蔱plication As An Alternative Approach For Large Segmented

Replication As An Alternative Approach For Large Segmented Telescopes Melville P. Ulmera , Robert I. Altkorna , E. D. Kibblewhiteb , Steven Varlesec a Dept. of Physics & Astronomy Dearborn Observatory 2131 Sheridan Rd, Northwestern University, Evanston IL 60208-2900, USA b Dept. of Astronomy and Astrophysics, University of Chicago, Chicago, IL c Ball Aerospace, Boulder, CO ABSTRACT

The next generation of optical/IR telescopes will require large numbers of co-phased segmented mirrors. Therefore, some form of replication technology is desirable to reduce costs. Electroforming has the advantage that it is a commercially developed technology for replication, and the technology has been widely used for making X-ray mirrors (e.g. XMM-Newton). Composite materials are appealing, since a great deal of development work as been done with composites as well. There are 3 areas that need to be addressed: replication with minimal stress so as to produce a high quality gure; attachment of support of the mirror segment so as to maintain the gure quality; and, thermal control requirements. Here we present a discussion of the requirements that lead us to select replication as the fabrication technology and the advantages of replication. We report on our rst results of making a concave mirror and testing support methods of ats. Keywords:

Electroforming, Composites, Segmented Telescopes, Optics 1. INTRODUCTION

There are a great many studies now being performed to design the next generation ground-based telescope such as CELT,1 20/20,2 Euro-50,3 and OWL.4 All of the projects other than 20/20 have a preliminary design with segmented optics similar to Keck5 and Hobby-Eberly6 for the primary mirror design. Regardless of the design, the bottom-line requirement for any new 30 100 meter class telescope is a low as possible cost and time to build, so that the project becomes feasible. For, given nearly unlimited resources and time, a 100 meter telescope could be built using the basic Keck technology with little innovation. The conservative approach (of CELT, Euro-50, OWL, etc.) has been to assume the mirrors will be stress gured and polished as was done for the Keck. This approach is based on the idea that since it worked, it can \simply" be scaled up. There are many reasons, however, for taking a step back and asking the full set of questions: What are the scienti c goals of the project? What requirements does this place on the primary mirror? What is practical in terms of adaptive optics? By taking into account these questions it is possible to apply systems engineering cost bene t analysis and devise an alternative design concept. For, there are several areas in which the basic Keck-like approach could be signi cantly improved to reduce cost and we are developing the strategy for achieving these cost reductions. We describe our basic concept design and preliminary fabrication studies in this paper. Further author information: (Ulmer) MPU : E-mail: [email protected], telephone: (01)847.491.5633 RIA: E-mail: [email protected], telephone: (01)847.491.7705 EDK: E-mail: [email protected], telephone (01)773.702.8208, Dept of Astronomy & Astrophysics, U of Chicago, 5640 S. Ellis Ave, Chicago, IL 60637, USA SV: E-mail: [email protected], telephone (01) 303.939.4444, Ball Aerospace & Technologies Corp., 1600 Commercial St, Boulder, CO 80301

2. SYSTEMS ENGINEERING AND SCIENCE DRIVERS

We have been working with a group which we dub ATILA (Adaptive optics primary Telescope Initiative for a Large Aperture), which is comprised at this time of astronomers within the state of Illinois. With ATILA, we want to advance our understanding of the early universe by measuring the space density of galaxies and their properties over 10 redshift intervals from z = 1 10. This requires measurements in the 2 m to 300 m range and a telescope with approximately 1000 m2 collecting area (37 m diameter). Furthermore, 0. 1 angular resolution is required, which requires adaptive optics (AO: for and introduction to and references to material on AO see refs. 7, 8). AO even in K-band (2.2 m) over our desired 5 10 diameter (necessary for making a survey) eld of view (FOV). For comparison, the diraction limit of a 37 m telescope at 2 m is about 0. 01. It happens that adaptive optics (AO) only has been successfully applied in K-band or long-ward, but this is perfect for our scienti c goals. Therefore tuning the telescope to the IR makes sense and alleviates problems with gure error requirements for the primary mirror segments and bandwidth requirements for the AO system. These considerations lead to the following general requirements: 00

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The so-called Fried correlation length at 2 m = 30 segment sizes less than 30 50 cm are needed.



The atmospheric turbulence variations that aect observations near 2 m lead to a bandwidth requirement of 10 Hz which leads to a sampling rate for a closed loop system of approximately 100 Hz.



The laser guide star (LGS) system for AO allows freedom of where to observe and naturally works to correct for turbulence in the lower atmosphere. A worst, this will produce 0. 1 images over a 5 10 FOV as the upper atmosphere will cause smearing of the image. On axis, i.e. within about 20 1 we expect to be able to achieve near diraction limit ( 0: 01) by a combination of a LGS and a natural guide star (NGS).

50 cm so that to achieve Strehl ratios of over 50%

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Segment gure accuracy of =20, which at 2 m is 100 nm.

By making the segments low mass (10 20 kg/m2 ) we can achieve another desirable goal: this is to apply the AO to the primary as this is the optimum place to apply the correction. This is because the primary is closest to where the atmospheric distortion actually takes place and the down stream mirrors that have been used in other designs are positioned at conjugate points that correspond to being below the earth's surface. Furthermore, the PAMELA project9 has already demonstrated that primary mirror adaptive optics works. 2.1. Desirability of Replication

The number of segments required is based on the Fried coherence length, as we have previously noted. Therefore, if we assume a segment of 0.5 meter diameter, corner to corner, we have an area of about 0.16 m2 so that for a 37 m diameter telescope we require 6,000 segments. This leads to a requirement of about 10 segments per day based on a requirement of being able to make all the segments in 3 years and assuming that 200 days per year are available to make segments. This requirement, plus the requirement that the segments be low mass leads to the consideration of replication. A replication process means that the segment does not have to be thick enough to be able to be ground and polished and, hence, can be made thin. To reduce the cost and time of production, replication will also be extremely useful. And, a by-product of making low mass system is that the overall telescope mounting system cost will be reduced. Furthermore, an additional cost saving can be gained by being able to design the pieces of the desired f/number of the telescope (1 or lower). In comparison, the `scaled-up \Keck-approach" tunes the f number to the ability to fabricate high quality 2 m diameter segments for visible band observations. In addition, the use of light weight segments means that the cost of actuators and the cost of the segments support and can both be reduced. To summarize, replication is desirable for several reasons: 1. reduced focal length

2. reduced areal density 3. reduced time to fabricate 4. reduced amount of grinding and polishing 5. reduced cost of improving image quality by primary mirror AO 6. reduced cost of the actuators needed for the entire system. 3. REPLICATION

The use of replication in optics has a long rich history10 which has been build upon to this day (see below) by both the X-ray and the optical (primarily) space community. As demonstrated above, it is clearly worthwhile take advantage of these developments for segmented ground-based telescopes. There are other advantages as well which we will describe below. The main drawback for replication technologies has been the inability to achieve the demanding =20 requirement. Electroforming has been able to achieve =2 =4 at 632 nm, but little has been published in the area of making segments for optical (\normal incidence" optics) mirrors beyond web pages11{13 and Ref. 10. Much more extensive work has been published for grazing incidence optics, e.g. Refs. 14{16 and references there in. Replication by composites work has also been carried out extensively (cf. Ref. 17, 18 and references therein), and again about =2 =4 is the best than has been achieved. Then, the work on \normal" incidence mirrors can be summarized as follows. Replication in general oers a signi cant advantage over classic grinding and polishing, in principle. The advantage is especially true if spherical mirrors are used, but as we discuss below this is not necessary. The gure quality that can be achieved for aspheres has been quoted as about =2 =4 at 632 nm, which is marginally acceptable at 2 m or longer. The issue of support and mounting for a speci c system has not been fully addressed. The groups have tended to focus on one technology such as: composites, electroformed metals, or injection molded Pyrex. In the following sections we describe many of the details that must be considered in applying replication optics to the tasks of producing the generation large segmented telescopes of the 30 100 meter class. By making a comprehensive design tree and by having a team that consists of composite and electroforming experts, we have an advantage over previous work. Previous work has tended to concentrate on a single technology that was primarily to address space applications such as making deployable systems. Particularly innovative approaches that we are going to use are: (1) to combine new composites (Elastic Memory Composite materials) with electroforming; (2) to use techniques of stress shaping the masters as is being developed for X-ray optics19 ; (3) to apply slight bending of the nal segments to change them from spheres into aspheres. 4. A REVIEW OF SEGMENTED MIRROR DESIGNS

There have been two approaches to segmented mirror designs that have been explored, but only one put in to actual practice for astronomical telescopes. The one that has been put into practice is to use of relatively massive large segments for Keck (1.8 meters in diameter corner-to-corner, 75 mm thick; segment mass 400 kg, areal density20 190kg/m2. Each of the 36 Keck segments has its own set of actuators and edge sensors that are adjusted to take out changes in the total mirror gure due to gravity and changes in the orientation of the telescope. It has been found that the system is stable enough that the phasing takes about 1 hour from start to nish and lasts for several weeks.21 A modi ed design that uses only spherical segments such as Hobby-Eberly,6 uses about 1 meter segments and corrects "slow" (days to weeks) changes of the combined segmented primary mirror gure. The \Keck" approach is \brute force" and requires adaptive optics downstream to achieve the ultimate power of the telescope. A modi ed version of this approach is to use even larger segments e.g. 20/202 but eectively

this is the same general concept: apply \slow" corrections (for changes that are mechanically/gravitationally induced) to the primary and apply all \fast changes" (for atmospheric turbulence corrections) downstream. At the other extreme is the approach mentioned which has not yet been put into use for large astronomical telescopes: PAMELA,9 which uses 7 cm segments (8 mm thick and mass of 40-45 gm for an areal density of about 10 kg/m2 ) and rapid enough (150 Hz) to correct for atmospheric variations that aect the performance the visible (about 600 nm). One one hand, the PAMELA project demonstrated the basic concept works. On the other hand, the work on PAMELA demonstrated that there are many details that need to be worked out such as the damping of the pieces and deformation of the gure when actuators are attached. The PAMELA test bed is only a 0.5 meter mirror with 7 cm segments. We plan to use larger segments. In addition, we will carry out a detailed study of the segment fabrication and support prior to the fabrication and use of segments in a test bed. At least two key issues have to be resolved: the mounting/alignment and keeping the entire segment sti enough so that 100 Hz AO closed loop control will have a negligible eect on the shape on the segment gure. This requirement of 100 Hz in tip/tilt motion means that the individual segments should not have resonant frequencies below 100 Hz. Replication processes won't be as useful as possible if the number of masters must be equal to the number of replicas. Also, convex masters are more diÆcult to test then concave ones. Below we address these items as they relate to the fabrication process. 4.1. Reducing the number of masters required

One approach to reduce the number of masters is to design a spherical mirror, then, in principle, all the masters are the same. However, this is not quite true (cf. PAMELA9 ) for mapping hexagonal segments onto the spherical shape of the mirror. For an aspherical/parabolic primary mirror design, the situation is even worse. A method (cf. Ref 17, 18) to make the exact size and orientation of the hexagon, is to start with a circular master with a radius that is larger by about 1% than largest segment for a given position on the primary mirror. Then, the replica can be made with masks (cf. Figure 1) aligned to the orientation required with respect to the radius of curvature of the primary gure and the location of the segment with respect to the optical axis of the primary. Furthermore, the masters can be made in spherical shapes and then bent slightly to convert the shape into an asphere (see Figure 2). This general concept is already being studied for the European XEUS mission19 (also J.-J. Ferme, private communication). 4.2. Related Issues

One of the major diÆculties in the \classic" Keck approach is polishing to the edge of the segments and making segments so that the gaps between mounted segments is negligible. This is naturally achievable via replication techniques as the master is made to the full circumference of the required hexagonal mirrors and then a mask is placed over the master before the replica is made Therefore, there is no distortion due to changes in stress that would result from cutting the piece after fabrication. Well de ned edges can also be easily produced in the same process. Making and testing 3 4 m diameter convex mirrors is not yet possible, yet these are the baseline for the next generation large telescopes. However, those carrying out replication techniques have developed a method of making convex sub-masters from concave originals and then making concave replicas from the sub-master (cf. Ref. 10). It will be an interesting and useful exercise, therefore, to scale up replication processes for making 4 m diameter high delity mirrors. If the replica concave mirror tests well, this will prove that the convex mirror has the desired gure as well and this will be a viable method of verifying the gure quality of the desired convex (secondary) mirror if high delity replication can be achieved.

5. OUR APPROACH 5.1. Areal Density and Figure

This can be reduced by using smaller diameter pieces that can be supported with with a relatively sti honeycomb or similar supports ( e.g. PAMELA9 ). The material of choice is yet to be determined as there seem to be at least 3: electroformed metal 50-300 m thick; (b) composites 1-5 mm thick; and, (c) glass 0.5 mm 5 mm thick. Besides the delity of the replicated optic, another concern is how to make the mirror sti enough so as to not deform under gravity nor to vibrate with such a large amplitude so as to render the segments useless for adaptive optics (approximately 100 Hz motion for the IR). Due to the availability of electroforming facilities and access to composite materials we are exploring these two options as we describe below. Since replication has not yet been able to achieve the =20 (or better) level even at 2m, we have a \fallback" position which is, instead of making a rigid segment panel, make one that is exible enough so it can have its gure, post replication, adjusted with \set-it-and-forget-it" actuators. Yet-to-be determined are the fabrication speed, the cost of this process, the mass and cost of actuators, and the response of the nal design to being moved at 100 Hz. However, this approach will also allow us to reshape the segment slightly from a sphere to an asphere and thus achieve the goal of requiring only one gure for the master. 5.2. Time to fabricate

Any replication process, be it electroforming, composites, or injection molding, can be done on the time scale of one segment per day per production \station." As long as the capital cost of a segment station is small compared to the total production cost, the stations can be duplicated. Electroforming, composite casting, or injection molding will be able to meet the goal of making up to 10 segments a day at aordable prices. 5.3. Initial Figuring costs

We have several ideas for reducing the cost of guring and polishing above and beyond the concept of using replication. The rst is to make the initial pieces slightly over sized so masks can be used to generate the end-point segment size. As mentioned in the above, we can make the initial master spherical in shape and then bend it to form the parabolic shape necessary. Instead it may be better to bend the replicas slightly after fabrication. Furthermore, by using the master/sub-master approach, as many as needed sub-masters can be replicated from a few initial (concave) masters. 5.4. AO and Actuator Cost Reduction

This has been addressed above but for completeness, we cover the issue here once again. The goal of AO is to greatly improve the image quality of the system up to the diraction limit. As it is the initial wavefront that experiences the phase lags due to atmospheric turbulence, the most appropriate place to apply AO is at the primary. This has not been done in the past (except for PAMELA) as the segments of systems such as Keck are too massive (and perhaps, more important, exceeds the Fried coherence length for  <  10m). Currently installed monolithic mirrors were not designed for \fast" closed loop AO either. Instead, all current \10-m class" telescopes apply the AO to a downstream mirror. to achieve the optimal performance, placing a closed loop tip-tilt system on the primary is the preferred methodology, however, as the alternative is the application of multi-conjugate adaptive optics (MCAO) which is going to be quite diÆcult (cf. ref. 22).

5.5. Hardware Development Progress Report 5.5.1. Introduction

Seed funding from our respective institutions that has allowed us to begin some preliminary studies. We have therefore begun to make some proto-type pieces and to test some mirror support concepts. The initial topics we are addressing are: how accurately can the replica be made? Can a sti enough support can be applied to the replica before its release from the master that will overcome residual stresses in the replica so as to produce a =20 gure? We also are exploring combining the best of both worlds of EMC material and electroforming by determining if EMC material has the ability to reproduce the gure of the master to higher delity than other composites. It may also be possible to build slightly compressive stress into the replication process so as to convert a spherical shape into the desired parabolic one. Preliminary results related to these topics are described below. 5.5.2. First Results

One project we have begun was to electroform 5 cm diameter concave mirrors from glass double convex lenses. First, the glass lens was coated with a conducting layer of chrome. Then the mirrors were placed in a xture and put in the electroforming bath. The lenses were left in the bath for from 5 to 20 hours depending on the desired thickness of the piece. We made 3 pieces in all: 50, 100 and 200 m thick. The pieces can be examined through the lenses prior to removal. The two thinner ones (50 and 100 m thick) are in the process of being back coated with EMC material while the other piece (about 200 m thick) was removed without a backing. The gure of this one mirror (called m3 hereafter) was compared against that of the lens. The peak-to-valley variation was found to be 7 microns for m3 gure over the piece, in contrast to about 0.6 microns from the lens (master). The m3 gure does not meet our speci cations for an optical or near IR, but this would meet the requirement for panels to be used in the microwave (lambda about 1mm) range. On one hand, the result seems consistent with reports of being able to meet the requirements for microwave radio dishes.11 On the other hand, this seems drastically worse than reports of being able to make electroformed mirrors good to about 1/2 lambda.11{13 However, only the simplest stress reduction was used to make this piece, i.e. ow of the electroforming bath

uid onto the part and and a relatively slow nickel deposition rate. By using feedback control and rotating the piece, we should also be able to achieve gures comparable to or better than those reported elsewhere. For, a

large portion (20% 30%) of the area of the mirror does have a peak-to-valley deviation similar to the master or about 0.6 microns peak-to-valley, or about 1 lambda.

At this writing the EMC material work has not been completed, but we expect the much thinner electroforms mentioned above to be dominated by the gure of the material. How well the EMC material will perform is to be determined, but it has exciting possibilities due to its ability to be reshaped under heating. In parallel, to begin to address the support issue, we used two honeycomb-like structures. We also used ats with a hexagonal mask. As an additional experiment, we have made experiments of how to attach the support to the at. The naive approach is to use epoxy, but epoxy does not have the same CTE as nickel and is not strong relative to shear. We have, therefore, been experimenting with placing the support onto the back side of the at while it is still on the master. And, then, the entire unit is placed back into the electroforming bath and plating is continued for about 5 hours. This is enough to produce 50 m thick coatings which have proved to have shear strength that is as large as the piece itself. For our rst try, we used supported a 10 cm diameter hexagonal at only at \hard points." The relevant xtures and honeycomb-like support are shown in Figure 1 These hard points are at the corners of 5 cm ona-side equilateral triangles (assembled to form a hexagon, cf. Figures 1,2). This con guration oset the main walls (200 m thick and 3 cm high, made of nickel) of the triangle from the back plane surface of the mirror (about 200 m thick) by about 500 m. This was enough to cause a warping of the 10 cm diameter electroform so that we do not recommend this approach. Next, we plan to use a honeycomb with 2 mm thick walls (1 cm high with cell sizes of 0.5 cm, cf. Figure 1 panel 3). The walls of this honeycomb t within 10 m or better of the back surface of the mirror. This support will work much better.

As of yet we have no de nitive results to report about the best replication technique to use, but we conclude that electroforming with proper backing, either with EMC material and/or with honeycomb is very promising. Furthermore, a rigid support system attached by \electroplate-welding" is one method that is capable of joining the master to a rigid support structure. 6. FUTURE DIRECTIONS

We need to scale up the pieces to 0.5 m diameter and to run tests to verify that the appropriate damping and actuator attachments can be applied to allow us to use the segment in a closed loop AO system for the IR (100 Hz). The damping and actuator requirements need to be matched with the support structure design. Other engineering work will need to include wind loading studies. The entire process then needs to be laid out and tested from making master to sub-master to making several copies. We also need to decide whether to bend the master, the sub-master, or the nal mirrors to aect the nal parabolic/aspheric shape will need further experimentation as well. In parallel with the perfection of cost reduction of the fabrication steps, once a few good segments are made, tests can be performed to advance the work done on PAMELA by using our larger segments, applying the technique to the IR, using a LSG to correct to lower atmosphere, and a bright star (e.g. Polaris) to correct for upper atmospheric turbulence. By making a functioning system with several 0.5 m diameter segments with separations approaching 37 meters, we will have produced a proof of concept for ATILA and be ready to propose to build a telescope. 7. SUMMARY AND CONCLUSION

We have shown that near a IR survey of the deep sky with a 1000 m2 telescope is feasible if based on replication techniques and the application of AO to the primary. Our own initial test at fabrication segments suggests that previous work can be advanced to meet the requirements of a =20 gure for  >  2 m. Another advantage of high- delity replication is the possibility of making 3 4 m diameter convex mirrors. A further advance will be to apply primary mirror AO down to 400 nm, where the diraction limited angular resolution for 37 m telescope will a truly impressive 2 milli-arc seconds. ACKNOWLEDGMENTS

This work was partially supported by NASA Illinois Space Grant, Ball Aerospace, Northwestern University and the University of Chicago. We thank M. Steele and M Farber for making the electroforms. The ATILA team (L. Thompson, P. I., D. A. Harper, R. Kron, J. Mohr, D. York, plus authors [E. Kibblewhite & M. Ulmer]) have all contributed to the concepts described in this paper. We thank G. Emerson for making measurements of the electroformed 5 cm diameter concave mirror. REFERENCES

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From left to right: The rst panel shows a at circular master held in a white square plastic frame, a xture and mask set up to hold the master, (center) and the \honeycomb" support, bottom; center panel shows the master mounted in the xture and the \honeycomb" support. The irhgt mos panel shows a more preferred honeycomb support. This honeycomb is more closely spaced and ts at against the piece. The corner-to-corner diameter of the outer hexagon in all 3 panels is 10 cm. Figure 1.

A schematic of how to make parabolic shapes while starting from an initial spherical mandrel. AL also shown are diagrams depicting the honeycomb support shown in the 2 left most panels of Figure 1, and how such a support would be used to support a mirror segment. Figure 2.