the teeter’s telescope 14.5-inch truss-dobthe teeter’s telescope 14.5-inch truss-dob layer fans...

Astronomy TECHNOLOGY TODAY 35

This article is intended to serve as acritical look at the Teeter’s Telescopestruss-style Dobsonian telescope. In par-ticular, I evaluate custom-build #70, a14.5-inch f/4.5 Ultra-Limited Editiondelivered to me in October 2010. Mygoal in writing this article is to documentthe build quality and point out detailsthat may be particularly interesting to tel-escope makers and advanced visual ob-servers. This article explores the telescopestructure only.

My experience in telescope buildingincludes assisting in building an 8-inchf/7.6 Dobsonian with a solid rotatingtube and rebuilding a solid tube Tectron20-inch f/5 Dobsonian into a truss de-sign. In addition, I taught telescope mir-ror-making for 17 years at North

Carolina State University and have 25years visual observing experience withtelescopes. I enjoy deep-sky observingwith an affinity for planetary nebulae.

Fit and FinishThe fit and finish of Teeter’s Tele-

scopes enjoy a very positive reputation.While the finish is easy to evaluate, thefit determines how the telescope will per-form.

A careful visual inspection of thewoodwork confirmed that all joints weretight and the hardware was mounted se-curely. I carefully measured hardware in-stallation and the squareness of thestructure with a drafting ruler and steeltape to make sure that collimation wouldhold to expected tolerance. The greatest

error in the centering of the spider in theupper tube assembly was one half mil-limeter. The greatest error in the center-ing of the mounted mirror in the cellfrom the sides of the mirror box was a lit-tle less than one millimeter on theground side of the cell. The primary cellwas placed perfectly parallel to the top ofthe mirror box. The distance from thetop of the mirror box to the bottom ofthe upper tube assembly was equal all theway around. Finally, the spider wasmounted precisely parallel to the bottomof the upper tube assembly.

Rob Teeter has made the cherry fin-ish a signature feature of his telescopes.While the cherry finish was gorgeous, Iwanted a lighter color that would reflectheat a little better and not show dust. I

The Teeter’sTelescope14.5-InchTruss-Dob

By Phyllis K. Lang

An Evaluation of a Teeter’s Dobsonian Structure

chose the light maple stain at Rob’s sug-gestion, and the result was beautiful.

The Mirror BoxThe mirror box and rocker box are

made of 3/4-inch birch plywood. It issolidly built and heavy for me to movealone. Fortunately, my husband is readyand willing to help with the heavy lifting.

My previous 20-inch Dob was also builtwith 3/4-inch plywood. I prefer the heavystructure because vibration from wind or hand movement is harder to induceand damps out more quickly than alighter structure. I also believe the stiffer structure holds critical collimationbetter.

A small fan is mounted behind the

primary to assist with cooling. It is an upgrade to the standard kit. It is neatlyinstalled with wiring fastened with wireclips and an LED-lighted switch just beneath one of the collimation bolts. The fan is a quiet and blows 32 cfm. It induces no vibration at all into the optical system. I highly recommend thisupgrade for use after the boundary

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36 Astronomy TECHNOLOGY TODAY

THE TEETER’S TELESCOPE 14.5-INCH TRUSS-DOB

layer fans have done their job.

The Primary CellThe primary cell is made of 1.0-inch

box-tube aluminum and 1.375-inchchannel aluminum. The welds are cleanand the riveting is tight. It includes an18-point floatation system for the 14.5-inch thin Pyrex mirror with adequate

travel on the collimation bolts. These collimation bolts have large knurled-steel knobs that turn very smoothly, allowing precise adjustments to primarymirror collimation. The cell is mountedfirmly to the 3/4-inch plywood mirrorbox. Image 1 shows the rear of the mirror box: cell, primary and rear fanshown

The Tailgate AssemblyThe cell hinges out and down to

allow the mirror to be removed. Image 2shows the Glatter Cable Sling System,the 18-point floatation system, and themirror retaining clips turned to permitmirror removal. The tailgate hinges onbolts buried in the sides of the mirrorbox. The tailgate is secured with 9/16-



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inch bolts through the mirror box andthe aluminum frame of the cell. This cellisn’t going anywhere, and neither is themirror when the clips are rotated to the“closed” position.

The Boundary Layer FansThe twin boundary layer fans pull

air into the mirror box, blowing down

and across the face of the primary mirror(Shown Image 3). Two holes are cut inthe mirror box opposite the fans to allowthe moving air to drain. I debated thevalue of boundary layer fans years agowith my fellow mirror-making and tele-scope-making buddies. We determinedthen, and I remain convinced now, thatbreaking the boundary layer with fans

improves the quality of images signifi-cantly when atmospheric seeing is excel-lent. Image 3 shows the mirror boxinterior and boundary layer fans.

These fans are intended for morethan breaking the boundary layer. I callthem turbo-blowers because they deliver110 cfm each and cool the mirror box in-terior. I can attest that running the fans



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for 30 minutes after setup and before ob-serving improves cooling of the primaryand mirror box significantly. The fansinduce vibration into the mirror boxstructure that definitely affects images.Teeter’s Telescopes does not recommendrunning these fans while observing, andI agree.

Push-To MotionI am a “push-to” observer so the mo-

tion on the altitude and azimuth bearingsis important to me. This telescope em-ploys Ebony Star laminate and generouslysized PTFE pads for the bearings. The az-imuth bearing includes a large PTFE padin the center. The motion in both alti-tude and azimuth is smooth. I have notdecided whether the amount of “stiction”is perfect or just a tad too much in ahigh-power (e.g. planet) observing sce-nario. This is a critical adjustment in myexperience – I once applied some ArmorAll-brand treatment to an azimuth bear-ing that resulted in an unusable tele-

scope. A friendr e c o m m e n d e dpure carnauba waxwhich I havefound to be a bet-ter solution. If I doanything to thebearings, I willapply a smallamount of car-nauba wax.

More Aboutthe MirrorBox

Nice touchestypical of this telescope include theclamps that secure the fan wires, flockingmaterial on the light baffle above the pri-mary mirror and soft rubber bumpers onthe mirror clips. Wiring is neatly con-trolled throughout the telescope. Theflocking material is trimmed neatly and isinstalled with no bubbles or creases. Thedust apparent in some of the photos is

stuck to the flocking material and can beblown out with an air bulb which I keepwith my observing equipment.

The mirror clips stay positioned tokeep the mirror from tipping out of thecell when the telescope is horizontal.They passed an unintended test of thison the telescope’s second night out: thetelescope tipped past horizontal while



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moving it with the wheelbarrow handles.There was no damage, but the mirrorslipped forward in its cell. Once it was

repositioned properly in the cell, the clipsgave ample clearance between glass andbumper. If a mirror fits tightly in the cell

or sling, the glass is pinched resulting inastigmatism.

My previous 20-inch f/5 Dob (ca.1989) had a seat-belt sling in it. Whilethe belt was serviceable for years, I hadproblems with it slipping. If the belt was-n’t positioned correctly, astigmatism be-came apparent in the image. Theold-style seat-belt slings were also sus-ceptible to stretching.

The Glatter Cable Sling System is arecently-developed solution to bothproblems and is available as an upgradeto the standard package. Image 4 showsthe mirror box interior and cable slingclose up.

I use digital setting circles (DSC) inmy “push-to” observing. The DSC swivelbracket (Image 5) is an upgrade that allows my DSC computer to be attachedwith Velcro-brand fasteners to an ad-justable, convenient mount on the tele-scope. I can operate the DSC withoutdetaching it and it remains at arm’s reachfrom the focuser. This is a recommendedupgrade if you use a DSC computer.

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Ball-&-Socket Truss-PoleConnectors

The ball-&-socket connectors for thetruss poles (Image 6 shows the MoonliteTelescope Accessories truss pole connec-tors) are a huge improvement over thefull-capture assemblies I used in the past.Truss poles slipped occasionally in thecapture-style assemblies causing realproblems at collimation time. The ballson the new telescope’s truss poles fitnicely in the sockets. My greatest concernis that the steel screws that tighten againstthe hard Delrin-brand-plastic balls willflatten out spots on the balls causing amisfit. Time will tell whether that be-comes a problem; otherwise, this is abeautiful solution.

The Truss-Ring SystemTeeter has an innovative system that

joins the upper tube assembly to the mir-ror box, and it can speed up setup by asmuch as 10 minutes. Truss-pole connec-tors are mounted to the upper tube as-sembly as usual, but the connectors onthe mirror box end are mounted to a 3/4-inch plywood ring that bolts onto themirror box. This allows the upper tubeassembly and the truss poles to separatefrom the mirror box as a single unit.Image 7 shows the truss-ring systemmounted on mirror box and Image 8shows the truss-ring system separatedfrom mirror box. The fit of the ring to themirror box is tight and is secured by largerubber-coated hand knobs. This system isreally convenient and rigid enough not to

affect collimation. Of course, if you needto break the telescope down further, thetruss poles can be removed from all con-nectors and stored separately.

The Upper Tube AssemblyThe upper tube assembly (UTA) is

also solidly built, but it can benefit from asingle, simple change.

The AstroSystems spider was cen-tered in the UTA and the secondaryholder was fastened tightly on the spider.Even so, the secondary holder could ro-tate within the spider with hand pressure,so I added a split washer between the nutand the flat washer that secure the secondary holder to the spider. Problemsolved.

The secondary holder has four collimation screws that are easy to adjust with fingers (no gloves). The secondary holder also has a secondarydew heater to help with heavy seasonaldew here in North Carolina. The wiringis neatly routed on the spider with particular attention to flatness. Theamount of diffraction induced into theimage is minimal due to this careful detail.

Image 9 shows the circle of flockingmaterial opposite the focuser in the UTAthat comes with the standard package. Iinserted flocking material into my othertelescope a few years ago, and I highlyrecommend this simple step to improvecontrast in reflecting telescopes.

Image 6

CollimationI tested the telescope’s ability to hold

collimation while moving it in altitudefrom zenith to horizon, and I observed atroubling shift of several millimeters inthe beam of a laser collimator within thecenter spot on the primary mirror. Ifeared that this was due to flexure in thecell or the tube structure. Having made

careful measurements of all components,it was not readily apparent what mightbe causing the shift, but it was largeenough to affect images noticeably.

I talked at length with one of mymirror-making friends who pointed outthat the problem may be in the focuseror the laser collimator itself. I verifiedthat the laser collimator beam was colli-

mated within the housing, so I re-moved the laser and inserted a Cats-Eye 2-inch Infinity XLKautocollimator. I watched the hotspotpattern in the autocollimator as I ro-tated the telescope on the altitudebearing. There was almost no move-ment, proving that the problem lay inflexure of the laser at the focuser, notin the telescope structure itself. Fur-ther experiments showed that whenthe collimator was in the focuser’s1.25-inch adapter, the shift was huge –up to a centimeter. When the collima-tor was in its own 2-inch adapter andfitted very tightly in the focuser, the

shift was tiny – not really measureable.

ConclusionI am delighted with this telescope.

Having examined it with a millimeterruler, a carpenter’s square, a laser collima-tor and an autocollimator, I am confidentthat I have a well-built, sturdy telescopethat should perform extremely well.



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the teeter’s telescope 14.5-inch truss-dobthe teeter’s telescope 14.5-inch truss-dob layer fans...

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