Melting Telescope Dish

On June 6th Andrew Kirk imaged the 40m circular dish of the Owens Valley Radio Observatory apparently drooping and melting into the ground like a Dali watch. ©Andrew Kirk, shown with permission.

Atmospheric
Optics

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed
I was driving by the Owens Valley Radio Observatory near BIg Pine, California, when I noticed the 40-meter telescope looked odd. A couple of minutes later I realized I had seen a mirage image of the big dish. So I went back to document it.

Sadly, I had only a pocket camera with limited zoom. I improvised, though, and photographed through one barrel of my binoculars to increase the zoom [inset image]. Both photos are from about 2 miles distant.

Amazingly, as I watched, the dish began to tilt upwards and within a few minutes it was pointing straight up. Had I passed a bit later I would have missed it.

I hope to see this again...with my better camera in tow.
 

Light rays from the dish are passing almost horizontally through air layers at different temperatures. Rays passing downwards (say) from a cool layer to a warmer one are deviated slightly. The direction of the deviation or refraction is always towards the cooler and denser layer.

Vertical air temperature profiles can be complex and produce combined image shifts and distortions.

Looming is when a distant object appears raised up but not otherwise distorted. A linear fall off in temperature with height that is more gradual than normal can produce it. A more severe temperature gradient causes the object to appear lower Sinking.

When the vertical temperature profile is non-linear then distant objects can appear distorted. Towering is the name for a vertically stretched image and Stooping for the reverse.

The telescope dish vertical stretching was likely the result of an abnormally warm air layer close to the ground that had a non-linear temperature fall off with height. The upper part of the dish is less stretched (or not at all) indicating a more normal linear temperature gradient higher up.

The sighting and atmospheric conditions had some resemblances to stages of an Etruscan vase sunset mirage or the inferior mirages seen over sun heated roads. Strictly speaking this one was a distortion rather than a mirage because the latter is technically reserved for when there is more than one image of the distant object.