(ATM) Planetary Telescopes - Wire Spiders

[CowanM@aol.com]

On Tue, 3 Oct 1995, Dean Ketelsen wrote:

> 
>       There was some mention a month or more ago in reducing spider
> diffraction by using thin piano wire to support the secondary.  Does
> anyone have experience or references in this method?  I would very much
> like to use this technique, but thinking about how to terminate/adjust
> the wire ends has me stumped.  Any ideas?
> 
>                                       -Dean
>


Yes!  Wire spiders can work great, be easily made, and cheap to construct as
well!  I hung a 1.52" secondary in my 8" f/5.75 using .010 electric guitar
strings.  Guitar strings are very strong, flexible (they stretch a long time
before breaking), and if you play guitar, just save your used high E's.
  Otherwise buy three (about a buck each).  Probably you can use thinner
strings to get even less diffraction, they come down to about .007, but the
cross- section (and strength) of those is half of the .010.  

The ball end on the string where it goes in the guitar nut solves half of the
termination problem.  I didn't want any hardware on the outside of the scope
to get caught on, so I made three low profile support blocks out of
rectangular oak moulding 3/4" by 5/16", about 4 1/2" long.  Oak is strong and
taps well.  Drill two 3/16" access holes all the the way through about 1/4"
in from the end (the strings will go in here).  For the aft end drill a thin
hole all the way through the 3/4" dimension, a 1" cotter pins run through it
to snag the string's ball end and provide some lateral play.  For the fore
end, get some 1 1/2" 8x24 thumbscrews, washers, and nuts.  Drill a hole to
fit the screw size through the upper half of the other end of the oak block;
the washer goes against the thumb end of the thumbscrew (on the right looking
down the top of the block).  Grind down the threads in the middle of the
thumscrews, fit it into the block with the washer in place and mark the
center.  Then drill a tiny hole (1/16", you'll probably need a cobalt drill
bit) through the flat part.  This will be the "tuning" peg.  

I used a hollow secondary holder made from two pieces of 1 1/4" PVC electric
conduit connectors.  (These are the couplers used to join pieces of pipe).  I
mitered one at 45 degrees to form the mirror angle, sanded everything down,
and then superglued the two together. On the outside end I superglued a brass
friction washer to provide a bearing surface for the strings.  On the mirror
end I made three holes spaced 120 degrees around the axis of the tube,
located about 3/16" (for the two closest to the mirror mitre) about 5/16"
from the edge.  In these holes I superglued some aluminum grommets.  

Note that these grommets (or whatever you might use) should be big enough to
pass the string ball ends (if you want to be able to take the secondary out
again without taking all the supports loose).  The distance from the washer
end to the grommets is about 2 1/4".  I power sanded this assembly down to
less than the secondary's diameter, and painted it flat black.  The secondary
mirror is RTVd (silicone glue, that is) on the mitered end.  Note that the
distance the strings run through the secondary is about half the span of the
terminations on the support blocks.  This is important for stability.  It is
also important to have a long run through the secondary support.  When
assembled, the only freedom of motion the secondary support has is in
rotation; and at that, in my telescope, even a hard rap on the tube at high
power shows up in a small motion of the image that dies down in a little more
than a second.  The stability is very good.   

Now for the fun part!  The support blocks are laid out 120 degrees around the
end of the tube, with a leg straddling the eyepiece hole (obviously!).
 Probably you want to center the secondary support on one of the blocks to
line up the mirror to your focuser and figure out where the blocks should be
mounted.  (I evolved the design over several versions so I don't know how I
lined the stuff up, plus I wanted a short tube for portability and the blocks
extend to within 1/4" of the end of my tube.)  Along the outside of the tube,
measure for two holes that will locate each block; these should be about 3/4"
from the ends of the blocks.  Make these holes, then hold the blocks where
they should mount on the inside, mark them through the holes, and then drill
them for woodscrews seperately.  (ie., don't try to hold them and drill them
at the same time!!)  

Figure out somehow how long the strings ought to be, cut them and make a
little hook in the end so you can pass it through the tuning peg and catch
it.  Install the blocks, leaving the cotter pins out. Taking the ball end of
a string, you can now pass it through the body of the secondary holder, and
with a bit of fiddling, thread it through the corresponding grommet hole.
 Snag the ball end with the pin (Duh).  Once you've got all three in place,
tighten up the strings a bit with the pegs.  The nut is used to hold the
tension, so the peg should tighten the string if you pull on the top of it.
 Don't make it too tight yet.  Adjust the secondary's postion by a process of
approximation.  Be sure to get it in the center (or offset if you like) by
letting out and taking in various strings.  Move the strings on the big
washer so they line up (down the tube) on the inside, and move the ball end
on the pin to line them up on the outside.  When everything gets close then
tighten up the strings alternately.  You can tell by pitch if they're the
same length.  My spider is tuned to about one octave above middle C.   

Collimation is now a piece of cake!  Looking through the focuser mount, you
just grab hold of the secondary support and shove it around.  It will slide
up and down the strings to provide fore-aft alignment and angle adjustment.
 The only thing you can't easily adjust is rotation, so be sure to build it
with enough care to get that part right.  

This design yields barely visible diffraction smears on planets, with that
nice 6-spoke effect on the brightest of stars.  It only costs about $10 for
the parts, and what's more, if you get tired of looking at stuff, the music
of the spheres is as close as the end of your telescope.  

Slightly hazy but truly steady skies,  

Mark Cowan