[ATM] 16in tridob rebuild 1st light (long)

[Dave Smith]

Just wanted to quickly report the results on the
rebuild of my trilateral dob.
I've had the beast out in the driveway the past three
nights tweaking and adjusting. The orange streetlight
glow in my 'hood is borderline unacceptable, but then
options are limited when you need your tools handy.
:O) I'm in Orange County, southern California; I
suppose it could be worse, but the temperature IS
perfect and there's little wind.

Jupiter was excellent at moments with detail
sufficient to see both major spots and adjacent
swirling patterns. Seeing was hit or miss and it took
some patience and averted vision to see the detail.
Moved on to several of the clusters in Scorpio and
Sagitarius. Glow nearer the horizon was troublesome,
but the images were very crisp as long as we didn't
push magnification. (16" f5.6, 3.1 inch secondary,
using different combos of 2 inch EPs: 52mm and 48mm
Rini's and a 19mm SWA Russel Optics with my 2x Pocono
Mntn Barlow). M13 and M57 were also excellent at near
zenith; definitely more contrast with the thinner
atmosphere/smog layer. M57 was ghostly blue with some
fine detail visible even at low power. VERY rewarding.

Now, commenting on mechanical performance...
In the rebuild I enlarged the ground ring diameter
from 28 to 35 inches and the alt bearing diameter from
20 to 30 inches. This was done to allow for a larger
mirror box, a more stable stance of the OTA in the
flex rocker and higher center of OTA rotation. The
full weight of the scope places formica bearing
surfaces directly on SS rollerblade bearings.
Definitely suboptimal. In the short 3 nights under the
stars I have divits and grooves in two of the 4
bearing surfaces (the ground ring and the main
altitude centerfin). They affect pointing even at low
power and the original fantastic smoothness is gone.
Sigh. I've also discovered that the flex rocker flexes
in one dimension too many. Where my bearings are
mounted at about 45 degree angles on the top surface
of the flex rocker I see that the wood is buckling
outward allowing the altitude surfaces to contact the
flex rocker. Nice unintentional mechanical brake! I
tried to quick-fix by reinforcing the joints of the
rocker, however it only reduced the rub instead of
eliminating it. Not sure how I'm going to fix that. I
can "carve" away the area that rubs in the flex
rocker, but I suspect the rocker will just stretch to
accomodate more of the OTA.
I'm still having problems being nose heavy. I've
reduced the top of the OTA to a single ring, a red dot
finder, a 2" homemade crayford and the most simplistic
truss clamps (a small piece of aluminum angle, a
1/4-20 carriage bolt and a hand nut). The spider
mimics Mel's wire version. My secondary mirror is
approx 1" thick. I used ball/socket joints to capture
the trusses at the mirror box and endring. They work
extremely well. Complicated trilateral angles are no
more. I swapped them out at the end ring though--too
heavy. Even eliminating this weight, she's still nose
heavy. I've slapped the extra 25 pounds of lead (from
tridob version 1.0) on the center fin level with the
ground ring. It's well balanced with the lead. No
spring virtual counter weights at this point, but I'm
planning on trading lead for springs very soon.
My truss tubes were adjustable painter's poles. Over
time and use hand tightening of the joints caused a
couple of the plastic joints to crack and fail. So,
due to the slippage and undesireable concommitant
decollimation, I cut the threaded plastic unions off
and JB-Welded the two joints of aluminum tubing
together (both are ~1" dia/thin walled). The OTA is
now rock solid and stable.
I have two fans installed in the mirrorbox and aimed
across the surface of the primary. They work well as
before.
My ground ring and centerfin already have the JB
Weld/putty epoxy threads in place and I've built up
the stepper motor assemblies--they're good to go, I
think.

All in all, I'm very pleased with the rebuild. The
floppiness of the original OTA is GONE. I need to
solve the flex rocker issue and line the bearing
surfaces with steel. That should solve my "issues".

And THEN it's time to motorize.
I'll be updating the website soon. It's partially
updated now, but no final images.

the SolidWorks digital version:
http://dsmith.no-ip.org/smithersscope/atm/solidworks.htm

and the page of images of the rebuild:
http://dsmith.no-ip.org/smithersscope/atm/Tri-Dob%20Reconstruction.htm

As always, I couldn't have done it without y'all's
help! Great group! Cool hobby!
Clear Skies!
Dave

 


       
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