Temperature differences between air parcels and between the telescope and its surrounding air can affect image quality and jeopardize scientific observations. These differences are caused by thermal turbulence and have a tremendous impact on the image quality or seeing.
As we’ve already seen, every element of the telescope has its own systems to minimize the impact of turbulence. However, both the telescope as a whole and the surrounding landscape must be considered in order to effectively improve the seeing quality.
To do so, EST engineers use thermal analyses and models for each tentative design of the telescope (and thus, each tentative design of each of its components).
The first step is the transient thermal analysis: a temperature map of the different parts of the telescope that measures the impact of solar illumination for several days in a row... for each of the design options.
These data are then sent to CINME (the International Centre for Numerical Methods in Engineering, with whom EST has a collaboration agreement), where they use computational fluid dynamics to add the influence of the wind colliding with the telescope and the nearby landforms.
These fluid dynamic models are usually developed for a specific time interval, and their importance lies in the fact that they provide EST engineers with optical parameters that measure the image quality to be achieved by a specific design.
There are three such parameters: the full width at half maximum or FWHM (the ability of the telescope to discern between two nearby points, also known as the angular resolution), the Fried parameter or r0 (the length scale over which the turbulence becomes significant) and the Greenwood time constant (the time scale over which the changes in the turbulence become significant: that is, how long the field of view will remain stable).
These parameters are then used to elaborate and characterize the complete thermal models for each of the tentative designs, giving EST engineers enough information to choose between them and issue guidelines for the height of the telescope, the best coatings for the structure or the location of the refrigeration systems.
“To come up with the current design, we have probably done 100 fluid dynamics analyses, and an uncountable number of thermal analyses with their respective thermal models. And even so, we keep doing them to make sure the design is as streamlined as it should", explains Nauzet Vega Reyes, EST thermal control engineer.
What is seeing?
Seeing is the term astronomers use for the relative optical quality of the Earth’s atmosphere, defined as the steadiness and absence of distortion of the image.
Poor seeing is caused by temperature differences in the atmosphere. Temperature modifies the air density, and this induces a change in the refractive index, making images appear blurry. In other words, each air parcel behaves as a different material that the light has to go through, so the final image we get is distorted.
Depending on the cause, astronomers talk about different kinds of seeing. Solar telescopes are especially vulnerable to internal seeing, generated by the telescope itself: being constantly heated by the Sun, the surface of solar telescopes can easily reach higher temperatures than the surrounding air, originating large temperature gradients between them and so thermal turbulence.
Local seeing is caused by both convection currents rising from the ground (heated by sunlight) and the effect of air flows colliding with the telescope and the near landforms (or buildings). In fact, one of the reasons for solar telescopes to be located atop high towers is to minimize the turbulence originating from the ground.
There is a third kind of seeing, high-layer atmospheric seeing, but we’ll cover it in a later post.
For more information, see:
- Vega Reyes et al., "Local seeing determination by thermal-CFD analysis to optimize the European Solar Telescope image quality," Proc. SPIE 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, 99121C (22 July 2016)