm spot size is similar towhat obtained by the ESA-EUSO Phase-A study[2]. Wedefine tight tolerances based on a spot size of 2.5 mm tosecure the possibility of using small pixel size detectors.Each component of optics has 3 degrees of freedom,namely, the axial displacement, the lateral displacementand the tilt. The tolerance requirements are shown inTable V.

V. LENS MANUFACTURING

We confirmed that a machine which has capabilityto manufacture a PMMA-000 lens can manufacture aCYTOP lens by using 20 cm in diameter test pieces[4].In June 2008, a large precision diamond turning machinewas installed by NASA/MSFC and RIKEN in Japan.The machine is able to manufacture lens up to 3.4 m indiameter lens. This machine manufactures three lenses(>1.5m in diameter) of PMMA-000 from September2008. These lenses are central parts of the full scalelens system (for more information, please see [5]).These lenses will be transported to NASA and theywill undergo to optical test by using NASA’s facility,if funding is available.

VI. OPTICAL FILTER

A. BG3 baseline filters

The JEM-EUSO optics uses band-pass filters (330 ÷400 nm) to cut photons above 400 nm wavelengths. Thefilters are set directly on the window of each photo-multiplier forming the focal surface detector. The Schott

BG3 absorption filter has been selected as baseline filterfor JEM-EUSO.

B. Advanced filters

The atmospheric fluorescence emission of interest forJEM-EUSO is in the three nitrogen lines (337 nm,357 nm, 391 nm). The BG3 baseline filter, however,transmits photons between 250 nm and 500 nm. JEM-EUSO observes nitrogen lines and background photons.Therefore, the signal-to-noise (S/N) ratio of detector isnot optimum due to the background photons. The ad-vanced filter is able to pass through only around the threenitrogen lines. The advanced filter is a multilayer filter,which has 25 pair-layers of Ta2O5/SiO2. We coatedthe multilayer and tested its transmittance performance.The performance of the advanced filter is shown in Fig.3. The advanced filter improves 1.4 times of the S/Nratio.

VII. PERFORMANCE

The encircled energy (EE) performances are shown inFig. 4. The EE of both designs, Baseline and Advanced,are almost same performance. However, the throughputcurves in Fig. 5 show how much higher the performanceof the Advanced design is compared to the Baselinedesign. The difference between the performances de-pends on the surface reflection and material absorbance.Advanced optics has better performance than Baselineoptics because CYTOP has better transmittance thanPMMA-000.

A. Performance of the HTV stowing type optics

We have a telescope design to stow within the con-figuration of HTV (Japanese HII Transfer Vehicle). Thediameter of optics is enlarged to 2.65 m instead of 2.5m in the original design. And minor segments of thethe two parallel edges of the optics are removed. Awidth between the two parallel edges is 1.9 m. The side-cut optics has ∼90% aperture of the original design.Performance of the side cut optics is shown in Fig. 6.This design keeps the performance of the original designto 15◦ field angle. The field of view of the side cutdirection is limited to ∼24◦ field angle without focalsurface detector.

B. Summary

Advanced optics has better performance than Baselineoptics because CYTOP has better transmittance thanPMMA-000. Furthermore, Advanced optics can select

4 Y. TAKIZAWA et al. JEM-EUSO OPTICS DESIGN

TABLE IVSPECIFICATIONS OF ADVANCED DESIGN.

1st lens 2nd lens 3rd lens Focal surfaceMaterial CYTOP PMMA-000 CYTOP See [6]

Lens type Curved double-sided fresnel Flat fresnel (precision lens) Curved double-sided fresnel N/ADiameter [mm] 2650. 2289.1 2650. See [6]

Radius of curvature [mm] 2227.784 N/A 4056.070 See [6]Thickness [mm]. 15. 10. 15. N/A

Weight (BEE) [kg] 150. 47. 144. See [6]

Fig. 3. Transmittance of the advanced filter with increasing incidence angle (0◦, 10◦, 20◦, 30◦ from left to right panels). Three vertical linesshow the three nitrogen lines (337 nm, 357 nm, 391 nm).

TABLE VTOLERANCE REQUIREMENT

Degree of freedom RequirementLateral displacement Less than ± 2.5 mm

Tilt Less than ± 2.5 mradAxial displacement Less than ± 2.5 mm

Fig. 4. Encircled Energy performance of Baseline (blue) andAdvanced (red) optics designs. Solid lines are the performance of 5.0mm spot size. Dash lines are the performance of 2.5 mm spot size.

Fig. 5. Throughput performance of Baseline (blue) and Advanced(red) optics designs. Each curve is normalized to the throughput ofthe Advanced optics value with 0◦ incident angle.

Fig. 6. Performance of the HTV stowing type optics, normalized withrespect to the 2.5 m diameter case (green line). Blue curve: verticalrays with the side cuts direction. Yellow curve: parallel rays with theside cuts direction.

smaller spot size (2.5 mm) than Baseline optics, becauseCYTOP presents a smaller dispersion of refractive indexthan PMMA-000. We will choose the design to beused, when the JEM-EUSO mission shifts to the nextphase, by taking into account productivity and scientificperformance.

REFERENCES

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ings. (2009).[6] Y. Kawasaki et al., [JEM-EUSO Collaboration], these proceed-

ings. (2009).