Real­time dome turbulence characterization through image motion and scintillation of passive targets

Whether it is to look up to the skies or down to the earth, the turbulent atmosphere plays a major role. Overwhelmed by its randomness, even the visionary Isaac Asimov once claimed:

“To make still bigger telescopes will be useless, for the light absorption and temperature variations of the earth’s atmosphere are what now limits the ability to see fine detail. If bigger telescopes are to be built, it will have to be for use in an airless observatory, perhaps […] on the moon.”

His future-telling ability as a science-fiction writer failed him. We have come up with ways of taming the atmosphere through decades of studying and predicting its behavior. Here, in Chile, two of the largest telescopes ever built are progressing toward their first light in the next lustrum: the Giant Magellan Telescope and (European) Extremely Large Telescope.

Giant Segmented Mirror Telescopes (GSMTs) will depend on wide field Adaptive Optics (WFAO) schemes relying on a more comprehensive characterization of the whole atmospheric volume above them—its three-dimensional distribution, and also speed and direction of wind stratification. The success of WFAO has pushed GSMTs to change the traditional scheduling of scientific programs, based solely on its quality, to a new approach where the state of the optical turbulence is considered: the Service Mode. To achieve this scheduling goal, many models have been introduced to forecast the mesoscale atmospheric turbulence and then predict relevant astro-climatic parameters; that is, acquiring some meteorological information, including the refractive index structure constant height profile, Cn2(z)C_n^2(z), at a given time above the observatory site, and then estimating a nowcast with a validity of at least one night. With all their capacity, these models are still unable to predict ground-layer turbulence, thus impacting directly on the performance and scheduling of all WFAO programs. But even with the eventual success new strategies, one last barrier still remains: dome turbulence. For most practical purposes, it is defined by its impact on the telescope seeing. It severely affects the performance of wide-­field instruments for (high-contrast) imaging and spectroscopy.

All the techniques implemented to asses it assume the classical Obukhov-Kolmogorov (OK) theory as valid. Nevertheless, in such a constrained space, convection and shear play a larger role, and thus OK turbulence will only partially contribute to the observed phenomena. A seeing value may be retrieved from classical theories but neither Cn2(z)C_n^2(z) nor the power-spectrum exponent (α\alpha) will account for the real dynamics inside the dome enclosure. Moreover, besides their differences, all these approaches use primary light sources: active targets (LEDs or laser beams) with controlled power output. Inside the dome, the presence of radiation sources must be restricted to reduce noise contamination to any scientific program run by the observatory. Therefore, monitoring the turbulence in situ at all times with any of these techniques may put observational data recollection in jeopardy. Some years ago, the idea of using passive targets illuminated by natural light, and distributed over a landscape, can assist into determining the path integrated Cn2(z)C_n^2(z) by studying their relative motion arose—non-cooperative targets have been proven useful in estimating it at a specific location.

The purpose of the present research project is to bring forth image motion and scintillation of passive targets to the assessment of dome turbulence. Providing an innovative platform, with low maintenance costs, by taking advantage of our deeper knowledge of the statistical properties of the fluctuations of the refractive index. PASsive TArgets for DOme Turbulence Monitor, PASTA.M, will provide observatories with detailed and complete depictions of the dome turbulence beyond the possibilities of current instrumentation. Therefore, while contributing to the ranks of instruments independently sampling the ground layer, it will help understand the information retrieved by its counterparts at the dome location.

Call for a postdoctoral position

As part of this research project, we are seeking for a young PhD graduate to cover an open postdoctoral position at the Center of Adaptive Optics of Valparaíso (CAOVA, Pontificia Universidad Católica de Valparaíso, Chile). The interested candidate is expected to perform research in one of these topics:

By covering one of these areas, they will participate in the the development of PASTA.M. In the upcoming four years of development, we are planing several campaigns at the Paranal Observatory and the postdoctoral fellow will be required to participate and lead in some of them.

Salary and benefits

Initially, it is between CLP $1,200,000 and $1,910,000 (USD $1,530 to $2430) starting between January 1st and March 31st 2022 (one year with extensions).

The postdoctoral fellow will have access to shared office space, a desktop computer, and a small computer cluster. Experimental research will be conducted at the Atmospheric and Statistical Optics Laboratory (well equipped to conduct the research topics announced). Additional money can provided to attend conferences or short research stays through CAOVa.

Commitments

The candidate is required to apply to the national postdoctoral grant (FONDECYT) at some point; thus, extending their stay for another three to four years. These grants provides further research independence, and additional travel resources.

During their stay, they should have one paper accepted or published in a high-profile journal.

Requirements for application

Candidates should

Applications will be received until February 20th 2022.

Further inquiries

Prof. Dr. Darío G. Pérez, dario.perez@pucv.cl.