From: Mark VandeWettering 
To: "Jerry B. Hillman" 
CC: atm{at}shore.net
Reply-To: Mark VandeWettering 




Jerry B. Hillman wrote:

> Hi all,
> I would appreciate suggestions on how to attack the edge on this mirror.  It
> seems to be resisting all my efforts.  Inside ROC and Outside ROC pics can
> be seen at
>
> http://www.convertabarn.com/Figuring.htm
>
> It will be the two pics listed in the right hand column.
>
> Clear skies, Jerry


I've had some limited experiences trying to reduce turned down edges with
subdiameter tools, so take this with a grain of salt, but here is what _I_
would do.

My entire philosophy of polishing is based upon a couple of simple ideas:

1. pick a goal
2. develop a plan to meet that goal 3. polish
4. test.  If you met the goal, then you are done. 5. if you are making
progress on your goal, return to 3. 6. otherwise, return to 2.

Goals may in fact have subgoals.  Early on, it isn't worthwhile to set a
goal of "producing a parabolized mirror".  "Straighten lines
on the Ronchi" or "reduce the turned edge" are worthwhile
goals.

Let's look at your mirror.  I agree that the turned edge seems a bit more
extreme than I would like to see, although it isn't terrible.  I didn't see
what frequency of Ronchi screen you've got, for a large mirror like that
I'd probably use an 80 lpi screen, or maybe even coarser, but in any case
you still have a lot of glass to remove from the center of the mirror to
get to your final parabola.  I see basically two goals that you could
choose between.

Reduce the mirror's turned edge or

Work on parabolizing.

If I was really going to work on reducing the turned edge, I'd probably
take the time to make a fullsized pitch lap, and polish with fairly short
strokes, center over center, with tool on top.  I'd expect this will take a
while, I'd probably go at least a half an hour in my first polishing
session before testing.
It will probably take several passes to really eliminate the turned edge. 
I'd expect the mirror to be pretty spherical or maybe slightly oblate after
all this.
Then you can shift to parabolizing.

But you don't have a full sized tool, so I might be tempted to pursue the
parabolizing a bit more and see what the overall correction looks like. 
I've had good luck parabolizing with star laps of roughly half the diameter
(I've done
this without the star lap, but it seems easier for me to get zones that
way). I'd probably start with a stroke which just went through the center
of the lap and mostly avoided the edge.  I'd go for probably about 10
minutes, and see if I'm making a hole.  At that point, I'd probably
consider a hole to be a good thing.
I'd alternate between fairly agressively stroking the center, and more full diameter
stroke to blend them out.  When you get the center deepened and blended out
to the edge, you might find that the apparent turned edge isn't really so
bad at all.

The best article that I've discovered for this kind of figuring is
Cerevolo's "High Finesse Figuring" from the old "Telescope
Making" magazine.  If you'd like,
I could scan it for you, and send you a copy.

        Mark

--- BBBS/NT v4.00 MP