Re: plop auto cell 36 and 54 points bugfix [was: Re: [ATM],mirror cell design]

[Dale Eason <atmpob@yahoo.com>]

--- Mark Holm <mdholm@telerama.com> wrote:
> Ken Hunter wrote:
> 
> >Remember that we are talking of glass expansion and
> >bending in the sub nano-meter range 
> >
> 
> Actually, it isn't sub nanometer.  If it were, meter
> class mirrors could 
> be mounted on the heads of three carriage bolts and
> we could get on with 
> observing.
> .....

Just wanted to complement you on a thoughtfull and
well written reply.  Thank you for taking the time to
do so.  Wish I could write like that.  

Maybe one of the misconceptions implied in the
previous article that you were responding to is that
the forces applied to the mirror by a properly
designed mirror cell are temperature sensitive. I
think that while the materials that a cell is made
from do change shape with temperature, the cell is
designed in such a way that the forces it applies do
not change enough to hurt.

I have been wondering if the same is true as the scope
moves through various angles and gravity exerts a
different load on the mirror and cell.  Does the
stiffness of the cell components matter? I have
reasoned that for my visual scope it does not as long
as the forces and their location on the mirror do not
change.  The triangles and levers may sag under
different loading but they will still apply the equal
forces necessary.  The focus my change because of the
sag but the shape will not change.

And as an aside I have been testing my 16" 1 inch
thick F5 mirror on an 18 point cell tilted back about
10 deg. and also on a vertical test stand. I have been
using my automated tester and with it I can do many
measurement sessions to a very good accuracy.  I have
seen a large measurable difference between the two
test stands.  The vertical stand did not give
repeatable results but showed an average strehl of
about .8x with a std dev about .1. It varied much as a
previous poster reported by whether the mirror was
leaning back against the stand, balanced on the two
pegs, or leaning forward against the mirror hold down.
 Using the tilted mirror cell was much better.  The
average strehl was .95 with a std dev of .005.  So it
looks like the mirror cell can support the mirror much
better in this situation.

I have also been star testing this thin and light
weight mirror and so far those tests are consistent
with my Foucault testing using the Cell.

It has been with the help of this lists members and
their web sites that I have been able to create my
ultralight 16" telescope.  It is not finished but will
be soon. I thank you all for showing me how to push
the envelope.

Hang in there Jerry Hillman you will get there too.


Dale Eason



	
		
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