Re: ATM What do these numbers mean?

[Anthony Stillman <atmer@flash.net>]

>...And all of you smart-alecks...


>Size            8-in Diameter
>Focal Length    900 mm  (f/4.3)
>Figure          20% parabolized (K=-0.2)
>Accuracy:       Lambda/8 P-V



Wes,

I think that focal ratio is closer to f/4.43, if you round up.  (I wonder,
what is the etymology of smart-aleck?)

As Richard points out (and has previously as well) the mirror is 20 percent
of the way to a paraboloid.  If you were to take knife edge readings, they
would be 0.20 times the values of a fully parabolized mirror, with the same
radius of curvature of course.

Peak to valley measurements (P-V) are meaningless.  Oh yea, in my opinion.
In theory the mirror deviates from the specified surface by no more than
1/8 wave.  That may sound good, and it may well be, but without more data,
like an RMS reading, or the observation that the mirror is smooth, or the
Sidel's or the...., you can't tell.  Also, lambda isn't specified.  It
could be 550nm (or there abouts) as is commonly assumed by ATMs.  It could
be the HeNe line at 633nm as is commonly used by manufacturers.  It could
even be the strong CO2 line at 10u, though I doubt it.

I once saw a manufacture advertisment that specifed an "accuracy" of +- 1/8
wave RMS at 633nm.  Yea, that's right, plus or minus.  Even if they ment
P-V and not RMS it still translated to an terrble mirror.

Anthony