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LAMA prototype telescope. Hickson, Paul 2004

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The LAMA prototype telescope Bruce E. Truaxa, Kenneth M. Lanzettab, Paul Hicksonc a Diffraction Limited Design LLC, 388 Wedgewood Road, Southington, CT 06489, USA, btruax@dld-Ilc.corn b Deparment of Physics & Astronomy, State University of New York at Stony Brook, Stony Brook, NY 1 1 794, USA, lanzetta@sbastr.ess.sunysb.edu C Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Raod, Vancouver, BC V6T 1Z1, Canada, hickon@physics.ubc.ca Keywords: Astronomy, Telescope, mercury mirror, adaptive optics, interferometry ABSTRACT As a step toward the Large-Aperture Mirror Array, the LAMA telescope consortium is planning the construction of a prototype telescope. Intended as a test bed for the required technologies, the LAMA Prototype Telescope (LPT) would be a coherent array of six 6. 15-rn liquid rnirrors. Like the LAMA telescope, each telescope would be provided with tracking optics, path-length equalization, phase tracking and adaptive systems. The beam combiner, consisting of six concave adaptive mirrors, would have the Fizeau geometry enabling wide-field interferometric imaging. In order to facilitate construction, testing and operation, the LPT will be located at or near a developed astronomical site in the continental United States. While the primary purpose of the facility is to develop and prove the LAMA telescope concept and technologies, it will also be a powerful instrument for scientific research. With a light-collecting area equivalent to that of a 15-rn telescope, the LPT would be capable of interferometric imaging with the resolution of a 20- rn telescope. The telescope would be provided with an infrared imaging camera. This paper describes the telescope design and discusses the main technical challenges that must be faced. 1. INTRODUCTION The Large-Aperture Mirror Array telescope (LAMA) is an ambitious project to build a close packed hexagonal array of 66 liquid mirror telescopes12. The individual telescopes will have apertures of 6.15 meter diameter. Each telescope will have adaptive wavefront correction and the images from the individual telescopes will be combined via interferometry to create a diffraction-limited image. The light collecting power of the LAMA telescope array will be equivalent to that of a 50-meter telescope with a resolution in interferometric mode equivalent to a 70-meter aperture. A unique feature of the LAMA telescope will be a system of tracking optics which allow the array to track objects within a 4 degree radius about the zenith. The ambitious goals of the LAMA telescope make it a difficult undertaking. The LAMA consortium has determined that the prudent first step will be a smaller prototype array consisting of 6 telescopes and known as the LAMA Prototype Telescope (LPT). The goal of the LPT is to develop and prove the technology that will be used in the LAMA telescope. Not only will the LPT act as a test bed for the optical and servo systems, but it will also provide useful test data onhow to construct the enclosure and manage airflow around the mirrors. Located in an accessible location in the United States, development and construction costs will be manageable. Once the LAMA telescope construction has begun, the LPT site will be used as a staging and testing facility for the optical subsystems prior to delivery and installation at the remote site envisioned for the LAMA telescope. As an added benefit, the LPT will be an important scientific instrument3. Equipped with an infrared camera the LPT will identify and study very high redshift supernovae. Other important projects could also be investigated if more instrumentation is made available. Second Bäckaskog Workshop on Extremely Large Telescopes, edited by Arne L. Ardeberg, Torben Andersen, Proceedings of SPIE Vol. 5382 (SPIE, Bellingham, WA, 2004) 0277-786X/04/$15 · doi: 10.1117/12.566272 267 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms 2. GOALS OF THE LPT PROJECT The LAMA telescope team has identified the 9 critical issues listed below, which need to be developed and/or proven for the project to be successful. The mission of the LPT project is to address and eliminate any uncertainty in these 9 critical items. 2.1 Achieve a high quality mirror surface in operation The imaging goals of the telescope require that the primary mirror surface be parabolic to 20 nm or better. The quality of the mirror surface is affected by the stability of the mirror rotation servo and by airflow over the mirror. Studies performed on earlier mirrors have shown that the mirror rotation rate must be stable to better than 1 ppm. The LZT has already demonstrated the ability to achieve this level of stability on a 6 meter mirror. The more difficult challenge will be to manage the airflow around the mirror. Airflow near the mirror has two sources, the rotation of the rotation of the mirror itself and environmental wind. The goal is to keep the air velocity at the mirror surface laminar and low enough to achieve a 2Onm surface with environmental wind velocities up to the 80% probability velocity at the selected site. The exact value is dependent on the thickness of the mercury and this needs to be determined. 2.2 Achieve diffraction limited imaging performance The goal for LAMA is to achieve a Strehl ratio of greater than 0.5 for each telescope at 1 .tm for 80% of the observing conditions. Reaching this goal requires that the primary mirror, tracking optics, relay optics and beam combining optics together produce a diffraction limited wavefront. The implications are that the optical design must have very low residual error, and the optical fabrication and mounting tolerances will be very precise. In addition, optical alignment must be done very accurately and it must be maintained as the optics move for tracking and optical path length compensation. 2.3 Automatic forming and stabilization of 6.15 meter liquid mirrors It seems that it should be simple to form a liquid mirror: Pour the liquid into a container, spin the container and a perfect parabola is formed. In practice forming a liquid mirror is a difficult task. The final mirror should have a liquid layer that is as thin as possible to rapidly damp surface waves4. The high surface tension of mercury combined with mercury's inability to wet most surfaces makes it difficult to form a large continuous surface, particularly if the layer is thin. The 6.0 meter Large Zenith Telescope and the smaller liquid mirror telescopes which preceded it have all been formed by manually spinning up the mirror using human muscle and a lot of finesse. Once spinning the mirrors are only halted for occasional cleaning to remove dust and other debris that accumulates on the mirror surface. Manually starting a mirror once per month after cleaning is an acceptable method of operation. With 66 mirrors and monthly cleaning, at least two mirrors per day will need to be cleaned and restarted making automatic startup a mandatory requirement. Automation of this process will require starting the mirror with sufficient mercury so that the potential energy of surface tension is overcome by gravitational potential energy. Experiments with the LZT indicate that this requires more than 3mm of mercury. Once the mirror has closed and created a continuous surface, it takes about 2 hours for an oxide layer to form. The oxide seals the surface and helps to stabilize the mirror making it possible to remove the excess mercury and reduce the thickness to less than 1 mm. The LZT telescope is testing these assumptions individually. The task of the LPT is to integrate and automate the process. 2.4 Develop tracking servo and software No currently available telescope control software packages are configured to handle the requirements of a zenith pointing liquid mirror telescope. The LPT requires either the adaptation of a current telescope pointing and tracking package or the creation of a new package designed specifically for this telescope. 268     Proc. of SPIE Vol. 5382 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms 2.5 Interferometric beam combination Successful interferometric combination of all 6 telescope images at 1 micron and longer wavelengths must be demonstrated. Achieving this goal will require the development of a method to rapidly lock all six telescopes to each other. This method must be scalable to the full 66 element LAMA telescope array. 2.6 Achieve greater than 90% observing efficiency Unlike other telescopes, the LAMA telescope and the LPT have a maximum observing time on any one object of 30 minutes per day. Typical observation will be between 20 and 30 minutes. The telescope will therefore observe an average of 12 fields per night. Maintaining observational efficiency of 90% (measured as a percentage of observable hours after factoring out bad weather and engineering time) requires that all 6 telescopes be capable of slewing to a new object and acquiring lock of the adaptive system and the interferometers within 2.5 minutes with a goal of 1 minute. Meeting this aggressive time goal will require that the absolute pointing uncertainty of each telescope be smaller than the capture range of the adaptive system or about 2 arc seconds. This is an aggressive goal for any telescope. The LPT has the advantage that it only needs to achieve this accuracy over an 8 diameter tracking field. 2.7 Achieve extrapolated reliability of better than 90% Achieving operational reliability of 90% for the full 66 element LAMA array requires that the reliability of any one telescope be better than 99.84% or less than one disabling failure every 2 years of operation (assuming 80% of nights are observable). Therefore the reliability of the 6-element LPT array must be better than 99%. 2.8 Develop an observatory structure with adequate wind protection and acceptable seeing Requirement 2.2 for a stable, accurate mirror surface during 80% probability wind velocity implies that the observatory enclosure must provide adequate baffling of the mirror surface from the effects of environmental wind. This runs counter to recent practice where telescope enclosures have become more open to eliminate the effects of dome seeing. The enclosure will need to be designed to both protect the mirror from ambient wind while providing adequate flushing airflow to reduce dome seeing to a level that can be corrected with the adaptive mirror. 2.9 Prove that mercury vapors can be kept within an acceptable range Once a liquid mercury mirror has formed and stabilized an oxide layer quickly forms5 on the surface of the mercury. The oxide layer seals the surface and prevents evaporation of the mercury. With previous smaller mirrors the oxide seal reduces the mercury vapors to background levels in 24-48 hours. With a large number of mirrors cycling through a regular mirror cleaning/reforming process special air handling will be required to avoid environmental contamination. 3. LPT PROPOSED DESIGN 3.1 Optics A proposed layout for the 6 element LPT array is shown in Figure 1. Figure 2 shows the optical layout of a single telescope. The key parameters of the individual telescopes are given in Table 1 . The six telescopes are arranged in a hexagonal array. The tracking optics will be approximately 18 meters above the primary mirror. The tracking optics assembly will be mounted on a hexapod translation stage with approximately two meters of lateral motion and 0.5 meter of vertical motion. The light from the tracking optics will be directed to the path length compensation system located in the gap between the mirrors. The path length compensation optics will send the collimated light to the pupil positioning telescope and finally to the beam combining mirror which doubles as the adaptive mirror. The beam samplers for the adaptive wavefront sensor and the interferometer will be located between the beam combiner and the focal plane (not shown). A more detailed description of the optical system is presented in the "LAMA — project overview" paper in these proceedings.6 Proc. of SPIE Vol. 5382     269 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms Fi gu re  1.  LP T Te le sc op e La yo ut  270     Proc. of SPIE Vol. 5382 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms Figure 2. LPT Optical Schematic Be•m Con1cünpr and A.aamve Mirror Compensation Proc. of SPIE Vol. 5382     271 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms The optical throughput of each telescope module will be important to scientific productivity. The path length compensation for interferometry and the pupil imaging requirements for interferometry and adaptive optics add many optical surfaces to the telescope modules. The proposed LPT design has minimized the number of optics needed to meet all of these requirements. Each module will contain 1 5 mirrors in addition to the primary, plus a beam sampler. All of the telescope mirrors except for primary will be coated with protected silver. An estimate of the throughput of a single telescope is given in Table 2. Parameter Value Mirror diameter 6. 1 5 meters Primary mirror f-ratio 3.0 Instantaneous field of view 30 arc seconds Tracking Range 4 degree radius centered on the Zenith Pupil Demagnification factor 20 Table 1 . LPTTelescope Parameters 3.2 Servo Systems The current design requires 20 unique servo axes. Of these 20 servos, the pointing and tracking software will control 16. These 16 axes will track the object on the sky, maintain approximately equal path between all of the telescopes, maintain the proper pupil position on the beam-combining mirror and position the parabolic compensator to properly remove the wavefront errors introduced by the primary mirror. Three additional servo axes will be used for high speed tip/tilt tracking and high speed interferometric path length compensation. These three servos will be controlled independently of the pointing and tracking systems but they will feed back their average positions to the pointing and tracking system to correct for long term drift. The servo axis will be for the rotation of the primary mirror. The primary mirror servo will be completely independent of the pointing and tracking system. The proposed error budgets for the 16 tracking servo axes and the primary mirror rotation servo are given in table 3. Wavelength (pm) Protected Silver Hg Beam Sampler Total .4 .955 .76 .9 34% .5 .978 .76 .9 49% .6 .978 .77 .9 49% 1.0 .98 .784 .9 52% 1.5 .983 .802 .9 56% 2.5 .988 .829 .9 62% Table 2. Estimated Light Efficiency ofLPT Telescope 3.3 Primary Mirror Support System Achieving a high quality primary mirror surface depends on precise and well-controlled mirror rotation, a rigid and accurate primary mirror support surface, a thin layer of mercury and well-controlled airflow over the mirror surface. These requirements are intimately related. A thin layer of mercury damps out disturbances more effectively making it less susceptible to distortions caused by airflow over the surface. The goal of the LPT will be to reduce the mercury layer thickness to less than 1 mm. In order to achieve such a thin, stable layer of mercury, the underlying support surface 272     Proc. of SPIE Vol. 5382 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms will have a shape which is accurate to better than 100 microns. Finally, a stable rotation rate is critical to maintaining a constant surface shape. The primary mirror support structure will be made from a center hexagonal segment and eight "petal" segments. Each of these 9 segments consists of a monolithic front skin backed by ribs in a triangular cell pattern. Additional stiffness and a more aerodynamic shape will be achieved by welding a bottom skin to each of the segments. Figure 3 shows the proposed construction for the center segment and one of the petals. The nine segments will be bolted and pinned together and the gaps between the segments filled with adhesive to form a continuous top surface. Each individual segment will be machined accurately allowing the segments to be bolted together to the required precision. Figure 4 shows how the segments are assemble to form a complete mirror. The entire mirror assembly will be mounted at its center to a Precision Instruments 23R air bearing. The air bearing will provide smooth, frictionless rotation avoiding the "rumble" of mechanical bearings that would disturb the mirror surface. The air bearing will also contain the drive motor and encoder required for the primary mirror rotation servo. Axis Description Range Accuracy 1 PM Rotation Continuous 1 ppm 2 M2 Rotation about horizontal axis degrees 0.25 arc seconds 3 M2 Rotation about vertical axis 0 4 M5 Rotation about horizontal axis TBD TBD 5 M5 Rotation about vertical axis TBD TBD 6 Tracking optics horizontal motion (North/South) 2.6 meters 20 tm 7 Tracking optics horizontal motion (EastlWest) 2.6 meters 20 j.tm 8 Tracking Optics rotation about vertical axis (perpendicular to optical axis) degrees 0.25 arc seconds 9 Tracking Optics rotation about horizontal axis (parallel to optical axis) degrees 0.25 arc seconds 10 Tracking Optics rotation about horizontal axis degrees 0.25 arc seconds 1 1 Tracking optics vertical motion 45 mm 20 jim 12 Path length compensator horizontal motion 1.5 meters 1 jim 13 Path length compensation mirror rotation about vertical axis degrees 0.25 arc seconds 14 Path length compensation mirror rotation about horizontal axis degrees 0.125 arc seconds 15 Pupil relay vertical motion mm 25 jim 16 Pupil relay horizontal motion (along optical axis) meter 10 jim 17 Pupil relay horizontal motion 10 jim Table 3. LPT Servo Summary Proc. of SPIE Vol. 5382     273 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms The current plan is to locate the LPT observatory on a ridge approximately 2 km south of Apache Point Observatory. The telescope enclosure itself will be positioned at the edge of the ridge, partially embedded in the ground. The side walls of the observatory will extend to the approximate height of the tracking optics, 18 meters above the surface of the primary mirrors. During observing the roof of the observatory will roll back on tracks completely opening the top of the enclosure. Figures 5 and 6 show a proposed schematic layout for the observatory. Not shown in these views are the internal wind baffles that are required to control the airflow in the vicinity of the primary mirrors. Figure 3. LPT primary mirror edge and center segements 3.4 Observatory Figure 4. Primary mirror assembly 274     Proc. of SPIE Vol. 5382 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms Focal Pla ,icRoom 2os Figure 5. Proposed Observatory Plan View Figure 6. Proposed Observatory Elevation View 1' It, Elevator to telescope level andlower level entry foyer Proc. of SPIE Vol. 5382     275 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms 4. BACKUP AND SAFETY SYSTEMS Protection of the environment as well as the telescope infrastructure will be important goals of the LPT. Each time a primary mirror stops and restarts the oxide layer on the top of the mercury is broken and mercury vapors are emitted into the atmosphere. Therefore it will be desirable to run the mirrors continuously, stopping them only when necessary for cleaning or maintenance. Reliability of the mirror drive system will be critically important. At most observing sites, the biggest reliability problem is typically interruptions in mains power. Typical of other observatories, the LPT will have a backup generator which will automatically come on line quickly after a power failure. The air to support the air bearings will be supplied by a system containing dual, redundant air compressors and filters as well as a large air storage tank capable of keeping the bearings floating for at least 15 minutes. During the period required to start the generator, the drive motors will not be powered, but because of the near zero friction of the air bearings the speed will not slow significantly during the time required to bring the generator on line. If the mains power and generator both fail, then after about 5 minutes an automatic braking system will bring the primary mirrors to a safe halt before the air reservoir is exhausted. Another possible failure mode is an imbalance in the primary mirror dish during the mirror forming process. If the mercury layer "tears" before oxidation and a hole forms in one area of the mirror, the rotating mass will become unbalanced. The resulting moment could exceed the limits of the bearing. To avoid damage to the bearing, the mirror support structure will sit freely on the bearing. The coupling between the bearing and the mirror will allow the mirror to tilt before the moment exceeds the bearing's limits. To avoid a mercury spill under these conditions, the outer rim of the primary mirror will ride a few millimeters above a series of safety wheels which will catch the edge and keep it from tipping far enough to spill any mercury. In the unlikely event that a mirror does fail and cause a mercury spill, the area containing the primary mirrors will be designed as a containment area. This area will trap the mercury and keep it from escaping into the environment. 5. SUMMARY The LPT is a critical intermediate step on the path to the full LAMA array. It will allow the technologies that are critical to the success of the LAMA telescope to be proven and refined at a convenient site in the United States. Once LAMA begins construction, the LPT site can be used as a staging and test area for telescope modules prior to shipment to the LAMA telescope for installation. The LPT will also be a powerful telescope in its own right capable of significant scientific work. Current estimates are that the LPT can be designed, built and commissioned for a cost of $10 million to $15 million. 1 Hickson, Paul and Lanzetta, Ken, "Large-Aperture Mirror Array (LAMA): conceptual design for a distributed-aperture 42-meter telescope", Future Giant Telescopes, Proceedings ofthe SPIE, Volume 4840, pp. 273-282, 2003. 2 Hickson, P, Borra, E. F. , Cabanac, R., Chapman, S. C., de Lapparent, V., Mulrooney, M., Walker, G. A., "Large Zenith Telescope project: a 6-rn mercury-mirror telescope", Advanced Technology Optical/JR Telescopes VI, Larry M. Stepp; Ed., Proc. SPIE Vol. 3352, p. 226-232 1998 3 Lanzetta, Ken, and Hickson, Paul "Science with the Large-Aperture Mirror Array", 2nd Bäckaskog Workshop on Extremely Large Telescopes, September 9-1 1, 20034 Tremblay, Grdgoire and Borra, Errnanno F. "Optical Tests of a 3.7-rn diameter Liquid Mirror: Behavior under External Perturbations", Applied Optics, Vol. 39, No. 36, 5651 Hickson, Paul, Cabanac, Remi, and Watson, Suzanne, "A study of mercury vapor concentrations at the UBC/Laval 2.7 meter Liquid-Mirror Observatory", 13 November 1993, University of British Columbia Internal Paper 6Hickson Paul and Lanzetta, Ken, "Large Aperture Mirror Array — Project Overview", 2nd Backaskog Workshop on Extremely Large Telescopes, September 9-11, 2003 276     Proc. of SPIE Vol. 5382 Downloaded from SPIE Digital Library on 20 Sep 2011 to Terms of Use:  http://spiedl.org/terms


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