Measuring the Corona with a telescope like a shrimp’s eye

The humble mantis shrimp is considered to have the most complex eyes of any animal. Their eyes don’t just look weird from the outside—they’re weird all the way through. Each eye turns independently of the other. And each one sees in 3D. Imagine being able to see 3D views of your surroundings in two different directions simultaneously. The middle of their eyes are divided by up to six rows of receptors that see in a huge band of light from deep UV to the far end of red. And a couple of those rows detect polarized light. And therein lies a similarity to a test of PolarCam in the telescope that NASA used to study the Solar Eclipse of 2017.

Mysteries of the Sun’s Corona

Scientists remain uncertain as to how temperatures rise in the sun’s inner corona, and the sources of the particles polarizing the light passing through the corona. Light is polarized when all the light waves vibrate in the same orientation. It happens when unpolarized light passes through a medium. (For more about polarization, visit our Polarimetry page.) In the sun’s inner corona, the light from the sun’s fusion passes through a medium of free electrons. The electrons are torn from their elemental nucleii by the huge amount of energy, and the light polarizes. If the degree of polarization can be measured, NASA could calculate the distribution of the electrons in that solar atmosphere. The problem is that instruments to measure polarization have, until now, been limited to a rotating filter placed in front of a camera. The filter passes linear polarized light, an image is captured, then the filter rotates, passing linear polarized light that is oriented in another direction, an image is captured, and then the filter rotates again, allowing a third orientation of light to enter, and another image is captured. The problem is that time passes between images—and light changes its position a lot in a short period of time. To best understand the event, NASA needs a way to capture several different orientations at the same time.

Seeing polarization with new eyes

During the eclipse last year (seen broadly from North America) NASA scientists trained a telescope on the sun, taking advantage of the moon’s occlusion of the sun to observe and measure the events in the inner corona. While a telescope can be fitted with a disk to block the brightness of the sun so that the dimmer corona can be observed, diffraction effects prevent the disk from being small enough to observe the inner corona. Only an eclipse provides the conditions to observe the inner corona, so the opportunity is uncommon and the time available is short. The telescope was fitted with 4D Technology’s PolarCam, a camera that is able to detect and quantify polarized light, using thousands of tiny polarization filters—now the telescope can see polarization in real time, like the mantis shrimp. The camera is able to view multiple orientations of polarization in every frame, as a video or snapshot. It allows a new capability while observing the sun: realtime measurements of polarization in the inner corona. As NASA’s Goddard Heliophysics Lab put it:

“Typical coronagraphs use a polarizer filter in a mechanism that turns through three angles, one after the other, for each wavelength filter. The new camera is designed to eliminate this clunky, time-consuming process, by incorporating thousands of tiny polarization filters to read light polarized in different directions simultaneously. Testing this instrument is a crucial step toward improving coronagraphs and ultimately, our understanding of the corona — the very root of the solar radiation that fills up Earth’s space environment.”
— NASA

During the lunar event, as the data were being recorded, everything appeared to go smoothly. The NASA teams are still evaluating the data and have not issued a report on their corona measurements, as of this writing. If all goes as planned, the PolarCam could be used in other solar telescopes, possibly even orbital telescopes, in the future. As far as we know, there are no plans to put mantis shrimp in orbit.

Further reading and viewing:
https://www.nasa.gov/feature/goddard/2017/studying-the-sun-s-atmosphere-with-the-total-solar-eclipse-of-2017
Watch the Youtube video of the experiment: https://www.youtube.com/watch?v=z9b7h47vBbg
https://en.wikipedia.org/wiki/Mantis_shrimp