It might be interesting to find a scene where you get the spot, put the camera on a tripod and go to taking shots, rotating the camera a bit each time. When you get the spot note the location of the sun or light source. Do that in several different locations and you may detect a pattern.

Or find a location that gives the spot and then install a lens hood or hold a piece of cardboard in different locations and see if anything changes.

When troubleshooting mechanical, hydraulic or electrical problems, I first try to find out when or where something happens. For example, I can differentiate the difference between a bad rod bearing, main bearing or thrust bearing by simply revving up an engine several times. When it knocks in relationship to the engine rpm’s gives the answer.

Hi Harry,
You Definitely want to get the D850. The 47mp resolution alone is worth it. I have the same Sigma lens and I have taken fantastic photos of the moon and aircraft. I got my D850 from B&H last year. When they had a great promotion. It is the best Nikon camera I have ever owned. I have a part time Photography business. Wedding, portraits and head shots. My other camera is a D500 and it was my main camera. Till I got the D850. It's incredible It's built like a tank and fully weather sealed. I have the MB D-18 battery grip on mine and the Ergonomics are excellent, plus the weight of the camera helps give better balance when using long lenses. I have been a Nikon camera user since 1979 when I got the photo bug in High School and got the Nikon FE as my first SLR.
You will love the 850 worth every penny.
Take care and good luck.

The D850 in the DX mode gives you 19.4 megapixels out of the 45 available in the sensor. This is roughly on par with the D500 and slightly less than the 24 megapixels of the D780. In reading your followup posts, it looks like you're using FX lenses already. If you shoot FX and crop to 20-24 megapixels, you'll be ahead of the camera you are currently using, and you have the option of shooting FX for portraits, landscapes etc with stunning results. For what it's worth, the D780 is far superior to the D850 for video due to its hybrid sensor that can use phase detect autofocus. The downside to the D850 is bulk and weight.

If you're going to buy a new camera these days, wouldn't looking at a mirrorless camera like the Z8 make a lot of sense. I guess it would depend on your budget. The Z8 and Z9 win hands down for autofocus performance and frame rate.

Are you saving too for FX lenses? If you don't have full-frame lenses, you're wasting your money on a full-frame camera, as the FX camera will need to operate in DX mode with a DX lens. Your Sigma is fine on an FX body, but the question remains: what other FX lenses?

Regarding the D780 vs D750, they're both 24MP FX cameras and the D780 was pretty much a waste of time by Nikon, ignored by most of their customers. Look at a used D750 if a D850 is out of your price range.

You probably should be looking at a D500 or D7500 for your airshow needs, the more logical upgrade from a DX D5100.