All-aluminum mirrors for the MAGIC Cherenkov Telescope

Nadia Tonello, for the MAGIC Collaboration

Why all-aluminum mirrorsTechnical characteristics of the mirrorsPerformanceSummary

DPG Tagung, March 2003 Nadia Tonello, MPI für Physik, München2

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, München3

The MAGIC TelescopeORM, 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, München4

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, München5

PerformanceReflectivity 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, München6

Focal spot studies

Shape and size of the spot:

measurement of the

effective radius of curvature.

Light distribution from the center of the spot

90% of the light is collected in a circle ~30% of a pixel ø

6 cm

Measurements taken at 2* focal length

DPG Tagung, March 2003 Nadia Tonello, MPI für Physik, München7

Assembling and alignment

The mirrors are assembled in panels of 4, pre-aligned and fixed.

DPG Tagung, March 2003 Nadia Tonello, MPI für Physik, München8

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, München9

SummarySize 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 / 959Panels installation state:98 / 245

DPG Tagung, March 2003 Nadia Tonello, MPI für Physik, München10

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.