{"id":1108,"date":"2023-06-09T07:49:49","date_gmt":"2023-06-09T07:49:49","guid":{"rendered":"https:\/\/library.iiap.res.in\/collaborate\/?p=1108"},"modified":"2025-06-20T06:30:50","modified_gmt":"2025-06-20T06:30:50","slug":"soft-actuators-for-the-thirty-meter-telescope","status":"publish","type":"post","link":"https:\/\/library.iiap.res.in\/collaborate\/?p=1108","title":{"rendered":"Soft Actuators for the Thirty Meter Telescope"},"content":{"rendered":"\n

Prasanna Deshmukh<\/strong><\/p>\n\n\n\n

India’s participation in the TMT project will provide Indian astronomers an opportunity to carry out frontline research in astronomy. Another major reason to participate in this scientific endeavour which integrates latest innovations in segmented mirror design, precision control and adaptive optics, is to bring home some of these engineering expertise through international collaboration. Primary mirrors (M1) Segments are the most critical components for the TMT project. There are 492 (574 including spares) segments of 1.44 m diameter each with a thickness of 45 mm. All the segments need to be hexagonally cut and polished to the roughness of about 22 \u00c5 RMS or to about 20 nm peak-to-valley (PV), and without any subsurface damage. All the segments are off-axis and aspheric. In order to achieve very high spatial resolution as well as sensitivity, all the 492 hexagonal mirror segments of the TMT must be precisely positioned within few nanometres with respect to each other to form a 30-meter hyperboloid primary mirror.<\/p>\n\n\n\n

The segmentation of the primary mirror enables us to construct large telescopes but on the other hand, brings an increased level of control challenges. A typical Segmented Mirror Telescope (SMT) equipped with Adaptive Optics (AO) has several control loops working at different temporal and spatial frequencies are responsible for the final image quality. The architecture of different loops of SMT depends on the amplitude and the frequency range of different disturbances acting on it. Major disturbances include: (a) the gravitational deformation of the telescope, (b) the thermal expansion of the telescope structure, (c) the wind-induced deformation of the telescope, and (iv) the wavefront deformation due to atmospheric turbulence. (Angeli, Cho and Whorton 2003).<\/p>\n\n\n\n

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Figure1 : Schematic of Hard actuator of the KECK (left) and Soft actuator of the TMT
(right). (Graphics adopted from (D. G. MacMartin 2003) and (Lorell, Aubrun, et al.
2006)).<\/em><\/figcaption><\/figure><\/div><\/div>\n<\/div>\n\n\n\n

The relatively slower main axis loop (telescope mount control) ensures that the telescope is pointing toward the field of interest in the sky. Typically it involves two axes of rotation namely altitude and azimuth, both running in the closed loop tracking mode at predefined track rates. The fact that the primary mirror is segmented demands all the segments to be co-aligned and co-phased initially to act as a single mirror surface and further needs to be maintained in the presence of the different disturbance. The initial alignment is typically done using an Alignment and Phasing System (APS) and once done this loop is disconnected for few weeks until realignment of entire M1 is necessary. The role of maintaining the mirror shape is done by the global control system which continuously monitors the edge sensor readings (two edge sensors per inter-segment edge) and commands the actuators below each segment (three actuators per segment) to correct for any deviation of edge sensor readings compared to the reference. Further, relatively faster loop at the individual actuators (local controllers) makes sure that incoming set points from the global controller are maintained continuously by the actuator. This local closed loop controller is necessary to maintain the position of actuators to the desired set point, in the presence of wind & vibration disturbances.<\/p>\n\n\n\n

The actuators used in the Segmented Mirror Telescope are responsible for providing the tip, tilt, and piston to the individual mirror segments. Such actuators can be classified into two broad categories: hard\/rigid actuator and the soft actuator. The actuators used in existing segmented mirror telescope such as W. M. Keck Observatory (Keck) (Meng, et al. 1990), Southern African Large Telescope (SALT) (Swiegers and Hitesh 2004), Gran Telescopio Canarias (GTC) (Lefort and Castro 2008), Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) (Xu, Xu and Jin 2003), and Hobby\u2013Eberly Telescope (HET) (Krabbendam, et al. 1998) , are hard\/rigid actuators and have extremely high axial stiffness. One of the main drawbacks of a hard actuator is, it supports very low control bandwidth, which results in poor active dynamic interactions between the actuator and the mirror segment and hence cannot suppress high frequency disturbances induced by wind and other structural vibrations. Whereas, the soft actuators proposed for upcoming SMTs like the Thirty Meter Telescope (TMT) (Lorell, Aubrun, et al. 2006), (J. Nelson 2005) and the European Extremely Large Telescope (E-ELT) (Jim\u00e9nez, et al. 2010) uses a Voice Coil Motor (VCM) that gives high bandwidth actuation. The soft-actuator is fairly straightforward to implement, relatively inexpensive, lightweight and compact. It has very few moving parts, capable of producing large forces, can provide large mechanical range, and does not require lubrication.<\/p>\n\n\n\n

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Figure2 : Soft Actuator of TMT M1CS. (Image: ITCC\/Prasanna Deshmukh)<\/em><\/figcaption><\/figure><\/div><\/div>\n<\/div>\n\n\n\n

The TMT actuator (Lorell, Aubrun, et al. 2006) is an off-axis mechanism with VCM as its prime mover and an offloader for taking care of static loads. The VCM and stepper motor based offloading mechanism are linked to the actuator output shaft through the lever arm, hence providing amplification in the force as well as in displacement. In the TMT actuator, for effective rejection of high-frequency disturbances, a passive eddy current based damper which uses two powerful magnets and a copper plate have been incorporated. The TMT actuators use flexural elements for vibration isolation and reducing the wear and tear due to friction. The local control loop runs at a very faster rate uses a very high-resolution optical encoder directly coupled with the output shaft of the actuator.<\/p>\n\n\n\n

All the 492 hexagonal mirror segments of the TMT needs to be precisely positioned with respect to each other to form a 30-meter hyperboloid primary mirror. The M1 control system (M1CS) performs this task, with the help of actuators that corrects for the segments’ tip-tilt and piston errors measured by edge sensors. Actuator corrections are critical to retain the shape of the mirror that are otherwise disturbed due to wind and vibrations. Each mirror segment will be driven by three actuators, and altogether 1,476 actuators are required to keep all the segments aligned. Each of these actuators are made up of several precision manufactured components put together to act as a soft actuator working to nanometric accuracies and can provide tip, tilt and piston to each mirror segment with an accuracy of 4 nanometers.<\/p>\n\n\n\n

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Figure 3: Actuators (P2e) manufactured at 4 industries in India. (Image credit:
ITCC\/Prasanna Deshmukh)<\/em><\/figcaption><\/figure><\/div><\/div>\n\n\n\n
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Figure 4: P2e Actuator undergoing Functionality testing at Southern Electronics,
Bangalore using ITCC\u2019s actuator functionality test equipment. (Image credit:
ITCC\/Prasanna Deshmukh)<\/em><\/figcaption><\/figure><\/div><\/div>\n<\/div>\n\n\n\n

India-TMT is responsible for the manufacturing of all the 1,476 (1,536 including spares)
actuators required for the TMT project. In 2018-2019, 20 P2e Actuators were successfully fabricated and functionality tested at 4 Indian industries (Indo Danish Tool Room (IDTR), Jamshedpur., Southern Electronics, Bangalore., Tamboli Engineers Pvt. Ltd., Pune., Amado Tools, Bangalore.). These actuators were shipped to TIOPO in the USA for further performance testing (at Jet Propulsion Laboratory (JPL), Pasadena, CA) and Accelerated Life Testing (at The Pilot Group, Monrovia, CA).<\/p>\n\n\n\n

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Figure 5: ITCC Team with Actuators (P2e) manufactured in India being shipped from
ITCC to TMTPO. (Image credit: ITCC\/Prasanna Deshmukh)<\/em><\/figcaption><\/figure><\/div><\/div>\n\n\n\n
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Figure 6: P2e Actuators installed on the Accelerated Life testing setup at The Pilot Group, California. (Image credit: TMT\/TPG)<\/figcaption><\/figure><\/div><\/div>\n<\/div>\n\n\n\n
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Figure 7: P2e Actuator (manufactured by Tamboli Engineers, Pune.) undergoing
Performance testing at JPL, California. (Image credit:TMT\/JPL)<\/em><\/figcaption><\/figure><\/div><\/div>\n\n\n\n
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Figure 8: Actuators being life tested in cold condition loaded into freezer at The Pilot Group, California. Custom control boxes shown at right. (Image credit: TMT\/TPG).<\/em><\/figcaption><\/figure><\/div><\/div>\n<\/div>\n\n\n\n

All 20 P2e actuators manufactured in India, underwent Functionality Testing at respective vendors place. All the Actuators passed the functionality requirements. Performance testing of 7 Actuators was done in Warm and Cold (-15oC) conditions, no issues were found and results were satisfactory. Out of 20 P2e actuators manufactured in India, Life Testing on 12 P2e Actuators (3 warm, 9 cold) was conducted at The Pilot Group in Monrovia, CA. All 12 actuators successfully completed the required cycles.<\/p>\n\n\n\n

The actuators manufactured in India have successfully undergone the performance tests and lifetime tests conducted at The Pilot Group in Monrovia and at Jet Propulsion Laboratory (JPL) in Pasadena, USA. Based on the outcomes of P2e Actuator Manufacturing and Testing, improvised P3 actuators design is realised and is planned to be manufactured in India in 2021.<\/p>\n\n\n\n

(To be continued\u2026)<\/em><\/strong><\/p>\n\n\n\n

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About the author<\/strong><\/p>\n\n\n\n

Prasanna Deshmukh<\/strong> is an Engineer at IIA. His research interests include primary mirror control systems for segmented mirror telescopes, control systems, and astronomical instrumentation.<\/p>\n\n\n\n

India-TMT M1CS Actuator Team:<\/strong> Prasanna Deshmukh, Viswanatha N, P K Mahesh, Varun Saraswat, Lalit Kumar, Jayakumar P.S, Jeevan V, Surojit Roy, S.Hari Prasath, Sudharsan K, Vaishaly, Snehashis Bhattacharya, Vaishaly Nigam.<\/p>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Prasanna Deshmukh India’s participation in the TMT project will provide Indian astronomers an opportunity to carry out frontline research in astronomy. Another major reason to participate in this scientific endeavour which integrates latest innovations in segmented mirror design, precision control and adaptive optics, is to bring home some of these engineering expertise through international collaboration….<\/p>\n","protected":false},"author":5,"featured_media":1116,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_kad_post_transparent":"","_kad_post_title":"","_kad_post_layout":"","_kad_post_sidebar_id":"","_kad_post_content_style":"","_kad_post_vertical_padding":"","_kad_post_feature":"","_kad_post_feature_position":"","_kad_post_header":false,"_kad_post_footer":false},"categories":[17],"tags":[],"_links":{"self":[{"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=\/wp\/v2\/posts\/1108"}],"collection":[{"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=\/wp\/v2\/users\/5"}],"replies":[{"embeddable":true,"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1108"}],"version-history":[{"count":4,"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=\/wp\/v2\/posts\/1108\/revisions"}],"predecessor-version":[{"id":3251,"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=\/wp\/v2\/posts\/1108\/revisions\/3251"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=\/wp\/v2\/media\/1116"}],"wp:attachment":[{"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1108"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1108"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/library.iiap.res.in\/collaborate\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1108"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}