Re: [ATM] Focal Length Accuracy

[Dominic-Luc Webb <dlwebb@canit.se>]

Probably the best answer to Ken would be for me to offer a
list of comparison values. Obviously we need to also keep
in mind accuracy vs precision. Based on the slight flatness
of the base (I am using this as a ring), I reckoned on an
effective diameter as discussed earlier calculating back
from measurements on a couple mirrors with ROC observed
in common optical tests (Foucault, Ronchi, etc).

This probably all comes down to how exact Ken's lathe is, and
how well calibrated his micrometer is against some standardized
reference. With luck, the ruler used for my optical testers are
calibrated against the same standard used on his micrometer. I
did not expect a perfect match for these reasons. I chose to use
the optical tester as my golden standard, and some people
objected to this. Trying both side by side recently, it is clear
that there is uncertainty about the exact point where you hit ROC
on an optical tester. With the current spherometer, there is a
quite specific point at which the guage stops. The guage of course
has some finite resolution with errors +/- a couple microns, but
this may be fixed regardless of sagitta. I am not sure on this
point. However, my judgement of the point of ROC was affected by
the focal ratio in the optical tests for otherwise similar
mirrors.

Dominic

_______________________________________________
ATM mailing list http://www.atmlist.net/