RE: [ATM] Grinding the back flat

[GARY FUCHS <gary@rcn.com>]

Hello Richard,

How do you test the edge to know if it's sufficiently polished? 

Thanks,

Gary Fuchs

---- Original message ----
>Date: Fri, 19 Aug 2005 12:20:36 -0700
>From: "Richard Schwartz" <richas@earthlink.net> 
>Subject: RE: [ATM] Grinding the back flat  
>To: "'Bob May'" <bobmay@nethere.com>, <atm@atmlist.net>
>
>It is not enough to grind the back, you must also polish it.  
And then you
>must grind and polish the EDGE!   Otherwise, the telescope 
will not work at
>all.
>
>-----Original Message-----
>From: atm-bounces@atmlist.net [mailto:atm-
bounces@atmlist.net] On Behalf Of
>Bob May
>Sent: Friday, August 19, 2005 10:42 AM
>To: atm@atmlist.net
>Subject: Re: [ATM] Grinding the back flat
>
>Any ol' chuck of glass or iron will do a good job grinding on the
>backside.  My "traditional" method is to use a pipe end cap for
>grinding to dig a hole in the glass and this also works well for
>flattening the backside of troublesom pieces of glass.  For a
>flatness check, just put some cheap paint on a piece of 
plywood
>or such material that is basically flat and rub the glass over
>the paint.  High spots on the glass will pickup the paint and
>show where you need to grind some more.  A perfect surface 
will
>be one that most of the surface picks up the paint.  This should
>get you well within a thousandth of an inch which will be good
>for a back surface.  For something better, finding another thick
>piece of glass will do the last little bit of flatness.  I'll
>also note that you can stop at 25 micron grit as this is going to
>be more than fine enough.
>Bob May
>bobmay@nethere.com
>http://nav.to/bobmay
>http://bobmay.astronomy.net
>
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