Re: ATM 6" f/6

[John Garland <jgarland@morgan.ucs.mun.ca>]

On Wed, 4 Jun 1997, Yvan Blom wrote:

> I am grinding my first mirror, an 6" pyrex which was intended to be an
> f/8 ... f/10. I was helped through my first pains (some chips broke off
> the edge) by several of you ATM'ers (thanks again).
> Mirror was always on top.
> 
> Last monday, a few local ATM'ers measured my mirror's sagitta and it
> turned out it would be an f/6.
> Their advice was that an f/6 is not easy to figure for a beginner (and I
> guess they're right...)
> Nevertheless, I'd love a short tube.
> 
> What is your opinion on:
>  1. If I grind with no 240 for an hour, will the f-ratio significantly be
>     longer ?

I kept TOT pretty much 75% of the time (to work the edge which had some
scratches from a glass chip), I lengthened my FL by about 6 inches
(~15cm).  So yes, you can affect FL at these grit sizes in my experience
[1 6" f5.77 mirror :-)   ].  Subsequent work brought me back to f5.77
which was not far from the f5.5 I was aiming for.

>  2. If I continue with f/6, what's the easiest/best way to figure it
>     to a paraboloid? 
>     Should I use round - square - other formed lap patterns ?
>     Approx. figuring time ?
See below.

>  3. Is it best to polish the mirror completely sperical before figuring
The "spherical" part is not the problem if you follow correct technique.
It is getting the surface fully polished.

>  4. Can someone provide me with a log for a similar 6" f/6 ?
I spent some 20 figuring steps, most of them going too far and turning
down the edge and/or--depending on your point of view--making a hyperbola.
In all cases the hyperbola/TDE could not be brought back to a parabola
with my limited knowledge of technique, though I did try.

I finally went from a sphere to an ~1/7 wave surface in about 4 steps by
being _very_ slow, using accented pressure to deepen the centre and
working the edge with short accented strokes (all these are in Texereau).
I found the classic parabolizing stroke unusable with my skills/lap/etc.
as it consistently led to the above-mentioned condition after only minutes
of parabolizing time.

I was also given some good advice by list members.  One letter in
particular about parabolizing 6" mirrors I will try to find and forward if
I still have it saved.

John Garland
jgarland@morgan.ucs.mun.ca