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KECK OBSERVATORY The world’s biggest optical telescope; Pt.2 Last month, we gave the background to the site selection and segmented design of the 9.84-metre Keck Telescope. The guid­ing force for the project was Jerry Nelson who had the job of promoting the concept & convincing enough people to give finan­cial grants to allow it to proceed. By BOB SYMES Ultimately, he was successful in convincing the astronomers and accountants that the challenge could be met, and the problems overcome. Armed with a $US70 million grant from the W. M. Keck Foundation, Nelson and his collaborators set to work. The Cali­fornia Institute of Technology and the University of California made up the difference in the projected cost of $US94 million. These two institutions will run the telescope through the California As6  Silicon Chip sociation for Research in Astronomy (CARA), an association inaugurated specifically for this purpose. Through CARA, they will allocate the major part of observing time, though the University of Hawaii will receive 10% of the time as co-ordinator of the science reserve atop Mauna Kea. On September 12th, 1985, the ground-breaking ceremony took place on the summit and the dome and associated complex was completed in October 1988. The tube and supporting structure was contracted out to the civil engineering firm of Schwartz and Hautmont of Tarragona, Spain and was also completed in 1988. It was erected on the summit in 1989. Understandably, the mirrors caused the major headaches. At every step of the way problems arose and had to be overcome. Since multiple mirrors, when used together, cause optical diffraction effects if they remain as individual round segments, it was necessary to construct hexagonal segments that nestle into each other to minimise the effect. Under certain circumstances, such as when two telescopes are used as an optical interferome­ter, it is these very diffraction effects that are used to ex­tract information about the object under study, but when the telescope is used on its own, the diffraction spikes can hide details that might otherwise be observed. A further effect of diffraction is that contrast is re­duced, thus further hiding subtle detail. Squares, triangles and hexagons are the only shape of mirror that can nestle together in this fashion. From the point of view of wasted material and keeping the shape as nearly round as possible to make figuring easier, a hexagon shape was chosen. And this is where the problems began. Normally, a mirror is ground and polished in its final (usually circular) shape. But a new technique, known as stressed mirror polishing, was to be attempted. In this method, the polishing table has a series of suction pads and rams which distort the blank before polishing begins. The mirror is then polished to a spherical figure, and when it is released from the table, the correct hyperboloidal figure would be obtained. Terry Mast, the University of California optician who over­ s aw most of the design and construction of the mirrors, deter­mined that the correct shape would not be realised unless the blanks were polished in the round and then cut to hexagons, rather than the other way around. The danger was that when cut, internal stresses in the blank would be released, thus throwing out the carefully created profile. Less of a problem, but still requiring careful attention, was that since each of the 36 mirror segments has one of six possible different surface profiles, dependent on where it will be in the final mosaic, the radial position of the hexagonal sides had to be in exact relationship to the figure. Optics fabrication Itek Optical Systems of Lexington, Massachusetts was chosen to fabricate the optics, as they had much experience in satellite optical systems. The first six segments were to be delivered by late 1987 and the following 36 (which included six spares – one for each position) were to be made available within two years. However, by late 1987, work was still being done on the first segment and by mid- 1988 the second was giving trouble. The feared stress-relief distortions had materialised and each segment had to be individually touched up under computer control, optically tested using a laser interferometer, and then touched up again, until the residual errors were within the ability of the This diagram shows the location of the 36 segment primary mirror, the secondary (2) and tertiary (3) mirrors and the Nasmyth (4) & Cassegrain (5) foci. The tertiary mirror is required for the Nasmyth focus but is removed to allow light to pass through a hole in the primary mirror to the Cassegrain focus. warping harness on the tele­ scope mirror mount to correct. As a result of this delay, in 1989 CARA contracted another optical laboratory, Tinsley Laboratory of Richmond, California, to take over the construction of half the mirror segments. Work was under way by February. Both the Tinsley and Itek blanks were “hexagonised” at the Itek works, and by mid-1989 two segments per month were being produced between the two contractors. By this time, it had been decided to forego the computer controlled zonal refiguring, since this was proving too slow, and it was hoped that the warping harness could cope with the now greater residual aberrations. In fact, the after hexing deformations have been reported to be as great as 1 micron. The warping harness is a series of adjustable springs on the support structure of each mirror. There are 30 such springs for each segment and when correctly set, they can reduce the residual aberration by a factor of up to 15. At least the delivery was easy, unlike the delivery of the great primary of the 5-metre Hale telescope which made a slow journey from the Corning Glass works in New York to the west cost on a specially constructed flatbed railcar. By contrast, the mirror segments for the Keck were shipped from Lexington, Mas­ sachusetts to Honolulu by Federal Express! They were then sent by barge to Hilo on the windward side of the Big Island and by truck to the summit. The relatively low weight of each segment made this method quite feasible, something that couldn’t be said of the massive Hale mirror. In common with most observatories, there is a re-aluminis­ing facility in the building, so that the mirrors do not need to leave the mountain when the reflecting surfaces need to be refur­ bished. In fact, they were delivered from the mainland uncoated and were aluminised just prior to installation. Each mirror is housed in a complex support that includes adjustable pads, feedback sensors and actuators, as well as the preset warping components. The requirement is that every segment is supported in such a way that all act together to form one 10-metre mirror. Each segment be in perfect collimation with all the rest in the mirror support framework and must be able to correct for the inevitable tube flex­ure of a structure as large as this when the August 1993  7 movement is in the order of 1mm, in increments of 0.004 microns. The position actua­tor consists of a precision ground screw of 1mm pitch. Shaft encoders allow the screw to be turned in increments of one ten-thousandth of a revolution. This 1mm per revolution displacement is further reduced by a factor of 24 by a ratio-reducing hydrau­ lic bellows unit. Capacitive feedback sensors This view, taken from within the tubular structure of the tele­scope, shows all the mirror segments in place. In all, some 36 hexagonal segments are used to create the primary mirror. telescope is slewed from one part of the sky to another. Flexure of the tube has been estimated to be in the order of 0.5mm as the telescope is pointed in different directions, and this flexure has to be reduced by a factor of 10,000 in order to maintain the perfect collimation required to give the sub arc-second images that the site is capable of producing. The actua­tors are also capable of detecting and correcting thermal changes in the mirror and support structure. Mirror support system In order to provide this required collimation, the mirror support system comprises passive and active support. The passive support is made up of a stainless steel hub and disc (the flex disc), which sits in a circular cutout in the rear of the mirror and prevents the mirror moving laterally from its assigned posi­ tion. Support for the mass of each mirror is by means of three “whiffletrees” evenly spaced about the mirror, and about two thirds 8  Silicon Chip of the way out from the centre – at the radial centre of mass of the segment. Each whiffletree contains a further 12 floating supports, giving a total of 36 floating supports per segment. The principal is similar to the technique used by thousands of amateurs for their home-made telescopes, only mechanically far more complex and, of course, on a completely different scale. Effectively, each mirror segment is able to tilt or shift to counteract the previously discussed errors. By the way, the word “whiffletree” comes from the days of stagecoaches, where the whiffletree was the pivoting wooden cross-arm attached to the drag spar. By pivoting, it compensated for any uneven pull by the horses on either side of the spar. Each mirror segment, thus being able to move freely within its lateral confinement, allows the active control system to tilt or move it toward or away from the focus in order to maintain collimation. Each segment has three position actuators associated with it, one on each whiffletree. The total Feedback for the actuators is supplied by temperature com­ pensated displacement sensors, consisting of parallel plates mounted on each mirror, with a third plate, called the paddle, attached to the adjacent mirror, placed between the first two. The change in capacitance induced by any relative shift between the two mirrors is detected and the resulting corrective commands are sent to the position actuators. Each internal segment has 12 sensors attached to it and each peripheral segment has six or eight, depending on wheth­er it is a corner or side segment. The sensitivity of this system is such that displacements of the order of 0.001 microns can be detected. Jerry Nelson states that there are actually 63 more sensors than are required to define the mirror shape, so there is sufficient redundancy to keep the telescope functioning to speci­fication even if there are some sensor failures, assuming those failures are randomly distributed around the various mirrors. This also gives the ability to switch out a (faulty) sensor that is returning readings that are substantially different from its neighbours, whilst still allowing the tele­scope to operate normally. This is similar to the multiple sensor “democratic” systems used on aircraft computer controls. He further comments that a great advantage of the active control chosen is that it relies on no external source to define its parameters. When the telescope is switched on, it corrects itself and is ready for work. This can be done at any time, day or night, or even with the dome slit closed. Thus, engineering calibration or work can be carried out when ever it is convenient. This contrasts with some active systems, where a star or artificial equivalent has to be viewed and its image analysed before the appropriate commands can be issued to the correcting mechanism. False incoming data, such as air-column or dome turbulence that scatters the incoming star image, is therefore entirely eliminated. The information received from the 168 sensors, the correc­ tive calculations and the correction commands to the 108 actua­tors are handled by 12 microcomputers under the overall command of a DEC Micro-VAX. Corrections are performed every half second, with a 10-second settling time required after a major slew of the telescope. One of the computers is dedicated to maintaining a log of all readings and subsequent actions, so that if anything goes wrong, its data can be analysed to isolate the problem. An example would be where a wire or actuator rod breaks. The computer would sense an alignment problem, send a corrective command, and fail to see a response from the displacement sen­sors. Obviously a runaway condition is then likely. Whilst such conditions can be trapped by the software, by keeping an activity log, the actual source of the problem can be quickly identified. Secondary mirrors There are two interchangeable secondary mirrors that result in overall focal ratios of f/15 and f/25. The f/15 secondary is 1.45 metres in diameter and is intended for work in the visible spectrum. The f/25 secondary is 51cm in diameter and is designed for observation in the infrared region. The f/15 secondary mirror was ground and polished at the Lick Observatory optical laboratories in Santa Cruz, California, under the guidance of master optician David Hilyard and astronom­ e r Joseph Miller, who described it as the most difficult grinding job they had ever undertaken. The mirror is made of Zerodur, is hyper­boloidal in figure and, because of the very small focal ratio of the optics, is highly convex (the radius of curvature is only 4.7 metres). As a result, special flexible polishing laps had to be devised, and progress constantly monitored with a laser profilometer, which could detect aberrations of the order of λ\2. After final figuring, testing by more elaborate optical methods indicated a figure of better than λ\15. The finished mirror was shipped to Hawaii on July 19th, 1991. The optical Great care must be taken in polishing & figuring the mirror blanks & this is done before they are hexagonised. Here an optical techni­cian uses a laser profilometer to check a mirror blank. combination of the f/1.75 primary and the secondary yield a final f/15 focus. This secondary will be used for observations at visible wavelengths. A further complication that occurred during the polishing of this mirror was its distur­bance on the polishing table during the San Francisco earthquake in October 1989. Luckily no damage was sustained and re-align­ment was successfully carried out. The f/25 secondary is made of nickel-plated beryllium. It was figured at the Lawrence Livermore National Laboratory near San Francisco, tested and finally plated with gold. It will be used exclusively for work in the infrared spectrum and has the ability to be used as a “chopper”, mechanically moving to alter­nately provide a view of the object being studied and the back­ground sky. In this way, sky readings can be subtracted from “object + sky” readings to give an “object only” output from the detectors. Each secondary mirror is housed in its own secondary sup­port which can be placed interchangeably forward of the prime focus as required. Both supports have the same external August 1993  9 This photo shows the complex support structure of the main mir­ror. Each mirror segment is monitored & adjusted by the comput­er control system twice every second. shape as the main mirror mosaic to minimise the effects of diffraction and also to minimise the central obstruction. They block only 9% of the incoming light. Both secondaries will deliver their light to either the Cassegrain focus behind the primary mirror – the central hexagon being left out to provide access to this focus – or via a flat ter- tiary mirror placed in line with the mechanical axis, and at 45 degrees to the light path, to a focus at one of six locations around the telescope. Two of these locations pass through the axis bearings to two Nasmyth platforms, where bulky or heavy equipment can be accommodated without affecting the fine mechani­ cal balance of the system. The other four are for lighter Table 1: Telescope Facilities on Mauna Kea Facility Size Primary Use University of Hawaii 24-inch Telescope #1 0.61m Optical University of Hawaii 24-inch Telescope #2 0.61m Optical University of Hawaii 88-inch Telescope 2.24m Optical/Infrared NASA Infrared Telescope Facility 3.0m Infrared Canada-France-Hawaii Telescope 3.6m Optical/Infrared United Kingdom Infrared Telescope 3.8m Infrared Carltech Sub-Millimetre Observatory 10.4m Millimetre/sub-millimetre James Clerk Maxwell Telescope 15m Millimetre/sub-millimetre W. M. Keck Telescope 10m Optical/Infrared Table 2: Facilities Planned Or Under Construction Facility Size Primary Use Second keck Telescope 10m Optical/Infrared VLBA Facility Subaru Telescope US-Canada-UK National Optical Telescope Radio 8.3m Optical/Infrared 8m Optical/Infrared Smithsonian 6-Antenna Array Galileo National Telescope 10  Silicon Chip Radio 3.5m Optical/Infrared instru­ments that can safely ride in the tube itself. In addition to standard observatory instrumentation, five major instruments are being built specifically for use on the Keck telescope to take advantage of its unique capabilities. They are: (1). The Low-Resolution Imaging Spectrograph (LRIS), a collimated array of four 2048 x 2048 CCDs imaging an area of 6 by 8 arc-minutes at prime focus. Used in the 0.4-1.0µm region of the spectrum, its angular resolution is 0.15 arc-seconds. (2). The High Resolution Echelle Mosaic Spectrograph (HIRES). This is similar in construction to the low resolution spectro­graph but the spectral resolution is 10 times higher and it work­s in the 0.3-1.0µm region. (3). The Long Wavelength Spectro­ graph (LWS), a 96 x 96 BIB (Bumped Indium Bond) array used in the 8-20µm region. (4). The Near Infra-Red Camera (NIRC). Covering the 1-5µm spec­trum, it uses a 256 x 256 indium antin­omide array with an angular resolution of 0.15 arc-seconds. It was developed at Caltech. (5). The Long Wavelength Infrared Camera (LWIC) for use in the 8-14µm spectrum. It uses a 20 x 64 BIB array from Hughes and, de­pending on wavelength, the angular resolution is 0.08 to 0.32 arc-seconds. On November 7th, 1991, the telescope was officially dedi­cated at a ceremony at the summit. At this stage, only nine mir­rors were in place but already the first official observation and concept-proving run had been made. The first image obtained was a CCD image of the galaxy NGC 1232 (Arp 41) in Eridanus. The re­sults were as encouraging as the design and construction team had hoped, fully vindicating the optimism they had shown in this radical new telescope. The image showed detail that had not been previously seen from ground based telescopes, and was a portent of what was to come once all segments were in place and the telescope fully commissioned. Although the warping harnesses had not yet been fully ad­justed, and seeing was less than perfect, Airy disc star images were obtained with dia­meters of 0.61 arc-seconds at the 50% energy level, and 80% of the light fell in a circle 1.6 arc-seconds AUSTRALIAN MADE TV TEST EQUIPMENT 12 Months Warranty on Parts & Labour SHORTED TURNS TESTER Built-in meter to check EHT transformers including split diode type, yokes and drive transformers. $95.00 + $4.00 p&p HIGH-VOLTAGE PROBE Built-in meter reads positive or negative 0-50kV. For checking EHT & focus as well as many other high tension voltages. $120.00 + $5.00 p&p GW QUALITY SCOPES 100MHz DEGAUSSING WAND Great for computer mon­­­it­ors. Strong magnetic field. Double insulated, momentary switch operation. Demagnetises colour picture tubes, colour computer monitors, poker machines video and audio tapes. 240V AC 2.2 amps, 7700AT. $85.00 + $10.00 p&p TUNER REPAIRS From $22. Repair or exchange plus p&p. 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GOS-6100 GOS643 GOS622 4 Channels 2 Channels 2 Channels 100MHz BW 40MHz BW 20MHz BW 500uV - 5V/DIV 1mV - 5V/DIV 1mV - 5V/DIV Dual Timebase to 2ns/DIV Dual Timebase to 2ns/DIV Timebase to 2ns/DIV Dual Timebase Trig Audio Trigger Level Lock Audio Trigger Level Lock Variable Hold-Off Variable Hold-Off Variable Hold-Off 20kV Accel. Voltage 12kV Accel. Voltage 2.2kV Accel. Voltage EMONA INSTRUMENTS NSW (02) 519 3933 VIC (03) 889 0427 QLD (07) 397 7427 Also available from: WA (09) 244 2777 SA (08) 362 7548 TAS (003) 31 6533 August 1993  11 Table 3: W. M. Keck Telescope Specifications Optical Design: Ritchey-Chretien Primary Mirror Secondary Mirror (f/25) Effective aperture 8.2m Figure Convex hyperboloid Maximum diameter 10.95m Shape Circular Light-collecting area 75-76 sq.m Diameter 0.51m Limiting magnitude ±28 Radius of curvature 1.82m Figure Concave hyperboloid Distance from primary 16.6m Number of segments 36 Focus behind primary 4.54m Radius of curvature 35m Equivalent focal length 250m (f/25) Focal ratio 1.75 Gap between segments 3mm Site Mauna Kea, HI Total weight of glass 14.7 tonnes Longitude West 155 deg 28 min 3 sec Position actuators 108 - 3 per segment Latitude North 19 deg 49 min 6 sec Whiffletrees 108 - 3 per segment Elevation of dome 4150m Displacement sensors 168 - 6-12 per segment Dome height 31m Active-control 0.5 second cycle Dome width 37m Setting time after siew 10 seconds Dome moving weight 635 tonnes Dome air exchange 5 minute cycle Observatory Individual Segments Number of aspheric types 6 Telescope mounting Altazimuth Number of each type 6 Max. telescope height 24.6m Spares of each type on hand 1 Telescope moving weight 270 tonnes Focal length tolerance 0.2mm Project cost $US94 million Shape Hexagonal Construction time 7 years Greatest diameter 1.8m Project headquarters Kamuela, HI Thickness 75mm Glass type Schott Zerodur Mean annual temperature 0°C Mass 400kg Average wind velocity 25km/h Clear night per year 250 Environmental Secondary Mirror (f/15) Figure Convex hyperboloid Average relative humidity Less than 10% Shape Circular Sub-arc-second seeing Greater than 50% of time Diameter 1.45m Radius of curvature 4.73m Distance from primary 15.41m Focus behind primary 2.5m Equivalent focal length 150m (f/15) across. These images were obtained at the prime focus since the secondary and tertiary mirrors had not yet been in­stalled. After these test images, the nine mirror segments, already greater in light collecting capacity than the 5- metre Hale telescope on Mount Palo­mar, were removed for safety so that work could continue on the as yet unfinished support structure. At this same November ceremony, 12  Silicon Chip the ground was turned for a second, identical telescope, the Keck II. If all goes according to schedule, Keck II is expected to be operational some time in 1996. In October 1991, the Schott Glassworks began delivery of the first of the 42 1.9 metre blanks required for the Keck II. Used alone, the second telescope will double the available observing time. Just as important, the two tele­ scopes, 85 metres apart, can potentially be used as an optical interferometer, giving a light grasp equal to a single 14.1-metre mirror but with the resolving power of a mirror 85 metres in diameter. In practice, however, this theoretical resolving limit is unlikely to be achieved but confidence has been expressed that a resolution of better than 0.01 arc-seconds is feasible. The light collecting area of the two mir- An optical technician monitors a diamond-edged circular saw as it cuts a mirror blank to a hexagon. Thirty six of these hexagonal seg­ments are used in the Keck mirror & the gaps between them are less than 3mm. rors will be greater than the world’s current 10 largest optical tele­scopes combined! By early 1992, when 18 of the segments were in place, the telescope already ranked as the largest optical reflector. Work had been slowed down by a snowstorm in November, hampering access to the summit and pro­gress once there, but finally, on April 14th, 1992, the last of the 36 segments was lowered into position. Designer Jerry Nelson, project manager Jerry Smith, facilities manager Ron Laub and Don Hall from CARA were all present for the final mirror positioning, the culmination of a 15year dream. Although the telescope is officially completed, shake-down engineering tests, alignment, tracking and ironing out the bugs inevitable in a project of this size are continuing before it is finally commissioned. The same can be said of the fine tuning required to optimise the new instruments to the telescope. This is expected to take about a year and will be under the watchful eye of operations director Peter Gill­ing­ham, recent­ly moved to Mauna Kea from the Anglo-Australian Observatory at Coonabarrabran, NSW. Most of the problems encountered earlier in the telescope pointing software seem to have been solved but further work is required to iron out troubles in the segment active control computer. From concept to completion, the Keck telescope has taken nearly two decades to come to fruition, during which time many valuable technological lessons have been learned. Its commissioning will have lasting implications for astronomy. New horizons have been opened up to keep researchers and theoretical astro­ physicists occupied for years. It also comes at a time when new and exciting data is being returned from the orbiting Hubble Space Tele­ scope. Both telescopes have the same limiting magnitude of about 28 but they can work independently or in concert to push the fron­tier of knowledge forward an order of magnitude from anything that has gone before. Hubble’s great strength is its superb location; Keck’s is its massive light collecting power. And Keck does it at 1/16th the construction and operational costs! Hard on the heels of the now proven design concepts comes confirmation that other telescopes of this kind are to follow – from the US, Japan and Europe. This is perhaps the greatest contribution of its designers, builders and the telescope itself – the heralding SC of a new era. Acknowledgments “Sky & Telescope” magazine; CARA; Caltech; Itek Optical Systems; Summit & facility support staff - especially Andy Pera­ la, Jerry Smith and Mary Beth Murrill. August 1993  13