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Re: [ATM] 16in tridob rebuild 1st light (long)
Hi Dave,
I like the ball and socket joints for the truss work. Where did you find
the wooden spheres?
-Anthony, Round Rock TX
> -----Original Message-----
> From: atm-bounces@atmlist.net
> [mailto:atm-bounces@atmlist.net] On Behalf Of Dave Smith
> Sent: Wednesday, August 22, 2007 5:51 PM
> To: ATMLIST
> Subject: [ATM] 16in tridob rebuild 1st light (long)
>
> 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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