Re: ATM help - foucault test interpretation

[Bill Marriott <btk@pacbell.net>]

Maynard;

There is a consensus that you have a spherical mirror at this point,
though as to whether or not it is polished out or not (4 hr. for an 8"
is not a lot of polishing time, generally one might say, 1" per hour,
though it is possible to knock that down with a lot of oomph) is another
question. Using a laser pointer, or some other concentrated light
source, take a good look at your edge, and if there is any fuzziness at
all, or bright points of light(bigger pits) keep on polishing, don't
worry too much about the figure, though you don't want to go too far
astray. get a Ronchi screen, and learn how to use it.

When the mirror is polished out and spherical, and with no disrespect to
comments about using sub-diameter laps, with tool on top, I think for
someone working (on what I am assuming is your first mirror), the
results could just as easily be disastrous as beneficial. I myself use
this method, especially on larger mirrors, but on a 8", especially more
or less f7, the classic methods, using a full size tool, with mirror on
top, will yield the safest, and fairly quick results anyway, curve on
your glass.

When you are fully polished out (if not already) and are spherical, then
check back in, and you should be able to generate the not so elusive f7
parabola, rather easily.

One reporters opinion

Bill Marriott
btk@pacbell.net
http://www.starlightoptical.com
telescope/optical stuff





Lawrence Sayre wrote:
> 
> Maynard Robinson wrote:
> 
> > page - a la texereau) I get the classic darkening > > before and after ROC and the overall darkening at ROC > > but the shadow does not show a distinct parabola > > shape as per the published shadowgrams on a lot of > > the sites I have surfed.
> >
> > There is a curved surface there but am I right in > > assuming that I have maybe
> > got a spherical surface.   I have polished for 4 hrs > > tool on top.

> > Thanks
> > Maynard ROBINSON

> Maynard,
> 
>  I've found it far easier to get the elusive parabolic
> curve via using a TOT sub diameter (my preference is > for the star shape,
> but this isn't totally necessary) lap, as opposed to > > the full sized lap which was used to polish out and > spherize.