1. The optical layout of future WFCT
YangRui
Yunnan University
The LHAASO workshop
18,February,2011

2. Outline The present telescope Fresnel corrector WFCT
Two reflecting mirrors WFCT
Discussion

3. The present telescope Single tessellated spherical reflector
Radius: mm
Focal length: mm
Spot: mm
Resolution: º
Field of view: º14º
Size of PMT cluster: 640 mm ×554 mm
Materials: float glass coated with
aluminum and MgF2
Reflectivity: %(UV)
(provide1ºpixels in an overall field of view)

4. Geometric size
24mm mm

5. Expecting parameters of future PMT
Diameter of PMT（mm) 40 19
25.4 29 38
Array
16×16
32×32
Size of PMT array（mm）
640×554
608×529
832×720.5
464×402
608×526.5
Resolution (degree) 1
0.5
0.75
Spot (mm)
FOV (degree)
16×14
12×10
Priorities
prototype 1a 1b 2 3

6. Image quality, especially for skew rays
Fraction of Enclosed Energy of existing telescope
PMT
Incident
angle 0º 7º
40mm
100%
60%
25.4mm
20%
19.0mm
50% 9%
Necessity for redesign the optical telescope
Small spot
Aplanat
Image quality, especially for skew rays

7. Demands
1/4 shipping container（inner room:
2.9 m × 2.5 m × 2.38 m）
Wavlength: nm—400nm
Field of view: 14º16º
Spot: mm or 25.4mm
Resolution: º
Cost-efficient

8. Fresnel corrector WFCT
19.0mmPMT
Total length: 2546mm

9. 19.0mmPMT

10. 19.0mmPMT Demands Simulation results Field of view 14º16º 13º15º
Resolution
0.5º
0.46º
Spot
19.0mm
Enclosed Energy
8595%

11. 25.4mmPMT
Total length: 2440mm

12. 25.4mmPMT

13. 25.4mmPMT Demands Simulation results Field of view 14º16º 17º19.6º
Resolution
0.5º
0.58º
Spot
25.4mm
Enclosed Energy
9098%

14. Main features of this design: The prime mirror: spherical reflector
same as prototype
Fresnel lens used as corrector
Focal length:
Aperture:  mm
Material: PMMA ( Polymethyl Methacrylate )
Good image
Mature technology： e.g ChengDu FSCREEN,
WuXi BHlens

15. Challenge: high UV transmittance
Manufacturing scheme of Fresnel lens: casting moulding or compression molding with optical plastic
Material: PMMA(73% transmittion for UV)
Total efficiency:  54%
Reflectivity of prime mirror: %(UV)
Transmissivity of PMMA: 73%(UV)
Imaging efficiency: 90%(skew rays)
Whether it can work well?

16. Two reflecting mirrors WFCT
Improving total efficiency
Total efficiency of two reflecting mirrors: 62%
prime mirror: %(UV)
Secondary mirror：83%(UV)
Imaging efficiency: 90%(skew rays)?

17. 832×720.5 mm
Total length: 2700mm
Even asphere:

19. Simulation results(25.4mm):
Field of view 14º16º
Resolution º
Enclosed Energy 100%(0º)  80%(7º)
Total efficiency for fringe rays
83%83% 80%=55%
The surface sag of secondary mirror is given by:

20. 608×529mm
Total length: 2700mm
Even asphere:

22. Simulation results(19.0mm):
Field of view 14º16º
Resolution º
Enclosed Energy 90%(0º)  65%(7º)
Total efficiency for fringe rays
83%83% 65%=45%

23. Main features of this design: The primary mirror: spherical reflector
The secondary mirror: even asphere
Radius:
Semi-Diameter:
Disadvantage : large obstructed area

24. Discussion Fresnel (PMMA) Two reflecting mirror Image good
Deformed (off-axis)
Energy efficiency
54%
45%55%
Obscuration ratio
10%16%
30%33%
manufacture
Mature,
cost-efficient
Asphere,
difficidult

25. Conclusions
Because of large obscuration, two-mirrors system is not good for this application.
Although practical efficiency will not so good as expecting, Fresnel corrector system is useful for imaging.

26. Thank you!