Several testing methods of non-imaging mirror segments ... · PDF fileof non-imaging mirror...

Several testing methods of non-imaging mirror segments surface parameters

Petr Schovánek et al.

Olomouc 2012

Univerzita Palackého v OlomouciPřírodovědecká fakulta

Oponent: RNDr. Pavel Pavlíček, Ph.D.

Kolektiv autorů:Petr Schovánek, Miroslav Hrabovský, Miroslav Palatka, Miroslav Pech, Dušan Mandát, Libor Nožka, Helena Hiklová

Publikace byla připravena v rámci projektu Investice do rozvoje vzdělávání

Tento projekt je spolufi nancován Evropským sociálním fondem a státním rozpočtem České republiky

1. vydání

© Petr Schovánek et al., 2012© Univerzita Palackého v Olomouci, 2012

Neoprávněné užití tohoto díla je porušením autorských práv a může zakládat občanskoprávní, správněprávní, popř. trestněprávní odpovědnost.

ISBN 978-80-244-3118-5

NEPRODEJNÉ

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Contents Several testing methods of non-imaging mirror segments surface parameters …….…………………………….……. 5 1 Introduction ………..…………………………..……….... 5 2 Shape testing of Reflecting Surfaces …………………….. 6 2.1 Testing by spot size ……………………………….. 6 2.2 Ronchi test ………………………………………… 7 2.3 Hartmann test ……………………………………… 9 3 Quality Testing of Active Surfaces Polishing …………….. 11 3.1 Visual control ………………………………………. 11

3.2 Form Talysurf Series 2 – instrument for measurement of surface roughness and shape ……... 11 3.3 The optical methods based on scattered light measurement ……………………………………….. 12

4 Testing of reflecting Layers ……………………………….. 13 4.1 Visual control ……………………………………...... 13

4.2 Micro holes control in intensive backside lighting …. 14 4.3 Adhesion testing …………………………………….. 15 4.4 Refelectivity measurement …………………………. 15 4.5 Surface distribution of reflectivity ………………….. 16 4.6 Scattering characteristic of thin films ……………….. 16

5 Conclusion …………………………………………………. 17 References ……………………………………………………. 18

Projekt je spolufinancován Evropským sociálním fondem a státním rozpočtem České republiky.

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Education of Scientists in the Regional Centre of Avanced Technologies and

Materials. CZ.1.07/2.3.00/09.0042

Several testing methods of non-imaging mirror

segments surface parameters

Petr Schovánek, Miroslav Hrabovský, Miroslav Palatka, Miroslav Pech, Dušan Mandát, Libor Nožka, Helena Hiklová Abstract. A short description of several testing methods used for the

control of non-imaging mirror segments is presented. These segments are a part of large optical detectors used for wavelengths from 170 to 300 nm. Tests are important for control of shape and reflection of mirror surfaces (mirror surfaces reflection).

1. Introduction

The aim of this text is to demonstrate several methods for testing of non-imaging mirror segments parameters. The described mirror segments are elements of large astronomical spherical mirrors and these mirrors create together with RICH detectors (Ring imaging Cherenkov detector) gigantic Pierre Auger Cosmic Ray Observatory in Argentina. The tested mirror segments have hexagonal shape and length of side is appro-ximately 35 cm. The completed astronomical mirror is approximately squared with the dimensions 3.5 x 3.5 m. The radius of the mirror is 3.4 m and it is designated for collecting of UV radiation. All of the mirror segments must be subjected to a rigorous control of surface quality and of their geometrical and optical parameters. These control methods are described here with in.

6 A textbook of the project RCPTM-EDU

2. Shape Testing of Reflecting Surfaces

2.1 Testing by spot size The tested mirrors are designed to collect weak optical signals and

transport them to the active surface of the optical detector. A typical size of the active surface of the optical detectors is in units of tens of millimeters. It is clear that the critical parameter of this type of optical systems is not optical resolution, but the ability of the system to collect the most photons of optical signal and transport them with the minimal loss to the detector. This fact is important for the design of testing methods of non-imaging mirrors segments. Classical control and selecting method of non-imaging mirrors surfaces is based on a measurement of the diameter of circumscribed circle area of the spot which contains a required part of the energy – for example 95 % – of light reflected by a controlled mirror surface. The principle of this testing is in imaging of a stigmatic point from the centre of curvature of spherical surface to the same point. The detector and a point source of light are located near the optical axis in the plane, which crosscuts this axis in the centre of curvature. The design of measurement is in Fig. 1.

Fig. 1. Scheme of the control method with spot-size measurement.

In Fig. 2a, 2b, there are two examples of this type of testing after PC processing. It is evident that a high quality surface produces a spot with a small diameter. On the other hand, an imperfect reflecting surface produces a large spot whose dimension may be larger than sensitive surface of detector.

Several testing methods

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Fig. 2a. Examles of two spot-size measurement. 95 % of to mirror reflected energy is in spot-size with diameter: D = 1,28 mm.

Fig. 2b. Examles of two spot-size measurement. 95 % of to mirror

reflected energy is in spot-size with diameter: D = 5 mm.

2.2 Ronchi test Let us arrange the control instrument in accordance to Fig.1. If we put

in front of the detector crosscutting the optical axis linear grating mask than we can see a shadowgraph of it on the detector. One part of the beam reflected on the mirror is overshadowed by a linear mask and this is the reason why this shadowgraph – Ronchigram – is created [1] (Fig. 3) (Ronchigram is not interferogram). If we apply an ideal spherical mirror and place the mask precisely in the plane in the centre of curvature than the whole surface of the tested mirror will be light or dark – this is an analogical image when the well-known knife test [2] is used. The shift of the linear grating mask in optical axis direction from the centre of curvature changes the view of the Ronchigram into the system of equidistant strips. These strips are always deformed in the Ronchigram which is generated by a real optical surface.


Fig. 3. Scheme of the Ronchi test with examples of mask’s picture from different mirrors.

Based on the analysis of deformation of this geometrical structure it is

possible to make a qualitative analysis of the reflective surface deformation. This is useful for surface classification and for the correction


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of aberations in the production of large optical surfaces as in astronomical telescopes.

Currently it is possible to make a quantitative classification of tested surfaces by using modern detectors and computers. The principle of this method is the same as in the case of a classical Ronchi test. What is new is in digital testing in the PC combination of Ronchigrams which are using of two different pictures of the greeting simultaneously. Principle is illustated in Fig. 4. In the Fig. 4a there are two pictures which are formed the same greeting in two mutually perpendicular directions. These two pictures are scan by CCD camera and than are fold into one picture by computer. The result is shown in Fig. 4b. After finding the intersections of the lines it is possible to calculating components of normal vectors if the parameters of a system are known. It is possible to make the relief of surface of tested mirror from this way obtained information. The result is in Fig. 4c.

a b c

Fig. 4a,b,c. Images to digital Ronchi test.

2.3 Hartmann test Another frequently used objective noncontacting method for testing of

mirrors segmnets is method based on principle of Hartmann test. The testing arrangement is similar to the previous case. The change is in the application of mask with a pinhole symetric system (Fig. 5.). In the plane in the centre of the curvature we can find a deformed picture of the mask with pinholes.

Simple Hartmann test from Fig.5 it is possible to significantly improve it by CCD camera, spatial modulator and beamsplitter (Fig. 6). The spatial modulator enables creation any of shape of Hartmann mask. The picture recorded by the camera is converted by morphological operations, in the next step, centroids of pinholes are located. In these points angular coefficients of normal of reflective mirror surface are estabilished and


then it is possible to create the searched shape of this reflective surface. The right part of Fig. 6. shows the deviation of real surface from ideal spherical surface which is obtained by the method described above. The maximal resolution of this method is in tenths of milimeter.

Fig. 5. Scheme of Hartmann test (on the left) and Hartmann mask’s

picture (on the right).

Fig. 6. Digitalized Hartmann test set up and 3D graph of differences between real and ideal surface.


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3. Quality Testing of Active Surfacesccc ccccPolishing

High quality optical elements for UV radiation are possible to make

only in the conditions of the best controlled polishing technology. Comparable mirrors, which operate in visible light very well, might not have the same quality in UV. The reason is in the dependence of scatter properties of polished surfaces on wavelength of reflected signal [3].

3.1 Visual control The basic method of testing the high quality polished surfaces is visual

control. The procedure of visual control is subjective, simple but in the manufacturing process is irreplaceable. This control is based on the magnifier observation of a testing surface in a collimated intensive beam of light. A technologist with experience is able to recognize and identify a number of kinds of manufacturing imperfections, surface defects, for example scratches, digits, striaes, cords or surface shape imperfections near the edges of optical parts. In the next step the technologist can make the suitable corrections right in the technological process to get better results immediately [4].

3.2 Form Talysurf Series 2 – instrument for vvvvv measurement of surface roughness and shape

Instrument Form Talysurf Series 2 by fy Taylor & Hobson is used for objective control of roughness of optical surfaces after all technological operations which are based on ablation of surface layer of glass (milling, grinding, polishing). This method provides high reliability and valid results for surfaces with various surface roughnesses. The testing of high polished materials requires an experienced operator in measurement and knowledge of this very sophisticated instrument. The results validity of measurement of high polished materials consists in correctly the chosen measuring parameters. The testing of surfaces with roughness in nanometers is complicated because the method uses pinot contact of surface with a pin. However, the results of this method are well conformed with optical methods.


Fig. 7. The plot of mirror roughness measuring with Form Talysurf

Series 2 (Ra = 1.7 nm).

3.3 The optical methods based on scattered light ccccc measurement

The important contributions for testing of surfaces are methods based on evaluation of light scattered by surface. CASI – Complete Angle Scatter Instrument is used for laboratory testing and µSCAN for testing in workshop both made by Schmitt Measurement Systems. CASI instrument is used for extraordinary precise testing of polished surfaces. The results with CASI are applied as a reference model for the other methods. In the principle, CASI takes measurement in tested surface and analyzes data to produce BRDF (biridectional reflectance distribution function), PSD (power spectral density) and surface statistics. CASI system for testing of mirror segments must be equipped with the special holder for large objects.


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Fig. 8. It is clear in the context of values displayed on the axes, the

CASI system is extremely sensitive. The values, which were the base for constructing of the BRDF curve, are very precise. In the figure there is a plot of BRDF (curve with wide basis) of polished SIMAX (PYREX) glass, which is the standard material for this type mirror segments. The roughness of the surface is Ra= 2.5 nm.

4. Testing of Reflecting Layers

An important step in production technology of mirrors for UV part of optical spectrum is a deposition of reflecting thin film system. The required high efficiency in UV is the reason for the application of different materials than for visible part of optical spectrum. The absorption characteristic of SiO2 which is commonly used for visible light, makes it impossible to use it for UV. This is the reason why MgF2 or HfO2 are usually used as materials for the production of protective thin layer for UV [5]. Tested properties of reflecting thin films are reflectivity, homogeneity, local defects, scattering characteristics and adhesion.

4.1 Visual control Visual control is the basic step of optical thin films testing. The

optimal conditions for this type of control are inside of a dark room. It is advantageous to use a powerful light source with a collimated beam. This is the way how to identify local defects on the surface, inside or under the thin film respectively (Fig 9).


Fig. 9. Area scratching of thin film (left part), over coated fingerprint and hard scratches (right part).

4.2 Micro holes control in intensive backside ccccc lighting

Visual control of micro-holes in thin films is very effective. The micro-holes are simple to find by a powerful light source that is placed on the backside of the surface of the controlled mirror. The hole appears to a lighting point on the dark field (Fig. 10). The dimension of the holes is in range from microns to tenth of millimeters. This dimension of the holes depends on the dimension of the particles of dust, which are the reason of its origin. The hole in thin film is created by implosion of film inside of the evacuated empty volume around the dust particle. This empty space is overlapped during the evaporating process by a thin film. The effect of implosion is caused by atmospheric pressure after the removal of the optical part from the evaporating machine after some time.

Fig. 10. Scheme of micro-holes method test. The detail in right part is

schematic picture of the layer damaged by micro holes.


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4.3 Adhesion testing Another important characteristic for the evaluation of thin films quality

is the adhesion on substrate. This is the procedure of testing: we glue about one centimeter of Scotch tape on surface near the edge of the mirror and we strip away the tape after. This method is simple and results are clear. The high quality thin film stays on the substrate without damage. This test was applied on each mirror.

Fig. 11. A detailed view of a brake-away part of thin film after the test

application.

4.4 Reflectivity measurement Control specimens, which are made together with each of segment, are

used for the absolute reflectivity measurement. The reason is an insufficient dimension of the measuring chamber of the spectro-photometer. The instrument VUV-VASE, spectroscopic ellipsometer made by J.A.Wollam Co Inc., is used for this test. The curve on Fig. 12 is created by combining the measurement results, which are recorded in a vacuum and in a nitrogen atmosphere. Initially obtained values of reflectivity are converted for normal incidence.

Fig. 12. Graph of reflectivity of prototype segment for RICH detector

of CBM (Coalbed Methane Project) project.


4.5 Surface distribution of reflectivity This testing method is based on comparative measurement of

reflectivity. The reflectometer has been developed for the Project Auger in Karlsruhe. The reflectivity is determined by comparing the signal reflected by a tested mirror with a signal of reflectivity etalon. The distribution of reflectivity is tested by repeating the measurement in many points. We put in an example result of measurement in 30 points on mirror segment for Pierre Auger project. The distribution of points in which is the mirror measured and distribution of reflectivity are in Fig. 13.

Fig. 13. The scheme layout of measured points with graph of reflectivity distribution.

4.6 Scattering characteristic of thin films The last parameter that we control on a mirror’s reflective layers is the

roughness of its surface. Optical thin films cannot increase markedly the primary surface roughness of mirror substrate. We use the mentioned above instrument CASI for this aim. The presented example is a plot of BRDF by prototype segment with Al+MgF2 reflecting layer for RICH detector of CBM project (Fig. 14). The calculated average value of roughness of the reflecting surface is in this case Ra= 2.96 nm.


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Fig. 14. The curve shows the BRDF values measured for signal

incident at 5 degrees from the normal angle. The diffused light is collected in the range of 10°. Laser light with wavelength λ=325 nm is used for testing.

5. Conclusion The manufacturing of mirror segments for near UV radiation is a

complicated problem which needs a sophisticated approach to technology. Mainly the roughness of reflecting surfaces and optical thin films technology for UV demands an extraordinary amount of attention and an experienced team. “Conditio sine qua non“ for top products are testing and measuring methods ensuring that the finished segments will provide trouble-free service in the Pierre Auger project.


References [1] Malacara D. [Optical Shop Testing], John Wiley&Sons, Inc. (1977) [2] Warren J.Smith-3rd ed. [Modern Optical Engineering], McGraw-Hill xxx(2000) [3] Stover, J.C. [Optical Scattering: measurement and analysis], SPIE (1995) [4] Karow H.H. [Fabrication Methods for Precision Optics], John Wiley&Sons, Inc. (1993) [5] Pulker, H.K. [Coatings on Glass, Second, revised edition], Elsevier xx Science B.V. (1999)

RNDr. Petr Schovánekprof. RNDr. Miroslav Hrabovský, DrSc.RNDr. Miroslav PalatkaMgr. Miroslav Pech, Ph.D.Mgr. Dušan Mandát, Ph.D.Mgr. Libor Nožka, Ph.D.RNDr. Helena Hiklová

Several testing methods of non-imaging mirror segments surface parameters

Výkonný redaktor: prof. RNDr. Tomáš Opatrný, Dr.Odpovědná redaktorka: Vendula DrozdováNávrh a grafi cké zpracování obálky: Jiří K. Jurečka

Vydala a vytiskla Univerzita Palackého v OlomouciKřížkovského 8, 771 47 Olomoucwww.upol.cz/vup

Olomouc 2012

1. vydání

ISBN 978-80-244-3118-5

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