From: "Tom Krajci" 
To: , 
Reply-To: "Tom Krajci" 


>From: "Mike and Sara" 

>Hello!  I am trying to decide whether to glue my F3 12.5 inch primary
to my
>9 point mirror cell or not.  I've created a little different cell....

Any diagrams/drawings/photos?  That will help explain things better than a
couple hundred words will.

>Instead
>of having the collimation bolts go to each triangle, I have each
triangle
>bolted to an underling board...

How are the triangles pivoting?  When you say "I have each triangle
bolted to an underling board..." it sounds like there is no provision
for tiling of the triangles to carry the mirror's weight equally on all
support pads.  That would be a bad idea.

>..and then that board attached to another board
>with the classic bolt/spring arrangement.  As a result, with
collimation,
>the distance between the triangles will not change - the entire mirror
>plane will shift.

Again, a photo/diagram will help here.  I am concerned about the overall
height of this arrangement...mirror mass..and how well or poorly the entire
mirror cell resists moments.  In other words if this is a 'tall' mirror
cell arrangement, a 'thick' mirror, and a 'not very stiff' arrangement of
parts/plates/bolts/etc. in the whole mirror cell stack...it'll bend over
when the mirror gets close to horizontal...and with such a fast
mirror...your collimation tilt requirements are much more stringent than
for a slower mirror.

>In such an arrangement, I thought I could glue one point of each three
>point triangle to the mirror, and allow the other two points of each
>triangle to float freely....

What do you mean by 'float freely'?  If they are resting on the back of the
mirror they are in a stiction/friction coupling with the mirror's back. 
That is not free floating...at least as I define it.  Then...as the mirror
is tilted so the scope is near horizontal...the mirror is supported by
three glued pads instead of nine.

>This would eliminate the problem of differential
>flexure between the mirror and polymer glue compound.

I do not understand what you state here.  Do you mean differential expansion?

>    However, I am concerned about how accurately centered the mirror
must
>be.  I have read in the archives that the mirror must be centered to
1/16
>inch or better with respect to the secondary in order to achieve
maximum
>performance for the system.  If this is the case, then having a support
>system that would allow the mirror position to be varied, such as the
two
>wire scheme I have seen presented in the archives, makes sense as
opposed >to fixing the mirror to the mirror cell.

Your mention of 'two wire scheme' implies an edge strap/wire support. You
did not mention this previously.  Again, photos/diagrams would help.

If you have a wire edge support...and some glued pads...you may have some
interesting results.  I recommend choosing one or the other. Either strap
only, or glued pads only, but not both.

>If, on the other hand, if I can glue the mirror to the mirror cell
>support, it would make the system much more convenient to use.

I find glued cells more convenient to use.  Mirror does not flop about in
cell, or knock against retaining clips, or move around against the
strap/wire.

>What are people's opinions regarding accuracy of centering a primary?

The faster the mirror...the tighter the collimation requirements. Whether
you line things up by tilting, or by lateral shifting...or a
combination...make sure your cell allows for
precise/repeatable/settable/non-slipping adjustments...or you will have
created for yourself a headache.  And yes, for a cass. you need control of
centering of the optical and mechanical elements with respect to each
other.  However, it may be easier to have provisions to laterally shift the
secondary mirror and it's cell, instead of trying to shift the main mirror.
 But make sure you install the main mirror as carefully centered as
possible in your tube.

Do you plan to focus by moving the main mirror, moving the secondary, or
with a focuser at the back of the telescope?

Good luck,
Tom Krajci
Tashkent, Uzbekistan


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