Re: [ATM] newbie question

["William Lee" <wjl12@comcast.net>]

That article is also available in the book "Scientific American: The Amateur
Astronomer" ($17.95 from Amazon:
http://www.amazon.com/Scientific-American-Amateur-Astronomer-Wiley/dp/047138
2825/ref=sr_1_1/104-7797941-1019955?ie=UTF8&s=books&qid=1189621691&sr=8-1 )
or, I suppose (since I can't find my CD-ROM), the Amateur Scientist on
CD-ROM (http://www.surplusshed.com/pages/item/m2071.html ).

Bill

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf
Of Jarvis Krumbein
Sent: Wednesday, September 12, 2007 12:55 PM
To: nweeks@prism9.com
Cc: atm@atmlist.net
Subject: Re: [ATM] newbie question


Nigel, there is an article in the November 1963 Scientific American on
making an aluminum mirror.  I made one from that article with some
success as did several friends.  I later tried to go larger with nothing
but bad results.  Aluminum does not act like glass during grinding and
polishing.  Using sheet aluminum doesn't work because the materiel has a
grain that is aligned in one direction which will result in astigmatism.
Using extruded rounds works up to a diameter of 4" but not larger.  This
is due to the uneven compression of the material across its diameter
resulting in uneven hardness.  I did succeed in making a 3.5" diameter
secondary for a Cassegraine but it was not the greatest and soon
abandoned.  I don't know whether cast aluminum plate would work but it
might be worth a try.  I still have several 8" diameter disks that I play
with on rare occasion but with no more success no matter what I try.

Read the Scientific American article before going any further.  If you
can't find a copy, let me know off-list and I will send it to you.

Jarvis Krumbein
jkoptic@juno.com

On Wed, 12 Sep 2007 14:40:45 +1000 Nigel Weeks <nweeks@prism9.com>
writes:
>
>
>  I've had a bit of a search through the archives for this list, but
> can't find any threads or comments, but I apologise if it's been
> asked
> before.
>  What are the pros and cons with making a primary paraboloid mirror
> purely out of aluminium?
>
>  I can see it's softness making it easy to machine, shape, scratch,
> and repair.
>
>  I can see it's thermal conductivity enabling it to reach ambient
> temperature very quickly, as well has heating from your hands when
> shaping it, possibly causing errors during grinding and finishing,
> requring a jug to prevent hand contact transferring heat.
>  I can see it's light weight being beneficial to the portability of
> scopes, especially if additional material is removed from the back
> of
> the mirror prior to grinding and shaping, so the back of the mirror
> has a honey-comb appearance.
>
>  Being an alloy, you could tap threads straight into the back for
> mounting and adjustment if needed.
>
>  Aluminium is a popular optic  mirror coating, so all the
> alloy
> mirror needs is the protective coating after polishing
>
>  It's tendency to oxidise may be a drawback, but the protective
> coating will handle this.
>  Over to the gurus! I'm about to go buy 2x 18"x2" chunks of alloy,
> one for tool, one for mirror, and start grinding.
>
>  Someone talk me out of it...
>  Nige
>  Nigel Weeks
>
>  Software Engineering
>  Prism9 Industries
>  email: nweeks@prism9.com
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>
>

_______________________________________________
ATM mailing list http://www.atmlist.net/

_______________________________________________
ATM mailing list http://www.atmlist.net/