Wide FoV IAC telescopes Initial Design Considerations

1. Wide FoV IAC telescopes Initial Design Considerations Vladimir Vassiliev Pierre-Francoys Brousseau Stephen Fegan (UCLA) Goal Problem(s) Designs: pros & cons Where are we going? Conclusions

2. If trend continues to 2020 and ~ 1000 sources detected => 4 – 5 sources per field of view km2 Telescope Target Parameters • Light collecting area: 40 m2(QE=50%) – 100 m2(QE=20%) • Effective Aperture: 7 m – 12 m • Field of View: 15 deg (0.26 rad) • Viewing Solid Angle: 180 deg2 • Image quality: 1’=0.017 deg (<2 deg) – 5’ (<7.5 deg) (?) • Wavelength range: ~0.3 – ~0.6 micron • Focal Plane Instrument: • Array of light sensors: ~1024x1024 • Pixel: 0.86’ per pixel • Plate Scale: 0.5mm per arcmin (0.5 m diameter II) • 3.8 mm per arcmin (3.3 m diameter mosaic of MAPMTs [H9500] -> 1.6 m [H? 32x32]) "Ground-based Gamma-ray Astronomy: Towards the Future"

3. Image quality problem (7.5 deg) • Whipple-like designs >5’ Spot Size [arcmin] f=F/D - number FP/D ratio [1] Would require f > 3 and focal plane size > D Plate scale is mismatched f=F/D - number "Ground-based Gamma-ray Astronomy: Towards the Future"

4. “Super-Etendue (throughput)” problem It is extremely difficult to maintain reasonable image quality and achieve high throughput factor by simultaneously having large aperture and large field of view. Traditional optical designs forbid this. "Ground-based Gamma-ray Astronomy: Towards the Future"

5. “Super-fast” problem • Duration of Cherenkov light flash is a few nanosec. Thus unlike optical telescopes the imaging cannot be improved through increased exposure. Because of this severely light limited imaging regime optical system of “1 km2 array” telescope must be composed from minimal number of optical elements. • Plate scale and FoV requirement are compatible with effective focal length 1.9 m (II, <) or 12.6 m (MAPMTs, <) suggesting f/0.19 – f/1.26 At present it seems that f/0.19: VERY expensive telescope, REASONABLE cost camera f/1.26: VERY expensive camera, REASONABLE cost telescope "Ground-based Gamma-ray Astronomy: Towards the Future"

6. Scalability • With the fixed FoV telescope design is scalable with the primary mirror diameter. However, this changes plate scale which may not be allowed due to limit on the number of optical elements in the system. X Replica of Newton's first 6 inch reflector Telescope and Camera R&D are coupled Telescope prototyping is affected "Ground-based Gamma-ray Astronomy: Towards the Future"

7. Classical catadioptric wide FoV telescopes 3 optical elements design • Schmidt-Cassegrain • Spherical primary mirror corrected by the Schmidt corrector plate, convex hyperbolic secondary mirror and a focal plane located behind the primary • Maksutov-Cassegrain • either a spherical or parabolic primary mirror in conjunction with a meniscus-shaped corrector plate at the entrance pupil. The meniscus-shaped corrector plate allows for the use of an easily fabricated spherical secondary mirror rather than the hyperbolic mirror required for the Schmidt telescope. Main disadvantage: does not scale up to large apertures (>2 m), since the corrector plate rapidly becomes prohibitively large, heavy, and expensive. "Ground-based Gamma-ray Astronomy: Towards the Future"

8. Primary aberrations / design requirements Fast (small f-ratio) systems are severely affected by spherical aberrations and coma. • Spherical ~1/f3 • Coma (1st order) ~q/f2 • Astigmatism ~q2/f1 • Field curvature ~q2/f1 Design requirements: Optical system consists of minimal number of optical surfaces Spherical and Coma aberrations free Tolerable Astigmatism and high order Coma "Ground-based Gamma-ray Astronomy: Towards the Future"

9. Single Mirror: Lessons • Parabolic mirror is free from spherical aberrations but suffers from Coma • Davies-Cotton design, a cleaver spherical aberrations free discontinuous mirror solution, reduces Coma but doesn’t meet large FoV specifications. • One mirror catadioptric design may be “aplanatic”, but it suffers from large Fresnel lens requirement "Ground-based Gamma-ray Astronomy: Towards the Future"

10. 2-mirrors telescopes • Two mirror designs • Cassegrain • Gregorian • Dall-Kirkham • Ritchey-Chrétien Spherical aberration and Coma free Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope with a hyperbolic primary and secondary mirror. Famous RCTs The two10m components of the Keck Observatory The four 8.2m components of the Very Large Telescope in Chile The 4m Mayall telescope at Kitt Peak National Observatory The 3.5m WIYN telescope at Kitt Peak National Observatory The 2.4m Hubble Space Telescope currently in orbit around the Earth "Ground-based Gamma-ray Astronomy: Towards the Future"

11. RCT & Schwarzschild theorem Traditional for Cherenkov telescopes Davies-Cotton reflector compensates spherical aberrations by discontinuous mirror. Discontinuous primary and possibly secondary need to be explored for reduction of aberrations in fast optical systems Generalized Schwarzschild theorem: “For any geometry with reasonable separations between the optical elements, it is possible to correct n primary aberrations with n powered elements.” (1905) Fp concave Fs s convex F=Fp Fs / (Fs + s - Fp) Traditional RCT design is inconsistent with small plate scale requirement Discontinuous primary and continuous secondary introduces comatic aberrations (!) "Ground-based Gamma-ray Astronomy: Towards the Future"

12. Non-traditional RCT & Abbe sine condition Aplanatic Highly aspherical non-conic mirror surfaces Astigmatism and high order Coma can be contained within specs for FoV ~15 deg. Focal Plane Size, FPS, cannot be made arbitrary small Fp concave Fs s concave F=Fp |Fs| / (|Fs| - s + Fp) F/Dp > 1/2 "Ground-based Gamma-ray Astronomy: Towards the Future"

13. Ray Tracing: Design Example of detailed ray tracing in modified RC design Dp=10m Ds=4.1m Df=1.6m A(0)=0.81 x pi D2/4 A(7.5)=0.55 x pi D2/4 Spot size can be a few arcmin at the edge of the FoV "Ground-based Gamma-ray Astronomy: Towards the Future"

14. Ray Tracing Simulations at 7.5 deg Violation of Abbe sin condition in attempt to reduce plate scale rapidly deteriorates imaging quality (>=100’ at the edge of FoV). "Ground-based Gamma-ray Astronomy: Towards the Future"

15. Ray Tracing: Spot size "Ground-based Gamma-ray Astronomy: Towards the Future"

16. 3 optical elements systems: RC-catadioptric Needs detailed performance optimization Plate scale can be further reduced Fresnel lens aperture can be made acceptably small, however, preliminary analysis indicates strong accompanying vignetting Not clear if Abbe sine condition can be satisfied and very fast systems can be made aplanatic 3 optical elements and Schwarzschild theorem insure high potential for aberration reduction. The prove is classical Schmidt-Cassegrain designs and its versions Ligtht loss and cost increases Fp Fs s Fresnel lens F/Dp < 1/2 (?) "Ground-based Gamma-ray Astronomy: Towards the Future"

17. Three-Mirror Telescope: Paul design Needs detailed performance study for fast IACT applications 10 deg2 FoV, < 0.5’’ image quality LSST: 8.4-meter primary mirror, 3.4-meter secondary mirror, 5.2-meter tertiary mirror. The light reflected by this tertiary mirror then passes through a 1.4-meter lens to the camera detector. "Ground-based Gamma-ray Astronomy: Towards the Future"

18. Large Aperture D > 7m PMT or MAPMT based camera SiPMs Avalanche Geiger discharge ? Emerging Options >1.5 m II ? "Ground-based Gamma-ray Astronomy: Towards the Future"

19. II To trigger To single camera Emerging Options: D < 5 m PMT Telescopes could be deployed individually or combined on a single mount Combine electrical signals from all cameras operating in single photon counting mode “Star-like” approach Combine optical signals (MMT, Keck, SALT,…) “Ashra-like” approach "Ground-based Gamma-ray Astronomy: Towards the Future"

20. Ashra Optics Primary Mirror: 1.8m FoV: 50 deg Resolution: 1 arcmin Cost-performance balance Modified Baker-Nunn optics "Ground-based Gamma-ray Astronomy: Towards the Future"

21. Cost Considerations • The largest challenge is to find cost-effective solution ! • Large aperture large FoV Paul or RC-catadioptric designs requiring large focal plane plate scale are most likely prohibitively expensive (>>$1M per telescope) even if designed with moderate image quality of 1’. • Relatively small aperture (3-4 m diameter) modified wide FoV RC telescopes with small focal plane plate scale (<1m per 15 deg) allowing high pixel density focal plane instrumentation (MAPMTS, IIs) may provide basic integration element for construction telescopes with effective 8-13m aperture. • (D= 3m , A1=7 m2, A7=49 m2 (8 m), A19=133 m2 (13m)) "Ground-based Gamma-ray Astronomy: Towards the Future"

22. 3-4 m RC advantages • It appears to be consistent with virtually all proposed in this workshop telescope array concepts (1km2, STAR, small telescopes for high energy regime) and with operation in wide FoV sky survey mode • It appears to be compatible with potentially low cost high pixel density focal plane instruments based on MAPMT mosaics, IIs, and possibly SiPMs and APDs. • It might be utilized as a basic element for integrated moderate and large aperture telescopes for 1km2 array or (<10 GeV) large aperture telescope concepts via combining optical or electronic images • Utilizing innovative engineering designs have high potential for cost effective solution "Ground-based Gamma-ray Astronomy: Towards the Future"

23. Conclusions • Design of the wide FoV large aperture IACT optical system is driven by the high throughput, lowest light loss, small focal plane plate scale, low cost, and moderate image quality of ~1’. • Due to short effective focal length of optical system required to satisfy these factors design of the telescope is highly sensitive to spherical aberrations and Coma. • Aplanatic modified RC design with relatively small aperture may provide adequate solution as integration element • Optical group needs to be formed to further explore this primary option as well as Paul and RC-catadioptric design alternatives "Ground-based Gamma-ray Astronomy: Towards the Future"

24. Newspaper Ad • The original Edmund Scientific Astroscan 3m diameter RC IACT is an inexpensive, high-performance telescope that is easy-to-use, maintenance-free and completely portable. Its wide field of view, beautiful images and ease of use make it an excellent telescope for beginner and expert stargazers VHE astronomer alike. "Ground-based Gamma-ray Astronomy: Towards the Future"