I recall the star test is recommended to be done at a defocused distance of 10λ inside and outside focus. Is there a way (a formula perhaps) to determine this distance for an f-6 scope (I also recall the distance was focal ratio dependent)? Suiter had a table in his book, but I no longer have it.
Also is there any latitude in this distance as the shorter focal ratios call for shorter distances and thus smaller defocused images? I know the rule of thumb of using, for example, a 6mm eyepiece or shorter for an f-6 focal ratio scope.
I'm not a star test fanatic since interpreting it is, IMO, difficult to do unless you are very familiar and experienced with it. But, my son has our 6", f-6 DOB which I recently replaced the secondary mirror with a better one from Antares, and I want to get an idea of its relative quality.
So any info is appreciated. More of a curiosity thing to determine if the better diagonal helps.