spooner@page.az.net (Michael Spooner) wrote: > I consider a flat for autocollimation indispensible (others are free to > disagree) for *good* optics. I've never done a null test. Is a 6" flat sufficient for a 6" objective, or does it need to be oversized? How do you judge when the null appearance is good enough? Are there foucault measurements of the null which give a wave rating? -Bill