🥳 Lab demonstration of the PIAAN: a milestone for RISTRETTO

15-03-2025 #

🥳 Lab demonstration of the PIAAN: a milestone for RISTRETTO #

The observation and characterisation of exoplanets is a difficult task: they are faint and they orbit around a much brighter star. To remove the blinding halo of the star, an optical system called coronagraph is used by astronomers. For RISTRETTO, it must be able to diminish the stellar halo by a factor of at least 10.000, while transmitting as many planet photons as possible at only 2 times the diffraction limit of the telescope (i.e. nearly the physical limit of any telescope). Many concepts were succesfully developped and tested over the years, but these needs are so specific that we had to develop a new concept, the so-called Phase Induced Amplitude Apodizer and Nuller (PIAAN).

Two components constitue the PIAAN:

  • A PIAA optics, that reformats the light distribution from the telescope, from a uniform to a gaussian one;
  • A 7-spaxel IFU, using a set of hexagonal lenslets to feed 7 single-mode fibers. Those fibers have particular spatial filtering properties that we exploit in order to cancel the stellar light.

The proper dimensioning of both components promises to reduce in theory the stellar light coupled into the fibers by a factor 1.000.000 (i.e. a star transmission of 0.0001%), while the light from the planet is nearly unaffected, with a transmission of about 70% (!), despite its apparent proximity.

Several IFUs have been made over the last 3 years thanks to a new technology, which allows to custom print lenslets on top of the fibers with a remarkable accuracy. Our tests have demonstrated the maturity of this recent technology for our purpose. The PIAA optics was received in early 2024, which allowed us to test the PIAAN on a dedicated, high contrast bench designed and integrated in Geneva.

4 years after the original idea, the PIAAN is effectively working! And this is a major milestone in the development process of RISTRETTO.

We demonstrated that we can extinguish the star by a factor of 30.000, which may look small compared to the theoretical limit, but is in agreement with our expectations knowing the current technological limitations. This was also allowed by the use of a Deformable Mirror (DM) and a new control strategy, that gives us extra degrees of freedom to manipulate the light and compensate for some defects inherent to the manufacturing process. The performance is a couple times better than required for RISTRETTO once observing at the VLT. Indeed, its performance will eventually be limited by the residual turbulence after the eXtreme Adaptive Optics system. The latter system is another cornerstone of the project, which is under study.

HCbench_picture_scheme Picture of the PIAAN test bench with both its components and the Deformable Mirror highlighted.

PIAAN_contrast_optim Measured star transmission T of the PIAAN. In orange, before using the DM, T0.027%. In green, after optimization of the DM, T0.0035%.