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18" f/4 Truss Dobsonian Telescope

complete.jpg (36928 bytes)


Construction Progress:


I've been working on the baffling and finally have gotten some of it documented. I started with the focuser and worked my way down the optical path. I chose to implement all of the large baffle pieces out of corrugated plastic sign board (coroplast) which is impervious to dew. All of the baffles snap or velcro in place in a couple of minutes. I determined the size and placement of the baffles by just looking through the focuser and getting rid of what didn't belong! 

I learned the following trick from Tom Osypowski and added my own twists to it along the way. To make the problem areas more obvious I pointed the scope into the garage with the primary end facing the bright concrete driveway. I draped the UTA with a dark cloth then looked down the draw tube to see where the problems might be located. I then put in the Paracorr and the Nagler31mm eyepiece and examined the exit pupil both naked eye and with a magnifier. With the UTA draped and the primary end facing bright background you can quickly see what needs to be worked on. I did this at a variety of altitude settings to make sure I wasn't missing anything. Once all this is finished I'll repeat the process outside with the scope facing the north sky to see what else needs work. I may need to flock the baffles after reviewing the results to improve veiling glare such as might be expected while trying to examine a dim object near a bright one just outside the FOV.

iris open.jpg (35761 bytes)Here's a couple of pictures of the iris diaphragm I'm using as a focuser baffle: one open to the full width of the drawtube and one partially stopped down. I got the diaphragm at Edmund Optics. Mine is the aluminum one with the 60mm bore.


iris partially closed.jpg (41569 bytes)In use, I close down the iris with the little tab visible at the bottom after focusing until the edge of the field appears to dim. You can see the contrast improve and then the field just becomes dimmer.


primary back.jpg (29893 bytes)I made a primary baffle system out of coroplast as well. Two pieces snap in place under the primary.  They cover the cell to the edge of the primary and leave the back of the primary open. These pieces are about 1.5" from the back surface of the primary allowing lots of airflow. You can also see why the next thing to do is to put flock paper on the azimuth plate! I'm actually going to flock a piece of coroplast and just set it in place as part of the setup routine.




primary top.jpg (31756 bytes)Here's a view from the top. Also in this picture you can see the little piece of EPP under the acrylic top which allows the mirror to breathe.


backend.jpg (42374 bytes) Three coroplast sides fit on top and are held in place with velcro. The height was empirically determined by shining a small LED while raising it. The required height was nearly the same as the end of the trunnions so I just added a little to bring it to the top. The truss poles, trunnions and the mirror cell hold the pieces in place with a few bits of velcro. If you're not using a shroud you might be surprised at how much of the ground you may be looking at... In this view you can also see the 80mm ball bearing fans I'll be using to cool the primary. 

dsc side baffle.jpg (31297 bytes)This shot shows the focuser side of the primary baffle and the DSC encoder mount. I used a piece of aluminum angle stock fit with a piece of plastic. A 1/4" hole through the aluminum and plastic snuggly fits the encoder shaft which turns freely on its ball bearings. Consequently the angle stock is easily held to the trunnions with velcro and removable for transport. The tangent arm is similarly secured with velcro. You can also see the top of the azimuth plate which will next be covered with flocking paper. A small corrugated lip, also flocked will be added to the "front" of the azimuth assembly (bottom right of the picture) to complete the baffling there.

inside with fans.jpg (37771 bytes)This view of the inside shows the fans mounted on the baffle. You can also see the acrylic cover over the primary and one of the polycarbonate/velcro hold-downs securing it to the primary. I keep the acrylic cover slightly lifted from the mirror by use of a expanded polypropylene shim to allow ventilation.


primary cover holddown.jpg (24854 bytes)Here's a close-up of one of the hold-downs. The fuzzy side of the velcro protects the mirror from the polycarbonate angle stock (wall edging) and wraps around the primary and mates with a piece attached to the bottom of the primary. Three hold-downs are used to secure the acrylic top. Works great!




uta.jpg (34843 bytes)Here's the UTA with everything in place.




secondary baffle.jpg (51858 bytes)I made a new secondary baffle to replace the foam core one. The foam core works but it isn't very rugged and is getting beat up looking. I made the new one out of 4mm thick corrugated plastic board used for signs; Coroplast is one brandname you might find. I cut it using the foam board as a template. Then in order to allow it to bend to match the curve of the bicycle rim, I slit it lengthwise along every fifth corrugation. I washed it well to remove any mold release then sprayed it with 3M-77 and allowed it to dry to touch. I then applied the flock paper that I had originally used on the foam board. To hold it to the bicycle rim I used the fuzzy part of a velcro strip which I applied to the backside then slit where it crossed the same corrugation cuts. The bicycle rim has the hook part of the velcro applied on the inside. Works great and should last a lot longer. Actually stiffer as well. 


hub detail 1 smaller.jpg (46359 bytes)A few pictures added to show some secondary details. The hub is a length of 3/8-16 threaded aluminum rod with plates at the top and bottom to terminate the wires. Here's the one at the top but shown upside down in the alignment fixture.

secondary adjustment detail smaller.jpg (44901 bytes)The bottom hub along with the secondary adjustment plate. The thumbscrews are ordinary cap screws with plastic push-on knurled tops.


hub alignment smaller.jpg (59527 bytes)The secondary alignment fixture with a couple of pieces of aluminum in place to provide right angles to align the hub axle against. . You can see the three mounting ears where the binder clips attach the truss pole pairs to the secondary ring. (The focuser support legs hadn't been finally sized and cut when this picture was taken and the focuser was being held by c-clamps.) 

binder clip detail smaller.jpg (43133 bytes)The top of the truss poles with the binder clips.


binder clip detail 2 smaller.jpg (47284 bytes) Here's another angle of the binder clip. 


truss mounting ear smaller.jpg (47945 bytes)The jaw of the clip just opens and the head of the 1/4-20 bolt is inserted into the mounting ears  on the ring. Works great!


primary cover smaller.jpg (51753 bytes)Here's a picture of the primary cover made of 1/2" acrylic with some ears made from polycarbonate plastic corner edging.  They're stuck to the acrylic top with exterior grade double sided foam tape. The cover just sits on the outer edge of the mirror. I've since added some velcro hook to the back of the mirror and velcro loop straps to the inside edge of the ears to keep the mirror cover in place when it is carried. I plan to cover it with some sort of white material.


Tore down the SpiderWire spider yesterday and could find no other explanation for lack of tension other than it stretched under continuous load. Replaced it with 0.020" unwound guitar string, added the split foam earplug dampers and checked the performance last night. Vibration wasn't eliminated but much reduced. I'm going to try increasing the tension to reduce it further. Lots of thin overcast moved in after midnight, bummer...

9-7: First Light!!!

Last night was first light for my 18" f/4 DOB and it was a great success! Here's the pics:

Jim Miller 16 closeup smaller.jpg (154140 bytes) Jim Miller 16 closeup2 smaller.jpg (159127 bytes) Jim Miller with 16a smaller.jpg (217522 bytes) 

I arrived at 5:30pm at the SVAS monthly star party at the Blue Canyon site. I set the scope up very easily by myself with no hitches just as planned. The only tool used is a allen key used to install the four bolts holding the two trunnions to the mirror cell; hard to forget only one tool. The whole scope breaks down into very manageable size and weight pieces and fits neatly in the trunk of my car, save for the poles which ride in the passenger compartment. As it began to get sufficiently dark to see the laser image I did a quick collimation of secondary and primary with the X-Y grid provided by the Lasermax collimator.

Shortly before sunset Tom Osypowski arrived, took the above pictures and helped me rough align the equatorial platform. I had been fumbling around trying to align the finderscope against some trees in the distance when Tom suggested using Venus! Much better idea! <grin> (Now you know why it took me six months to design the mount...and another six to build it.) By that time Polaris was visible and we did a quick tweak on the platform alignment then turned on tracking. With tracking enabled we could now jack up the power on Arcturus in the still bright sky to do a fine tweak of the primary collimation. Result was the the scope was ready to go as soon as the sky was.

Lots of people passed by to ask questions about the scope and the platform as we were setting up and it was a lot of fun answering them. Items of interest: nice integration of platform and scope, complexity and cost of aluminum construction (quite low actually), bicycle rim upper stage, minimal secondary holder, binderclip secondary rim attachment, open structure (no tube or shroud), CF truss poles, smooth movement (powdercoat on teflon), foam core board baffle. Tom got quite a few questions about the platform and I think he might get additional platform order or two down the road as a result. ;-)

My wife and daughter are not (yet) astro-nuts but came up for about an hour or so to see the scope and the stars and it was great to have them there to share the experience. They've put up with a lot of scope technobabble and distracted mumbling by me over the last year and I'm grateful for their patience.

The evening was a real treat for me as I had never spent more than a few minutes at anything larger than an 8" DOB and most of my viewing has been done with my Meade 5" newt. Tom suggested and found several targets that would show off the optics such as the Swan, Lagoon, Dumbell and Saturn nebulae. We also looked at M13, M15, M51, M31 (and neighbors) which all looked beautiful and many other objects which I've forgotten since last night. (Next viewing equipment will be an observation plan and a small tape recorder...) I never knew planetary nebula were robin's egg blue until last night...they were all dim smudges in my previous scope experience!

Optics proved excellent: Swayze mirror and Galaxy secondary. Plop designed 18 point cell and RTV'd optics worked great with no astigmatism ever observed. I definitely need to get more power than my 5mm provides; Santa now has my request for a 3-6mm zoom. The Nag16mm is a great finder and nicely frames the Swan and lots of other objects. The 31mm w/ Paracorr was wonderful on M31 and companions and is generally a treat to just sweep the Milky Way and see beautiful rich pinpoint fields from edge to edge.

Tracking was awesome: Osypowski builds a mean machine! The usage model of hand-moved star hop (DSCs to be added later) and let the platform keep it in view really pleases me. The unusually low CG of the scope allowed Tom to build in 2+ hours of tracking into the platform which is very nice. The low eyepiece height meant that I (5'9") never took more than two steps up the little ladder even with the platform underneath and tilted away.

About the mechanics:
The unsupported trunnion noses posed no problem at all. They are substantial and combined with the lightweight of the scope result in no observable vibrations.

The Moonlite ball and socket pole connectors at the bottom, although not designed for that location, worked great: no tools required!

The binderclip connection to the secondary rim worked great with no slop and no tools.

The powdercoated, cast aluminum trunnions (Obsession) on teflon pads spaced at ~90* provided a firm yet very smooth feel: no stiction whatsoever yet no movement while changing eyepieces. Still need to add some ballast for the Nag31 though...

Setup and takedown is really nice with the heaviest piece being the mirror/cell at 40lbs and only four allenhead bolts to secure the trunnions to the cell. Everything stores neatly in the car as planned. Even though really tired last night at 2am takedown was very easy and uneventful. Total weight of only 100lbs including platform makes this easy on the muscles.

Baffle made from black foamcore board (steamed off the paper) and attached with velcro works great opposite the focuser: light and stiff.

Thumbscrews on the secondary adjustment and big disk primary collimation knobs (homemade) on 3/8-24 threaded rod make for very smooth collimation.

The only flaw in the mechanics is the secondary rotational oscillation. Even after some work on damping and a settling time of under a second it is still annoying to have the secondary start shaking the image, particulary when working above 300x and trying to fine focus. This will be the first thing to be worked on.

Things to do as a result of first light findings, aka peeling the onion:
Fix the secondary oscillation, finish the baffling (focuser, primary), add another finder (one is not enough).

Things not yet started:
DSC installation

Many thanks to Tom Osypowski for his generous assistance in construction advice, design feedback, vendor suggestions and loan of his milling machine. And of course his beautifully crafted equatorial platform and wonderful help in making this a successful first light!

Many thanks also to Bruce Sayre whose website, CAD ideas and email exchanges got me started down the aluminum, minimalist path and believing I could actually do this.

7-8: Got everything put together at the primary end. It feels good to get here finally. Here's some pics:

bottom complete smaller.jpg (58409 bytes)Here's the full assembly. 


bearing smaller.jpg (55979 bytes)Just the bearing on the Equatorial Platform 


platform smaller.jpg (62368 bytes)Tom Osypowski built Equatorial Platform


back detail smaller.jpg (53802 bytes)Back view of primary end.


pad detail smaller.jpg (45797 bytes)Bearing detail.


corner detail smaller.jpg (50034 bytes)Corner detail showing Moonlite truss connector with modified knobs


top detail smaller.jpg (45519 bytes)Top detail showing truss pole collar and extra knobs added to handle tension.


secondary bits smaller.jpg (69451 bytes)Now working on secondary. Shown are truss poles, JMI focuser, bicycle rim and piece of MDF to do the fixturing.


I've gone to plan B on the secondary and using a bicycle rim to save time at the cost of a bit more weight.  I was also concerned about the hot CA sun softening the epoxy on the CF/nomex structure.  I'm going to build it up and test it anyway after the scope is working. Update: Per George Sparr, ACP owner, cure temperature for both truss poles and CF/nomex plate is 250F so there should be no problem in sunlight or even in a hot car.

4-13: Cell support was welded about six weeks ago and came out quite nice. Cell parts were fabricated using a miter saw and fine tuned with a belt sander. Collimation bolt holes and inserts were fabricated before welding. Cell support was jigged (forgot to get a picture) on a piece of MDF. 

cell support top.jpg (35740 bytes)Picture after welding shows temporary bolts to protect holes during welding.


cell support.jpg (39143 bytes) Another view of the welded assembly.


azimuth fixture.jpg (36913 bytes)After measuring cell support the azimuth assembly was similarly cut and jigged.


azimuth welded.jpg (37857 bytes)It came out satisfactorily from welding. Note that corners are nicely shaped due to sanding back at the joints and the welder doing a fill and grind.


corner prep.jpg (36197 bytes) Example detail before welding.


trunnion stalk sizing.jpg (57793 bytes)Now working on cutting the stalks to mate with trunnions.


template detail.jpg (34497 bytes) I made a template from plywood to get the proper dimension to allow for the teflon, its support plate and the laminate.


compression wedge.jpg (32140 bytes)Here's a picture of one of the compression wedges that will allow tuning of the teflon bearing position.


secondary material.jpg (42510 bytes)Also did some testing of the secondary material edge finish. You can see the honeycomb and the spackle filler in this pic 


1-15: Mirror Arrived! Steve Swayze met his ship date within 1 week! Thanks Steve!

Design Changes

4-13: Replaced delrin plugs in stalks with compression wedges to hold teflon. Simpler to deal with, much cheaper and easy to make.

1-9: Focuser position TBD.  All six truss poles shown.  Conceptual spider info now shown.

1-1: 24" Obsession trunnions instead of homebrew.  Mirror support now 1.5"sq x 0.125 wall tubing throughout.  Trunnion saddles now 1.25x2.5x0.125 stalks welded to 1.25sq x 0.125 wall tube box. [trunnions not yet shown as half circles.]  Outer wall of stalks left higher to provide lateral support to trunnions as platform tracks. 

11-28: Removed T-nuts holding primary; primary will be RTV'd directly to cell triangles.  Spherical bearings in cell will be epoxied into bar rather than press fit.  "Box" incorporating saddle bearings will now be a welded assembly rather than screwed for simplicity of construction.  ALT encoder will now be direct drive rather than friction drive. Secondary ring edge finish changed.

10-18:  Replaced glued nylon plugs at truss tube ends to 1.5" AL tube sections glued to outside of tubes and umbrella connectors.  Simpler, same weight, repairable, no machining, better adhesion.

9-?:   Changed from spring latches to binder clips for secondary ring to pole attachment.


I chose 18" f/4 to keep the eyepiece height around 6' or less at the zenith.  I'm 5'9" so that should work fine.  I chose early-on to use composite materials for the truss poles (3/4" carbon fiber (CF) from ACP) and secondary ring (1/2" thick CF/Nomex honeycomb from ACP) to minimize secondary weight.  This helps minimize the need for counterweights.  To ensure transportability in any car the trunnions are made removable.  Finally I chose a Tom Osypowski Equatorial Platform after review of the guiding and tracking alternatives.  Although the scope could sit on an ordinary ground board the design is intended to integrate directly with the TomO EP.

I borrowed heavily from designs and information from Bruce Sayre, Greg Babcock, Dave Sopchak and Tom OsypowskiThe Dobsonian Telescope by Kriege and Berry (K&B) was used as a technical reference throughout.  The ATM list has been a valuable source of information.  Thanks to all!

Here's a few renderings of the scope so far.  

Perspectiveperspective.jpg (136433 bytes), Toptop.jpg (200108 bytes), Frontfront.jpg (89668 bytes), Sideside.jpg (73815 bytes), Tilted45.jpg (162642 bytes), Horizontallaid down.jpg (118386 bytes).  

This was done on DesignCAD 3DMax to see if there are any interference problems (none found...yet).  Here's the DesignCAD file.  Upperspace has a free DesignCAD viewer here.  

For a 3D view

If you'd like to see it in 3D here's a VRML file of the design at azimuth. Here's one at 45°. Instructions for obtaining a free VRML plug in posted below.

Cell and Primary

The primary will be a 18" f/4 1.625 thickness with standard coating from Swayze Optical, Inc.  1.625" was thickness chosen to keep mirror/cell assembly weight down.

The cell support structure is the central element of this scope.  It is the item to which all of the Optical Truss Assembly (OTA) is directly attached.  Although this comes at the cost of some loss of compactness it allows for an OTA which is independent of rocker (or other) stability concerns and allows the design to be extremely simple to fabricate.

The cell support (red) is a standard design per K&B using 1.5"x1.5" square AL tube (0.125 wall) ladder rungs welded to 1.5"x1.5" (0.125 wall) tubular side rails.  The cell is a 18 point design based on PLOP output.  The 3/8" AL cell bars [not yet shown] will have epoxied-in stainless steel (SS) spherical Teflon-lined bearings to allow movement of the bars and triangles.  The 3/16" AL cell triangles will provide an RTV mounting surface for the mirror; spacers will be used to ensure ~1/8in RTV thickness at each apex.

Collimation is provided by 3/8"-24 collimation bolts threaded through the bottom and middle ladder rungs (K&B) and supporting cell bars on spherical bearings (Sayre).  Aluminum plugs will be inserted into the rungs before welding to provide additional thread meat.  Locknuts will capture the spherical bearings on the collimation bolts yet allow sufficient angular motion.  Cell support side tubes similarly plugged will be drilled, tapped and fitted with SS threaded inserts for repeated rocker attachment/removal.  The top rung of the ladder will only be used to carry the third truss mounting block.

Truss and Connections

The truss is a CF six pole design (Babcock, Sayre) selected for weight and stiffness.  The 3/4" ID x 0.030 wall (72" raw, cut to exact length at assembly) truss poles are mounted directly to the mirror cell using Moonlite ball and socket truss connector blocks.  The truss blocks are at 120 degree spacing and moved out from the primary sufficient to ensure that the truss poles don't violate the optical path (they don't, whew!).  

The secondary end of the poles will be joined by offset angle brackets and mated to the secondary ring (yellow) with binder clips ala Sopchak for tool-less assembly.    Truss blocks can be shimmed if necessary for focuser positioning.  Hardware attachment at either end of the CF tubes is done via 1.5"long 1"ODx.049"wall AL tube sections epoxied to ends of CF tubes and fitted with tube inserts (Obsession Telescopes).  Poles will remain paired at the secondary apex for ease of assembly, disassembly and storage.


The trunnions are 24" diameter semicircular arcs cast in aluminum from Obsession Telescopes.  These will be milled on the outer surface to 1.062" width from their rough width of 1.25".  The extra 0.062 provides clearance between the saddle bearing wall and the cell support on each side.  The large 24" diameter was chosen to allow the center of rotation to be coincident with the estimated CG.  Subsequent CG calculations with the trunnions included indicate an estimated CG slightly below the axis of rotation for resulting in a positive margin at the secondary.  Since the trunnions are designed to be removable their large size will not present a problem for weight or storage.  

The trunnions will be attached directly to the cell with 1/4"-20 FH machine screws during setup.  A stop will be provided to prevent motion past vertical.  Provision will be made on each axis for parking.

The trunnions' bearing surface will be covered by laminate.

Provision will be added to the side of the cell for attachment of additional counterweights if necessary.

Saddle Bearings

The trunnions are supported by four stalks (dark blue) rising from the azimuth plate stiffeners.  These stalks are 1.25"x2.5" (0.125" wall) rectangular tubes and are welded to the 1.25x1.25 (0.125 wall) square stiffeners which are welded to form a box below.  Each stalk's outer wall is left higher than the remaining stalk and provides a lateral stabilizer surface for the milled trunnion surface to ride against.  This wall will be lined with 0.125 thick delrin.  Expansion plugs fabricated from 1" AL U-channel and 3/4" square AL bar tapped for 1/4-20 draw screw are inserted into the top of the stalks to allow positioning of the bearing. The 1/4-20 also passes through a 1.125" square 1/8" thick AL plate upon which the teflon is applied.

Teflon pads are sized to achieve K&B recommended 15PSI.

Care was taken in choosing the saddle bearing height to ensure the primary does not collide with the AZ encoder!


This scope's AZ plate 23.375"x23.375"x1/8" thick (orange) will sit directly on a Tom Osypowski Equatorial Platform.  The AZ plate will have a central 1/2" hole to engage a 1/2"dia pivot pin and ride on three Teflon pads centered on a 9.5" radius, all provided by the Platform.  The Platform provides a declination axis for fine tracking which has +/-1/4" of movement by moving one of the three pads.  The plate will be made from 1/8" aluminum since the low supported weight (~75lbs) and 1.25" sq tubing stiffeners to which it is welded should ensure adequate stability.

Secondary Ring

The secondary ring 24"ODx20"IDx1/2" thick (yellow) will be fabricated from a 24"x24" prefabricated CF reinforced Nomex honeycomb plate from ACP using a carbide bit router.  Hard points required in the plate will be made by drilling from one side of the plate, relieving the honeycomb underneath the edge of hole and backfilling with glass fiber filled epoxy.  Edge treatment will be with Red Devil spackling sanded and covered with a thin layer of epoxy and painted.  

Focuser mount consists of a 1.5" AL angle bracket to which is welded two pieces of 3/4" angle.  The 1.5" piece mounts to the secondary ring and the 3/4" pieces are separated to match the width of the focuser.  The focuser mounting screws pass through a 1/16" aluminum plate and then into the 3/4" angle pieces. The 1/16" plate provides a reaction surface for the focuser alignment grub screws.

A Sayre designed three vane spider and hub will support a 3.5" secondary mirror from Galaxy Optical.

The focuser mount  will be located TBD° up from horizontal for more comfortable viewing near the horizon.  The bottom of the focuser mount  will pickup (method TBD) a pole of the nearby pair to provide additional focuser stability.


DSC will be Sky Commander which provides DOB Equatorial Platform support.  AZ encoder fits directly in pivot pin provided by the Osypowski platform.  ALT encoder will be mounted on a piece of angle AL which will bridge one of the trunnions thus allowing the ALT encoder to be mounted at the center of rotation.  Another piece of angle fixed at one end to the encoder body and at the other end to the saddle will provide a reference.  The reference arm and encoder will remove for storage and transport.


Primary cooling if necessary will be provided by fans mounted on the sidewall of the mirror cell.  Battery will be mounted underneath the primary on supports on the rungs.


Baffles will be provided for the primary, opposite the focuser and at the base of the drawtube.


For travel the azimuth plate will ride on the Platform and the mirror cell will nest inside the sidewalls of the azimuth plate riding on blocks of EPP as a cushion.  This will provide a compact, low height assembly.  A mirror cover will be provided.  Trunnions ride separately in the trunk and the truss poles will ride in a bag in the passenger compartment.


Here's the Excel spreadsheet with some estimates for the weight and balance as well as Teflon pad size estimates.  [needs updating for obsession trunnions and tubing replacing plate]  Free Excel viewer from MS.

Feedback wanted!

I'd like to start cutting aluminum by Jan 14, 2002 and I'd love some feedback.  Please send all feedback to jim at jtmiller dot com.  Thanks in advance to all!

VRML Viewer

If you need a VRML viewer for your browser go to Cortona.  Click on the link to go to the installation page then follow the instructions for "Manual Installation".  It is less hassle for most people.  Make sure to also load the VRML 1.0 converter after you load the Cortona plug in.

Once the viewer is installed you should be able to click the Telescope link above and after a bit of a wait some of the scope should come into view.  Hit the "Fit" button in the lower RHS to fit it into your screen.  Hit the "Study" and "Rotate" buttons then click and drag to rotate the scope.  Be aware that if you have a joystick attached to your computer you may be getting input from it which causes the scene to move on its own.  Either null the joystick inputs or just use the joystick to fly around instead of the mouse.