Experts from DTU Space have spent the summer upgrading NOT (Nordic Optical Telescope), an Earth-based telescope located 2,400 metres above sea level on La Palma in the Canary Islands. Thanks to the upgrade, the 30-year-old telescope will now be even better at identifying exoplanets—planets orbiting other stars than the Sun.
A spectrograph on NOT has been improved to increase the precision of the equipment. This means that NASA and MIT have now bought observation time on the telescope on La Palma in collaboration with the exoplanet group at DTU Space. In addition, data from NOT will complement NASA’s data from the TESS space mission, which is also looking for exoplanets.
“It would cost around DKK 30-40 million to build a new spectrograph, whereas our improvements only cost DKK 2 million. So, it’s definitely a cost-effective way to update the telescope,” says Lars A. Buchhave, Professor at DTU Space, who is behind the implementation of the project, which has been funded by the Carlsberg Foundation.
“And the fact that NASA and MIT have agreed to buy observation time on the telescope—which is a bit of a rarity—is a great recognition of the quality of this improvement work.”
Collects light from stars with fibre-optic cable
Roughly speaking, the telescope collects light from distant stars and sends it into a spectrograph via a fibre-optic cable. The spectrograph includes a so-called grating—a glass structure that diffracts the light. Inside the spectrograph, the light is split into different wavelength areas. Researchers will then analyse these areas and look for signatures from exoplanets.
The different wavelengths of light emitted from distant stars contain information about the exoplanet that may orbit a particular star. By analysing the light, you can obtain knowledge about, for example, the exoplanet’s mass and atmosphere.
TESS is used to identify exoplanets from space and determine their size. The mass of the exoplanets is determined using follow-up data from a number of Earth-based telescopes, including, of course, NOT. Based on this knowledge about mass and radius, it is possible to calculate the planet’s average density, and thus get an initial idea of what the planet is composed of.
It is also possible to examine an exoplanet’s atmosphere and, among other things, gain insight into its chemical composition and thus compare it with other planets.
Three important elements improved
A small team of technicians and researchers from DTU Space have been in the Canary Islands to carry out the upgrade: major improvements of FIES, the NOT telescope’s spectograph.
The upgrade of FIES includes three main areas: its grating has been installed in a new pressure chamber, and it has been fitted with new optical fibres and a new calibration source.
One of the significant improvements has been to make the spectograph’s grating less sensitive to changes in atmospheric pressure, which disturbs the measurements.
A unique solution
Scientists and engineers at DTU Space solved this challenge by developing a unique solution.
The mass of an exoplanet is identified by measuring small Doppler shifts in the host star’s light, which occur because the planet ‘pulls’ on the star when it orbits it.
These measurements can be translated into radial velocities, which are used to determine the weight of the mass that impacts the star. In this way, an exoplanet can be ‘weighed’, so to speak.
But similar shifts occur in the grating of the spectrograph as a result of changes in atmospheric pressure where the telescope is placed on Earth. This impact can be 100 times greater than the impact from the exoplanet, which makes it difficult to carry out measurements with satisfactory precision.
But Lars A. Buchhave and his colleagues have calculated that it is possible to achieve a much smaller impact—and thus more precise measurements—by isolating the grating from the surrounding atmospheric pressure.
This has been achieved by constructing a sealed metal box to place the grating in, after which it is filled with neon gas, which causes much less change to the refractive index of the incoming light than atmospheric air does. This ensures better precision in determining the mass of exoplanets.
“Now we just hope that it works as intended, and that the grating together with the other improvements will make the hunt for exoplanets even more exciting,” says Lars Buchhave.
“So far, that seems to be the case.”
A challenging engineering task has been solved, ensuring a more efficient hunt for exoplanets
Engineer Niels Christian Jessen (right) and Assistant Engineer Michael H. Avngaard (left) from DTU Space have been in charge of upgrading the essential parts of the telescope at La Palma to make it even betting at hunting for exoplanets.
They have improved the spectrograph, which splits the incoming light from stars into different wavelengths. Among other things, they have built a special metal box (pictured) and a frame that now houses the grating (a special glass structure), which is kept under pressure by means of neon gas.
The installation itself was a challenging task. The removal and relocation of the grating to the new metal box was one of the most critical moments of the upgrade. The task required great caution and a special tool (pictured), as the grating/glass structure was not to be touched during the installation.
Fotos: Joonas Viuho/NOT)