RE: ATM Another parabolizing stroke question

[Ric Rokosz <bz737@freenet.toronto.on.ca>]

On Fri, 2 Jul 1999, Bertapelle, Ken wrote:

> 
> For my next big project I want to make a 20" dob, including the mirror.
> The biggest mystery for me is the final figuring.  I've been watching
> and saving all the recent traffic on this subject, but I still don't
> feel like I really understand it.  I've also done a fair amount of other
> reading, including the books by Texereau, Berry, Kriege & Berry.

The article in Kriege and Berry is pretty good on using a sub-di. lap.
Another good article is the one in TM by Ceravolo.That technique is one I've 
used to do 
some pretty fast mirrors (f/2.8 and 3.3 ) and feel would make life easier 
for newbies.I think it's called "High fineses Figuring" in #39.pp41 on.
That article needs to be one of the first to be reprinted in ATMJ for sure.

Basically you use a stokes similar to the one in K&B - but it's 
elliptical in shape not straight back and forth.You stay away from the 
dead center.You move the stroke instep with the walk around.This produces 
(for me at least) a very smooth blending action and resultant smooth surface.
On the f/2.8 18 inch mirror I used a 4 inch lap -the edges have to be cut 
in the now famous star-shaped pattern to aid the blending.As an aside it 
took something like 22 hours to put the figure on that mirror;used a null 
test.

The 18 inch had a 3 inch hole in the centre so I had no choice but to 
stay away from the dead centre- just had the center of the lap touch the 
edge of the hole.I found it's better to have a small hill in the centre 
in the very final stages than a hole for obvious reasons.A couple of 
short stokes will bring it down.

If you test often and the fact you're using a small lap,makes it  
very unlikely that you will over shoot the mark.No worries about a TE 
neither.

There is a saying on the list -"grind more worry less" well in figuring 
with a small sub-di lap - just start and worry less.Sometimes you can 
read yourself into a fog.
For a 20 inch mirror I'd say use a 6 inch lap,with a well cut star 
pattern on it,not hard even a bit on the soft'ish side.

Ric
Toronto