1. All-aluminum mirrors for the MAGIC Cherenkov Telescope
Nadia Tonello, for the MAGIC Collaboration
Why all-aluminum mirrors
Technical characteristics of the mirrors
Performance
Summary

2. Nadia Tonello, MPI für Physik, München
Motivations
Reduce the weight of the mirrors:
limited deformation of the main frame,
rapid positioning for GRB alert
Parabolic (isochronous) mirror profile:
many different radii of curvature, difficult to make it with glass mirrors
Heating system:
avoid problems of dew (ice)
Reduce costs
SOLUTION: diamond milled all-aluminum mirrors 50 x 50 cm2 sandwich construction
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München

3. Nadia Tonello, MPI für Physik, München
The MAGIC Telescope
ORM, La Palma
(Canary Islands)
Mirrors area
239 m²
Camera
576 pixels
FOV 4°
Energy threshold
30 GeV
Positioning time
< 40 s
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München

4. Technical view of a mirror
All-aluminum 50x50 cm ² sandwich = light weight (~ 4kg)
Original development at MPI (Munich), production of improved raw blanks in Padua (I)
Diamond milled reflecting surface
Heating system
Light and stiff honey-comb structure
The mirrors are mounted in panels before the installation
1.0
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München

5. Nadia Tonello, MPI für Physik, München
Performance
Reflectivity vs. wavelength
for a coated mirror
Roughness = 4.1 nm
87% on av. between 300 and
650 nm (sensitivity region of the PMTs)
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München

6. Focal spot studies
Shape and size of the spot:
measurement of the
effective radius of curvature.
6 cm
Light distribution from the center of the spot
Measurements taken at 2* focal length
90% of the light is collected in a circle ~30% of a pixel ø
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München

7. Assembling and alignment
The mirrors are assembled in panels of 4, pre-aligned and fixed.
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München

8. Installation on the telescope
The panels are installed in the main frame .
The active mirror control allow to align the panels
(MG talk)
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München

9. Nadia Tonello, MPI für Physik, München
Summary
Size of a mirror
0.25 m2
Weight of a mirror
3.5 kg
Reflectivity
(incl. coating)
≥ 85 %
Roughness
4.1 nm
Average spot size (90%)
1 cm 
Mirror production state:
750 / 959
Panels installation state:
98 / 245
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München

10. The new technology mirrors are a good alternative to glass mirrors.
Conclusions
Production of light weight mirrors (weight :1/2 of glass mirrors) .
Optical quality matches glass mirrors.
Technology of diamond milling allows to change easily the focal length during production (for parabolic profile).
Cost of diamond milling = 290 € ( incl. quartz coating)
Prototypes installed in HEGRA CT1 show good quality after 5 years exposure.
Control of the optical surface of the telescope with AMC (M.G. talk)
Preparing next generation of 1 m2 mirrors of aspheric surface.
The new technology mirrors are a good alternative to glass mirrors.
DPG Tagung, March 2003
Nadia Tonello, MPI für Physik, München