It's about the same angle of view that a 400mm lens on a full frame camera has vertically. (wide field astrophotography can be fun: https://nt4tn.net/astro/hor

It's about the same angle of view that a 400mm lens on a full frame camera has vertically. (wide field astrophotography can be fun: https://nt4tn.net/astro/horse4.jpg https://nt4tn.net/astro/horse4.jpg ) VRO has a 10313mm focal length (vs 8.4m diameter, so f/1.234-- though with a huge central obstruction) illuminating a 630mm focal plane.