NPF update: Light-weight mirror development in Chile
A. Bayo, P. Mardones, S. Castillo, G. Hamilton, C. Lobos, L. Pedrero, C. Rozas, N. Soto, H. Hakobyan, C. García, M. R. Schreiber, W. Brooks, S. Zúñiga-Fernández
NNPF update: Light-weight mirror development in Chile
A. Bayo a,b,d , P. Mardones b , S. Castillo a,b , G. Hamilton c,d , C. Lobos a,b , L. Pedrero b,c,d , C.Rozas b,c,d , N. Soto b,c , H. Hakobyan b,c,d , C. Garc´ıa b,c,d , M. R. Schreiber b,c , W. Brooks b,c,d , andS. Z´u˜niga-Fern´andez a,b,ea Instituto de F´ısica y Astronom´ıa, Facultad de Ciencias, Universidad de Valpara´ıso, Chile b N´ucleo Milenio de Formaci´on Planetaria - NPF, Valpara´ıso, Chile c Universidad T´ecnica Federico Santa Mar´ıa d Centro Cient´ıfico Tecnol´ogico de Valpara´ıso, CCTVal e European Southern Observatory, Santiago de Chile, Chile
ABSTRACT
Planet Formation research is blooming in an era where we are moving from speaking about “protoplanetarydisks” to “planet forming disks” (Andrews et al., 2018). However, this transition is still motivated by indirect(but convincing) hints. Up to date, the direct detection of planets “in the making” remains elusive with theremarkable exception of PDS 70 b and c (Haffert et al., 2019; Keppler et al., 2018; M¨uller et al., 2018). Thescarcity of detections is attributable to technical challenges, and even for the rare jewels that we can detect,characterization (resolving their hill spheres) is unachievable. The next step in this direction demands from nearto mid-infrared interferometry to jump from ∼
100 m baselines to ∼ Keywords:
NIR observations, MIR observatinos, interferometry, CFPR, ground-based, planet formation
1. INTRODUCTION
Planet formation is arguably one of the hottest topics in observational (and theoretical) astronomy. Thousandsof fully formed planetary systems have been confirmed and small and large scales structures, hinting probablythe presence of young planets, seem ubiquitous among protoplanetary disks (e.g. Andrews et al. (2018)). Despiteall these exciting results, the direct detection of planets “in the making” remains elusive with the remarkableexception of two point sources: PDS 70 b and c (Haffert et al., 2019; Keppler et al., 2018; M¨uller et al., 2018).The scarcity of detections is attributable to technical challenges, and even for the rare jewels that we can detect,resolving their hill spheres to understand the key aspects of their formation is an unachievable endeavour.For this next step to happen, given the extremely small scale and complicated environment involved, near tomid-infrared interferometers are the only viable option, but these have to jump from ∼
100 m baselines to ∼ Further author information: (Send correspondence to A.B.)A.B.: E-mail: [email protected], Telephone: +56 32 250 8311 a r X i v : . [ a s t r o - ph . I M ] D ec n this wavelength domain (3-10 µ m), the cost of the telescope is intimately linked to the diameter of itsprimary mirror, and even if an exponential relation does not hold for segmented primaries, the primary mirror(and its weight) is still the main driver of the cost of the facility.Our approach to tackle this problem is to use Carbon Fiber Reinforced Polymer (CFRP) as the base tobuild segmented primaries taking full advantage of the lightness of this material, its strength, its relativelysmall expansion coefficient, and the opportunity that it brings to “serialize” the process of astronomical mirrorproduction.In short, the process of building a CFRP mirror is displayed in Fig. 1 and consists of the following steps:1. Building of a convex “mold” (mandrel) of the opposite curvature than that of the final mirror (but withsimilar or better surface roughness than the goal one).2. Laying-up and replicating such surface with some kind of reinforced carbon fiber sheet/gel/etc.3. Curing the copy / replica.4. Detaching the replica from the mandrel and “double” coating to maximize reflectivity and protect thesurface.Figure 1: Main steps of the CFRP Mirror replication method. For more details on the lay-up step and thecontrol through the process see Castillo et al. and Soto et al.Once a reflective high quality replica is produced, it needs to be integrated with the rest of the telescopethrough a suitable support cell.Of course, every one of the previously mentioned steps is meaningless without a set of measurements andquality control processes.Since the previous SPIE edition we have made substantial progress in all these areas and we are submittingaccompanying papers focused on lay-up, and control and metrology (see Castillo et al. and Soto et al., respec-tively). This paper “simply” aims at reporting on the general progress of the project, suggestions for supportcells being designed for light mirrors, and rough estimates of the cost model for our replicas.
2. MANDRELS
The performance of different materials for mandrels has been tested in the past two years. Our range coveredfrom the previously used steel mandrel that induced large scale deformations and offered itself non-optimalroughness, to different internationally and locally produced glass and glass-ceramics recipes. In Table 1 we showa summary of the materials analyzed and conclude that the best results ( ∼
10 nm roughness) are systematicallyobtained for silicate or even regular crown glass.The largest optical-quality mandrel produced by us so far has a diameter of barely 30 cms, but a 0.5 m one isbeing polished in our lab with a current measured roughness of a very few tens of nm (not included in Table 1).able 1: Summary of the performance as mandrels for different materials explored in our laboratory.GlassKind Silica Crown Pyrex BK7 UNK Pyrex Crown CrownSize (mm) 150 200 200 70 190 500 500 250 − ) 0.5 8 3.25 3.3 3.25 8 8Roughness (nm) 10 20 12 10 10 13 10 25Shape spheric spheric spheric flat spheric spheric spheric sphericHardness(Mohs scale) 7 7 7 7 7 7 7Ceramics Metal Marble Gypsum CFRPKind glass-ceramic aluminium steel Type 4Size (mm) 100 50 50x50 500 75 200 10 − ) 2.5 2.5 23 12 7.2 18 -0.8Roughness (nm) ∼
100 70 120Shape flat spheric flat spheric spheric spheric flatHardness(Mohs scale) 2A very important advantage of CFRP replicas is precisely its replicability, but, obviously, the processesinvolved infer stress on the mandrels. We have been monitoring any deterioration of the mandrels and estimatethat several tens of replicas can be obtained from a single mandrel before it deteriorates (see Table 1). A currentline of research in this aspect deals with mandrel reparation and we plan on reporting on it during 2021.
3. LAY-UP
As previously mentioned, the goal to “obtain a negative copy” of the mandrel with a composite material (such asCFRP) is to obtain a high-fidelity replica. Such replica could / should exhibit the same characteristics in shape(although opposite curvature) and roughness than the mandrel. Unfortunately, print-through and other kindsof “pattern transfer” from the individual carbon fibers present in composite materials can result in a “wavinesssignature” ( ∼
100 nm periodic patterns) that can seriously harm the utility of such replicas for interferometry.This fiber print-through problem is not an unknown one and several approaches already exist in the literatureto mitigate its effects. The most common one consist of incorporating additional layers of epoxy or even metalsthat will “smear out” the unwanted pattern (see for example Steeves et al. (2014)).During the past two years we have obtained very promising results to circumvent this problem by not onlyapplying additional epoxy layers, but tackling the problem early on, from the lay-up strategy. Regarding thelatter, for further details check Castillo et al. (in this same volume), but in short, the proposed method basicallyconsists on changing the orientation of a number of layers at the beginning of the lay-up, maintaining threesimple considerations: symmetry, balance and quasi-isotropy.These changes in lay-up resulted in a significant improvement at high and mid-spatial frequencies, usuallycorresponding to roughness and waviness (see Castillo et al. for details). . CURING
Temperature and pressure control are vital in handling the viscosity of the epoxy either present in pre-impregnatedCFRP or used independently in the replica process (and to reach the glass transition temperature of the resin).We have been working on optimizing a pure baking and autoclave-curing sequence to maximise the fidelityof the replica at all spatial frequencies. Unfortunately, the autoclave that provided the best results in terms ofoverall shape is a “wet” one that resulted in humidity pockets developing in between layers and therefore yieldingsub-optimal roughness performance. (a) Sketch of the different layers present in the vacuumbag during the first curing cycle in the oven. (b) Picture of a replica in the process of baking in theoven.
Figure 2: Example of step one on our two-step curing process.While we are in the process of receiving a large (1m in diameter capacity) autoclave, we have proceededwith a two step curing methodology: a higher temperature one where the pre-impregnated CFRP is cured (seeFig. 2 for a sketch of the layering process involved in the “baking”), and a secondary room temperature curingwhere additional layers of epoxy are added and imperfections at the smallest and intermediate frequencies arecorrected.This two-step approach provides virtually one to one copies from the mandrels in terms of roughness andwaviness, however, first order aberrations are still present. Our preliminary approach to solve this problem isprototyping a simple “one-time-active” support cell that serves also as a device to integrate the mirror in differentoptical setups (see section 6).
5. UNMOLDING AND COATING
Different release agents and application procedures have been tested through the last two years in order to findthe right balance between optimal unmolding, residual-free surfaces, and minimum damage to the mandrel.As mentioned before, the practically negligible small scale surface degradation in the whole replicating processand the large number of replicas that we obtain from a given mandrel before considering reparation (see Table 1),suggest that we have achieved that balance (see Soto et al. for details on our quality control and metrologyprocesses).Regarding coating, we are following two paths: evaporation and sputtering. Although the latter shouldprovide longer lasting results (based on glass coating experience), the life-time / deterioration of the reflectivityof the CFRP replicas (in particular on top of the additional resin layers) is still a matter of study.So far, we have experimented with copper (that poses adherence problems, both, on replicas with layersof extra resin and those without) and aluminium on ∼
20 cm replicas with single target strategy. Aluminiumpresented no adherence issues and the usual thickness of 100 nm yielded good reflectivity, although, as mentionedbefore, long term changes in reflectivity due to deterioration (mostly oxidation) of the surface have not yet beenstudied. By the end of 2019 we took some of our first coated replica, and some uncoated (see Fig. 3) to ESO’s a) First CFRP replica with successful aluminium coating inour lab. (b) Metallic uncoated replica transported toParanal Observatory.
Figure 3: Aluminium coated and uncoated CFRP replicas.Paranal Observatory, to be exposed to the dust and wind of the Atacama desert, and we plan on studying theirdeterioration during 2021.The purpose of this experiment is not only to characterize the adherence of dust to the coated surfaces and thepossibility to clean it without affecting reflectivity, but also to monitor any changes on uncoated / unprotectedsurfaces that may arise from extreme changes in humidity, for instance due to pockets of humidity locked withinthe layers in our lab (that is located in a very high humidity region). Our preliminary assessments in Paranalregarding surface roughness of the uncoated replica is that there is not significant deterioration.Even though we have not been able to characterize the deterioration of the coating, previous experience withglass suggest that a protective layer is key in ensuring long-lasting highly reflective surfaces, therefore we havealso experimented with two target sputtering coating.In this two targets approach, aluminium was still the material of choice for the reflective layer and silicondioxide that of the protective layer. The choice of SiO as protective layer was motivated by the literature, givenits good characteristics for the near-infrared (further tests for mid-infrared absorption, etc need to be carried outstill). Different thicknesses for the protective layers are still being tested with the plan to compare reflectivityover time for replicas with 50, 100 and 150 nm of SiO (over our “standard” 100 nm layer of Al).Our goal for 2021 is to commission a 50 cm coating machine and repair the 2 meter sputtering machineavailable to us, as well as to advance on two target coating characterization.
6. SUPPORT CELLS
As explained before, achieving a perfect copy of the mandrel at all spatial frequencies is something still out ofour reach. However, we have found ways to obtain excellent results concerning waviness and surface roughness,with the “only” pending issue being that of first order aberrations, mostly astigmatism directly linked to theorientation of the fibers in the “last” layer of CFRP sheet.While we are still tackling this problem from the manufacturing side (via additional optimization of thelay-up process an “a posteriori” resign layer additions), we are also developing a cell system with two main goalsn mind: to provide support to the CFRP mirror (and therefore serve as a key element for the integration ofour mirrors in different optical setups) and to be able to correct any first order aberration. This cell is mainlycomposed of 3 parts:1. A ring-type piece that acts as a boundary condition, keeping the perimeter of the CFRP mirror correctlyshaped.2. A body with a thread that fits with the ring-type piece, keeping the CFRP mirror in between those andalso acting as a base for the actuators.3. An array of threaded bolts with spherical heads as actuators.Figure 4: 3D CAD model of a third generation prototype of a CFRP mirror support cell.We have so far produced three different versions of this supporting cell 3D printing with different materialsto characterize accuracy of the correction and stability of the latter. This characterization is at its early stagesand therefore we cannot offer proper quantification of the image quality improvement yet.
7. COST MODEL
A main motivation to carry out this research on CFRP mirrors is the hypothesis that these mirrors will proveto be of similar quality than the “traditional” glass-based one but with a much lower cost.Even though the size of the replicas that we are producing are still of very moderate size, we performed theexercise to build a parametric cost model to test weather the possible market prize will go in the right direction.Accounting for human resources, machinery investment, mandrel acquisition, etc., our model places the costof a ∼
20 cm replica “ready to go” well below 450 USD; and a rough estimate for a mount that could handle sucha light-weight primary (barely 200grams) yields a figure of less than 750 USD.In principle, there is no reason to assume anything but linearity regarding the scalability in the cost of thereplica process itself, and if that is the case, CFRP mirrors can definitely be the change in paradigm needed forfacilities like PFI to become a reality.
ACKNOWLEDGMENTS
All the authors acknowledge financial support from Iniciativa Cient´ıfica Milenio v´ıa N´ucleo Milenio de Formaci´onPlanetaria. A.B acknowledges support from FONDECYT grant 1190748, A.B. and N. S. acknowledges supportfrom ESO Comit´e-Mixto and QUIMAL funding agencies. M.R.S., S.C and C.L acknowledge support from theALMA-CONICYT fund. S.Z-F acknowledges financial support from the European Southern Observatory via itsstudentship program and ANID via PFCHA/Doctorado Nacional/2018-21181044. eferences
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