dobstuff’s lightweight dobsoniankits dobstuff.pdf · 62 astronomy technology today dobstuff’s...

60 Astronomy TECHNOLOGY TODAY

In the last couple of years I’ve shiftedmy focus from personal observing to pub-lic outreach. Concurrently, I started usinga digital setting-circle system (Sky Com-mander) to help find some harder-to-hop-to objects with an audience used tochannel-surfing rapidity. My trusty 6-inchf/8 reflector continued to serve me well,mounted on a Half-Hitch alt-azimuthmount, which is indeed a luxurious way tomount a scope.

One of the things I noticed doing out-reach was that most children needed somesort of a ladder to get to the eyepiece. Thisoften made them uneasy, and inevitablysome of them would wobble, try to grabthe scope, which of course would swingaway. Anyway, I needed to spot them andenlist their parents to help as well. I beganto wonder if a scope that didn’t require aladder for the little ones might be a goodidea.

Around the time that this idea wasgelling in my head, I found out I had beenaccepted to the Astro VIP (Volunteer-in-Parks) program and would be a guest of theU.S. National Park Service at Capitol Reef

National Park in south-centralUtah for a month in the summer of2011. I began to think that maybea bit more aperture might be agood idea – to really take advantageof those deep, dark skies – as wellas a good opportunity to put mykid-friendly scope plans to the test.Enter an old Meade 10-inch f/4.2mirror bought on Astromart.comfor a song andDennis Steele’s PalmDesert, California-based telescopecompany, Dobstuff.

About DobstuffRather than offer a line of

stock telescopes, Dennis’ businessmodel for Dobstuff focuses onwhat he calls “scope makeovers.” Dennisfigured that the first ungainly decades ofthe Dobsonian revolution littered thecountry with tons of overbuilt, very heavytelescopes, and that their aging ownerswould be increasingly unwilling or unableto drag them into the backyard, let aloneto a dark-sky site far from the ever-en-croaching light domes of our cities and

suburbs. His makeover service takes the op-tics from these scopes (and to varying de-grees some of the other components likesecondary holders and focusers, dependingon what’s usable and what the ownerwants) and puts them into a tidy, light-weight and well-thought-out tri-parallel-strut Dobsonian.

The Dobstuff website is rich with im-

By Doug Reilly

A Big, But Lightweight Scopefor Public Events

Dobstuff’sLightweightDobsonian Kits

Image 1 - The author’s Dobstuff 10-inch kit in f/4.2configuration.

ages of made-over scopes and testimonialsfrom customers satisfied with Dennis’weight-reduction program. Dennis also of-fers kits, which are basically all the woodenparts and components needed to constructa telescope. Buyers need only sand and fin-ish the parts, glue them together and as-semble the scope.

Dobstuff ’s prices are surprising giventhe quality of the wood and componentsDennis uses, and especially so if you con-sider the prices some premium-telescopemakers asked for similar designs in the re-cent past. In fact, Dobstuff ’s scopes neatlyfill a market niche for portable Dobs thatexists between the mass-produced and thehigh-end premium (and often custom-made) truss Dobs.

Dennis’ telescopes utilize three alu-minum poles in a parallel-strut configura-tion. Further description of the designwon’t do as well as a single image (seeImage 1).

The Tri-Parallel-Strut DesignI was curious about the origins of the

tri-parallel-strut design. Just as Dobstuff ’sscopes seemed to occupy a middle-groundin the Dob market, their design alsoseemed to occupy a middle ground be-tween the classic Berry-Kriege truss Dob,and all its variants, and the superlight

“travel” scopes that seem to be all the ragein Europe (which appear to have a com-mon ancestor in the innovations of France’sPaul Strock). The tri-parallel-strut designrepresents a common-sense, no-more-than-necessary approach that reveals somethingdeeper than the “bigger is better” Ameri-can design philosophy that gave us theHummer for soccer moms. So where did itcome from?

My only conclusive finding is that I’mno private investigator. One of the prose-lytizers of the design, Steven Overholt,traced out its lineage in his immenselyquirky and entertaining self-publishedbook, A Telescope is a Space Ship. Accord-ing to Overholt, the first parallel-strut tel-escope to make a splash in thestargazing-public consciousness was JimSteven’s four-strut, 17.5-inch Dobsonian.The “splash” in question was the discoveryby Tom Bopp of Comet Hale-Bopp in1995, using Jim’s scope. By the late 1990s,Albert Highe was making tri-parallel-struttelescopes, and Michelle Plettstone startedselling them commercially. Dennis Steelewas in the same telescope-making circles asAlbert and Michelle. Further diggingthrough Albert’s archivedWeb page revealsthat he was inspired to explore the parallel-strut design by Ron Ravneberg’s 8-inchtravel telescope, Alice, made in 1990.

Ravneberg himself cited earlier work byThane Bopp, Dick Suiter, Tom Burns andBob Bunge.

So the tri-parallel-strut design seems tohave no clear inventor; rather it coalescedfrom the work of many people. I’m happyto throw out the “Great Man in History”theory in this case; I like the idea that thissprang not from a single genius but ratherfrom the collective work of many ATMsworking in isolation and community, bor-rowing, developing and sharing their ideas.Nobody owns the design, but plenty havefound it to be a sturdy way to construct alightweight telescope.

My Design GoalsI wanted a large-aperture scope (in this

case, I settled on 10-inch – remember, Iwas reared on a modest 6-inch – and 10inches represented a feast!) with a fast-enough focal ratio to keep the eyepiece nearenough the ground for most children toview flat-footed and without a stepladder.

I also wanted to keep the scope ascompact as possible, seeing as I would haveto load it up into a NissanVersa, along witha lot of other gear, for my trek cross-coun-try to Capitol Reef. Dennis makes what hecalls an “EasyTransportTelescope” option,which basically cuts the storage size of thetruss poles in half by introducing an inter-

Image 2 - Brandon Geisel’s (Tripodpads.com) tough, vibration-reducing urethane tripod pads were installed in place of more-standard Dob feet.

Image 3 - The Dobstuff kit arrived safely protected in two well-packaged boxes.

Astronomy TECHNOLOGY TODAY 61


DOBSTUFF’S LIGHTWEIGHT DOBSONIAN KITS

mediate wooden ring. Broken down, youhave six shorter poles instead of three full-length ones. I wanted to take this a stepfurther: What if you could nest the upperset of three 1-inch diameter aluminumtubes into the lower set of 1.5-inch poles?

Dennis usually recommends the 1.5-inchpoles for a 10-inch scope, but since onlyhalf of the structure would be made fromthe smaller tubing, he concluded that sta-bility would not be overly compromised.

This solution presented a challenge,

however. The usual manner of attachingthe poles to the rings (threaded inserts)wouldn’t work in the case of the top of thelower poles, since the tube needed to beopen to nest the upper poles, so we had tolook for an alternative manner of affixing

Image 4 - The author’s colleague, Peter Spacher, cuts triangle sec-tions from the sideboards.

Image 5 - The main components of the kit are prepped for multiplecoats of Ikea’s user-friendly Behandla finish, a mixture of beeswax,linseed and tung oils.

the lower poles to the rings.We found it inan idea utilized in a 10-inch travel scopemade by ATMer Eric Shrader: a captivewooden clamp. Dennis thought the ideathrough and came up with a tidy way ofintegrating the captive clamps into the in-termediate ring.

The craziest idea I had was to try to im-plement an eyepiece turret in the uppertube assembly. I had an older Vixen turretdesigned for the straight-through viewingpopular with Japanese refractor owners,with three ports for 0.975-inch eyepieces(18-mm, 12-mm and 9-mm Orthos weremy choices) and a single 1.25-inch port (fora 20-mmPentax XL). I mated the turret viaa 1.25-inch nosepiece from Borg with alow-profile 1.25-inch helical (non-rotating)unit, also made by Borg for its line of mod-ular refractors. I purchased the focuser aswell as a 1.25-inch eyepiece adapter fromAstro Hutech in California. Ted Ishikawawas, as always, helpful and prompt.

At some point in the design phase, Istumbled upon an ad for Brandon Geisel’surethane tripod pads (Tripodpads.com).While designed for tripods, obviously, Ihad another thought: Dob feet. I knowthat some people use hockey pucks andother people use rubber furniture feet andstill others use blocks of wood, but Ithough the urethane pads would have someadvantages – namely the same vibration

dampening qualities that make them usefulfor tripod-mounted telescopes. Brandonwas kind enough to donate a set of pads forthe cause. You can see them on the scope inImage 2.

The BuildA few weeks after Dennis wrapped up

the design phase, I found two large boxeswaiting on my doorstop (Image 3). Oneheld the truss poles and hardware, the otherthe wooden pieces. Dobstuff uses the in-dustry-standard furniture-grade Baltic-birch plywood. The pieces were cut outcleanly and precisely and, as advertised,needed very little sanding. Dennis hadasked me during the design phase if Iwanted circular cut outs in the rocker box.Although I thought these were a good ideato reduce some weight and provide somehand-holds, I wanted a different design,something to set my scope apart. So therocker box pieces arrived solid.

I got some help from Peter Spacher,the physics-lab director at the small collegewhere I work (Image 4), who has a ma-chine shop at his disposal – very conven-ient! I drew a pattern of three triangles oneach sideboard and rows of smaller circleson the front board.While Peter milled outthe triangles I cut out the circles using a 2-inch Forstner bit on a drill press and thenadded some holes to the azimuth base

board and themirror box’s bottom board aswell (Image 5).

The standard for most wood finishersthese days is polyurethane. While I don’thave anything against it (rub-on poly isvery easy to use), I have in general been try-ing to limit my exposure to nastier chemi-cals and complicated cleanup. I also wantedmy daughter to be able to help me with theproject (she was eight years old at the time),so found a product from Ikea called Be-handla, which is essentially a mixture ofbeeswax, linseed and tung oils, and a bitsafer to handle than other finishes. Ikeaeven lists it as food-safe. It also has a smellthat I find to be quite pleasant, goes onwith a clean rag, dries quickly, and gives thewood a nice luster without making it looklike it’s been encased in Lucite. So far it hasproven durable, and it can be easily re-freshed if need be.

After the pieces were sanded, I createda “bed of nails” out of some scrap plywoodand thin finishing nails (Image 6). Thiswas designed so that I could coat the entirepiece of wood and place it on the nails todry. I put five coats of the oil on the woodbefore I was satisfied with the finish.

I glued the rocker box together andheld it fast with clamps (Image 7) andglued the Ebony Star laminate to the bot-tom of the rocker box and altitude bearingswith contact cement. There was no getting

Image 6 - The author constructed a “bed of nails” for suspendingthe components as the newly-applied finish coats cured.

Image 7 - The rocker-box pieces are held in place with clamps asjoint glue sets.



around that nastiness! One standard tip forworking with contact cement bears repeat-ing: Coat both the Ebony Star and the sur-face it will go on with contact cement andlet both pieces dry. Then and only thenstick them together. You’re more likely toget good adhesion that way. I added screwsto the ends of altitude bearing strips (al-though one of them has since separatedfrom around the screw and is calling outfor more glue).

My only mistake was not spendingmore time squaring the rocker box. Thesides are not exactly parallel and this cancreate a little unevenness in the altitudemotion. A little time tweaking the Teflonpads and this flaw’s effect was minimized.Had I to do it again, I would work harderto ensure the sides of the rocker box wereperfectly parallel to one another. Live andlearn – and move on to other mistakes.

One of the best investments I made inspecialty tools (about $25, I think) was fora simple handheld pipe cutter. This is basi-cally a clamp with a set of wheels on oneside and a wheel blade attached to a screwon the other. Put it around the tube,tighten the clamp to engage the blade andspin it around the pipe, tightening theblade with each go-around. It’s a brilliant

little device and invaluable for trimminground tubing.

AlthoughDennis makes mirror cells ina wide variety of types, I had a universal cellfrom Compact Precision Telescopes left-over from another project, so I reused that.All I needed to do was drill three holes inthe base of the mirror box for the collima-tion bolts, and the mirror was ready to in-stall (Image 8).

The trickiest part of putting togetherthe telescope was adjusting the balancepoint. The altitude bearings need to be lo-cated properly on the side of the optical-tube assembly (OTA) and this took sometrial and error. I had drilled a series of holes1.0 inch apart on the wooden slats thathold the bottom of the altitude bearingsonto the mirror box (the upper parts arefastened to the poles with clamps, so areeasy to adjust).

Also leftover from a previous projectwas a curved-vane secondary spider fromProtoStar. This is a single-curve design thatyou don’t often see implemented on open-tube telescopes. I tried to make a go of itand found that a few L-brackets from thehardware store worked out nicely, provid-ing just enough tension and proper spac-ing. You can see the results in Image 9. The

curved-vane spider is one of my favoritethings about the telescope, neatly diffusingthe diffraction caused by the vanes to abarely perceptible haze around only thebrightest of objects. But to be honest, Inever much minded the diffraction spikescaused by the more-standard three or fourstraight vanes.

You’ll note also in Image 9 the angle-iron focuser board and a KineOptics HC-2 “Helical Crayford” focuser. I didn’t havethe turret assembly entirely worked out atthat point, but had theHC-2 lying around,so it did the trick in a pinch. By the way,the HC-2 is a very effective, super-light-weight focuser.

Shortly after I completed the scopeand had first light, I implemented the tur-ret (Image 10).

At my request, Dennis included hard-ware to connect my Sky Commander en-coders as well as a nifty post for mountingthe Sky Commander computer. I ran thecomputer’s control cables through the postto make it look a little neater. The scope’sbalance point was well below the altitudebearings, so I added a right-angle correct-image (RACI) finder on aTEC base to theupper tube assembly, along with the classicTelrad. Using ProtoStar’s awesome flocking

Image 8 - A universal primary-mirror cell from Compact PrecisionTelescopes (leftover from another project) was reused with theDobstuff kit. Three holes were drilled in the base of the mirror boxto pass the cell’s collimation bolts.

Image 9 - Also leftover from a previous project was a curved-vanesecondary spider from ProtoStar. L-brackets attach the spider tothe upper-tube assembly.



board, I crafted a baffle for the mirror boxand another for the upper tube assembly.

The Scope in UseFirst light was great fun. The scope

performed as I had hoped, and I had to sit

down to see through it comfortably! I dida test run of the scope’s portability duringa punk show at our theater a week or solater; the scope was easy to transport andset up, and movements were smooth. Inmy city’s terrible downtown light pollution,

I struggled to even find Saturn. A time-lapse movie of my setting the scope up andtaking it down is available athttp://www.youtube.com/watch?v=Sc0SyABRn3M.

A few weeks later, I packed the scope

Image 10 - An eyepiece turret from a Vixen refractor carries an as-sortment of eyepieces on the f/4.2 version of the scope.

Image 11 - A JMI Reverse-Crayford focuser was ultimately intalledwith the f/7 version of the scope. A Borg helical focuser providesultra-fine focus adjustment and serves as a focus-tube extension.



into my Versa for the long trek to Utah,and four days later I pulled into the parkto begin my month of service. The scopespent a lot of time in my car and withstoodthe bumpy roads well. One thing I learnedis that rutted dirt roads seem to have amag-ical effect on fasteners – I had to retightenjust about every screw on the scope.

During my first public stargazingevent at the park, I saw that my primarydesign objective had been met: childrencould look through the scope and almostnever needed any sort of boost. I waspleased. But then an opposite problem re-vealed itself: I had the scope set on Saturnand a lady who was a bit advanced in agemoaned audibly as she bent down to lookthrough the eyepiece, her knees making anoise that sounded much like an old boatcreaking on the water. Luckily she got backup again. I invited guests to use my ob-serving chair, but found that not everyonecould sit and stand easily; most refused touse it and simply bent over instead.

This flaw was entirely my fault. I hadpaid attention to the needs of young ob-servers while not considering the needs ofolder visitors and, moreover, failed to rec-ognize that I’d often be dealing with bothold and young in a single group.

Take TwoThis aside, the scope and I had a great

month in Utah. People had a good timeunder the stars and nobody’s knees actuallygave out.When I got home, however, I sawan ad on CloudyNights.com for a 10-inchf/7 mirror, and I jumped on it.

The three-pole design is very easy toreconfigure. In terms of hardware, I pri-marily needed a new set of poles and in-serts to lengthen the OTA. I had to forgothe nesting pole idea though for worry that1.0-inch poles might not be rigid enoughfor an OTA of such length.

The longer focal length and resultingOTA also introduced a different balancepoint, and suddenly I needed extra weighton the bottom end. This was solved by at-taching a 12-volt battery to the mirror box

and lightening up the upper tube assembly,and that meant removing the RACI finderand the turret. I also added a spring ten-sion on the side opposite to the DSC-en-coder arm; I’ve used such springs for yearsto increase the friction a little bit on the al-titude axis, and this one provides just theright feel. In order to keep the new OTAas short as possible, I choose a higher-pro-file focuser. My solution was to use a JMI1.25-inch Reverse-Crayford focuser (RCF)with the Borg focuser and 1.25-inch nose-piece acting as both fine-focus control andfocus-tube extension.The RCF-mini, as it’sknown in the JMI catalog, is a brilliant lit-tle piece of engineering, and I can see whythey are popular with ATMers makingsmaller-aperture travel scopes; well-made,precise, smooth, tiny, and lightweight(Image 11).

The original Meade 10-inch f/4.2 pri-mary mirror seemed to be quite good. Mysecondary was undersized and I believe thatled to a bit too much light fall-off, and Ifelt therefore that the scope never delivered

to its full potential. The new longer mirror,which came originally from APM in Ger-many, changed everything: It is one of themost stunning optics I have had the op-portunity to use. So my shorty, deep-skykid’s scope has become a great planetaryscope that even some adults need a littleboost to look through when at zenith(Image 12). The best-laid plans!

The longer OTA really helped me testjust how far the three-pole design could bestretched.The f/4.2 version showed almostno flexure at all, but would the f/7 versiondo? I placed a laser collimator into thescope with the OTA pointed close to thehorizon, collimated it and then raised itslowly up to zenith, all the while watchingthe laser centered on the primary and fold-ing back on itself. Any flexure of the struc-ture would show as the laser drifting off. Ifit did drift, it was not noticeable. My con-clusion: The three-pole design is VERYrigid! I have seen it with my own eyes andI am a believer.

The extra length and weight were also

Image 12 - The f/7 version of the scope issignificantly longer than its initial f/4.2configuration, but remains sufficiently rigidto firmly hold collimation.

Image 13 - The author’s daughter assistedin applying the Behandla finish to the woodcomponents and in assembling the com-pleted scope.



a greater challenge on the basic engineer-ing of the scope. I’m happy to say it re-mained very smooth, precise, and eventhough there was slightly more settle-downtime at the eyepiece than with the shorterversion, I’d still call its performance quitegood. I’m not sure to what extent the ure-thane pads are contributing to this, al-though I can say that, especially with themuch longer OTA swinging around, I ap-preciate the larger footprint the pads pro-vide, which makes the scope much lesslikely to topple over.

As mentioned earlier, I work for a col-lege and had a chance to try out the f/7 ver-sion of the Dobstuff scope this fall whilehelping out with the introductory Astron-omy classes’ observing nights. While notchildren, the students all had to use astepladder at some point or another, but itwas just a step or two, and all of them didfine. I’ll be back to spotting the littler kidsonce the star-party season starts up again,but they’re far less damageable whendropped, and the adults will not be secretlycursing me. Indeed, they’ll be too busylooking at all that detail on Jupiter.

ConclusionI couldn’t be more pleased with the

versatile Dobstuff kit. Dennis Steele is anexcellent craftsman, and his design is wellthought out, very sophisticated, and repre-sents what I think is one of the best bangsfor the buck out there in Dobsonian tele-scopes at present. I would recommend hiskits without hesitation for anyone with abeginning-to-intermediate level of woodfinishing skills. My 8-year-old daughtereven got involved in the fun of putting it alltogether (Image 13).

The bottom line: in my mind, thethree-pole design used by Dobstuff oftensupplants its more complicated kin andrepresents a happy balance between porta-bility and stability. I recommend the de-sign, and I recommend Dennis Steele’sDobstuff as a source for great information,inspiration, parts, kits, or even a completetelescope makeover.



dobstuff’s lightweight dobsoniankits dobstuff.pdf · 62 astronomy technology today dobstuff’s...

Documents