8" mirror grinding: SPHERICAL ENOUGH TO CELEBRATE
This site is my secret weapon: https://www.bbastrodesigns.com/ronchi.html
It’s a slightly janky javascript app made by some old telescope maker in Oregon, and it lets you get quantitative measurements from all those Ronchi pictures. Very important.
Looks like my outer zone focuses light at around 0.066 in from my desired radius of curvature, and inner zone focuses light at around 0.1 in. (There’s probably big error bars on both those numbers). Only the relative number of “inner zone focuses 0.1-0.66in = 0.03 in closer than outer zone” really matters.
Since the focal length is half the radius of convergence, that means my mirror focuses space light to 0.015" of where a spherical mirror would. Not bad!
CLOSE ENOUGH! SPHERE-ISH ACHIEVED!!
But my goal is to focus space light. A sphere will bounce light from center point back to itself, but to focus parallel light rays from space to a single point, I need a parabola.
Jean Texereau’s book says the formula for “parabolic abberation”, how different a parabola is from a sphere, is that the zone with radius h from the mirror should focus a distance Δp = h^2 / R further from the radius of curvature. Tape measure and eyeball says as I move my tester backwards, the ronchi switches from bands get wider to bands get smaller at 89 + 1/16", so that’s the radius of curvature of my mirror (which is twice the focal length). By that formula, my goal is to have the very edge focus h^2/R = (1*8/2)^2/89 + 1/16" = 0.1796" further from the front.
Above I calculated that the edge focuses 0.03in closer than the front. Wow, I went so far past parabolic in my quest to get to spherical. I think I’m ready to parabolize.
To parabolize, I have to deepen this curve around six times its current amount. That’s doable! The open astronomy discord recommended I start by smoothing out the division between the two zones with long strokes, and then begin using a different stroke to deepen the center. Let’s do this
Total grinding time: 21 hours