From: "John Murray" 
To: 
Cc: "Amateur Telescope Makers" 
Reply-To: "John Murray" 


Hi Dennis,

> First, you had mentioned that the polishing lap was squeeling. They
> do that when everything is working just right. They wear ear plugs on
> the good days at Zambuto Optical.
> On the all steel Big-o-Matic with aluminum laps and fresh pitch, the
> shrill sound makes you think that all the enamel will crack off your
> teeth.
You got that last bit right..........

>
> On to figuring.
> I would keep the sessions to 5 or ten minutes at the most and sneak
> up on the parabola. If you overshoot a zone, everything else needs to
> come down.
> On this last 8 inch mirror, we did about 20 - 5   minute sessions.
> Each time we compared the results to the display in John Upton's
> Ronchi for Windows program. Towards the end, we printed out
> the display and traced the bands on the surface of the mirror. It is
> difficult for the eye to pick up the difference between a one wave
> and  1/4 wave  mirror otherwise.
Ahh, That makes sense. I'll start with the 5 minute sessions then.......


>
> Hopefully, you have cut a slot in your overarm so you can adjust the
> figuring lap location to the center of the zone that needs
> correction.
Errr, slot? Using the original MoM I thought to work the zones that the
entire overarm assembly slid rearwards. Is this something I've missed. Can
I have more details thanks?

>
> You probably want to press your need diameter lap as a star shape
> which will blend the zones. Otherwise, the circular figuring lap will
> leave a ridge at its edge.
No probs with that...........
>
> Start at the mirror center and as each zone gets to the correct
> depth, move the lap back and work a new zone to the correct depth. I
> do a quick test each time.
A quick test each time? By this do you mean stop figuring, test the blank
with NO cool down (or only a short cool down)and then start again? No quick
presses etc etc?


> As the quill meoves back towards the pivot hinge, the lap arc will
> shorten. YOu will probably have to take a small amount of eccentric
> out of the arc each time that you reposition.
Move assembly or quill back and reduce the stroke size..........ok got that.

> Do not let the edge of the lap cross the edge of the mirror. It will
> turn down quickly. This process assumes that he edge of the mirror is
> correct and that all other areas will be lowered.
> It may take several iterations from center to back to get things just
> right.
So the extremities of the petal in the star lap are not to go over the edge
of the blank. I would have thought no work at the edge at all would lead to
a TUE.

> I used three pounds on a light aluminum weight. Your heavy polishing
> lap may dig in pretty quick. Use short sessions. You will probably
> need a 25 to 30% lap for final touch up.
>
> Use very thin cerium. Like skim milk. Use a very well pressed lap.

Not a problem, I'll keep an eye on that.
>
Cheers

John Murray
Whyalla
South Australia
Check our web page at http://astronomy.sa86net.com ATM and M-O-M stuff.

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