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Mount and Mirror (MM) Working Group Report

This report presents the sizes of well-known telescopes and discusses the choice of telescope size based on energy threshold, field of view, optical resolution, location altitude, type of light sensors, and available financial support. It also explores mechanical constraints, design criteria, and prime focus optics feasibility.

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Mount and Mirror (MM) Working Group Report

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  1. Mount and Mirror (MM) Working Group Report Presented by Razmick Mirzoyan On behalf of the MM working group Max-Planck-Institute for Physics, Munich R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  2. Sizes of few well-known telescopes DISH Ø 5m = small Ø 10m = medium Ø 20m = large Ø 30m = huge HESS II – 600m² 6° - 12° wide 4° - 5° normal FoV new MAGIC – 239m² HESS I – 108m² HEGRA – 8.3m² R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  3. Size of a telescope • Currently we have quite some experience with constructing telescopes of sizes ranging from (3 – 17) m in . • Within next 2 years we will start collecting experience also with a  28m H.E.S.S. telescope. R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  4. Choice of the size of a telescope • The choice of the size of a telescope is dictated by the desired a) energy threshold b) field of view (FoV) c) optical resolution in the given FoV d) location altitude e) also, type of the used light sensors (future trend) f) last but not least : available financial support R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  5. The estimated energy thresholds The Ethreshold shall scale with the  of a telescope as E ~ 1/{}² R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  6. IACTs comparative table R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  7. Comparison example In optical astronomy the cost of a telescope scales ~ {}2.6 It seems that the cost of an IACT can be scaled ~ {}2.2 R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  8. Mechanical constraints Space frame, dimensions, weight,… materials: • Steel: very big experience, thermal expansion O’K, stiff, heavy • Aluminium: innovative, expensive, twice higher thermal expansion than steel, light. No corrosion. • Carbon Fibre reinforced plastics: innovative, thermal expansion is just 7 % from that of Al, very stiff, very light. Avoid excessive heat. ~ 2 times more expensive than steel. R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  9. Exapmle CFRP frame: MAGIC-I Deformation of the mirror dish Camera support, bending vs. elevation Possible hysteresis of deformations R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  10. CTA Design criteria • Stereoscopic telescope system (>> 4 telescopes) • Alt-Az mount on rails • Davies-Cotton for small dishes, parabola for large dishes (> 10m) • Segmented mirror facetts, possibly ~ 1m size for large dishes • F/D > 1 • Robust, simple, reliable system (lifetime > 10 years), robotic operation desired, keeping maintenance/operation effort low • Survival winds > 160 km/h • Consider protection during ice, snow, below 0 temperatures R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  11. Design criteria • Weight of the camera (will depend on the level of integration of electronics inside the camera) • Positioning and pointing accuracy • Stiffness of the structure and/or AMC system • Safety concept • Human safety • Protection of the instrument • Redundancy in power, drives R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  12. Mechanics • Tracking precision: like optical telescopes • Positioning accuracy, speed • Fast slewing: (MAGIC:180°/50sec; H.E.S.S.: ~twice slower; VERITAS: 1°/1sec) • Drive type • Gear • Tooth rims • Friction R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  13. Arrival Time Front • No arrival time distortion for the parabolic design • Special 2 component optical telescope designs may provide no (or almost no) arrival time distortions • The wide-spread Davies-Cotton design provides ~ 3-4 ns time smearing for existing telescopes The RMS of the pulses from g, hadrons and µ after PMT and jitter simulations Mirzoyan, Sobczynska, et al., 2006 R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  14. Light sensor type • Classical PMTs: currently used in all IACTs: Best Photon Detection Efficiency (PDE): ~ 30 % • HPDs with Ga AsP can offer interesting alternatives. PDE ~ 45 % (QE: 50-55 %) (rather expensive) • SiPM: mature product in 3-5 years time. Can offer PDE ~ 60 % R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  15. Field of View Observations of the Crab Nebula with H.E.S.S.; astro-ph/0607333v1 • Currently H.E.S.S. has the largest • FoV of all existing IACTs. • Still, as one can see from the left • figure, the effective field of view is • „only“ ~3° in  (@ 3db=0.7 level) • One may think either to provide a) • a „large enough“ FoV, so that it can • include inside the largest possible • gamma source ( 5°  ?) or b) really • large FoV of (10-20)°  for the all • sky survay. R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  16. Prime focus optics: what is feasible ? Schliesser & Mirzoyan, Astrop. Phys., 2005 3´ optical resolution R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  17. Wide FoV • Also, the (10-20)° FoV can offer the advantage of very high optical angular resolution of ~ 1 arc minute everywhere in the FoV. • Simulation results indicate further improvement of the angular resolution, especially at low energies, when moving from currently used (3-6)´ towards 1´. • A proper design can offer almost no distortion of the arrival time front. • Schmidt design: can offer interesting solutions for Wide FoV • Interesting recent design, Ritchey-Chretien: Vassiliev, et al., astro/ph 0612718 R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  18. Wide FoV R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  19. Which mount is better ? • The above question is very much justified: • A heavy and stable mount, with marginal deformations: approach chosen by HEGRA, H.E.S.S., VERITAS • A light-weight mount that allows for small deformations, which are actively correctd for: approach chosen by MAGIC. • In the 2nd case the telescope consists of (mount + active mirror correction (AMC) system). R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  20. Example Active Mirror Control (AMC) System Star adjust option of AMC: On figure left one can see ~12 % of all mirrors in the Camera centre, the rest are Defocused. All the mirror can be Checked in 8 such figures. This will take ~ 10 minutes time + some off-line work. In addition, the reflectivity of all individual mirrors can regularly be measured. R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  21. Which mount is the best ? • We have collected positive experience with Alt-Az H.E.S.S. and MAGIC type mounts on rails. • A combination of a stiff mount + an AMC system could offer an interesting solution. This can be interesting for large and huge telescopes, especially for large zenith angle observations. • We have enough expertise to cope with this problem. R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  22. H.E.S.S.-2 mirrors H.E.S.S.-I mirror R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  23. H.E.S.S.-2 mirror candidates R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  24. H.E.S.S.-2 mirrors R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  25. All-Al MAGIC-I mirrors The MAGIC-I mirrors have size 0.5 m x 0.5 m. 4 of them are set together, forming a panel 1 m x 1 m. MAGIC-I uses 956 mirrors of size 0.5 m x 0.5 m. They are grouped in 239 panels of 1 m x 1 m. The single 0.5 m x 0.5 m mirrors are obtained by gluing two aluminum sheets on a “honeycomb” structure. After rough pre-milling, a diamond “fly-cutting” of the surface will generate a spherical surface of a desired radius, having a microroughness of about 100 Å R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  26. Upgrade of manufacturing process of MAGIC mirrors made in Padua Monolithic 1 m x 1 m mirrors do not need a support panel and the pre-alignment. A sandwich formed by two aluminum skins with inserted an “honeycomb” structure is assembled. The aluminum skins are pre-curved and glued on the honeycomb using a concave spherical mould. Afterwards a diamond milling of the surface is performed to refine the spherical surface. Very positive test results! R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  27. Cold Slumping Approach New INAF-OAB technology in collaboration with Padua and with Medialario Techn. – foresee the cold shaping of a sandwich made with 2 thin glass sheets (1.7mm) glued on an Al honeycomb (20mm) structure. While the radius of curvature of the mirror is large and the thickness of the glass sheet is rather thin, it can be conformed to the shape of a convex master by means of vacuum suction. • Advantages of the glass-faced panels: • Low cost. Lightweight. Doesn’t need polishing either of the glass/master. • These characteristics permit to have: • Competitive costs compared to other technologies • Superior performance • High production speed R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  28. Mould and panel produced in Medialario Tech. Aluminum master 1040 x 1040 mm Front and rear of a produced segment Size = 985 x 985 mm Weight = 10 Kg. Nominal radius= 35 m 1m x 1m mirror tests at ETH R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  29. Point Spread Function 90% of the light inside 0.9 mrad 80% of the light inside 0.6 mrad Achieved parameters already within the MAGIC-II specifications R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  30. Developments with potential • Dielectric mirror coatings can offer an interesting alternative: • one can „boost“ the mirror reflectivity in the entire wavelength • range 300-700 nm to ~ 100 %. Needs development work. • Aspheric mirrors are another interesting development direction. • They can offer the advantage of better PSF. R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

  31. Conclusions MM • Telescope Mount: quite some experience with several types, that can be used in CTA. • Mirrors: experience with several types, the cost varies between ~(1.2 – 2.5).-k€/m² for large quantities. We will need to use a robust technology, the reflectivity shall not seriously deteriorate within 5-10 years. • Wide FoV telescope designs may offer interesting solutions, we need to explore their potential. R. Mirzoyan: CTA Paris Meeting; Mount & Mirror WG Report

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