Re: [ATM] Push Pull Lens Cells

["Bob May" <rmay@nethere.com>]

Having just built a refractor, I know your pain.  One of the
problems is that it seems that there is no standard diameter for
glass for refractors and you need to do the cell fairly accurate
to the glass.  In addition, the thickness of the glass also
matters, unlike mirror cells.
As a result, most cells get made from looking at other cells that
have been made and copying the basics of the design.
I built a simple solid type cell (no adjustments for tilting the
glass) from plywood that I stacked after cutting on a router
table to the diameters needed and it works alright.  Better is to
make the cell from metal but that means a lathe fo a decent size
( I built a 6" clear apeture telescope which means that the
glass, is of course, larger than that) and my little 6" lathe
that I have isn't big enough to turn anything that large - I'd
need to have at least a 4" lathe at the minimum!  After that, it
is also a bit of a problem of getting material to turn a piece
from as most of the cell is empty spce in the middle which means
that you need to really find a ring rather than getting a big
chunk of something and hogging out the cell from that.  85
percent or more scrap just isn't cheap!
Fancier cells usually have a set of three pushpull screws around
the outside to move the part holding the glass so that it can be
put "square on" to the focusre and another sectgion that the
screws go against that attaches the cell to the rest of the
scope.  This mmeans two large chunks of metal are needed to
produce the cell which makes things even more expensive.
When working in aluminum or steel, yo, the cheaper metals, you
can bend a ring our of flat stock but that takes a rare piece of
equipment that you're going to have to build yourself, a metal
bender with three wheels in it.  I'd not consider brass due to
its expense unless you are really willing to spend that kind of
money on a lens cell.  Brass also works well in bending and its
only problme is that it work hardens if you bend too much so you
will want to heat it up while working it to get it soft again.
Needless to say, you want to weld the joint after initial bending
so that the metal is solid all the way around.
Have fun!!!  It's gonna be an interesting trip to make a lens
cell!
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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