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Table A.1

Carlina specifications.

Parts Characteristics Specifications
High level Low level OHP prototype

Primary mirror Numerous spherical mirrors, all with the same curvature radius. –  Mirrors aligned on a sphere (coherencing). – The numbers of mirrors and their positions have to be adapted to the science targets. Piston and Tip-Tilt of each mirror must be controlled with an accuracy of one λ. – 3 mirrors; R = 71.2 m – Bases: 5 m, 9 m, 10.5 m – Mounts stability and micrometric screws accuracy  ≈ 1   μm

Focal gondola Used to obtain stellar images: Constituted with a spherical aberration corrector (Mertz), a guiding system, and a focal instrument (densifier, AO, etc.). – Track the star on the focal sphere (R/2). – The drift during the tracking has to be smaller than one fringe per exposure time. – Have to be positioned at the correct focus. – Maximum tracking velocity – Velocity drifts  < λf/B per exposure time – Focus tolerance (spots shifted by half their size) – Maximum tracking velocity  = 2.6 mm/s – Maximum velocity drifts  = 1.7 mm/s for 1 ms exposure time – Focus tolerance  =  ± 0.6 mm

Metrology gondola Used to align the primary mirrors:
The Tip-Tilt of each primary mirror is adjusted by superimposing all the return images of a source at the curvature center, while the piston is adjusted by searching the white fringe. In practice, the source and the metrology CCD are placed on the ground and the metrology gondola is made of a convex mirror attached at the bottom of a girder (Fig. 8).
– The top of the girder gondola (Ω) must be stable, in a such a way the return beam goes on the field mirror at the ground (Figs. 3, 4). – The altitude of the girder gondola must be accurate to focalize the metrology source near the center of the diluted primary mirror (also on the field mirror). – The maximum tolerable displacement (“horizontaly”) of Ω is
– The position tolerance of the metrology gondola in altitude (vertical displacement) is:
– ΔΩ(x,y) =  ± 2   mm – Δz =  ± 2.5   mm with ro = 100   mm, rfield   mirror = 300   mm, R = 71.2   m and fmetrology = 1 m

Holding system The Holding system carries the metrology gondola, and the focal gondola above the primary mirrors. It can be made of cables under an helium balloon, or cables attached between mountains, pylons, etc. – At least at the altitude of the curvature radius R of the primary mirror above the primary segments. – It uses material with high tensile strength and Young’s modulus: Kevelar, Carbon, etc. – The holding system can be stabilized. – At OHP, a helium balloon at 120 m altitude tightens a tripod of PBO cables. – The top of this stabilized tripod is at  ≈ 71   m above the ground.

Notes. R is the curvature radius and f = R/2 the focal length of the diluted primary mirror; rfield   mirror is the radius of the field mirror; fmetrology the focal length of the metrology mirror; r0, the atmospheric Fried parameter ( ≈ 100 mm at OHP); B the baseline; the wavelength λ = 0.5   μm.

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