Light from distant galaxies passes through telescope lenses like through a stack of window panes. Each pane is coated with a thin film to reduce glare. But this film acts as a polarizing filter: one component of the light wave passes a bit more freely, the other a bit worse. As a result, the wave rotates slightly, and the telescope sees a different picture than what is in the sky.
Scientists built a computer model that traces the path of light through the lenses, accounting for all coating layers. It turned out that in a typical telescope for studying the cosmic microwave background (the faint microwave echo of the Big Bang), the polarization angle rotates by 0.05–0.5 degrees depending on the point in the field of view. It's as if the compass needle subtly deviates while you walk along a map.
This rotation is dangerous because it mixes two different signals: the temperature (brightness) map and the polarization map. If the error is not corrected, false spots will appear in the data, resembling traces of primordial gravitational waves. And that is one of the main goals of modern experiments.
The new calculation method allows predicting in advance how the coatings will distort the signal in each specific telescope. Then the correction can be applied even before observations, like aligning optics before a shoot. This is an important step toward making future maps of the cosmic microwave background truly clean.
A polarization rotation of just half a degree is comparable to the angle the minute hand sweeps in five seconds, but for finding traces of the universe's birth, even such a tiny amount is critical.