Re: [ATM] Updated pictures of 8"f6 mirror............

["John and Bernadine Murray" <onslow@cyberone.com.au>]

G'day Mike, 
	I'll respond below each snip..........

Good point.  The mass of the substrate is important.  I have no idea how
hard the pitch is.
John, how long does it take the channels to close under normal polishing?
(If you said this in an earlier email, then I missed it.)
I know you said you are using Gugulz 64 which should be OK, but we have to
verify that it is indeed working properly.  Should take a few hours at ~22C
(72F).

The laps made with a lap mould from pitchlaps.com. Followed the instructions
ok. After a few hours of work the channels are about 50% closed. 


What I see in the series 7 photos is a classic oblate condition in which the
outer 1" - 1.5" of the mirror turns up.  Why does this happen?  Well, the
transition zone between the raised outer zones and the "flat" central region
is a low spot, and that's where the "active region" of the lap stops on the
forward and back stroke, and where it causes the most wear.  So, if it is
indeed a 1/4D stroke, that's 2" long, so the active region of the lap is
~1/2" in from the edge of the lap for the particular conditions you are
working under.

I agree.......


The stroke you are using is likely too regular.  One thing you can try is to
move the active region around by doing strokes of different lengths.
Remember the 1/3D that we all think of as the normal stroke is only and
AVERAGE, and in reality I make strokes from 1/6D to 1/2D while doing "normal
strokes".  This moves the wear pattern around.  I recommend working both MOT
and TOT with strokes of more random length.

I have done a bit of mot when doing the 1/4 stroke. I think a bit of mixing
it up is also called for. Up to the time of the 1/4 stroke all work was tot.

Scratching the facets of the lap can help lap contact and may really help in
getting things more spherical.  I recommend ~10-15lbs of downward force to
the mirror as you make the polishing strokes.  This helps keep the pitch in
contact.  As for speed, I recommend that one forward and back movement takes
about 2 seconds.  Keep the lap/mirror fairly wet - don't let it dry out.

Yep, doing that. I use a fine grid mesh to microfacet the lap before a
polishing session. I was initially working to get spherical which the
session 4 photos showed and became concerned about the tde that was evident.
>From there consulting the stellafane site and other sources they all
suggested mot 1/4 stroke. Well I did that with the obvous results. Ah well,
Me and Manuel discussed tonight about getting spherical again and if there
is some tde present then so be it.......  Shock horror and awaiting
indignant protests now....!!!!!!!! After that we'll try the parabolising
strokes and see how we go. Currently a 200mm 20mm thick ply disk sealed with
epoxy + pitch + 2.5kg weight on it. What weight that is I'm not
sure..........


Cheers 

John Murray
Whyalla
South Australia

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