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

["Jerry" <wa4guu@verizon.net>]

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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