NASA has learned how to dynamically eliminate picometer distortion from telescope optics

In the “clean room” of the Goddard Space Flight Center, the technology uncovers the segmented mirror of the observatory of James Webb in preparation for the alignment test in the summer of 2016. Photo: NASA / Chris Gunn

To find and determine the characteristics of dozens of exoplanets similar to Earth, a very stable space telescope is required, the optical components of which move and distort the picture by no more than a few picometers - this is less than the size of an atom. New generation tools are also required to guarantee this level of stability. A year and a half ago, NASA allocated funding to a research group at the Goddard Space Flight Center and 4-D Technology to create a high-speed interferometer that should provide peak telescope stability. Such a task could not be solved by anyone before.

As in all interferometers, here the light beam is divided into several coherent beams. Each of them goes its own way, and then they unite again, creating an interference pattern that can be used to determine the phase difference of the interfering beams at a given point in the picture. So you can register the slightest movement or displacement of the material. Such an interferometer was used to adjust 18 mirrors of the observatory of James Webb.

NASA decided that measuring only the surface of the mirrors is not enough. Therefore, at the Goddard Space Flight Center, together with 4-D Technology, we developed an advanced dynamic laser interferometer that simultaneously detects displacements not only of mirrors, but also of their mounts and other structural components, operating in conditions of vibration, noise or air turbulence. The tool was four orders of magnitude more accurate than any such technique at the time. Shortly after its creation, the tools were immediately used in the laboratories, clean rooms and test chambers of all project participants.

But this, too, was not enough to carry out LUVOIR (Large UV Optical Infrared Surveyor) type space missions. The concept assumes that large mirrors with a diameter of 8–18 m cover simultaneously the ultraviolet, visible and infrared wavelength ranges. LUVOIR telescopes will be able to analyze the structure and composition of the surface of exoplanets, as well as remove weak star stars, to give an idea of ​​how planets are formed. Moreover, such telescopes will be able to determine bio-signatures in the atmospheres of remote exoplanets: the content of CO ₂ , CO, molecular oxygen (O ₂ ), ozone (O ₃ ), water (H ₂ O) and methane (CH ₄ ).

Shooting simultaneously in different spectra of LUVOIR will help to understand how the UV radiation of the parent star regulates atmospheric photochemistry on habitable planets.

On January 25, 2018, the research team at the Goddard Space Flight Center announced the creation of a tool that will make it possible to achieve peak telescope accuracy. This is the first of its kind unique instrument of this kind - speckle interferometer (speckle interferometer).


The Goddard Center Optics Experts Babak Salf (left) and Lee Feinberg (right) with the help of engineer Genie Eli-McMahon (from the center) developed an Ultra-Stable Thermal Vacuum System (Ultra-Stable Thermal Vacuum system), which will be used for testing interferometer measurements with an accuracy of 12 picometers

Scientists have demonstrated that the new interferometer is capable of dynamically recording displacements on a 1.5-meter segmented telescope mirror and its supporting structure with an accuracy of 25 picometers.

Such atomic scale shifts in certain areas of the mirror may occur due to temperature changes or as a result of “inaccurate” transportation from Earth, when the launch vehicle accelerates with an acceleration of 6.5 g. Scientists say that even a shift by one atom will affect the accuracy of measuring the atmosphere and the surface of remote exoplanets.

Developers are now going to test the interferometer in an ultrastable thermal vacuum setup — and see if it can register displacements of 12 picometers, that is, 1/10 of the diameter of a hydrogen atom.