We went to see the White-tailed Lapwing at Blacktoft Sands. I have a primitive system of taking photographs through my telescope.
I have problems getting the camera aperture into the right place for a photograph and you get odd effects like the one below.
My first thought was the Airy Disc for an aperture because it is a bright central disc. But it is a true image around the edge instead of the required black circle minimum. So is it to do with the eye ring of a telescope. The eye ring is put in place so that the eye is positioned at the place where all of the light from the objective lens is focussed: it all passes through a disc at that point. The total image disc is smaller for the this effect. I need to come up with a different way to test this.
