Alan Adler's New Pusher system
Further Notes On Flex Cell Construction
Solid Rubber Pusher Ring:
I've found that an easy way to make the rubber pusher ring is to glue
1/2" square "O-Ring Cord" in a circle on the pusher plate with a dot of
cyanoacrylate (instant) adhesive about every inch. It's essential to
draw or scribe precise circles on the pusher plate to align the rubber.
Compression tends to push it up a bit on the inside of the curve. So
after the glue has dried (about an hour) sand the face of the rubber
ring flat on a large sheet of coarse sandpaper which has been attached a
flat surface with double-backed tape.
The O-Ring Cord that I use is McMaster-Carr #9700K16. Price is $1.81
per foot. My 8" scope needed about 25 inches.
Sponge Rubber Pusher Cushion:
I first used the same McMaster sponge material for both the puller and
pusher cushions. But the pusher sponge is under greater PSI loading
that the puller. After a year, my pusher took a permanent set and
eventually became too thin to maintain proper flex. I've now found a much
better material which is sold only in fifty foot rolls of strip. But a
roll only costs $13.58. The material is self adhesive on one face, 1/4"
thick and 3/4" wide. McMaster-Carr Natural Sponge Rubber Strips
#93625K64. I also find that stability of flex is far superior with this
pusher cushion.
(I still recommend the original 8647K45 (3/8" thick Neoprene/EPDM/SBR)
sponge rubber sheet for the puller cushion. It takes a glue bond
reliably, which can be a problem with many rubber products).
I made a round plywood mandrel and glued it to a sheet of stiff poster
board. The mandrel is one inch smaller in diameter than the mirror to
match the inside diameter of a 1/2" wide pusher cushion.
Cut a strip of sponge to a length which easily fits around the mandrel.
It's important to have a very easy fit here because there's a tendency
to stretch the material slightly which results in an undersized cushion
ring. Temporarily leave the protective paper backing on the adhesive
and cut a second strip which is about an inch longer than needed to wrap
around the first layer.
Now go back to the first strip and wind it gently (no stretch) around
the mandrel with the adhesive facing out. Trim it to the exact length
which gives a clean butt joint where it closes on itself with no
stretch. Hold it in place and remove the paper backing near the butt joint
only.
Now take the second (longer) strip with some of its backing removed
about eight inches from one end. Position it (adhesive facing inward)
over the first butt joint and stick it together. When doing this keep
upper and lower edges even and flush. Work gently around (no stretch)
until the two strips are stuck to each other forming a complete ring, 1/2"
wide (radially).
Now try the ring on the rubber pusher ring. Are they the same
diameter? They will be if you didn't stretch the sponge. But more likely
there is a slight diameter difference. You can live with that as follows.
Joining the Sponge to the Solid Rubber:
Align any part of the sponge ring with the solid rubber ring and tack
it with a drop of cyanoacrylate. Now tack it 180 degrees away, first
positioning the sponge so that the excess or shortage is equal on both
sides. Now tack at 90 and 270 degrees. If the diameter was close to
correct, you might be able to stop there. But if it bulges, tack it at
45, 135 etc. Keep going until it is radially flush with the solid rubber
ring.
Do NOT just work continuously around when tacking the sponge ring to
the rubber ring. That will leave excess on one side which would lead to
astigmatism. Tacking opposite sides as described above avoids
astigmatism.