From: Jerry 
To: "Bill T." , ATM List

Reply-To: Jerry 


    Ok. That is where I've heard that before. Yea well...... Texereau is
French.

    Looking those words up in an English dictionary indicates either
word as a correct adjective to describe a mirror having the form of a
parabola. In fact by the definition of  "paraboloidal" the
surface need not be a figure of revolution. If the curve defined by a
single intersecting plane was a parabola, it is a paraboloid. So if you
take a sheet of paper and bend it from one side to the other into a
parabola, it is a paraboloid even without bending across the other
direction. It is interesting to note that figures 11 and 12 on the same
page in Texereau have captions using the word "parabolic".

    I am glad that Texereau put that in his book however, even if I
think he is wrong.  It causes discussions such as we are having here and it
actually does cause more thought to be given to the nature of the surfaces
we put on mirrors. And even some that we don't. By definition a mirror
could be made that across one diameter is a parabola and across the
diameter at 90 degrees to the parabola is made spherical. If smoothly
transitioned from one to the other diameter,the surface would also contain
the entire family of prolate ellipses. This mirror could be described by
any of the adjectives "parabolic or parabolidal", "spherical
or spheroid" and "elliptical or ellipseoid".  What a
quagmire of  adjectives!  I hope no one ever designs a scope using one of
those.
 It would cause more confusion than the term "Herschelian" telescope.

Pardon me but I am not going to run this through the spell checker.

Jerry

Bill T. wrote:

>
>
>> I've heard this one before but "parabolic" is correct. And so is
>> "paraboloid".
>
>
> Look, "parabolic" is OK for talking about sliced up dunce caps, but
> something as wonderful and perverse as the surface of a telescope
> mirror deserves something a little better, for instance
"paraboloidal."
>
> Now I've got you!  Texereau, page 17, 6th line up from the bottom.
> Hah!!!
>
> Bill T.
>
>
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