Copyright 2003 Society of Photo-Optical Instrumentation Engineers. This paper will be published in Precisie Portaal and is made available as an

Transcription

1 Copyright 2003 Society of Photo-Optical Instrumentation Engineers. This paper will be published in Precisie Portaal and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for fee or for commercial purposes, or modification of the content of the paper are prohibited.

2 Design guidelines for thermal stability in opto-mechanical instruments Peter Giesen *, Erik Folgering TNO TPD, Delft, the Netherlands ABSTRACT Opto-mechanical instruments are sensitive to temperature effects. The optical performance will be influenced by temperature variations within an instrument. Temperature variations can occur due to environmental or internal heat sources. Assembly at a different temperature than eventual operation of the instrument can also influence the performance. This paper describes principles to minimize thermal disturbance of optical performance. The thermal behaviour of a system can area-wise be divided in heat source, heat transfer area and place where the optical performance is affected. Placement of the heat source is critical. Using a large thermal capacity, the influence of the source will be minimized. Heat transfer can be controlled by insulation or by good conduction, the latter minimizing the thermal gradient along the thermal path. Thermo mechanical effects on the optical performance can be controlled using a thermal centre, a combination of materials with different expansion properties, low thermal expansion materials and scaling effects of the optical design. TNO TPD designs and manufactures opto-mechanical instruments for space and astronomy. The design guidelines described are commonly used in these instruments. Several examples of the application of these design guidelines are presented in this paper. Keywords: Thermal management methods, passive athermalisation, dimensional stability, thermal centre. 1 INTRODUCTION Most opto-mechanical instruments are sensitive to thermal influences: the light path changes with changing ambient temperature of the components in the system. Some instruments, e.g. those using interferometry, are more sensitive than others because of high stability tolerances. In an interferometer, optical path length differences of nanometer level are relevant. Thermally induced deformation of optical components can have disastrous effects on their performance. In reflective optics, a small displacement of the reflecting surface will induce a change in optical path difference (OPD) twice that size. Changes in temperature occur during change of environment, e.g. between day and night. The environment during assembly, alignment and calibration in the laboratory may be different from the one where the instrument will operate. Temperature changes can be large. Some instruments must even operate in a cryogenic environment. Besides misalignment, an instrument can even be destroyed due to high internal stresses, since different materials behave differently to temperature changes. Some materials expand more than others, which can cause high strain and stress. In Figure 1 an example is given of a glass lens glued in a metal mount. The mount has a higher coefficient of thermal expansion than the lens, so during cooling of the system, the stress in the glass increases and results in breakage. * giesen@tpd.tno.nl; phone ; fax ;

3 Figure 1: Differences in Coefficients of Thermal Expansion (CTE) between glass and metal have a big influence on the stresses in the glass. This paper contains design guidelines for stabilizing and minimizing the effects of thermal expansion, with emphasis on deformation and not on stress and breakage. Examples will be given based on opto-mechanical instruments developed at TNO TPD. 2 DESIGN GUIDELINES In order to analyse thermal behaviour of an optical instrument, the system of heat transfer can be divided (see Figure 2) into three thermal subsystems: 1) the heat source or heat sink, 2) the heat transfer, and 3) the disturbance on the optical path. SOURCE HEAT TRANSFER DISTURBANCE Figure 2: System of thermal behaviour. All thermal subsystems can be used to reduce and control thermal influences on the instrument. 2.1 Source A source can either be hot or cold (relative to the instrument). A source can be an actuator, a light source or a change in room temperature. The choice of actuator or light source and control of ambient temperature is often limited. An actuator or light source may be placed outside the opto-mechanical set-up, or even outside the room. A light source could be linked to the set-up through fiber or light guiding. Thorough knowledge about the sources in the system is essential for a good thermo mechanical design. 2.2 Heat transfer Heat will be transferred between the source and the position where it can disturb the optical performance. This transfer takes place by radiation, conduction or convection. To minimize the disturbance because of deformation, the heat transfer can be minimized by blocking the path, so called insulation. One can use materials with low thermal conduction, and minimize mechanical contacts. Components in mechanical contact with each other will usually have a larger thermal resistance than the solid components. In a vacuum environment, the contact resistances are even higher because only radiation occurs.

4 On the other hand, a good thermal conductivity results in a more homogeneous temperature in the instrument. All components will change equally and so deformation is better controlled. Housing of instruments must then be made of one material that has a high thermal conductivity e.g. aluminium. A third method is to use a large thermal capacity. The effects of change in temperature will be minimized since the source is too small to heat the system significantly. This can be done by using a large thick aluminium plate for mounting the optical components. 2.3 Disturbance To minimize disturbance on the optical path, there are many options, which can be divided into four categories: 1. design in such a way that deformations of the system do not disturb the path, 2. use a combination of materials that compensate the thermal expansion, 3. use materials with a low thermal expansion, and 4. use the fact that the optical path is insensitive to scaling effects Insensitivity to deformation All materials change in size if temperature changes. However, a position in the material or part can be defined that will not move with change of temperature. This is called the centre of thermal expansion (or thermal centre). With well constrained systems, a thermal centre can be designed (see Figure 3). The system must not be over-constrained, but must allow uniform thermal expansion. In Figure 3, a body is connected to a base frame via 3 contact points. A spring applies a force to ensure that the body is in contact with all 3 contact points. Spring Base frame Thermal centre TC Figure 3: The thermal centre will not move if the body expands uniformly. [1] If an optical component will be placed at the TC (in Figure 3), it will remain stable with respect to the mechanical base frame. A kinematic coupling with three balls in three V-grooves (see Figure 4 left) will experience well-defined thermal expansion if the temperature of the top plate is heated with respect to the base plate. The six degrees of freedom (DOF) are paired at each ball. In this case the top plate has its thermal centre in the centreline. Another configuration; (see Figure 4 right) kinematic coupling with one tetragon socket that constraints 3 DOF, a V-groove that constraints 2 DOF, and a flat that constraints 1 DOF, has a line of thermal expansion through the socket. The mounting with three V-grooves is often more useful, because the centre of the mount is the centre of expansion, and all ball contacts are in one plane.

5 Thermal centre Figure 4: left (and middle) kinematic mount with 3 balls and 3 V-grooves; right kinematic mount or Kelvin clamp. The thermal centre is indicated. An object can be fixed using 3 leaf springs, as shown in Figure 5. If a temperature difference occurs between the object and its frame, the centre of expansion is at Tc. Because of the flexures, the temperature difference and/or the coefficient of thermal expansion can be large without serious effects. Figure 5: Thermal centre created with 3 leaf springs Combination of materials All materials have different Coefficients of Thermal Expansion (CTE, α). Some are large, like rubber and plastics (α = ppm). Some are medium, like aluminium (α = 23 ppm) or steel (α = 10 ppm). Some have a low CTE like Invar (α = 1-2 ppm) and wood (α = 3-5 ppm). Some have almost zero expansion, like low expansion glass (α = 0.05 ppm). Some have a negative CTE, like some ceramics (α = -5 ppm). By combining materials that have different thermal expansion coefficients, an almost zero expansion length of the construction can be achieved. The concept of combination of materials is not new: It has been used in pendulums for clocks. Galileo Galilei ( ) used his heart rate to time the oscillations of the chandelier in the cathedral at Pisa. He observed that the period remained constant as the amplitude of the (small) oscillations damped down, thus establishing the isochronously property of the pendulum. The period of the pendulum depends on its length, but the length of a metal pendulum rod changes with the temperature. As the temperature goes up, the length increases and the clock controlled by the pendulum runs slower. In 1715 the Englishman George Graham attached a mercury column to the lower end of the pendulum rod. When the

6 temperature went up, the length of the mercury column increased, keeping the centre of mass of the system the same distance from the pivot point. The CTE of mercury is 182 ppm. In 1725 the Englishman John Harrison invented the gridiron pendulum, another solution to the temperature compensation problem. Brass and iron rods, with different temperature coefficients of expansion, were fastened alternately top and bottom in a gridiron pattern to keep the centre of mass at the same point [4]. He compared his clocks with movement of the stars. His clocks did not deviate more than one second per month. This was a good achievement since the best clocks in the world of that time deviated about 1 minute per day [2]. Figure 6: schematic drawing of an Iron grid pendulum Low thermal expansion materials A variety of ultra low expansion materials is available, but unfortunately not always useful. E.g. ultra low thermal expansion glass ceramics are brittle and therefore cannot be used as construction material. Invar will drift because of mechanical stress. Also, low expansion properties are useful near room temperatures, but at other temperatures, the materials will not behave as such Scaling effects Because of thermal expansion of a structure, optical components will move with respect to each other. But the curvature of a mirror also scales with temperature. If material of the structure and that of the mirrors in the opto-mechanical apparatus are the same, the effects of temperature on the optical system can be minimized.

7 3 THERMAL STABILITY IN SCIENTIFIC INSTRUMENTS 3.1 HIFI HIFI (Heterodyne Instrument for far Infrared) is a cryogenic space instrument built by SRON in the Netherlands. TNO TPD contributes to the optical and mechanical design of the instrument. TC 1 of the 3 leafsprings Figure 7: HIFI instrument. Left complete design, right the optical components. A thermal centre (TC) is defined such it is near the centre of the telescope of the satellite. The instrument must be cooled to extremely low operational temperature (15 K) and the instrument is integrated at room temperature. The structure is made of aluminium to ensure that the whole structure will shrink by the same amount. In addition, minimum thermal stress is introduced to the instrument. The structure is made of only a few parts that are fixed to each other to prevent a temperature gradient between the parts. The disturbance of the optical system is minimized because all components and structural elements are made of the same material, so the optical system will scale with temperature. A thermal centre is created using leaf springs; therefore the light of a telescope will always enter the instrument at the same position. Another advantage is that the leaf springs prevent internal stress since the material of the base plate on which the instrument is mounted has a different CTE and temperature than the instrument itself. 3.2 ESO-VLTI The Very Large Telescope Interferometer (VLTI) owned by the European Southern Observatory (ESO) exists in the coherent combination of four VLT Unit Telescopes and of several moveable 1.8m Auxiliary Telescopes. A key technique for optical aperture synthesis (interferometric) instruments is the co-phasing of beams of individual telescopes. The optical path difference between the telescope wave fronts must be within 10 nm. The delay line must operate in an environment that varies in temperature +/- 2.5 C. The room it operates in has a vertical temperature gradient as well. Several principles described in the previous chapter are used here. (1) First heat sources like electronics are placed at positions that cannot influence the cat's eye reflector. (2) To prevent gradients in the Cat's eye housing, isolation in the heat transfer path from the stage with actuators is realized by mounting the Cat's eye housing on the stage on V-grooves. (3) Minimal mechanical contact also means low thermal conduction. Besides the housing, also the mirrors are mounted by 3 balls and V-grooves (see Figure 9). (4) Firstly to isolate the mirror from the structure and prevent a temperature gradient in the mirror. (5) Secondly to ensure that the thermal centre is on the optical axis and the disturbance on the optical system is minimized. (6) The housing and the mirrors are all made of the same material, so homogeneous expansion will not result in degradation of optical performance.

8 V-grooves Figure 8: Left: optical path of the Cat 's eye reflector. The three V-grooves to mount the housing to the stage are indicated. Right: Cat's eye reflector is placed on the stage for large (60 m) displacements Balls of M1 Balls of M2 Figure 9: Thermal centre is achieved with a kinematic mount for both M1 and M2 mirror. 3.3 Optical delay line For several more optical aperture synthesis studies, TNO TPD developed optical delay lines. Even stability in the sub nanometer range is sometimes required. For a thermally very stable optical delay line, a structure was built that uses a combination of materials with different coefficients of thermal expansion. A delay line, according to the cat's-eye approach presented in Figure 10, exists of a tube with 2 mirrors. A collimated beam will enter the tube, the first mirror will focus the beam on a second one, which reflects back to the first one, and the beam will leave the delay line again. The second mirror is placed on a piezo. The OPD can be controlled at sub nanometer level. To ensure that the delay line does not depend on temperature, length L0 must be kept stable (see Figure 10). Consequently (L1 α1 - L2 α2) must be zero, with α1 and α2 the CTE of the materials. The design can be optimised by varying L1 and L2 and choosing a pair of construction materials.

9 collimated beam material 1 Low expansion glass M1 material 2 Piezo with M2 Figure 10: Principle of thermal compensated delay line. The expansion of material 1 is compensated with the expansion of material 2. CTE of material 2 > CTE of material 1. The tube of the delay line is made of titanium and the second mirror is mounted on an aluminium rod. The position of mirror M2 with respect to M1 remains the same, if the temperature changes. M1 is made of Zerodur and therefore the curvature will not change due to temperature changes. In real designs, deformation of other parts like M2 and actuator must also be taken into account. Figure 11: Optical delay lines. (Assembly of two delay lines. The collimated beams will enter through the front holes) 3.4 GAIA GAIA is the Global Astrometric Interferometer for Astrophysics and will be launched in The aim of this ESA space observatory is to measure with extreme precision the position and speed of billions of stars and to create a three dimensional map of the universe. The satellite measures the angles between stars using two telescopes set at a fixed angle of 106. Measurements must be accurate to 15 nanoradians. Since it is not possible to achieve the optomechanical stability, required to attain this level of accuracy, the raw data must be corrected to take account of tiny movements of the telescopes. To do this GAIA, is equipped with a metrology system to monitor the angle between the telescopes. This system consists of two interferometers that must be capable of measuring an optical path difference of 50 picometers ( m), half the diameter of an atom. TNO TPD built the GAIA OPD test bench to demonstrate the feasibility of the metrology system on the GAIA satellite. The system consists of two interferometers that can measure an optical path difference of 40 ± 15 pm.

10 To realize the high stability, the following principles are used. (1) As few as possible thermal sources near the set-up (only the detector and 2 actuators are near the set-up). (2) To minimize heat transfer, the set-up is placed in a vacuum chamber. No heat from outside can reach the set-up through convection. (3) The set-up is built on a large aluminium base plate. This high thermal capacity causes that temperature changes can only slowly occur. The total mass of the base plate and set-up is 335 kg and is 1.4 m long. (4) Since only one material is used, the set-up will expand uniformly, and thus scales uniformly. (5) A combination of materials is used in the imaging telescope for compensating the focal length. (6) Radiation shields are used to eliminate heat transfer from the vacuum chamber to components. (7) By monitoring the temperature of the set-up, the right moment of the day can be predicted to reach the highest measurement accuracy. Wall of the vacuum chamber Telescope Detector Base plate Figure 12: GAIA Test bench for metrology 4 REFERENCES 1. M.P. Koster, Constructieprincipes, Twente University Press, Enschede, Dava Sobel, Longitude, Walker and Company, New York, D.G. Chetwynd, "Selection of structural materials for precision devices", Precision Engineering, Volume 9 number 1, 3-6, Internet: 5. Mierlo, H.A. and Nijenhuis, J.R., "Design of the VLTI Cats Eye for optical delay lines", proceedings of the 3rd International Conference of EUSPEN, Volume 1, 95-98, Roger A. Paquin, "Dimensional Stability", SPIE, Volume 1335, Roger A. Paquin, Daniel Vukobratovich, "Optomechanics and dimensional Stability", SPIE, Volume 1533, Paul R. Yoder Jr., "Optomechanical Design", SPIE Press, Volume CR43, 1992