From: "John Murray" 
To: "Amateur Telescope Makers" ,
"Mirror Matic"
        
Reply-To: "John Murray" 


G'day Bob,
        The penny drops now. Several sessions ago, Dennis Rech suggested that I
go
straight to the figuring stage. Until I get the belt that I require in, I
thought I'd perservere with the shorter polishing sessions. It'll be a
coupla days so the question is Would it be worth continuing what I am doing
except instead of 20 minute polishing sessions, do maybe 5 minute sesions
and hopefully get on top of the ring without the oscillation.

Cheers

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




> -----Original Message-----
> From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Bob
> May
> Sent: Saturday, 26 July 2003 2:51 AM
> To: atmlist
> Subject: Re: ATM Latest polishing session photos........
>
>
>
> John, the first rule with polishing and figuring is that you have to turn
> the mirror a lot slower than with grinding.  Fast turning in
> polishing will
> creat rings of zones because the friction of the glass-pitch
> interface will
> heat the glass and pitch to some degree.  This causes a problem as the
> heated glass and pitch will expand.  This really causes problems as the
> expanded glass will then support the lap from the rest of the
> glass and thus
> you will then plane that part of the glass down until it is the
> same height
> as the rest of the glass (which isn't warmed from the friction!) and the
> friction evens out and this means that the heated part of the glass is no
> longer getting the heat input that it has been getting and guess what
> happens - that part of the surface then cools and the glass retreats from
> the lap surface, leaving the rest of the glass to take the
> friction, heating
> the glass and so forth and it keeps oscillating until you stop the
> polishing.  I'll note that heavy weights on the lap will also do the same
> thing at a lower speed.
> Thus, if you are wanting to get the pits out of a surface, attach heavy
> weights to the lap and turn fast and they will go away quickly.  Then you
> have to slow down and remove the weights to get the artifacts
> from the rapid
> polishing removed.  I'll note that many of the Ronchi tests show that fast
> heavy polishing action where the rings have started oscillating with the
> heat of polishing.
> Bob May
> http://nav.to/bobmay
> bobmay{at}nethere.com
> NEW! http://bobmay.astronomy.net
>
>
>

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