Special chains enable the precise alignment of a state-of-the-art telescope on La Palma.

The LST-1 Cherenkov telescope on the Canary Island La Palma is one of the most modern receiving systems for gamma rays that reach us from distant galaxies over many millions of light years and hit the earth's atmosphere. With the help of this high-tech telescope astrophysicists are hoping for new insights into the most energetic processes of the universe such as supermassive black holes or supernovae. One of the challenges for the telescope is the research of so-called gamma-ray bursts (GRB), the origin of which remains unclear to this day. The energy emitted by such a gamma-ray burst is millions of times stronger than that of a supernova.

GRBs are extremely transient events, usually lasting only a few seconds, but with large fluctuations: Some of the GRBs shine for up to one minute - some for only fractions of a second. The LST-1 is to observe the longer shining GRBs. For this purpose, the telescope weighing 115 t must be precisely aligned to any position in the sky within shortest time. For the vertical alignment of the telescope, Wippermann developed the components for an extremely robust as well as precisely working drive solution.

Mission:

The LST-1 Cherenkov Telescope (Large-Sized Telescope) on the Canary Island of La Palma, which went into regular operation in 2021, is the first of eight large, latest-generation gamma-ray telescopes worldwide to be built as part of the international CTA (Cherenkov Telescope Array) observatory at two sites (on La Palma and in Chile). The observatory will also include 30 to 40 twelve-meter telescopes and up to 70 three-meter telescopes. With a mirror diameter of 23 meters and a total height of 46 meters, LST-1 is the largest telescope in this project. The challenges: For detection of the very transient gamma radiation, the telescope must be aligned to any point in the sky with highest precision and within just 20 seconds. The Scientists and technicians at the Max Planck Institute for Physics in Munich are responsible for the drive required for this vertical alignment. In an early stage of conception, the planners contacted the chain specialist Wippermann to find a suitable drive solution for the vertical alignment. The basic factors for the impressive specifications:

Robustness

Designed for a total weight of over 50 t of mirror weight and transmission of enormous torques
Highest wear resistance for minimum maintenance intervals
Resistance to temperatures between -25°C to +40°C

Precision

High movement speed (maximum 20 seconds to any position)
Highest precision with alignment accuracy <14 angular seconds (0.0038°)
Slip-free movement of the sensitive telescope

Solution:

Due to the high weight of the telescope and the required alignment precision, a tangential drive in the form of a lantern gear design was the obvious choice. For safe absorption of the high forces the chain is firmly bolted to the drive arc of the telescope like a rack.

Since chain replacement is only possible with great effort by design, the chain must be extremely low-wear and designed for a very long service life. Therefore, the chains and sprockets required an individual, customized special solution that reliably meets the requirements for load-bearing capacity and wear resistance for this maximum load application. Wippermann developed both a special chain based on the D652 (32B-2) double roller chain according to ISO 606 as well as four specially adapted sprockets to be used on the impressing gamma telescope: A specially tuned heat treatment process gave the drive products a high surface hardness and, as a result, the best wear resistance for a long service life.

Lantern gear drive with fixed drive unit

With the lantern gear drive, the drive unit (red) is fixed at one point and moves the telescope with the aid of the drive arc.

Segments of the drive arc for the lantern gear drive

The Wippermann Duplex roller chains are fixed to the steel profiles by means of special attachments: Every third outer fitting plate is positively connected to these attachments via extended bolts.

Custom made sprockets

Drive with custom made sprockets with a tooth geometry that has been optimally designed for the used chains to provide a slip-free drive.

Enormous load on the drive system

The drive arc with the roller chains is attached to the moving structure of the impressive parabolic mirror. When moving the 50 t mirror, the chains and sprockets have to absorb enormous forces.

High-precision movement without slippage thanks to the perfectly coordinated lantern gear drive system

Another challenge was the precision of the lantern gear drive: The sensitive telescope system consisting of the 400 square meter parabolic mirror and the 2,000 photodetectors must be moved without slippage at all times. Furthermore, the astronomy researchers needed an adjustment accuracy of <14 angular seconds. For your information: An angular second is the 3,600th part of one degree. Therefore, the drive specialists at Wippermann had to perfectly adapt the double roller chain and sprockets to each other.

An essential aspect for slip-free operation and precision is the perfect lantern gearing of the sprockets, which enable a positive drive without wrapping. For this purpose, a special tooth geometry of the sprockets was calculated by Wippermann's engineering department and the precise manufacturing with tightest tolerances was done on a state-of-the-art CNC machine. Thanks to the consulting and manufacturing expertise of the chain specialist Wippermann it was possible to manufacture an extremely powerful drive system for this unusual application. And other telescopes with this drive solution from Wippermann are already in the pipeline. This provides the astrophysicists involved in the CTA observatory all around the world the opportunity to take a very close look in the future: at active galactic nuclei, supernovae, pulsars, black holes and other phenomena in space.

^{Photos: Max-Planck-Institut für Physik München}