ESO and Fokker Space Sign Contract about VLTI Delay Line

The VLTI Delay Lines

ESO PR Photo 08/98 [JPEG, 102k]

Schematic representation of the VLTI Delay Line, showing the retro-reflector on its moving base. For more details, please consult the technical explanation below.

This highly accurate system will be developed in close co-operation with the Dutch institute TNO-TPD (Netherlands Organization for Applied Scientific Research - Institute of Applied Physics). The most innovative feature of the Delay Line is the new control strategy, a two-stage control system, based on linear motor technology, combined with high accuracy piezo-electric control elements. This enables the system to position the so-called cat's eye reflector system with an accuracy of only a few nanometers (millionth of a millimetre (nm)) over a stroke length of 60 metres.

Within radio astronomy, interferometric techniques have been applied by Dutch astronomers since many years. They will now be able to contribute with their extensive knowledge of such systems to the next generation of astronomical interferometric instruments within the present collaboration.

About Fokker Space

Fokker Space is well embedded in the Dutch aerospace infrastructure, thanks to close relations with the Dutch Space Agency (NIVR), the National Aerospace Laboratory (NLR), the Delft University of Technology and other Dutch space industries and institutes like TNO-TPD (Netherlands Organization for Applied Scientific Research - Institute of Applied Physics). Fokker Space has also entered into strategic partnerships in Europe, Russia and North America. These facts, combined with the long lasting relation with the European Space Agency ESA and with the Dutch Government imply that Fokker Space has secured a solid base for continuation of its business far into the next millennium.

Some technical details about the VLTI Delay Line

In order to enable a useful combination of the light beams from the individual telescopes of the VLT (that is, to produce interferometric fringes at the focal point), the optical path length differences must be corrected by the Delay Line system.

These differences are caused by:

• the static geometric path length difference between the telescopes in a certain configuration;
• the diurnal motion of the astronomical source during observation due to Earth's rotation; and
• the rapid path length variations due to atmospheric disturbances and/or mechanical vibrations along the optical path length.

The VLTI Delay Line system consists of a retro-reflector mounted on a moving base. The optical design of this 'Cat's Eye' is of the Ritchey-Chretien type that reflects the light very effectively. For this particular application, the 'Cat's Eye' is not a corner cube with 3 perpendicular mirrors as is the case in the reflectors on cars and bicycles; it is in fact a telescope with a mirror at the focus that sends a light beam back in a direction parallel to the one it came from.

The moving base enables the Cat's Eye to travel along a 60 metres long rail track, thereby providing optical path difference corrections of up to 120 metres, as required for the VLT telescope configurations at Paranal.

The necessary, rapid path length corrections are performed by a fine positioning loop in which a piezo crystal (mounted on the backside of the Variable Curvature Mirror M3) is used to correct the fast optical path variations as measured by a Fringe Sensing Unit (FSU). The latter provides a signal to the Delay Line system via a fast link to the Delay Line Local Control Unit. An optical datalink to the Cat's Eye on the carriage ensures the transfer of data to the Piezo controller.

The carriage is driven by a Linear Induction Motor. The coils for the motor are mounted on the floor of the Delay Line Long Support Bench and the magnets are mounted on the bottom of the carriage. The metrology system (to measure the carriage position) consists of a laser-interferometer whose beam follows the same path as the light beams from the telescopes via the Cat's Eye.

The main design parameters are shown here:

Optical Path range	above 120 m
Optical Path resolution	better than 20 nm
Optical Path stability	better than 14 nm over any 0.01 sec (in the visible spectral range)
	better than 50 nm over any 0.05 sec (in Near-IR spectral range)
	better than 225 nm over any 0.3 sec (in Thermal-IR spectral range)
Absolute position repeatability	50 micron (over full length - 60 metres)
	1 micron (over observation length - 3 metres)
Maximum velocity:	0.5 m/sec
Maximum velocity errors	1 micron/sec
Maximum power dissipation	15 Watts

Note:

This Press Release is issued jointly by ESO and Fokker Space on the occasion of the signature of the contract for the VLTI Delay System which took place at Fokker Space in Leiden (The Netherlands) on March 12, 1998.

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