How do engines make a GPS for the Milky Way possible?

1. How do engines make a GPS for the Milky Way possible?
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What's going on in the Milky Way? We know surprisingly little about this, as we literally
have a hard time seeing the forest for the trees. The Moons project, coordinated by British
astronomers, is set to change that significantly.
When exploring our own disk-shaped spiral galaxy, astronomers have a fundamental
problem: Earth is not exactly in the middle, but it is in a place on the disk of the Milky Way.
If you want to look into its centre from here, or even beyond to the other side, countless
stars get in the way. From an earthly perspective, it has often been difficult or impossible to
determine where they are on our common disc. One of the regions about which we know
particularly little is the bulge in the centre of the galaxy, where heaps of stars and clouds of
gas cluster around a suspected black hole.
A large-scale astronomical project should close many knowledge gaps in the near future.
Eight institutes from several countries are involved in its implementation. The client is
the European Southern Observatory (ESO). This scientific organization operates some of
the world's most powerful telescopes in Chile's the Atacama Desert. They include the Very
Large Telescope (VLT) at the Paranal Observatory, with a mirror diameter of 8.2 meters.
The aim of the project is to equip the VLT with a new instrument for recording the optical
signals from space. This is a spectrograph that can simultaneously record a large number of
cosmic objects in the visible and infrared parts of the spectrum. His abbreviation describes
the whole project: Multi-Object Optical and Near-infrared Spectrograph, Moons. It is
coordinated by the United Kingdom Astronomy Technology Center (UK ATC) in Edinburgh,
Scotland.
New possibilities thanks to spectrography
"With a quality still camera, you can swap lenses. With an astronomical telescope, it's the
other way around - we have excellent lenses from the VLT and will be swapping out a
currently connected 'camera' for our Moons," explains Dr William Taylor, a scientist at UK

2. ATC. With its new technology, Moons opens up completely new possibilities for observing
the sky, even if it does not produce large-scale images in the traditional sense, but instead
focuses on many details in great detail.
It works like this: The huge lenses and mirrors of the VLT are swivelled as before to the part
of the sky that you want to examine. Now, in the moons, the ends of exactly 1001 optical
fibres are aligned with individual objects in this cosmic region. Instead of imaging the entire
selected area like a camera, the new instrument focuses the fibres on specific points in the
universe. And these points are not simply photographed either, but their light is broken
down into its individual parts, ie different wavelengths, using prisms.
"We get a lot more information with this method than we do with a simple image," explains
Taylor. "For example, we learn about the chemical composition of the observed object. We
can also calculate its dynamics - the speed and direction of its movement.
Because Moons captures the near-infrared spectrum, we can accurately analyze the redshift
that light from distant objects undergoes on its way to us." As a star moves away from Earth,
the wavelength of its light lengthens. Part of visible light is thus shifted into the invisible
infrared range, which is still close to the visible spectrum.

3. Thousands of objects in view at once
So far, it has been possible to observe at most around a hundred objects individually with
the available technology, and this is only in the range of visible light. With Moons, not only
does this number increase tenfold, but the depth of information also increases many times
over. Within the Milky Way, it will thus be possible to look much more precisely between the
trees and get a clearer picture of the entire forest.
"Indeed, one of the goals of our project is to create a 3D map of the Milky Way, with which
a kind of GPS navigation in our galaxy will be possible. In addition, the Moons technology
allows us to go very far with a previously unattainable resolution - and thus also very far
back in time - to look back. We will be able to approximate the Big Bang to within a few
hundred million years." This gives the scientists an insight into the early childhood of the
universe. Although you can already look there today, the picture with Moons is now much
sharper and more detailed, explains Dr Taylor. "We will also be able to map the universe at
unprecedented depth."
To do this, the astronomers want to target many millions of objects over a period of about
five years. This can only succeed if the 1001 optical fibres of the spectrograph can be
aligned to the cosmic targets quickly and largely automatically.

4. This task is also performed by many fibre positioning modules (Fibre Positioning Units, FPU).
They each have two drive units consisting of a stepper motor and a low-backlash spur gear.
The rear Alpha unit moves the central axis. The front beta unit is mounted eccentrically on it,
which at the same time moves the fibre tip.
By combining the two axial movements, each FPU covers a circular area in which the fibre
can be oriented at will. This area partially overlaps with the areas of the adjacent FPU. In this
way, every point in the detection area can be controlled.
In order to ensure the required precision and to avoid collisions between the FPU tips, the
systems must work with high repeatability. The high-quality stepper motors come from
Faulhaber Precistep, the backlash-free gears from the Faulhaber Mini motor contribute to
the accuracy of the positioning. The Faulhaber subsidiary MPS is responsible for the
mechanical design of the modules.
Spectrograph with extreme accuracy
"We received a lot of valuable input from all three companies involved in the Faulhaber
Group," reports Dr Steve Watson, who is responsible for the development of the FPU at UK
ATC. "Without their specific know-how, we would not have been able to develop this central

5. module in this form and above all not to produce it in the required quantities." In addition
to the speed when aligning the optical fibres, the highest level of precision is also important.
"We achieve an accuracy of 0.2 degrees and position repeatability down to 20 microns,"
Watson continues. "In view of the length of the FPU and the modular design, these are
outstandingly good values. The correct alignment in relation to the focal plate on which the
modules are arranged is also retained in all positions."
The high precision and extreme reliability of the components make it possible to keep the
controls simple - another condition for the smooth operation of the spectrograph. Complex
electronics and control logic would present a major hurdle to quickly driving 1001 units
simultaneously. Thanks to the high quality of the components, precise alignment is achieved
with simple open-loop control. In addition, the technology must be very robust and
practically maintenance-free in order to fulfil its tasks without interruption over the planned
ten-year lifetime of the plant.
project manager dr Alasdair Fairley is already looking beyond such technical questions:
"Work on Moons is progressing well. We expect to be able to install the spectrograph in the
summer of 2021. Commissioning will take about half a year, so we expect to start mapping
in early 2022 We are confident that the FPU will remain fully functional for the ten years
without maintenance."