Re: [ATM] Newport conical blank?

[Mark Cowan <toolontop@yahoo.com>]

Hi Rob:

I'll give you a slightly different perspective.. ;)

--- polaris@gsinet.net wrote:

> 
> Hi,
> 
>   I'm returning to the list after a long hiatus.
> During this time I have been 
> horse-trading various scopes and now have a 5" apo. 
> I'd like to build another 
> nice dob to go with it, 10-11" aperture.  I
> currently have a 14.5" commercial 
> dob which I am selling due to back problems.  Back
> injuries will limit me to a 
> newt of more modest size.  I have seen conical
> mirrors being offered by Bob 
> Royce but, being on a limited budget I am thinking
> of grinding my own.  I've 
> ground a few smaller mirrors before, and took a 16"
> through 3u AlO.
> 
>   So .... I have noticed that Newport offers conical
> mirror blanks for sale.  
> I am considering ordering one, of maybe 10" diam.  
> I'm just wondering how big 
> a deal it is to do a conical mirror.  Am I getting
> into a can of worms?
> I'd like a mirror which is a little lighter than
> normal and cools quickly.

I don't see that Newport offers anything strictly
conical back (i.e. not cass style) beneath 12.5". 
They cost a little more than a standard flat
full-thick blank with about a 1/3+ weight advantage.  

Offset this by how you're going to support the back if
you work it tool-on-top with subdiameter tools.  But
not everybody does that. ;)  

But Newport's standard thin 12.5" at 1.5" thick weighs
a few lbs more than the conical blank, costs less, and
will cool uniformly which is something you might want
to consider...

There "thin" blanks are my standard thickness now;
"very thin" would be below .8" or so on a 12.5", still
quite doable.  Support becomes more of an issue but
PLOP is free and a 9 point cell (or 6) is not hard to
construct.

Against all this is the desire of people on the list
to learn all about your trials during a conical back
project - but I won't go there. ;)

Mark Cowan
Salem, OR USA

> 
>   Would I be better served grinding a very thin
> mirror instead?  
> 
>   What kind of problems might I have grinding my own
> conical mirror?  I have 
> the focault tester and compounds already.  
> 
> Thanks,
> Rob O.
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
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