Re: [ATM] Focal Length Accuracy

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

Not at all bad to practice this if you are planning to go for
more complicated designs. Some of them require a very specific
curve to be generated onto a very specific thickness of glass
and with very tight tolerances. Some of the folded & catadioptric
systems have this problem, for instance. Of course, in going
for these projects, you usually start by not removing too much
glass to start with; that alone could bring disaster.

On a similar note, I recently bought a ring type spherometer
from Mike Hunter and decided to calibrate it. I am not sure
how other people do this. It was machined to specific
tolerances, but calculating back from the "known" ROC (from
optical tests) did not yield the expected ring diameter. It
came down (I think) to whether the ring was landing on the outer
or inner edge, etc. I ended up using an "effective" ring
diameter, calculating back from a couple of mirrors with "known"
ROCs. This brings me back to our earlier discussions about the
accuracy of a spherometer, now that I actually have one in my
hands with a decent guage.

How does one calibrate a spherometer such that you would
put more weight on the readings from this than a Foucault or
other optical test where you measure the point where light
is actually focusing?

Dominic


On Wed, 23 May 2007, Anthony Anconetani wrote:

> Good morning Jim,
>
> You have asked why I want the focal length to be as close to 36 inches
> as possible.
>
> No particular reason really. Just practice. 8-)
>
> -Anthony

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