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PRIMA hardware development at MPIA

The cat's eye retro-reflector optics for the PRIMA Differential Delay Lines (DDLs) have been designed at the MPIA, and have been manufactured with help from industrial partners. The DDLs allow a different amount of OPD to be applied to the two different light beams coming from one VLTI telescope. A pair of DDLs are situated after each of the Main Delay Lines (MDLs) within the VLTI.

Two PRIMA DDLs sit in each vacuum vessel. Each DDL consists of a cat's eye telescope sitting on a translation stage. The vacuum vessels themselves are mounted on a supporting optical table in the interferometric laboratory of the VLTI.

In the current phase only four VLTI Main Delay Lines (MDLs) will be supplied with Differential Delay Lines DDL(s), namely MDL #3, #4, #7, and #8. (i.e. vessels #1 and #3 will initially be empty)

The hardware contribution from the MPIA consists of the delivery of the DDL-optics, i.e. the 4 cat's eye systems and the vacuum windows for the vessels.

The coarse stroke for OPD-correction is 70 mm (provided by a translation stage), the fine stroke (provided by a piezo behind M3 of the cat's eye) is 10 μm.

There are four holes on the front side of the cat's eye – two in the horizontal plane for the input and output beams from the telescope – and two in the vertical plane for the metrology beams (here two beams are covered by the alignment targets).

Two optical cat's eyes in a vacuum vessel (in this image the cat's eyes are still equipped with alignment targets for the telescope input and the metrology beam)

Cat's eye optics on their translation stage.

The light-purple adjustment plate serves as the interface plate to the stage as well as providing the possibility for adjusting the optics on the translation stage.

Cat's eye telescope seen from the front.

Cat's eye telescope seen from the rear side.

The light-purple adjustment plate is equipped with several tools for fine adjustment of the optics on the translation stage. Also shown are three of the six magnets which prevent the optics from moving during minor earthquake events.

The cat's eye is a high-precision optical system which has been designed by Peter Bizenberger and manufactured by Axsys.

A cross-section through the cat's eye, and the adjustment plate beneath the cat's eye (light purple).
The following specifications have been met:
Beam diameter23 mm
Field of view in pupil10 arcmin
Separation of input and output beams120 mm
Wavefront error< 20 nm RMS
Tilt of input/output beam< 0.5 arcsec (on axis)
Differential tilt0.75 arcsec
Wavelength range0.6—28 μm
Overall transmission> 95% for λ > 1.0 μm
Maximum weight for optics8.5 kg (including plates)
Minimum eigenfrequency> 200 Hz

View from rear side of M3 showing the housing of the three-stage piezo actuator

MPIA Interferometric Laboratory with clean bench (inside transparent curtain) where the cat's eye telescopes were tested before delivery to Geneva for final integration

Photograph of the DDL in the laboratory at MPIA
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last update: 26-09-2013
editor of this page: Ralf Launhardt