Re: ATM Sphere First?

[mjc5 <mjc5@psu.edu>]

	Don't fear the sphere! For a first mirror especially, it is a good idea
to hit a sphere first, then go to the parabola. In this way you will
gain valuable testing experience at a point where it is not so critical
if you make a mistake. As you take the mirror towards the sphere you
take your readings as a guideline, but they won't get you in as much
trouble if you make a mistake.

	Another advantage of going to a sphere first is that you get a better
feeling for what the parabola actually is and how to reach it from the
sphere in this manner. If you start from some odd figure and try to work
it towards the parabola, you are at the mercy of your testing ability,
and your logic process (sounds bad, but I'll bet all of us have
accidentally worked a mirror in the wrong direction at one time or
another) You are also at the mercy of your developing knowlege of what
mirror stroke does what.

	By spherizing first, you are getting control of your process and
getting a good polished mirror in the bargain.

	Once you get to a sphere, parabolization is pretty easy. Of course this
depends on the F ratio, but I still think it makes better sense to go
from a *known surface*. There is only one spherical surface, and it only
looks like one thing in the tester.

	Now for more experienced workers, it is possible to go more directly to
a parabola. I'd save that for my second or third mirror.

	- Mike -

	After
James Lerch wrote:
> 
> Hello again :)
> 
> When polishing, should you go to sphere first and then head towards
> parabalizing?
> 
> I ask because my mirror has a center region that focuses longer than the
> edges.  I've been polishing MOT with 1/3 COC strokes and the long center
> zone is growing towards the edges and I will eventually reach a sphere.
> 
> However the thought occurred to me that maybe I could do some parabalizing
> strokes to knock that center down a little quicker.  A contrary thought is
> that I will just end up with a mess and have to head back to sphere anyways.
> 
> I'm a gambling man, and exchanging several hours of work for some knowledge
> doesn't scare me, but I would rather prefer an educated opinion....
> 
> Thanks,
> James Lerch
> St. Petersburg, FL
> http://lerch.yi.org/atm for my latest ATM adventures