[ATM] 16" Tri-Dob Success

[Dave Smith <smithersscope@yahoo.com>]

Greetings my friends,
After a few years of quasi-lurking, I wanted to post a
preliminary success--and express a heart felt THANKS
to everyone on the list for support and guidance.
Regrettably my second 13" trepanned and ground
homemade mirror-to-be sits safely in a drawer in the
garage. Some day it will be a real mirror. Meanwhile,
I wanted to get out under the stars when the weather
permits. So I picked up a 16" x 1" plate glass mirror
from Mark Cowan at Obsidian Optics for a song off ebay
and followed Mel's pattern to build a tri-dob mount.
I've taken pictures of the scope and am slowly
building a website to share what I've learned. I'll
post that address as soon as I can get it up and
running. I just changed ISP's at home. One of life's
little challenges.

I built a couple of finderscopes from copy lenses I
salvaged. All the aluminum came from the dumpster. The
truss poles are the extendable painter's poles for $10
each at Home Depot. I'm using the screw on paint
rollers, minus the rollers, at the connections at the
end of the poles to tie into the endring. I just
removed the rollers and put the heavy gauge wire into
the vice, inserted a bolt and hammered the wires into
loops. I use hand nuts to lock everything down. I'm
using a simple endring on the OTA. (Oh the joy of
routing circles with a router and jig! Circles rule!)
It was originally just aluminum plate, but I had to go
back and reinforce it with plywood to eliminate
flexure. I broke down and bought a sheet of Ebony Star
for the bearings. (choke, cough, cough.) It was,
however, a worthy investment now that I've compared it
to regular formica. They DON'T compare. Homemade
Crayford focuser, also built from scraps and salvaged
stuff. Very smooth. This one is my second iteration.
My brother now uses my first one on his 8" dob. He
never realized how bad rack and pinion was until he
tried the Crayford. It had a milled surface for the
shaft to roll on. I've learned that milling is
completely unnecessary. I now have the focuser shaft
riding directly against the outside of the drawtube,
and I can easily adjust the tension on the shaft to
compensate for the weight of the barlow/EP. So far it
has performed really well.
I'm using Mel's pattern for a wire spider. It's
difficult to describe...needs pictures. I captured
short sections of allthread at 120 degree positions on
the endring running parallel to the incoming light
path by tapping the aluminum endplate. Then I tapped
six short sections of aluminum tubing to mate onto the
ends of the allthread, above and below the endring. At
the extreme ends of those aluminum caps I drilled
holes aimed at the secondary mirror. Into those holes
I inserted small eyebolts. The eyebolts, in turn,
capture the wire that passes to and from the
secondary. The aluminum caps are adjustable along the
axis of the light path (assuming you can disconnect
the eyebolts that hold the wires--which I can), so you
can coarsely adjust both the secondary position AND
angle in this manner. The secondary is mounted in the
traditional way with three springs for fine adjustment
of the angle. The eyebolts also allow me to adjust the
lateral position of the secondary and center it in the
optical light path. Adjusting the tension and
centering the mirror was a nightmare until Mel
suggested making a "fixture" (a physical connection
between the secondary mirror and the endring) to hold
the secondary in place until the wires were in place
and tensioned. Thank you Mel! Whew!

I realized all my efforts to be able to adjust the
secondary were overkill when, during my first setup, I
had the epiphany that all I needed to do was adjust
individual pole lengths. Criminy! Can anyone be that
dense?

Setup time is minimal...maybe 5 minutes to assemble
and 3 to 5 to collimate. I was amazed that collimation
is pretty close, even if I completely disassemble the
scope. My homemade laser colimator works great, though
I need to rebuild it--the length is all wrong and it
has wires hanging out the end...too impatient to make
'that' look good. Functional was acceptable during the
build. They're actually not as difficult to make as I
envisioned.

The scope performs beautifully. It was extremely top
heavy--no surprise, owing to the thin 1" thick f5.6
mirror. Focal length is 89"+. I had to add 25 pounds
of lead to the bottom of the mirror box (5 five pound
deep sea weights). That hurt. The mirror was only 15
pounds. I think my entire OTA now weighs just under 40
pounds. I use a 5" tall ground ring that has a
footprint diameter just shy of 3 feet and the flex
style rocker box. (These two pieces weigh in at about
6 or 7 pounds total.) This places the mirror mount
practically on the ground. I still need a 6 foot step
ladder to view most objects. I used a 12 point
flotation cell made of rectangular bars, ala Tom
Krajci's recommendation. I don't have the mirror
siliconed in place yet...cause I'm still tweaking
things below the mirror. I also adopted Tom's virtual
counterweight spring system. Fantastic! The motion is
smooth and stable. I had to add three springs at each
side of the mirror box--it looks goofy--but they were
the only springs I could find that had the right
spring coefficient! Lessons in leverage. Thanks Tom!!
I mounted two fans to the mirrorbox. One is positioned
below the mirror and blows across the flotation cell
directly onto the back of the mirror. I have been
unable to determine if this helps. The second muffin
fan is mounted on the brace that connects the altitude
bearings--that is the altitude trunnions and shelf
that fold down onto the mirror face during transport.
The fan is angled (20 to 25 degrees) or aimed down
onto the center of the face of the mirror. This one
works like gangbusters. Point the scope at Jupiter,
focus, defocus, watch the heat waves dance. Turn on
the fan and the waves start washing off in one
direction. After a couple of minutes they're gone.
Focus back in on Jupiter and it's crisp. This fan
gives me a slight vibration that I can feel in the
truss poles and barely detect at the eyepiece, but I'm
going to remedy that by replacing the foam at the
muffin fan mount with something more pliable.
Because of limitations with my cheap eyepieces I've
only been able to push the magnification to about
200x. That'll change soon. 
Seeing is another thing and altogether out of my
control.

The Hercules cluster at 100x made all the pain and
frustration in building this monster vanish.

Side by side against Celestron 10" and Meade 11" RCTs
there simply was no comparison. Increased focal length
is no match for aperature (well, unless you can do
CCD, I suppose.) What a bright and beautiful image.
Whoooohooooooo!

My wish list now is real simple:
Motorize.
Upgrade the EP's.
Get to some dark skies.


How can I ever thank all you good folks!??
My hat is off to all of you!
Keep up the good work!
Dumpster Dave


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