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Re: [ATM] Updated pictures of 8"f6 mirror............
G'day Jerry,
I think you may be right there. One thing I neglected to mention is
that it is a full size lap made from 2 discs of 20mm plywood epoxied
together and sealed with epoxy. I also have from the start been polishing
with a 2.5kg weight on the lap. The lap is made using a mould from
pitchlaps.com. The roughness is becoming more noticeable. Series 4 polishing
was done 1/3 coc tot with 2.5kg weight with a stroke time of around 2 - 3
seconds back and for. The lap was well pressed as in all of them but looking
back the sessions 5 - 7 were polished in the morning at around 25 degrees in
the workshop. Numbers 2 - 4 were polished at around 21 - 22 degrees in the
workshop, also the sessions were a bit humid after rain and I recall reading
that humidity is just as important as temperature. Apparently evaporation
from the edge can affect the polishing at the edge. I am using Gugolz 64. I
also have gugolz 73 available 3 yrs old unopened in the tin. I won't use it
just yet though. I think perhaps time to go back to tot 1/3 stroke again and
try and reduce the roughness and get back a bit closer to spherical. Also,
have the temperature a bit closer to 22c although that's a bit hard in oz in
summer...;-)) Ah well, win some lose some.;-))
200mm diameter
1240 focal length
Moving source
Picures are at
http://users.cyberone.com.au/onslow/
Cheers
John Murray
Whyalla
South Australia.
-----Original Message-----
From: Jerry [mailto:wa4guu@verizon.net]
Sent: Monday, 27 November 2006 3:10 AM
To: 'John and Bernadine Murray'; atm@atmlist.net
Subject: RE: [ATM] Updated pictures of 8"f6 mirror............
G'day John
The shorter strokes are taking the mirror more oblate and adding
some roughness. I think you were doing better with the 1/3 stroke. If you
go more oblate you are almost certain to see the edge get worse.
The series 04 images show the best surface and a better edge. I
suggest you go back to what you were doing that produced that. A little
more of that would have removed the hill. If not, even with the center hill
it was a much better place to start to a parabola. The edge was better and
the hill would go away rather fast with some parabolizing strokes.
An experiment you might try would be to add 5 pounds of weight to
the back of the lap. Your 8" lap probably weighs about 5 lb. Another 5 lb.
would get you to around 10. You would have about .2 lb per square inch.
James Mulherin has suggested to me that .2 to .25 lb. per square inch is a
good pressure for machine figuring. That would seem to me to apply to hand
work also.
Not having compared results of different pressures before, only
using the natural weight of the lap, I can't say if it will help. Guess I
was busy trying other things. I will try that on the next mirror. If you
were to try this you should be careful to center the weight on the lap.
Jerry
-----Original Message-----
From: John and Bernadine Murray
Evening All,
I had a look at the mirror just now before I head off to n/shift ;-(
No shimmy was noticeable so I took the 7th series of pictures now instead of
waiting for the morning. The tde is still there but seems to have slightly
decreased. Been using a 1/4 coc mot stroke in 3 8 minute sessions. A 3
minute press with 20kg on top was performed between each session...
200mm diameter
1240 focal length
Moving source
Picures are at
http://users.cyberone.com.au/onslow/
Cheers
John Murray
Whyalla
South Australia.
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