3 Laboratory results

The interferometer was aligned and tested in the laboratory with an artificial star using a laser and a quartz-iodine bulb for white light. After the alignment was completed the imaging capabilities of the interferometric array were tested by replacing the pin hole placed in front of the white source with a multi-hole mask simulating a multiple star. Figure 3 (left) shows a simulated triple system and a cluster of 6 objects (right).

Owing to the unequal thickness of micro-lenses in the array, used initially without immersion, the images obtained at this stage were not correctly phased and exhibited a speckle pattern rather than the expected interference peak. As expected, the speckle pattern was observed to become double when observing a binary artificial star, thus confirming that large hyper-telescopes will be usable for speckle interferometry when not phased adaptively.

In such large hyper-telescopes adaptive piston phasing will however be desirable, in addition to adaptive optics within the sub-apertures. We recall that speckle interferometry does not have an equally good imaging performance for complex objects.

The first micro-lens array can be removed if the sub-pupil size is small enough that diffraction suffices to fill the facing micro-lenses of the second array. This also facilitates somewhat the phasing and makes the alignment easier at the cost of an increased difficulty in controlling the size of the diffraction lobe on the micro lenses. We discuss this effect in Sect. 5.