How hard is it to grind a mirror to exact Focal Length?

[Stonius]

Any instrument makers out there?

I wonder how hard it is to grind a mirror to an exact focal length?

I'm curious because I have an RC scope where the primary-secondary spacing is critical (to the mm).

If I platesolve an image to derive the focal length, do I adjust the mirror spacing to put it in line with the specs? Or do I assume that there is some variability in mirror production and this is normal as long as images are good? Or that the platesolver is approximate, not exact?

Best

Markus

[Lognic04]

For a dob, mirror makers let the FL vary widely. But if one wanted to grind a mirror to a specific spec it wouldn't be hard to get it +/- 5mm with a spherometer and flash polishing.