NavList:

Don Seltzer, you wrote:
"Could it be the fish eye distortion that is typical of a GoPro camera?"

I had the exact same thought, right down to imagining that this was someone in the woods of Russia, out hunting, wearing a GoPro camera. It may well be the case, but after a while I had myself convinced that this would have to be the opposite of a fisheye, often called a "pincushion" distortion. But here we're getting into some real trivia on a photo of a cute cat. That's very "Internet" in so many ways! But the actual answer to this question seems definitely off-topic for NavList.

For the general topic of photographic celestial navigation, this sort of distortion does nicely illustrate a general point: you need "plate constants" in order to do this right. Photographs are not mirrors of reality. Greg Rudzinski has done many experiments with photographic sights using the basic principle that there is a simple linear relationship between angles and pixels. For some lenses at some level of accuracy, this is good enough. But we really should worry about second and even third order distortions in multiple directions. That sounds like major work, but these can be measured with a single photograph and some off the shelf analysis software. All that's required is a photo of some suitably "busy" starfield (or some other standard set of targets). When you take an image of a starfield and run it through one of the astrometry.net tools, the analysis actually works out those distortion coefficients. Since the legacy of the mathematical analysis is astrometric photography on glass plates, these numbers are generally known as "plate constants" even today. If we can get plate constants for our digital cameras, and if those constants are reasonably stable (I don't know how likely that is with normal cameras), then we can do some highly accurate angular measurement bypassing sextants entirely. This is on my ten-year list ...things I have been thinking about doing for ten years, haven't yet, and may not for another ten years.

Overheard at the last meeting... "Hello, my name is Frank. And I am a procrastinator. It has been one week since I last tore up my to-do list..."

Frank Reed

PS: For my fellow procrastinators, you can file your taxes (that's US federal and state income taxes, for our non-US readers) by the end of the day on April 18 this year! Three whole days of glorious, free procrastination.