PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 1

JEM-EUSO lens manufacturingKouki Maekawa∗, Yousuke Hachisu∗, Hitoshi Ohmori∗, Kazutoshi Katahira∗,

Yoshiyuki Takizawa∗, Yoshiyuki Takahashi†∗, Alessandro Zuccaro Marchi‡ and Roy Young§for the JEM-EUSO collaboration

∗RIKEN, Japan†University of Alabama in Huntsville, USA

‡CNR-INOA Firenze, Italy§NASA/Marshall Space Flight Center, USA

Abstract. We have been manufacturing full-scaleJEM-EUSO lenses by using a large ultra-precisionturning machine. This machine is able to manufac-ture up to 3400 mm in diameter. The JEM-EUSOoptics has two double-sided spherical fresnel lensesand a precision fresnel lens. Each lens consists ofa center part and an outer ring part. The centerpart is about 1500 mm in diameter. The optics teammanufactured central lenses of the PMMA opticssystem. We describe in this paper the progress ofthe lens manufacturing.

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

I. INTRODUCTION

The JEM-EUSO[1][2] telescope is composed of twodouble-sided spherical fresnel lenses, a precision fresnellens, and focal surface detectors as shown in Fig. 1. Eachfresnel lens is 2650 mm in diameter and collects near-ultraviolet photons (330 nm ∼ 400 nm) with a ±30◦ fieldof view and an angular resolution of 0.1◦. Table I showsspecifications of fresnel lenses for the manufacturing.The lens 1 and the lens 3 are double-sided sphericalfresnel lenses. The lens 2 is a precision fresnel lens.A fresnel structure and diffractive structure are appliedto both sides of the lens 2. The role of the diffractivesurface is to reduce the color aberrations by usingthe diffractive effect. Since diffractive lenses have theopposite dispersion of refractive lenses, diffractive lensescan cancel the color aberration of refractive lenses. Thelens 1 and the lens 3 will be made of either PMMA-000 (baseline design) or CYTOP (advance design). Thelens 2 will be made of PMMA-000. PMMA-000 is aspecial grade UV transmittance poly-methyl metacrylate.CYTOP is an amorphous, soluble perfluoro polymer,its transmittance is 95% between UV and near IR. Inaddition, the refractive index dispersion of CYTOP issmaller than PMMA-000, therefore, CYTOP reduces thecolor aberration effect as compared with PMMA-000.The optical characteristics of CYTOP are superior toPMMA-000 (for more information, please see [3]).

In 2007, the authors had conducted basic cuttingexperiments by using both CYTOP and PMMA withsmall samples (20 cm in diameter) using a normal sizemachine. As a result, we verified that both CYTOP and

Fig. 1. Schematic view of the JEM-EUSO optics.

PMMA showed the same surface roughness under thesame manufacturing conditions[4]. The next step is toestablish a technique of the manufacturing of a largesize fresnel lens.

II. A LARGE ULTRA-PRECISION TURNING MACHINEAND THE FIRST CUTTING TEST USING A 30 CM FLAT

WORK PIECE

In this study, a large ultra-precision turning machinewas installed by NASA/MSFC and RIKEN. This ma-chine is composed of a rotary table supported with the oilhydrostatic bearing and linear axes X, Z, and rotationalaxis B. The diameter of the table is 3400 mm. Thismachine is able to manufacture fully a 2650 mm fresnellens. The resolution of linear axis of X and Z is 10nm. The authors experimented with manufacturing of aPMMA flat work piece by using this machine. Its sizeis 300 mm in diameter. A fixture supported the workpiece and was put on the center of a rotary table witha bolt fixture. Fig. 2 shows an experimental overviewof cutting process. Manufacturing conditions were asfollow, spindle rotation speed was 50 rpm, feed rate was0.2 mm/min, and depth of cut was 0.005 mm per pass.The cutting fluid mist and a single point diamond tool(R = 0.5 mm) were used. The measurement results ofcutting surface by NewView (Zygo, ×50) is shown inFig. 3. Surface roughness in tool feed direction was 13.8nm (RMS), which satisfies the requirement value of lessthan 20nm (RMS).

2 K. MAEKAWA et al. JEM-EUSO LENS MANUFACTURING

TABLE ISPECIFICATIONS OF FRESNEL LENSES FOR THE FIRST MANUFACTURING.

Lens 1 Lens 2 Lens 3surface front side back side front side back side front side back side

Material PMMA PMMA PMMABase form Spherical (R = 2278 mm) Flat Spherical (R = 4052 mm)

Lens diameter [mm] 2650 mm 2200 mm 2650 mmGroove Height [mm] 1.0 1.0 1.0 700 nm 1.0 1.0Groove Width [mm] 296∼5 80∼1 119∼3 0.5∼0.01 53∼1 179∼2

Fig. 2. A picture of cutting process (PMMA-000 workpiece is 30cm in diameter) by the large ultra-precision turning machine.

Fig. 3. Measurement results of a cutting-surface (Zygo,NewView×50). Surface roughness in tool feed direction was 13.8 nm(RMS), which satisfied the requirement value, less than 20 nm (RMS).

III. MANUFACTURING OF 1500 MM DOUBLE-SIDEDSPHERICAL FRESNEL LENSES

Even though the manufacturing target is 2650 mmdouble-sided spherical fresnel lenses, it is difficult tomanufacture a whole lens at a time because the machinedoes not have enough height to hold a curved lens.Then, we divided the lens into two parts, a central lens(1500 mm in diameter) and an outer annular lens. Thispaper reports the manufacturing of the central lenses, thelens 2 and the lens 3. The lens material is PMMA-000.Parameters of these lenses are shown as Table I.

Fig. 4. A picture of slumped PMMA-000 plate. its diameter is 1500mm.

IV. FABRICATION OF CURVED BASE PLATES FOR THE1500 MM DOUBLE-SIDED SPHERICAL FRESNEL

LENSES

We fabricated spherical curved base plates by usinga hot press method. This fabrication helped reducingthe time to fabricate spherical fresnel lenses. First, wefabricated both convex and concave wooden molds thathad similar curvature radius. The molds and PMMA-000plate set up on a press mold machine. Next, a PMMA-000 plate was heated up in an oven to soften the PMMAand it was pressed on the wooden mold while cooling.Fig.4 is a picture of a slumped PMMA plate with adiameter of 1500 mm.

Fig.5 shows the process of fabricating a fresnel lens.A fixture was set up on the rotary table and the PMMA-000 plate was set with bolt fixtures. We used a singlepoint diamond tool (R = 0.5 mm) to fabricate a fresnelstructure with 2 axes, X and Z, control. After fabricatingof one side, the lens was flipped over and the other sidewas fabricated, the spindle rotation speed was 40 ∼ 50rpm and depth of cut was 0.01 mm per pass. For theinitial process, the feed rate was set as 0.5 ∼ 10 mm/minand we used an air flow for cooling the diamond tooland cleaning chips. For final process, we set feed rate as0.2 mm/min and used the cutting fluid mist for cooling.

V. RESULT OF FABRICATION

Fig 6 shows an appearance of a fresnel lens 3. Itachieved high transparency and the picture demonstratesthat it has a function of a telephoto lens. However,the groove pattern was not clearly fabricated in theouter lens region. The reason was because the amountof the cutting fluid mist was not adequate and the

PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 3

Fig. 5. Overview of the fresnel lens fabrication process. A 1.5 m indiameter slumped base plate was set on the machine.

temperature increased at the cutting point. And otherreason is that the tool could not follow the right trackon the lens because the table rotation speed was faster.The table rotation was unstable by the oil temperaturechange for the hydrostatic bearing of the rotary table.These problems were solved by controlling conditions,such as, the amount of cutting fluid mist, the nozzleposition, the spindle rotation speed, the temperature ofhydrostatic oil and the pitch of the table movement. Wefabricated the fresnel lens 2 under the optimum conditiondeveloped from the lens 3 manufacturing. Fig. 7 is apicture of the lens 2. It is better mirror-surface than thelens 3. The NewView (a surface roughness measuringinstrument) could not be used, because this lens is toolarge. Therefore, we used an AFM. The measurementresults by NewView were considered as an evaluationstandard. We clarified the correlation of the results ofNewView and AFM with PMMA-000 sample; 13.8 nm(RMS) by NewView was 30 nm (Sa) by AFM. Fig.8 shows the result of the surface roughness measuredby AFM on the lens 2. The surface roughness of thelens 2 was about 24.7 nm (Sa). We estimate the surfaceroughness of the lens 2 is smaller than 13.8 nm (RMS),which satisfies the requirement value of less than 20nm(RMS). The authors demonstrated that manufacturingof large double-sided spherical fresnel lenses would beachieved.

VI. CONCLUSION

This paper reports that we created large double-sidedspherical fresnel lenses by using the ultra-precisioncutting method with a single point diamond tool. We willmanufacture three central lenses by the end of July 2009.The sizes of these lenses are 1500 mm in diameters.Afterwards, we intend to send these lenses to NASAwhere an optical test of the 3-lens subscale system willbe conducted, if funding is available.

Fig. 6. A picture of fabricated the lens 3 (1.5 m in diameter).

Fig. 7. A picture of fabricated the lens 2 (1.5 m in diameter).

Fig. 8. AFM measurement of the surface roughness on the lens 2.

REFERENCES

[1] T. Ebisuzaki et al., [JEM-EUSO Collaboration], these proceed-ings. (2009).

[2] F. Kajino et al., [JEM-EUSO Collaboration], these proceedings.(2009).

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

[4] Y. Takizawa et al., 30th ICRC, Mexico (2007)