Note: images available at

Less than one month after “First Light” for the fourth 8.2-m YEPUN
telescope (ESO PR 18/00), another
special moment occurred at ESO’s Paranal

This time, it was the first truly “underground” event, in the
168-metre long Interferometric Tunnel that has been dug beneath
the platform at the top of the mountain. As one staff member remarked
on this occasion, it was something like “the first scheduled trip of
the Paranal metro”!

With the successful integration of the first Delay Line on
Monday, September 25th, 2000, ESO has accomplished another important
step towards the VLT
Interferometer (VLTI)
. It will be followed by the integration
of the second Delay Line by the end of November and the third is
scheduled for February 2001; both are now in their final development
phase in Europe.

“VLTI First Light” is then expected to take place soon thereafter,
by means of two small special telescopes (“siderostats”). The
combination of the light beams from two of the 8.2-m Unit Telescopes
will happen in mid-2001.

The VLTI Delay Lines

The VLTI Delay Lines form essential parts of this very
complicated optical system. They serve to ensure that the light beams
from several telescopes arrive in phase at the common interferometric
focus. Details about how they function may be found in ESO PR 04/98.

In order to achieve the necessary performance, ESO has worked with
two Dutch contractors, Fokker
and TNO-TPD – Netherlands Organization for Applied
Scientific Research – Institute of Applied Physics
, to arrive at a
totally new Delay Line concept. Another Dutch participant in the VLTI
project is the Nova-ESO VLTI
Expertise Centre (NEVEC)
, cf. ESO
PR 14/00

The installation at Paranal

The last twelve months have been very busy for the integration
team, with much preparatory work at the VLTI buildings for the final
installation of the Delay Line systems.

The assembly of the translation mechanisms for the first two Delay
Lines in the tunnel started in mid-2000. This included the alignment
of their rails and supports to the extreme accuracy of about 0.25 mm
over a total distance of 66.7 metres (PR Photos 26a-b/00). To
achieve such an unusually high precision, ESO – in collaboration with
the French company FOGALE – developed a measurement system that
is based on the water-level principle.

The delicate assembly and alignment of the critical sub-systems of
the Delay Line were undertaken with the support of Fokker Space
and TPD/TNO (PR Photo 26e/00). Also for this,
state-of-the-art methods were required in order to ensure a stringent
performance of the system. This includes optical alignment of the
optics with an accuracy at the arcsec level and positioning of the
linear motors at the 0.01 mm (10 µm) level.

The Delay Line is one of the key systems in the VLT
Interferometer. It is responsible for the compensation of the length
of the optical path that is different from the individual

Extreme accuracy needed

In the case of the VLT, this accuracy of the path length
compensation must be within a tolerance of only 0.05 µm (0.00005
mm) over a distance of 120 metres. The present concept by ESO and the
Dutch contractors is based on a retro-reflector (a “Cat’s Eye”)
that is fixed on a carriage that runs on two stainless steel rails
(PR Photos 26c-d/00).

The motion on these rails is performed by a 60 metres linear motor
and a piezo-transducer element. They are controlled by a laser
metrology system that measures the instantaneous distances betwen the
mirrors with the required accuracy.

This carriage is 2.5 metres long and weighs 250 kg. The total
friction force is less than 50 grammes, thanks to the extreme accuracy
of the rail alignment and special ball bearings. Because of this, the
total power required for the Delay Line operation is only about 15

The mirrors of the retro-reflector are made of aluminium by
REOSC (France). They have been coated with a single layer of
gold for the best possible reflection at infrared wavelengths.