[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
No Subject
Well, it's about time I contributed to this group (and help kick it off
for the new 'management'), so:
The spider discussion is timely for me, as I have been doing some thinking on
a spider/diagonal holder design for a 10" f/6.4 I am building.
I have been considering two approaches, tensioned wires, or an open
vane that should offer most of the benefits of the tensioned
wire approach, yet be easier to construct and mount.
The open vane is, basically, just a standard solid vane design - but I
will machine out the central area of the vane, leaving just the ends
and a thin strip of material at the top and bottom edges of the vane.
I will use fairly thin material and will make the edge strips something
on the order of 1/8" wide. This design should be stiffer than a wire
approach (due to the greater cross section relative to a wire) and will
eliminate the greater complexity of mounting/tensioning the wires (though
some good ideas for this have been detailed on this ATM list).
I have also given some thought to improving on the standard (Novak)
diagonal holder. Assuming the diagonal/spider is located accurately
in the tube (axially), the only adjustments required are two rotations -
tilt (perp to optical axis) and rotation (about optical axis).
The current diagonal holders (Novak, etc) offer adjustment (or rather,
misadjustment....:-() with 4 degrees of freedom (actually all 6, but
two of the translations are a side effect of the rotational pivot not being
at the optical center of the mirror). This is due to the ball/socket and
3 pull screws used for mounting the holder. Tweaking the 3 screws can
certainly be done, but I would much rather constrain the holder so that
it only adjusts in the needed two axis' (and removes the coupling with
the other unnecessary axis').
What I will do is machine the holder to mount directly to the spider, but
still allow rotation (I will eliminate the long threaded shaft and nuts
that allow axial placement on the novak style). For tilt adjustment,
rather than use a pivot I will use a flexure and two screws (in an opposing
push/push action). To make the flexure, I will machine a piece of
round stock so that only a central web of material remains (like an I-beam).
This approach results in high stiffness and is simple (esp if you have
access to a lathe/mill).
I'll try and show this (my ascii diagraming skills are questionable...):
<html><pre>
.
. .
. .
. .
. . |
. . |
. . |
Diagonal --> . . |
. . |
. . | <-- Fiberglass Tubing
. . |
. . |
. . |
. .| |
. =====================
[] | [] <-- Machined Base
[] | []
=====================
[] []
[ ] [ ]
/\ /\ /\
screw flexure screw
</html></pre>
I am thinking of making the diagonal holder itself out of fiberglass
tubing that is somewhat smaller in OD than the diagonal, then just bonding
the diagonal to it in 3 places using silicone adhesive (I will slightly space
the mirror off the angled surface of the fiberglass tube so it is supported
and held only by the 3 areas of adhesive). As a failsafe (in case the
adhesive lets go....) I will bond a safety line to the back of the mirror
and attach the other end to the diagonal holder base.
This should have the effect of minimizing obstruction (no part of the
holder extends outside of the digonals minor axis size) as well as
minimizing the metal in the optical path (use fiberglass for
the holder). The open vane spider also minimizes the amount of metal
in the path.
Has anyone had any experiences (good or bad) with bonding diagonal
mirrors to their holders?
Bob