Re: [ATM] John Dobson

[Dale Eason <atmpob@yahoo.com>]

No need to bother John with that question.  Many on
the
list understand the issue.

As you see you can do it either way.  There is nothing
special about the box.  Put the bearings at the
balance
point of your scope.  If that is in the middle then
you
have to build something onto the tube to place the
bearings there.  Thus the box in the middle.  Putting
the box at the bottom end will help hold both the
bearings and the mirror with one thing.  But that only
works if your scope will balance there.

Regarding your Foucault tester and Mask.  You will
need
more openings or a mask with additional different
openings to test you mirror.  You need openings out to
the edge.  Others on the list have given you good
links to web sites explaining this.  Use those links.

Second individual Foucault images will not be enough
to tell you or us what the figure on your mirror is. 
You must use the technique as described by several
other to measure the mirror.  This involves taking
several readings at different position of the tester.
Then feeding those reading into an analysis program
like TEX or FigureXP or Sixtests.

Concerning the camera.  It doesnot not matter.  It is
hard at first to get everything lined up. Also the
camer lens needs to be right up at the knife edge or
it will not see the complete image of the mirror.

Movements of the knife of only .001 of an inch into
and out of the beam will causes the shadow to change
drastically.  So it helps to have the tester on its 
own stand and the camera on its own tripod. So when
you bump the camera it doesnot move the knife.


Dale Eason


--- David  Williams <dwilliams1128@earthlink.net>
wrote:

> Hello List
> 
> Does anyone here know about John Dobson?
> 
> I hear he was born in China and moved here to
> America, and he lived in a monistery where he was
> kicked out for building telescopes!!  
> 
> Does anyone know how to contact him?  I need some
> advice on the rocker box for making my Dobsonian
> telescope.  I've seen some of them have the box
> include the whole bottom half of the sono tube and
> some of them just include the middle where they are
> balanced.
> 
> David 
> Bye
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 

--- David  Williams <dwilliams1128@earthlink.net>
wrote:

> Hello List
> 
> Does anyone here know about John Dobson?
> 
> I hear he was born in China and moved here to
> America, and he lived in a monistery where he was
> kicked out for building telescopes!!  
> 
> Does anyone know how to contact him?  I need some
> advice on the rocker box for making my Dobsonian
> telescope.  I've seen some of them have the box
> include the whole bottom half of the sono tube and
> some of them just include the middle where they are
> balanced.
> 
> David 
> Bye
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
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