P.M. Gray, S.C. West & W. Gallieni
Steward Observatory, University of Arizona, Tucson, AZ 85721
A.D.S. Italia s.r.l., Lecco, Italy
Proceedings of SPIE conference on Optical Telescopes of Today and Tomorrow, 2871, 1996
ABSTRACT
1. INTRODUCTION
2. SUMMARY OF SECONDARY MIRRORS
3. MMT F/9 SECONDARY
4. HEXAPOD FIVE AXIS POSITIONER
5. MMT F/5 SECONDARY
6. MMT F/15 ADAPTIVE & CHOPPING SECONDARIES
7 LBT SECONDARIES
8.ACKNOWLEDGEMENTS
9. REFERENCES
ABSTRACT
The 6.5 m upgrade of the Multiple Mirror Telescope (MMT) will include a
number of new secondary mirrors. For first light, there will be an f/9
Cassegrain secondary manufactured from a 1.0 m diameter Hextek
borosilicate honeycomb meniscus blank. This f--ratio is designed to
match that of the present MMT to allow the use of existing instrumentation for
first light. This will be followed by the wide field f/5 secondary
combined with a refractive corrector which includes an atmospheric
dispersion corrector (ADC) to give a 1° unvignetted Cassegrain
field. The f/5 mirror is made from a 1.7 m diameter lightweighted
machined Zerodur blank.
Two f/15 0.64 m diameter secondaries are being designed. The first of these is an adaptive secondary consisting of a thin 2 mm thick shell faceplate with 300 voice coil actuators and associated capacitive displacement sensors. A chopping f/15 secondary is planned using a rigid lightweight blank such as silicon carbide.
The 8.4 m Large Binocular Telescope (LBT) will have two secondaries for each of the two primaries. For first light, 0.87 m diameter f/15 Gregorian adaptive secondaries are planned. These concave mirrors will use the same thin shell faceplate, voice coil actuator and capacitive sensor technology currently being developed for the MMT f/15 adaptive secondary. A pair of 1.25 m diameter f/4 Cassegrain secondaries will be built next. These will be used together with refractive corrector optics to give a 1° field.
These mirrors are being polished and tested at the Steward Observatory Mirror Laboratory (SOML) using the recently completed Secondary Fabrication and Test Facility. Stressed lap polishing is used to achieve the fast, highly aspheric surfaces and testing is done with the computer generated hologram (CGH) test plate technique.
Each of these secondaries requires a support system, five axis actuation and thermal environmental control. The in--house development of this number of secondaries enables an integrated design approach. As much as possible of the development, design and hardware costs will be shared between secondaries.
This paper describes the designs which are being developed for the support, actuation and thermal control for each of these secondaries.
Keywords: secondary mirrors, five axis, support, Hexapod, adaptive secondary, MMT, LBT
1. INTRODUCTION
The performance specifications of modern 8 m class large telescopes put
stringent requirements on all parts of the optical system. The optical
tolerances and support requirements of large primaries present perhaps
the greatest challenge. However, the optical fabrication, support,
actuation and thermal control of secondary mirrors plays a critical
role in the overall optical performance of the telescope. Big fast
primary mirrors require large highly aspheric secondaries. These must
be lightweight, but yet be well supported to meet the strict telescope
optical quality requirements.
Fast primary f-ratios require precise secondary optical axis alignment to micron accuracy. Five axis actuation of the secondary mounting is required to actively correct for gravitational and thermal distortion of the telescope structure. This positioning mechanism is required to operate smoothly with minimal backlash and hysteresis for micron step active corrections and over larger coarse focus and recollimation adjustments. Although frequent active offsets will be achieved closed loop via optical information from low order wavefront sensors, it is desirable to achieve reasonable absolute positioning accuracy open loop for initial setup. The support and actuation structures must have sufficient stiffness to resist dynamic wind loading.
Thermal control of the secondary mirror surface is essential, as for the primaries, to match the enclosure air temperature to avoid local seeing effects. Further, secondary mirror blanks with non--zero coefficient of thermal expansion (CTE) require thermal control to avoid temperature gradient induced figure distortion.
The optical manufacture of secondary mirrors at Steward Observatory is being carried out in--house using the Mirror Lab's secondary polishing and test facility 1,2. For manufacture the secondaries require polishing cells which not only need to provide a suitable support during the polishing operation, but also include a separate test support system which mimics the optimum support geometry chosen for the final telescope support. Further, to increase the speed and accuracy of testing, a thermal control system similar to the telescope version is desirable. Thus it possible to coordinate and combine many aspects of the design of the polishing/test mirror cells with those of the telescope cells.
The development of a series of secondary mirror supports has meant that it is possible to develop and share many design details from one secondary to the next. Despite the fact that each of the secondaries has different blank material and construction as well as variations in requirements and specifications, the best overall approach has been to over--engineer to some extent in order to reap the benefits of commonality.
2. SUMMARY OF SECONDARY MIRRORS
| Telescp. F-Ratio | Dia. mm | Mass kg | Blank Material | Optical Config. | Comments |
| MMT F/9 | 1030 | 77 | Hextek Borosilicate | Convex Cassegrain | First light Existing Instruments |
| MMT F/5 | 1715 | 288 | Lightweight Machined Zerodur | Convex Cassegrain | Wide field 1.0° 3 lens corrector |
| MMT F/15 IR | 642 | 15 | Silicon Carbide | Convex Cassegrain | Keck--type IR chopper |
| MMT F/15 AO | 642 | 116 | Zerodur backplate Thin shell faceplate | Convex Cassegrain | Adaptive shell Voice coil-capacitive sensor |
| LBT F/15 AO | 871 | 214 | Zerodur backplate Thin shell faceplate | concave Gregorian | Adaptive shell Voice coil-capacitive sensor |
| LBT F/4 | ~1250 | ~113 | Lightweight Zerodur or Hextek | Convex trapped Cassegrain | Wide field 1.0° 3 lens corrector |
3. MMT F/9 SECONDARY
For the first light and during the initial observation period of the
6.5 m converted MMT, existing instrumentation will be used which has been
designed for the six combined beam f/9 Cassegrain focus.
The mirror blank for the f/9 secondary is a lightweight borosilicate honeycomb which uses the ``blown tube'' technology developed by Hextek3. The blank is made by sandwiching, between two faceplates of 9 mm thickness, tubes which are expanded by air pressure to form hexagonal cells. After forming the honeycomb structure, the blank was reheated and slumped in a mold to form a meniscus shape with a thickness of 150 mm. The 1016 mm blank weighs 70 kg. After delivery the backplate was bored with a 19 mm diameter holes in the center of each hexagon cell to allow ventilation air to be circulated inside the cells. The rib structure formed by the blown tubes is very fragile. Unfortunately, no special ribs or mounting flanges were provided inside the honeycomb structure since the design of the support system was not finalised when the blank was ordered. These factors made the design of a reliable support difficult.
The support scheme selected uses technology similar to that employed to support the large borosilicate honeycomb primary mirrors. A distributed set of pneumatic force actuators is servoed via a load cell connected in series with a defining hardpoint. The load cell output controls an air pressure regulator connected to a one third sector of support actuators. A total of 18 actuators are used for axial support divided into three sectors. The location of the actuators was determined from geometry given by patterns analysed by Nelson4 with slight adjustments to coincide support locations with honeycomb rib intersections.
The actuator cylinders use low stiction Bellofram rolling diaphragms diameter 35 mm with a piston design which pulls up for positive pressure. A working pressure of 0.7 bar will be used. The axial support actuators are attached to the mirror through 32 mm diameter Invar pucks glued to the backplate of the mirror with a silicone adhesive (Dow Corning 6093-Q3) which has sufficient shear compliance to decouple the slight CTE mismatch between borosilicate and Invar. Short lengths of stainless steel cable between the pucks and actuators provide moment and lateral force decoupling.
The lateral support of the blank on the backplate is difficult to implement in a simple support system since, when horizon pointing, the overturning moments induced by supports not aligned with the center of gravity (CoG) require adjustments to the axial support forces. The fragile construction of the Hextek honeycomb makes it difficult to attach supports which are aligned with the internal CoG. The most rigid attachment point is the edge of the front and backplates. A spreader bar is used which spans between Invar pads glued to the rim of the front and back plates. The Invar spreader bar is athermalised with a short length of steel to compensate for the CTE mismatch between Invar and borosilicate. The lateral force actuator is connected on the spreader bar to coincide with the CoG. A total of 18 lateral support actuators are located around the mirror rim. At horizon pointing, half of these pull from the top and half push from the bottom. These are divided into three sectors which are servoed via three fixed lateral defining hardpoints as per the axial support. The top pull actuators are decoupled from the spreader bar attachments using a short cable length (as for the axial supports) while the bottom push actuators incorporate a ball bearing decoupler. The defining hardpoints must use a release mechanism to allow the mirror to be safely supported when the support system is switched off. The release mechanism needs to repeat the mirror position to the micron level for good optical alignment.
4. HEXAPOD FIVE AXIS POSITIONER
To maintain the necessary optical alignment it is necessary to provide
full active control of not only the focus Z motion but also XY lateral
position and tilt orientation. To achieve the required
collimation specifications these motions must be precise at the micron
displacement level with total travel sufficient to compensate for
telescope flexure and thermal distortions of several millimeters. The
positioner must also have sufficient dynamic and static stiffness under
various operating conditions of wind buffeting and mirror chopping or
adaptive correction. Several alternative positioning mechanisms were
considered such as slide arrangements and flexure pivots. It was
necessary that the positioner fit in the restricted space inside the
secondary hub and be sufficiently modular so that it could be remounted
and shared between several secondary mirrors.
The final choice was the six linear actuator ``Stewart Platform'' type mechanism known as a Hexapod. This type of actuator has been used for a number of applications in telescopes5,6,7,8. The company ADS Italia which has experience with these type of actuators was contracted to perform the detailed design work for a suitable Hexapod actuator for the MMT secondaries.
The Hexapod design specifications were as follows:
Dimensions (actuator): diameter 64 mm x length 300 mm
Dimensions (Hexapod assembly): outside diameter 400 mm x height 340 mm
Total Weight: 60 kg.
Drive Mechanism: roller screw, diameter 20 mm x pitch 1 mm
Drive Motor: In-line direct coupled brushless DC motor, torque 2.5 Nm
Encoding: in-line direct coupled optical encoder, 2000 cts/rev
Additional Absolute Encoding: side mounted LVDT, Invar coupling strut
Positioning Accuracy: ±2 µm (XY), ±5 µm (focus)
Total Travel: 5 mm (XY), 10 mm (focus)
Actuator Joints: orthogonal flexures
Stiffness: 2 kg/µm, equiv. resonance with 180 kg load = 50 Hz
This Hexapod will be shared between the MMT f/9 and the f/15 adaptive secondary. For the f/9, the top mounting flange of the Hexapod is attached directly to the lower mounting ring of the secondary hub (see Figure 1). The three attachment points at the lower end of the Hexapod couple directly to the back of the mirror cell behind the three defining hardpoints of the mirror support. This provides a direct load path from the Hexapod to the mirror to maximise the overall stiffness. For the f/15 adaptive mirror, the Hexapod must be located higher on a mounting flange halfway up the secondary hub (see Figure 2). This allows additional space behind the secondary for the adaptive mirror actuators and associated electronics.
The main control and power electronics for the Hexapod actuator is located in a thermally isolated enclosure on the outer top ring of the telescope. Both this enclosure and the Hexapod actuators are thermally controlled actively with coolant lines feeding the top of the telescope.
5. MMT F/5 SECONDARY
The wide 1.0° field f/5 secondary is targeted towards
multi-object fiber spectroscopy with an additional requirement to
produce superlative image quality over a smaller 0.5° central
zone. A three element corrector above the Cassegrain focus gives
unvignetted field of 1.0° with polychromatic 0.6 arcsec image
quality. A prismatic atmospheric dispersion corrector is also included
below the corrector. A multi-object fiber positioner, Hectospec, with
300 fibers feeding a bench spectrograph is being built by Fabricant et
al. at SAO.
The large mirror blank required for this wide field secondary was designed by Fata and Fabricant 9. It was machined from a solid Zerodur blank by Schott. The plano-convex blank has an outside diameter of 1715 mm with a center thickness of 200 mm. The 94 mm AF hexagonal cells were machined with a flat bottom cavity through 68 mm diameter backplate holes with an undercut to give a 13 mm backplate thickness. The remaining ribs are 8 mm thick and the faceplate has a typical thickness of 15mm, thick enough to resist polishing presure. An outer ring of odd polygon shaped cells makes the transition from honeycomb pattern to the outer circle. The finished lightweighted mass is 288 kg.
The dual function of this secondary as both a wide unvignetted field with moderate image quality for fiber spectroscopy and over a reduced field, as a high quality imager, put stringent requirements on the support and actuation design. The large diameter and high aspect ratio of the blank, combined with tight specifications for support--induced optical deformations, requires the use of a reliable, predictable distributed support system. The five axis actuation for active collimation must meet the micron type accuracy with the large mass of the secondary and support. The support and actuation system has been designed to meet these goals. Following the philosophy of sharing designs between several secondaries, this support and actuation was extended to the somewhat simpler requirements of the smaller f/9 secondary.
The support system will be very similar to that being built for the f/9 mirror. A larger number (27) pneumatic actuators will be used to cope with the increased mass and provide a more uniform distributed support. The actuators will be identical to those being used for the f/9 secondary with the same inside--out, pull up positive pressure design. The actuators will be divided into three sectors. The pressure in each sector being controlled via an air regulator servoed from a load cell. Three hardpoints, each with load cell, provide the rigid positioning at zero load through the servoed force support. Each load cell includes a breakaway coupling which disengages the hardpoint above a safe working load but must repeatably re--engage with high accuracy.
The lateral support is achieved with a similar arrangement of edge support spreader bars bridging between front and backplates. Attached to the spreaders are the same type of pneumatically controlled actuators arranged around the top and bottom rims. A set of three lateral hardpoints and load cells define the position and control the pneumatic actuators.
The same type of six--legged Hexapod actuator will be used for five axis control. To cope with the larger mass and size, the f/5 Hexapod must have linear actuators with increased stiffness. The Hexapod geometry must also be modified to provide a larger attachment diameter to match the optimum location on the mirror cell. As for the f/9, the three Hexapod attachment points will be designed to coincide exactly with the three hardpoints of the support system. This direct load path optimises the mirror support stiffness.
Active thermal control will be included in the f/5 support system. This will allow the mirror temperature to be matched to the enclosure air temperature to within the ±0.2$°C specification. The thermal system will also remove heat generated by the Hexapod actuators and associated electronics. The implementation of the thermal system is made easier by the fact that a temperature controlled coolant supply must be fed to the secondary hub area because the requirements of other secondaries such as the adaptive f/15.
The large secondary with its associated support and actuation has a considerable mass. The small secondary hub and narrow spider vanes are optimised for minimal IR--emitting cross--section. In order to support the f/5 secondary, an additional set of four bracing struts are swung into place to link the f/5 secondary hub extension to the top frame of the telescope. These must be locked in place with sufficient accuracy such that the now over--constrained secondary hub is displaced by an amount which can be compensated by the five axis Hexapod actuation.
6. MMT F/15 ADAPTIVE & CHOPPING SECONDARIES
Two infrared alternative f/15 secondaries are planned. The first of
these is the adaptive optics version which uses a thin continuous face
sheet driven by 300 voice coil actuators controlled by capacitive
displacement sensors 10. This adaptive secondary mirror is the core
of the 6.5 m MMT adaptive optics system 11.
The faceplate consists of a 2 mm thick Zerodur/ULE
shell 12, diameter 642 mm with the convex aspheric on one side and a
spherical rear surface. Behind the faceplate, separated by a gap of
100 µm is a reference 71 mm thick backplate with matching spherical
surface. The capacitive sensors are formed with metallic film rings
surrounding the actuator holes. The actuators consist of a voice coil
winding at the tip of a thermally conductive rod. The coil is in close
proximity to a small rare--earth magnet glued to the back of the
faceplate. The rear end of the actuator rods are clamped in a metal
plate heatsink with internal coolant channels. The heatsink plate is
thermally isolated from the glass reference backplate. The backplate
provides the reference for the displacement sensors. To provide
sufficient access for actuators a cantilevered central
tube supports the backplate. This central support results in a
maximum gravity deflection of 2.7 µm. This provides a sufficiently stiff
reference for the adaptive actuators to close loop on a reference star,
however to use this secondary open loop from lookup table requires the
backplate deflections to be repeatable. Prototypes to test various
aspects of the adaptive secondary technology have been assembled and
tested13,14,15.
Another key component of the adaptive optics system is the artificial star laser projector (see Figure 1). At the top of the secondary hub is a large lens doublet and fold flat. This optical system is aligned to give a diffraction limited 0.5 m diameter collimated beam. The 589 nm laser is fed from an off--telescope location, along the elevation axis and up the telescope structure via fold mirrors. The laser projector optics are permanently mounted on the telescope thus the top of the secondary hub is always occupied. The optics for the laser projector have been built and are currently used as the laser guide star for the FASTTRAC II adaptive optics instrument for the six mirror MMT11.
A chopping f/15 secondary is also planned. Two alternatives are being investigated. The first is a more conventional fixed chopping mirror from a lightweight silicon carbide or beryllium blank. The chopper mechanism would be a Keck Lockheed type. The second alternative is to make a simplified version of the adaptive mirror shell with a thicker faceplate (6 mm) and a smaller number of actuators with a larger 500 µm capacitor gap which would permit a chop throw of 20 arcsec.
7 LBT SECONDARIES
The two secondary mirrors planned for LBT are the f/15 IR adaptive Gregorian
secondary and the wide field f/4 trapped Cassegrain. Both these are at
concept design stage, however their design follows on from the MMT
secondaries 16.
The adaptive secondary will use the same technology of thin shell faceplate, voice coils and capacitive displacement sensors as is being built for the MMT f/15. Although a large diameter (871mm), the concave Gregorian will be easier to manufacture and test. An artificial point source can be used with the concave mirror to provide a straightforward means of calibrating and tuning the adaptive shell.
The wide field f/4 secondary has very similar dimensions and specifications as the MMT f/5. It is considerably smaller diameter because of the faster f-ratio and the raised position of the trapped Cassegrain focal plane. A different challenge arises however to design, manufacture and mount the trapped Cassegrain instrument and wide field corrector optics above the primary mirror.
The mechanical design of the LBT structure includes an innovative concept for support and interchange of the secondary mirrors. Instead of the more traditional telescope structure and spider vane approach, LBT uses pivoted truss ``swing arms'' to move selected secondaries and instruments into position. All secondaries can be permanently mounted and aligned on the telescope and brought into position in minutes. This requires a locking mechanism which preloads the structure in the operating position with good repeatability and low hysteresis. The swing arm structural design required a stiff truss cantilever which had low cross sectional area along the optical path and high resonance frequency for wind buffeting. The final designs 17 achieved <2 % obstruction and 20--30 Hz resonances comparable with more traditional spider vane systems.
8. Acknowledgements
This paper is compilation of the past work of several groups who have been
working for a number of years on the planning and design of the various
secondaries for the MMT and LBT. Dan Blanco whilst working on the MMT
Conversion project was involved with the procurement of the Hextek
blank for the f/9 secondary. Dan Fabricant and Bob Fata of SAO were
responsible for the investigations, design and procurement of the
large, lightweighted Zerodur blank for the f/5 secondary. Piero
Salinari and his group at Arcetri Observatory, Italy have been
collaborating for several years on the adaptive secondary with Roger
Angel, Michael Lloyd--Hart, Buddy Martin and other at Steward
Observatory and Dave Sandler, Don Bruns and others at Thermotrex Corp.
John Hill, Warren Davison, Piero Salinari and the LBT project group
have worked for many years on the LBT secondary mirrors and support
structure.