Holger Riewaldt a,b. , Martin Lastiwka b. , Nathan Quinlan b. ,. Kevin McNamara b. , Xin Wang b. , Anita Enmark a. , Mette Owner-Petersen a. , Andrew Shearer b.
Status of the Euro50 Project Torben Andersena, Arne Ardeberga, Holger Riewaldta,b, Martin Lastiwkab, Nathan Quinlanb, Kevin McNamarab, Xin Wangb, Anita Enmarka, Mette Owner-Petersena, Andrew Shearerb, Chengyu Fana, Dan Morarua a Lund Observatory, Sweden; bNational University of Ireland, Galway ABSTRACT The Euro50 is an extremely large telescope for optical and infrared wavelength with a 50 m primary mirror. It has a segmented, aspherical primary mirror and an aspherical, deformable secondary in a Gregorian layout. A tentative conceptual design exists and has been documented in a study report. Recent activities have concentrated on the science case for extremely large telescopes in the 50 m class and on identification of potential technical “show stoppers”. The science case investigation has identified four fields of particular interest. The studies of critical technical issues have concentrated on atmospheric dispersion effects for high-resolution adaptive optics for extremely large telescopes, and on the influence of wind and other disturbances on wavefront control. Wind load on the telescope, the primary mirror and the enclosure has been studied using wind tunnel measurements and computational fluid dynamics. The impact of wind on the total system has been investigated using an integrated model that includes the telescope structure, the primary mirror segment alignment system, the secondary mirror alignment system, and single conjugate adaptive optics using the deformable secondary mirror. The first, tentative results show that wind disturbances may be significant and that the task of correcting for wind residuals may be at least as large for the adaptive optics system as that of correcting for atmospheric aberrations. The results suggest that use of extremely large telescopes for observations of earth-like planets around nearby stars may imply a considerable challenge. Keywords: Euro50, extremely large telescopes, integrated modeling, adaptive optics, wind, exoplanet detection.
1. INTRODUCTION The Euro50 is a proposed astronomical Extremely Large Telescope (ELT) for optical and infrared wavelengths (see Figure 1) [1-10; 14-16; 18; 23-25; 28; 29]. The telescope will have a 50 m segmented primary mirror with 618 hexagonal segments. Each segment is two meters from side to side. The Gregorian optical configuration together with the post-focus adaptive optics system is shown in Figure 1. Both the primary and the secondary are elliptical for an aplanatic design. Adaptive optics is integrated into the telescope system and the secondary mirror is deformable with over 3000 actuators. Rigid-body motion of the secondary mirror in five degrees of freedom is controlled using a floating suspension of the secondary mirror unit and force actuators with feedback from an inertial reference unit with accelerometers and rate gyros. Below the primary mirror, there is ample space for wavefront sensors, relay mirrors and a second deformable mirror. Observations are possible in the seeing limited regime and with single or dual-conjugated adaptive optics. There is an elaborate live optics system that combines primary mirror segment control, secondary mirror form and position control, telescope main servos and adaptive optics into an integrated system for wavefront control of the entire telescope system. The Observatory at Roque de los Muchachos is a possible site for the Euro50.
2. STATUS A thorough design study has been carried out and is documented in a study report [1]. With the concept for the Euro50 rather well defined, it has been elected to concentrate on two specific fields, further exploration of the science case, and studies of technical “show stoppers”. Exploration of the science case is of high importance because it has an overall impact on the specifications and the design of an ELT, such as the Euro50. Studies of earth-like planets around other stars constitute a good example. A
ground-based ELT optimized for such observations will be significantly more costly than an ELT without this capability. Hence, it is important to define the science case well in advance because of the large impact on design and cost.
Figure 1. Overall conceptual design of the Euro50 in its enclosure (left) and optical layout (right).
As potential technical “show stoppers” we have selected two specific fields. The first of these is the influence of atmospheric dispersion on high-resolution adaptive optics. Typically, observations take place in one band, for instance the K-band, whereas wavefront sensing may be established at 589 nm using laser guide stars. Due to atmospheric dispersion, the deformable mirror can in principle only compensate for the atmospheric fluctuations at one wavelength (589 nm), setting limits on the spectral bandwidth achievable for adaptive optics observations.
Figure 2. Typical average wind speed profile as a function of height over ground. The Euro50 is shown for size comparison. It can be seen that for the ELT generation of telescopes, the wind pressures will typically be 2-3 times higher than for traditional, smaller telescopes.
The second critical area that has been studied is related to the wind load on the telescope. The Euro50 telescope will be nearly 100 m high and the wind load is significant (see Figure 2), even with an enclosure to protect the telescope against wind buffeting. At the same time, wavefront aberrations must be kept at a low level, down to about 10-100 nm, depending on the observing wavelength range. Considering that the wind deformation of the structure may be in the millimeter range, this sets hard requirements on the control systems for the primary mirror, for the secondary mirror position and form, and for telescope pointing (“live optics”). Essentially, the live optics must be capable of suppressing disturbances by five orders of magnitude, dynamically and statically. Hence, the issue is two-fold. Firstly, the static and dynamical wind loads must be determined with reasonable accuracy. This has been achieved by performing in parallel a wind tunnel study and a Computational Fluid Dynamics (CFD) study. Secondly, the consequences of wind must be studied. The foremost tool for this is integrated modeling, which combines different technical disciplines to establish a global simulation model of the complete telescope system, including adaptive optics. In the following, a summary of each of the above activities shall be given. Reference is made to dedicated papers within each of the fields [4; 5; 16; 25; 28].
3. SCIENCE CASE A large amount of ambitious and important science programmes require the performance of a 50 m ELT. Among them, the work on planets has a high profile and attracts very high interest. Nine years after the first identification of an exoplanet [21], we try to understand the complete chain of processes leading from initial gas collapse, through the forming protostar, the growth of a circumstellar shell, the following protoplanetary disc and the production of planets. With current telescopes, we can observe various steps of the development. However, while we have a reasonable overview of the ensemble of events, we can neither follow nor understand the detailed and decisive processes. In addition, while we tend to agree on a number of conclusions, not least regarding the development and time scale of the protoplanetary disc and the events leading to the formation of the first giant planets, we are unable to reconcile these processes and find a satisfactory explanation of their joint activity, even if impressive efforts are made [11-13]. Currently, we know more than 100 planetary systems. Our observations have already produced some statistics, which, albeit plagued by observational bias, provide some insight into the general nature of the systems and their planets. Even taken the bias into account, we are confronted with a number of facts that we cannot explain. These facts concern the nature of the orbits as well as of the planets. We are puzzled by the extremely tight orbits of some giant planets, by the frequent signs of migration and by the existence of orbital groups and gaps. Already the first system detected took us with surprise, and so it has continued. It is very hard to reconcile theory with observations, even if sophisticated work is abundant [19; 20; 22; 26]. Our initial presumption that the solar system could be regarded as a reasonable template for the general distribution of planetary systems is today highly doubtful. Existing telescopes, not least the VLTs equipped with full AO, will undoubtedly increase the number of planetary discs and systems known and studied. They will also provide us with much more details concerning all parts of the processes leading to planets as well as regarding many of the planets found. Still, however well we improve and equip the VLTs, they will not suffice in our quest for the most interesting details and specifically not for studies of Earth-like planets and signatures of life. There is every probability that our neighbourhood is the home of hundreds of Earth-like planets. However, without an ELT, we will never be able to analyse them for their nature and the existence of life. Highly connected to the formation of planetary discs and planets is the formation and initial evolution of stars. Again, we believe that we can grasp the grand design of the processes, and, again, we miss some of the most fundamental and interesting details. As an example, the first events leading to the birth of stars in giant molecular clouds are shrouded by the mixture of gas and dust providing the starting material for star formation [17]. Narrow-band imaging and highresolution spectroscopy at NIR and MIR wavelengths would highly improve our understanding of the processes. With a 50 m ELT, these observations could be made.
Another process of high importance but little understood is the formation and evolution of galaxies. The initial phases of galaxy formation are close to unknown, and the subsequent phases are understood to a very limited extent only. We have been able to study a number of events involving interaction of two or more galaxies. However, we have vague ideas at best of the extent to which galactic encounters and exchanges influence the structure of normal galaxies. Even less is known concerning the implications of such interactions for the large-scale star formation processes determining the evolution of galaxies. The reason for our lack of insight is a pronounced dearth of observations of the fundamental signatures of galactic evolution. Stellar clusters are prime sources of information on evolution and key tools for systematic studies of the evolution of galaxies [10]. In the near future, VLTs, equipped with full AO, will provide us with the possibilities to study star clusters in the Local Group of galaxies. However, the Local Group does not provide even a marginally satisfactory sample of galaxies for our purpose. To get a meaningful sample of data, we must reach the Virgo and Fornax clusters of galaxies. This can be achieved with a 50 m ELT.
4. ATMOSPHERIC DISPERSION Atmospheric dispersion seems to be a relatively overlooked problem in relation to ELTs. Basically, dispersion results in two effects, (i) the optical path-length measured in waves along a given ray will be different for different wavelengths and cannot be compensated for all wavelengths by the same deformable mirror, and (ii) rays associated with different wavelengths will be refracted differently (dispersed) by the atmosphere and hence an incoming polychromatic ray will be split into different colors following different paths through the atmosphere. We have made some studies of the consequences of the effect (i) since it is believed to dominate. Assuming AO compensation to be perfect at the sensing wavelength, observation at other wavelengths will result in degraded performance related to Strehl ratio, contrast and resolution. The degradation will depend on the outer scale of the atmosphere. As an example, the curves of Figure 3 show the logarithm of the contrast, defined as the ratio between the corrected peak intensity divided by the background intensity at the peak position. Perfect correction takes place at 589 nm. Even close to the wavelength of optimal correction there is a severe reduction in contrast, which may affect the prospects for detection of earth-like planets orbiting nearby stars. This is the subject of investigations in progress. Use of laser guide stars for adaptive optics presents a problem for the control loop in that it may be necessary to rescale the optimal control to other wavelengths, which means that the control must be “open loop”. Since also the Strehl ratio degrades when moving away from the wavelength of optimal correction, the bandwidth of good correction will be limited, even using rescaling. One consequence is that the demands on the design of an atmospheric dispersion compensator can be relaxed because performance anyway is compromised. These problems are dealt with in [25].
Figure 3. Ratio between the corrected peak intensity divided by the background intensity at the peak position for different choice of entrance aperture diameter. Left: outer scale 100 m, right: outer scale 20 m.
5. WIND LOAD 5.1. Wind Tunnel Measurements To study the static and dynamical wind loads on the enclosure and the Euro50 telescope, a wind tunnel measurement was carried out in a boundary layer wind tunnel in Galway, Ireland. The wind tunnel setup is shown in Figure 4. The models were manufactured in scale 1:200 to match the conditions of the wind tunnel. The static and time varying pressures on the enclosure and the telescope primary mirror were measured in a large number of points. The timevarying pressures were recorded as time histories and their power spectral densities were determined by postprocessing. Due to limitations in the measuring equipment, it was not possible to record pressure time histories at several locations of the primary mirror simultaneously, so the spatial pressure distribution on the primary mirror has not been determined. Further details can be found in [28] and in [27]. A comparison of some results from the wind tunnel measurements and the CFD computations can be found in Section 5.3.
Figure 4. Model of Euro50 telescope and enclosure in the boundary layer wind tunnel.
5.2. Computational Fluid Dynamics A CFD analysis has been carried out using the code CFX5.6. A Large Eddy Simulation (LES) model has been used to describe the turbulence. The mesh with about 900 000 volume cells is shown in Figure 5. The analysis has been performed on a cluster computer with 32 2.4 GHz Xeon processors and it took 79 hours to simulate 300 seconds of real time. The static and time-varying pressures on the enclosure and the telescope primary mirror were determined as a function of time. In particular, the CFD model has given valuable information on the spatial distribution of the pressure load on the primary mirror. Reference is made to [28]. 5.3. Comparison of Wind Loads A more detailed comparison of the wind loads determined by wind tunnel measurements and CFD calculations can be found in [28]. In general, there has been good agreement between the wind tunnel results and the CFD results. Here, an important and representative result shall be presented, see Figure 6. The power spectral densities for a number of points distributed over the primary mirror are shown in gray, and the averages in thick, black lines. It is seen that, in spite of the highly different methods applied, there is good agreement between the results. The RMS of the pressure variation over time is about the same for the two approaches (~30 Pa). The peak is located somewhat below 0.1 Hz for the wind tunnel results whereas it is somewhat above 0.1 Hz for the CFD computation. However, the difference is not drastic. The high-frequency part of the CFD curve is lower than that of the wind tunnel curve. That may be related to the choice
of CFD volume cell size, which cannot be very small for reasons of computation time. All in all, the agreement is good and the results seem reliable.
Figure 5. Mesh used for the CFD calculation of the turbulent flow near the enclosure and the primary mirror of the telescope.
Figure 6. Power spectral density of wind pressure at various locations of the primary for the telescope pointing 60 degrees away from wind and with an elevation angle of 60 degrees. Left: Wind tunnel result scaled to Euro50, right: CFD result.
6. INTEGRATED MODELING Integrated modeling is the most important tool for studies of ELTs and for prediction of their performance and limitations. Reference is made to [5]. It is also an excellent tool for studies of the influence of wind. It has been found that adaptive optics will not only compensate for atmospheric effects but also for a range of wavefront error residuals from the telescope. As mentioned above, the live optics must typically suppress wind disturbances by about five orders
of magnitude, and the simulations carried out so far seem to indicate that the task of correcting for wind disturbances by adaptive optics will be at least as demanding as that of correcting for atmospheric aberrations. For computational reasons, at the time when the simulations were performed, it was not possible to simulate the full system with a adaptive optics wavefront sampling rate at the level required. The sampling time was 18 ms, which is 510 times longer than required. The results shown here are therefore only tentative at this time but they do give indications on the nature and magnitude of the problems to solve. Figure 7 shows typical results from the first tentative simulations. The figure to the left is valid for wind disturbances without any correction by live optics whereas the plot to the right covers the same load case with full correction by the segment alignment system, the secondary mirror rigid-body motion control system and adaptive optics. It can be seen that the residuals are of the order of 20-30 microns, clearly unsatisfactory for observations at 2.2 microns. The residuals seem to be two-fold, firstly low spatial frequency residuals originating from wind deformations of the telescope and secondly residuals from the lack of proper dynamical edge phasing of the individual primary mirror segments (“sawtooth effect”). Adaptive optics with a continuous deformable mirror will not easily correct for the saw-tooth effect. Although the results shown here are somewhat disappointing, there will clearly be a significant improvement once the sampling interval of the adaptive optics system is reduced as indicated above. However, wind load on the telescope still appears to be critical for any ELT. Figure 8 shows similar results for atmospheric aberrations and no wind (for the K-band). The residuals include waffles from the reconstructor algorithm. A plot of the point spread function corresponding to the right-hand part of Figure 8 can be seen in Figure 9. Again, these results may be improved once the wavefront sensor sampling interval is reduced. It seems within reach to achieve a Strehl ration of about 0.58, which is the error budget value for this part of the system, with a goal of 0.4 for the complete system including single conjugate adaptive optics.
Figure 7. Optical path differences at the exit pupil of the telescope. Left: Wind disturbance with no correction by the primary mirror segment alignment system, the secondary mirror rigid body motion control, and adaptive optics control of the secondary mirror. Right: The same load case, however with all components of the live optics system, including single conjugate adaptive optics, engaged. There are residuals of the order of 20-30 microns.
7. CONCLUSIONS The Euro50 studies have lately concentrated on the science case and on identification of areas that could possibly jeopardize total system performance. Four science fields of particular interest for extremely large telescopes have been identified. Also, we have concluded that atmospheric dispersion may be critical for high-performance imaging with
adaptive optics for extremely large telescopes, where only relatively narrow spectral bandwidth observations with required performance may be possible. Early tentative simulations also show that wind load on the telescope, even with a protective enclosure, may be critical. System residuals from wind tend to smear out the center peak of the point spread function outside the peak. This may compromise the possibility of observing earth-like planets orbiting near-by stars. Further studies are needed in this respect and close collaboration with instrument designers together with a careful evaluation of the science case are needed to achieve a compromise that maximizes the scientific return of an ELT. Studying earth-like planets orbiting nearby stars with an extremely large telescope will pose a particular challenge.
Figure 8. Optical path differences at the exit pupil of the telescope with atmospheric disturbances only. Left: No correction by live optics, right: full correction by live optics including single conjugate adaptive optics.
Figure 9. Point spread function corresponding to the optical path difference plot in the right-hand part of Figure 8.
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