The Alt-Az Initiative
An Experiment in Purposeful Evolutionary Acceleration
Meeting the challenge of increasing the number of small research telescopes by lowering their costs through technology transfers and quantity production is taking place across a broad front, albeit at a rather slow pace. This natural technical evolutionary process (Basalla 1988) could be accelerated, however, through purposeful tech transfer “bridging mechanisms” between the large and small alt-az telescope development communities. These mechanisms include enhanced inter-community communications, as well as the encouragement of tech transfer initiatives, tech demo telescopes, and quantity production of unusually low cost, small research telescopes. We consider, below, a pilot project “experiment” in the purposeful evolutionary acceleration of small research telescopes.
The Alt-Az Initiative was established in June 2007. The Initiative has been self-funded, with individuals and small firms donating time and materials. The Initiative focused, from its inception, on the development of “advanced technology,” lightweight, low cost, 0.5 to 2.0 meter alt-az research telescopes. To provide even more focus, a specific goal, a “challenge” was established early on, to develop a lightweight, transportable, 1.5 meter alt-az research telescope. From its inception, the Initiative also concerned itself with instruments, dedicated scientific research programs, and automation (as well as remote access and networking).
Special emphasis has been given by the Initiative to those instruments and research programs which would most benefit from low cost 1-2 meter telescopes/instrument combinations. These include faint object time series optical and near IR photometry, time series fiber-fed low and medium resolution spectroscopy, and high speed photometry.
Special emphasis has also been given to the involvement of undergraduate students, who have participated since the Initiative’s inception in research, development, conferences, and publications.
Considered separately below is each of the five key elements of the Initiative:
Enhanced communications Tech transfer initiatives
Tech demo telescopes Transfers to quantity production
Student participation in research and development
Although the bulk of the Initiative’s communication is via email and telephone, Initiative members have gathered together in person at eleven workshops to date. The two founding workshops were held in June 2007 in Portland (Sidereal Technology) and San Luis Obispo (California Polytechnic State University). A large workshop followed in October in Dallas, and there have been a number of workshops since then, including workshops in Hilo with Gemini and Subaru engineers. The next workshop is June 6-7, 2009, in conjunction with the American Astronomical Society’s summer meeting in Pasadena, California.
The Alt-Az Initiative has also sponsored two major conferences, one held in San Luis Obispo in June 2008 and the other in Hawaii in January 2009. These four and five-day conferences, respectively, were well attended, and each conference included several workshops and some three dozen talks, many of which will appear as chapters in the Initiative’s forthcoming book. The Initiative’s next conference will be in Hawaii, February 8-12, 2010.
Not only are the Initiative’s workshops and conferences open to all, but developments are being shared with the larger astronomical community through published papers, articles, and a low cost, forty-chapter book, The Alt-Az Initiative: Lightweight Telescope Developments and Scientific Research Programs. This book is in final editing and will be available from the publisher this summer (2009). This book is divided into eight sections:
Light Weight Optics
Observatories and Enclosures
Automation, Networking, and Remote Access Small Telescope Research
Research as Undergraduate Education
Examples of Undergraduate and High School Research
Details on the Initiative’s workshops, conferences, and publications, as well as useful references, links, etc. are available on the Initiative’s web site, www.AltAzInitiative.org.
Initiative Galileo’s Legacy Conference at the Makaha Resort, Hawaii, January 2009
Initiative Small Telescope and Astronomical Research (STAR) Conference at San Luis Obispo, June 2008 (Both pictures on the left)
Galileo’s Legacy was followed by an insider’s tour of the 8-meter Gemini Telescope on Mauna Kea on the Big Island of Hawaii, January 2009.
Tech Transfer Initiative
So far, a dozen technical initiatives have been completed or are underway. We will consider by way of illustration, example initiatives drawn from three areas:
Telescope drives and controls
Lightweight primary mirrors
Active primary mirror control
Telescope Drives and Controls During the early stages of the Initiative, several members visited Magdalena Ridge Observatory and were given a very thorough tour and explanation of the modern, 2.4-meter alt-az telescope by its Chief Engineer, Elwood Downey, and its Lead Engineer from EOS Technologies, Kevin Harris. The 2.4 meter MRO telescope employs direct drives. The telescope itself acts as the bearings and “frames” for the azimuth and altitude motors. A ring of permanent magnets in both altitude and azimuth are the motors’ rotors and similar rings of coils that act as the stators. Magnetic forces between the rotor and stator move the telescope based on the output of high resolution, on-axis encoders, with the motion determined by a servo algorithm. There are no gears, friction rollers, belts, or any other mechanical reduction devices to introduce periodic errors or compliance that could reduce the telescope’s stiffness and natural frequency. As noted in Bely’s (2003) classic book, The Design, and Construction of Large Optical Telescopes, “A direct drive, which eliminates all mechanical systems, is the ultimate choice.”
While the stators, rotors, and rather computer-controlled electronics are all commercially available, they can be more expensive than many some modest-aperture telescopes. Initiative member Dave Rowe devised a very low-cost, axial flux, direct-drive motor for telescopes. Normal direct drive motors are radial flux motors. The radial “one ring inside another” configuration is energy efficient, but involves an expensive arrangement of magnets, coils, and soft steel to contain the flux. Rowe realized that energy efficiency was not important for modest aperture alt-az telescopes, and devised an axial flux motor, with a simple coil ring placed on top of a permanent magnet ring. The axial flux direct drive motor only costs about $300 per motor in magnets, wire, and steel, and is extremely easy to build.
Direct-drive, direct-position-reading servo systems are much more difficult to control than ordinary brushed DC motors and require not only high-resolution on-axis encoders as inputs but high-speed, computation of the control system filters a feedback algorithm. Initiative member Dan Gray at Sidereal Technology designed a low-cost control system that handles two“brushless,”AC synchronous, direct-drive telescope motors. Gray also wrote all of the real-time, embedded firmware for the controller, and the PC-based command-and-control software.
Lightweight Primary Mirrors Both large mountaintop alt-az telescopes and their diminutive lowland brethren share a common obsession with lightweight primary mirrors. Without lightweight mirrors, telescope weights and hence costs rapidly get out of hand. For small telescopes, especially in the 1-2 meter aperture range, transportability and assembly are vital issues if 18-wheelers and cranes are to be avoided. Initiative members have been addressing the challenges of lightweight yet low-cost primary mirrors for small alt-az research telescopes from several perspectives.
Several Initiative members have been investigating the use of foam glass as lightweight spacer/structural material between the top and bottom glass plates of a primary mirror. Pittsburgh Corning makes Foamglas, a rigid insulating material used in LNG ships, many industrial applications, and even under heated concrete runways in cold climates such as Alaska. Andrew Auregema has been machining concave surfaces in the tops of Foamglas blocks, while David Davis has been slumping and fusing glass plates to Foamglas in his kilns. Attendees at the recent Galileo’s Legacy conference in Hawaii were intrigued when David pointed out that Foamglas sandwich mirrors float on water. He then proceeded to toss a Foamglas mirror blank on the floor without damage. Soda-lime Foamglas comes in a number of densities, and there is also a low coefficient of temperature expansion borosilicate version of Foamglas.
Initiative members are investigating the use of low shrinkage adhesives to fasten pre- slumped top plates (and flat bottom plates) to Foamglas cores as an alternative to kiln fusion. A group in Italy under the direction of Giovanni Pareschi is also working on Foamglas mirrors, and the Director of Development at Pittsburgh Corning, Steve Badger, has been generously supportive of these Foamglas mirror developments.
Initiative member Peter Chen has been developing very lightweight carbon fiber replica mirrors for several years and is now achieving success with this approach. Initiative member Kiran Shah is experimenting with polyurethane replica mirrors. Shah’s mirrors have a solid polyurethane front surface supported by a dense polyurethane foam body that is formed in a mold.
David Davis has been experimenting with slumped meniscus mirrors. He builds his own kilns and does his own grinding, polishing, and figuring by machine. Work is also continuing on the development of sandwich mirrors with glass spacers by Tong Liu at Hubble Optics.
Active Optical Correction Initiative members Mel Bartels and David Davis are both interested in seeing how far thin, slumped meniscus mirrors can be “pushed” with respect to both aperture and thinness. Soda-lime or borosilicate float glass can only be procured in thicknesses up to 25 mm. As apertures of meniscus mirrors this thin increase, however, an aperture will eventually be reached where, similar to the situation with mountaintop alt-az telescopes, corrective forces under computer control will need to be applied to the primary mirror if it is to retain its proper figure as the telescope changes its altitude and hence the gravity vector acting on the mirror.
In a visit to the Gemini North eight-meter telescope, Initiative members discussed the Gemini’s “voice coil” active mirror support system with Chris Carter, then Gemini’s control system engineer (Chris recently joined the Thirty Meter Telescope developmental team). Chris suggested that for smaller telescopes, a voice coil support system to “tweak” the mirror into proper shape as altitude changed could be made at low cost. Two undergraduate electrical engineering students at California Polytechnic State University are now developing low-cost electronic controls for voice coil meniscus mirror adjustment. They are also, in a parallel effort, developing the electronics for small stepper motor mirror tweaking. In a visit to the Gemini North eight-meter telescope, Initiative members discussed the Gemini’s “voice coil” active mirror support system with Chris Carter, then Gemini’s control system engineer (Chris recently joined the Thirty Meter Telescope developmental team). Chris suggested that for smaller telescopes, a voice coil support system to “tweak” the mirror into proper shape as altitude changed could be made at low cost. Two undergraduate electrical engineering students at California Polytechnic State University are now developing low-cost electronic controls for voice coil meniscus mirror adjustment. They are also, in a parallel effort, developing the electronics for small stepper motor mirror tweaking.
In both cases, our only concern is to make very low frequency (less than once per second) adjustment of the primary mirror to correct for the lowest order Zernike terms, especially astigmatism, as a function of altitude. The requisite adjustments can, we believe, be calibrated “offline” on a number of bright stars distributed in altitude with a coefficient-determination algorithm that minimizes the spread of the stellar image. During operation, the current altitude of the telescope will be noted, and the required settings interpolated from a lookup table. This approach is used by the eight-meter Subaru telescope with recalibration of their look-up table only required about once a year.
Finally, there is a third approach we are considering for active optical correction, and this is the use of a semi-passive bimorph mirror. These mirrors consist of two thin disks, one of active piezo material coated with a thin layer of conductive metal on each side, and the other a thin front surface mirror. The two disks are bonded together with a low shrinkage adhesive. The back metal coating on the piezo disk can be divided into sections. When a high voltage (100 to 350 volts) are applied to the sections, the piezo materials in each section contract or expands some amount, depending on the applied voltage and its polarity. The combination of these expansions and contractions can warp the thin mirror disc in a modal manner. A computer controls these voltages through digital-to-analog converters and high voltage operational amplifiers.
Instead of using actuators to apply forces directly to the primary mirror itself, a much smaller, semi-passive bimorph mirror near the instrumental payload is distorted via applied voltages in such a manner as to cancel out the gravity-induced distortions in a lightweight primary mirror. These corrections can be applied in a manner similar to what was discussed above, i.e. via a lookup table versus altitude.
Instrument Clusters One of the Alt-Az Initiative’s members, Gary Cole, has been focused on the concept of developing lightweight automated science instrument clusters to maximize the number of available observing modes on a single telescope. This in turn multiplies both the research and educational value of the facility.
The core of this project has been the development of a programmable, lightweight, 4 way instrument selector that can tie together, both optically and mechanically, a complete suite of science instruments including the new 50mm CCD cameras. This device is just now entering production from Optec, Inc.
As an example, the prototype 20-pound cluster provides: target acquisition, imaging, Sloan band photometry, deep R~400 survey spectroscopy, R~10000 spectroscopy, JH band infrared photometry, and broadband dual-beam imaging polarimetry. The selector and instruments operate as IP network devices. The data is gathered in FITS formats. A single laptop operates the entire cluster along with the telescope under the control of an automated observation scheduler.
Assuming a typical 1000 hr/year observing program and five-year lifetime, the amortized cost of this instrument cluster is less than 5$/hour. The range of both research and education opportunities provided by an instrument cluster such as this one on a 1-meter platform is very exciting.
Optec SSP-4 NIR JH Band Photodiode Photometer Automation Currently, near-infrared imaging cameras remain quite expensive ($50K and up) because these cameras cannot be fully implemented in silicone chips alone, requiring a hybrid combination of materials. While efforts are underway to reduce the cost of these cameras, a much lower cost, and easier to handle option for JH band near IR photometry is available in Optec’s SSP-4 photometer for only $3K. The SSP-4, developed by Optec, in cooperation with the American Association of Variable Star Observers, is ideally suited for observing long-period Mira and other variable stars.
Initiative member Dan Gray has automated the filter changing and flip-mirror functions of the Optec SSP-4 to allow fully automatic operation. Dan and Initiative member Russ Genet made a test run on this modified SSP-4 at the University of Oregon’s Pine Mountain Observatory in July 2008. Dave Rowe analyzed the resultant data.
Initiative member Andrew Aurigema machined the top surface of Foamglas® material from Pittsburgh Corning. David Davis then fused thin glass plates to the Foamglas to form a lightweight mirror blank.
Chris Carter and Russell Genet examine one of the Gemini’s spare voice coil actuators used to maintain the shape of Gemini’s actively corrected primary mirror. Chris suggested that active mirror support would be a good tech transfer candidate.
Initiative member Greg Jones’ low-cost, semi-passive bimorph mirror.
The Optec SSP-4 near IR photometer (left) modified for automated operation, was evaluated on Dan Gray’s automated 14-inch alt-az telescope (right) in a test run at Pine Mountain Observatory.
Instrument cluster developed by Gary Cole. Permanently mounted, yet remotely switchable instruments increase system versatility and, compared to mounting and demounting instruments, greatly reduce the costs of operation and maintenance.
Left, 3-D drawing of the lightweight programmable instrument selector that will be offered by Optec, Inc. Right, bench testing of the lightweight instrument cluster prototype.
The axial-flux direct-drive motor devised by Initiative member Dave Rowe. An 18-inch diameter motor was readily assembled from parts and materials that cost only $300. The azimuth assembly on the right includes the motor, bearings, and high-resolution tape encoder and read head.
The low-cost direct drive servo motor controller devised by Initiative member Dan Gray. An onboard microcomputer performs the requisite trigonometric and other calculations at high speed to control the current through the motor’s coils.
An ultra-lightweight, experimental carbon fiber composite mirror designed and fabricated by Initiative member Peter Chen. A two-meter telescope composed of a number of such mirrors in a multiple-mirror configuration could provide the ultimate lightweight alt-az telescope.
Initiative member Tong Liu and the lightweight sandwich mirror he developed for the Cal Poly 18 tech demo telescope. Glass spacers are fused between two thin glass plates.
Tech Demo Telescopes
The objective of the Initiative’s first tech demo telescopes was to combine together various elements from individual technical initiatives and to gain experience as a group in building and operating a complete system. A 1.5 meter, dedication-mission demonstration telescope is under consideration.
The modest aperture (18 inches) Cal Poly 18 (CP 18) alt-az telescope was designed and built not only as a technical demonstration but also for eventual use by student researchers at California Polytechnic State University. The telescope’s drive system has no gears, belts, or friction wheels; instead, as described earlier, direct-drive motors and
high-resolution encoders are completely integrated into the bearing assemblies and telescope’s structure. In altitude, for instance, a ring of permanent magnets is firmly mounted to the OTA while an opposing ring of coils is mounted on the inside of a fork arm. The electronic control system has been designed to operate these brushless motors in a high precision mode.
To achieve the highest possible closed-loop servo bandwidth, the structure was designed—by Cal Poly students using finite element analysis as well as traditional analytic tools—to have an unusually high natural frequency. The direct drive system and stiff structure effectively counters wind gusts when such a telescope is operated out in the open or within a roll-off roof observatory.
The Cal Poly 18 alt-az telescope designed and built by Initiative members, including undergraduate students at California Polytechnic State University. Final fabrication and assembly was completed at Sidereal Technology in Portland. On the right are the coils for the altitude direct-drive motor.
Transfers to Quantity Production
PlaneWave Instruments CDK 700 Telescope Construction has begun at PlaneWave Instruments by Initiative members Rick Hedrick and Joe Haberman and their associates on a production, 0.7 meter, corrected Dall Kirkham telescope with Nasmyth foci. The telescope employs direct drive motors and control system electronics and software that are similar to and based on the Initiative’s experience gained with the Cal Poly 18 demo telescope described above.
A Nasmyth optical configuration with two foci (optional second focus) was chosen to place instruments and eyepieces at a convenient height, to make the telescope insensitive to even large changes in instrumental weight (counterweighting is not required), and to minimize the telescope’s moment of inertia in altitude. The corrected Dall Kirkham (CDK) optical system, designed by initiative member Dave Rowe, was chosen for its wide, well-corrected flat field, generous back focus, and its relative insensitivity to secondary mirror lateral misalignment (the secondary mirror is spherical).
The 28-inch primary is relatively easy to manufacture, as is the 11-inch spherical secondary. The two-element corrector employs all spherical surfaces. The well corrected field supports the largest commercially available CCD sensors available today and should continue to do so well into the future. The large, flat field is very useful for a variety of off-axis guider configurations.
Sidereal Technology Direct Drive Controller The controller electronics and software for both the Cal Poly 18 and PlaneWave Instruments CDK 700 were designed by Dan Gray, and are manufactured by Sidereal Technology. For precision telescope motion, brushless direct drive motors must be operated in an AC synchronous, servo-control mode. The position of the telescope is sensed with ultra-high precision encoders on the two telescope axes, the position error is calculated, and the requisite torque feedback is computed. The three phases of the motor are then energized based on this feedback torque by way of pulse-width modulated switches. This control system is ASCOM compliant, so it can be used with a number of overall telescope/observatory high-level supervisory software suites.
The PlaneWave Instruments CDK 700 telescope. The Nasmyth configuration facilitates instrument changes without rebalancing and also provides a convenient height for visual observing. Shown is an overall telescope drawing.
The CDK 700 azimuth drive assembly is more than a motor. It is a 3 phase 24 volt AC direct drive torque motor, with an integrated high-resolution encoder, and an integrated 22.5” bearing. This assembly is both the base of the telescope and the drive. On the left is a drawing of the assembly that illustrates how compact it is, while a piece of the assembly being machined is shown on the right.