High Altitude Balloon David Reynolds

In 2018 David Reynolds sent up a balloon to high altitude with a 90° field of view. The lens has a very slight barrel distortion, however, even when the horizon is in the part of the screen with the slight bend canceling some the curve, the horizon is still curved as the globe predicts.

David has a gallery of screenshots including a calibratiopn photo and a screenshot of the horizon below center of frame with the curve still present: https://imgur.com/a/qCIF5Wk

Horizon below center frame with line added for reference. Click to expand.