Reconstruct cosmic evolution history

Space with its infinite expanses fascinates mankind. Scientists have always been "reaching" for the stars, hoping to gain insights into elementary questions about our existence. A few years ago, researchers from the Sloan Digital Sky Surveys (SDSS) collaboration presented the first 3D map of the known universe. Now it is to be refined by spectroscopic investigations and provide new insights. Collecting data from millions of objects is a Herculean task that can only be accomplished precisely and in finite time with the right automation technology. Small "robots" and approx. 2,000 brushless DC servomotors from Faulhaber help to do this.

Over the next five years, SDSS V scientists plan to observe and analyze more than four million stars and about 300,000 black holes. Data will be collected with two large telescopes from Apache Point in New Mexico and the Las Campanas in Chile (Figure 1). "With the dual perspective from the northern and southern hemispheres, we can look in all directions in the sky," explains Jean-Paul Kneib, professor of astrophysics at the Ecole Polytechnique Fédérale de Lausanne in Switzerland.

This "view" first passes through optical fibers in the telescopes, which act as light receivers and point precisely to each celestial position. The high-precision alignment of the optical units is performed by 500 small robotic actuators per telescope. Brushless servo motors and precision gears from FAULHABER drive each of the two robot axes (Fig. 2). The optical fibers in the telescopes can thus be precisely aligned with specific objects in the universe in order to observe individual stars or the luminous accretion disks of black holes in a targeted manner.

Formation of the heavier elements

In addition to reconstructing the history of the Milky Way, the scientists want to trace the formation of chemical elements and decipher the inner life of stars. They also want to investigate the formation of planets and answer many of the unanswered questions that still exist about black holes. A mapping of the interstellar gas masses of the Milky Way - thousands of times more precise than before - is also planned. With the new findings, scientists hope to describe the "self-regulating mechanisms of galactic ecosystems" "This is what we have already used the two telescopes for in previous SDSS projects. But with SDSS V, we are now making a real quantum leap in terms of the efficiency of the observation and the amount of data collected."

Faster alignment creates new opportunities

For a measurement, the telescopes are first aligned in the direction of the observation objects. The precisely aligned optical fiber then takes a closer look at individual points in the overall view. "Previously, we had to have special plates made for each of the different observation tasks. Preparing each plate took several weeks. Then the fibers were fixed in the plate by hand - a very laborious and time-consuming process," reports Jean-Paul Kneib. With the new technology developed for the SDSS V, changing the fiber configuration for new objects takes no more than a minute instead of weeks (Fig. 3). This allows researchers to respond quickly even to unforeseen cosmic events, so they can target them from the start.

For example, if other telescopes detect an event such as a new supernova, one of the actuators can align its optical element to it with virtually no delay. A detailed analysis of the physico-chemical processes of this "hotspot" at a previously unattainable early stage of supernova evolution is thus possible. The generated measurement data can then be compared with existing theories and refine the models. "Since we save an enormous amount of time with automatic alignment, we can observe many more objects and make correspondingly more individual measurements," explains Jean-Paul Kneib. "This effect is further potentiated by the high precision. The diameter of the optical fiber is one hundred micrometers. The diameter of the point of light striking the telescope from an observed cosmic object is about the same. The more precisely these two small areas overlap, the more light yield we have for our measurements and the faster we get valid results."

Optical fiber precisely adjusted

The small robot actuators each consist of two longitudinally arranged slender cylinders with a curved extension at the front end. The rear, thicker cylinder sits in the telescope's detector plate. It forms the alpha unit and rotates the central axis of the robot. In the direction of the axis, the beta unit is mounted eccentrically at the front. It moves each of the three fiber tips also on a circular path: for the light in the visible spectrum, the light in the infrared spectrum and the calibration fiber in the curved extension (Fig. 4). Together, the two axial motions can thus position the optical fiber tips within a circular area with an accuracy of 5 micrometers as desired. The circles covered by the robots partially overlap with the circular areas of their neighbors. In this way, any point in the sky can be automatically aimed at within the telescope's detection range.

High accuracy due to sophisticated mechanics

The high accuracy is ensured by Faulhaber motors and gearboxes, as well as the mechanics developed specifically for this application by Faulhaber subsidiary MPS. Both robot axes are driven by brushless DC servo motors with diameters of 12 and 6 mm, respectively, from the 1218 ... B for the Alpha and 0620 ... B for the Beta axis. Matching planetary gearheads transmit speed and torque to the robot mechanics.  Integrated encoders report the exact rotational position of the motors to the controller (Fig. 5). "To achieve the required precision, we had to eliminate play in the system," explains Stefane Caseiro, who was responsible for designing the mechanics at MPS. To do this, the engineers replaced, among other things, the usual coupling between the gear shafts and the mechanical axes of the robot with clamp connections. A compression spring provides a base load to make the gearbox backlash-free. "Just finding the right spring took several months," recalls the MPS engineer.

Finding the right partner for this new development took Professor Kneib's team much less time. "There are not even a handful of manufacturers in the entire world who can produce miniature motors with the required quality and durability," says the astrophysicist. "Faulhaber was naturally on the short list of companies we asked for a quote. We had already worked successfully with MPS on an earlier project. The physical proximity to these specialists is of course also an advantage - it takes just under an hour and a half to drive from the university in Lausanne to MPS in Biel. In addition to the high quality and the good joint experience, it was a very decisive argument that Faulhaber, with its subsidiary MPS, can supply drive and mechanics from a single source."