Introduction: 10-inch Dobsonian Telescope
This instructable documents the design and construction of a 10” reflector telescope. It is not intended to be a complete step-by-step instruction set, but rather to document some of the design details and methods that were used.
There are a lot of online sources filled with good information on how to build a basic Dobsonian telescope. This design had a few additional requirements that made it slightly different than most “standard” Dobsonians; it was going to be used by children, so it needed to be durable, extremely stable, as well as low enough to the ground for kids to look through the eyepiece, ideally without need for a step-stool.
My hope is that other builders can benefit from or improve upon these designs. As a disclaimer – this was my first build of a telescope, and I am not a professional optical designer, so I don’t purport to be an authority, but I have successfully seen the project through to completion and can report that it works!
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Step 1: Optical Tube Assembly
The optical tube assembly is comprised of two major components, the rocker box and the tube. The rocker box is constructed of ¾” plywood, and supports the primary mirror cell as well as the altitude bearings. It is equipped with a hinged rear panel (the “tailgate”) that allows installation of the mirror cell, as well as access to the mirror when needed.
The tube itself is a concrete form tube, made from 0.095” thick cardboard. The tube is reinforced with a series of plywood baffle rings on the interior, as well as a plywood focuser support and the secondary mirror holder.
Looking in detail at the rocker box, there are several non-obvious features to the design. The tube itself is held into the rocker box by a pair of semi-circular clamps; the lower half of each clamp is glued and screwed into the rocker box, while the upper half of each clamp can slide in a pair of dados cut into the interior of the box, and is secured with a pair of threaded rods and nuts. This design allows for the tube to be rotated to re-orient the position of the focuser if desired, and also allows for fine-tuning of the position of the focuser relative to the mirror. The tube has a total of +/- 45mm adjustment from a nominal design position, which was intended to be enough to cover any errors in calculating the nominal position of the focuser.
The focuser itself is a 2” Crayford design. While many Dobsonian designs secure the focuser directly to the tube with threaded fasteners, this design uses a plywood support structure inside the tube with captive T-nuts. This has two purposes: first, it provides additional structural support to the focuser, so that in use if the focuser is bumped, grabbed, or otherwise abused, it is unlikely to damage the cardboard tube itself. Second, it makes installation of the focuser easier, as the captive T-nuts allow for all installation to be done from the outside of the tube – there is no need to reach into the tube to hold a nut and washer while driving the screws that secure the focuser.
In a similar fashion, the secondary mirror spider is supported by a plywood ring structure rather than being screwed directly to the tube. This provides a very rigid support for the mirror and prevents vibration when the scope is in use, prevents the possibility of the spider deforming the tube when the attaching fasteners are tightened, and ensures that the secondary mirror is reliably located in the center of the tube.
The baffles inside the tube are simple plywood rings, secured to the tube with “plugs” of epoxy. This was done in lieu of screws for two reasons: first, to avoid the possibility of splitting the plywood rings, as screws would go into the edge-grain of the plywood; second, to avoid the necessity of having numerous exterior fasteners on the tube, which would detract from aesthetics and have a tendency to snag on clothing during transport of the tube. During construction, the rings were first positioned individually, then a series of eight ¼” holes were drilled through the cardboard of the tube until the wood surface of the ring was exposed. A small amount of epoxy resin was then applied to the hole, bonding to both the ring and the tube. When cured, the epoxy was sanded flush with the surrounding tube, producing a strong, vibration-resistant joint. The only downside to this construction method is that the rings are permanently bonded to the tube and cannot be removed or serviced, but this was considered an acceptable tradeoff, as it seems unlikely that this a need for removal would arise. The interior of the tube was painted with matte black paint, after first being scuff-sanded and then primed with a layer of shellac. This was done because the plastic moisture-barrier on the interior of the tube is fairly smooth and slippery, and it seemed unlikely that latex paint would bond to it reliably. Shellac tends to bond well to almost anything, and provides a good sealer/primer layer for subsequent painting.
The tube was primed and then painted with ordinary latex semi-gloss white paint, to ensure that it would be easily visible in the dark, and to provide a layer of protection for the cardboard should it be exposed to moisture. A future upgrade will include wrapping the tube in white self-adhesive plastic drawer liner (“Con-tact Paper”) to provide a more robust moisture protection surface. The exterior of rocker box was similarly painted with semi-gloss black latex paint, and provides a nice contrast to the white tube, while the interiors was painted with flat black to reduce the possibility of internal reflections.
Step 2: Primary Mirror Cell
The primary mirror cell was constructed from ½” Baltic birch plywood. This was the only component on the scope that used this premium material, as it was deemed important to ensure dimensional stability in the mirror cell.
The cell is a nine-point design, with the locations of the support points calculated using the open-source “GUI PLOP” software. The triangular supports ride on spherical bearings which were press-fit into counter-bored holes; these bearings in turn are secured to the base structure of the cell with hex cap screws and loc-nuts. The use of a nine-point cell and spherical bearings is probably overkill for a 10” mirror, but ensures that the mirror is evenly and reliably supported.
The base of the cell is anchored to the rocker box with three 3/8” bolts that pass though springs and are secured with the collimation adjustment knobs. The springs are located in shallow counter-bores to keep them concentric with the bolts and ensure that the cell is supported evenly.
The actual support points where the mirror is bonded to the cell are ¼” dome-shaped stick-on silicone rubber pads, usually used to keep lamps or other objects from scratching furniture. These pads were surrounded with a ring of clear RTV silicone, and the mirror was then set on the supports while the RTV cured. The nine contact points were judged sufficient to support the nine-pound mirror without the need for a sling support, but three retaining lugs were added to eliminate any possibility of the mirror becoming free from the cell should the silicone adhesive fail. The support lugs are lined with pool-table felt to ensure that they don’t scratch or bind on the edges of the mirror.
All parts for the primary mirror cell were cut on a CNC router for expediency, but could be readily duplicated using non-CNC equipment if needed.
Step 3: Secondary Mirror Holder and Spider
The secondary mirror spider was constructed from steel. The four rectangular vanes are welded to a central segment of tubing, into which an aluminum reducer bushing is press-fit. The rectangular vanes are affixed to a plywood support ring that fits into the optical tube. This arrangement is designed to maximize rigidity and prevent vibration of the secondary mirror when the telescope is in use. Because the rectangular vanes are welded to the center tube, they do not need to be held in tension to maintain their position, which makes mounting and positioning easier.
The mirror holder is fabricated from a central piece of 3/8” threaded rod, to which are affixed a pair of circular plates that function as a tilt-adjustment mechanism. The upper plate has three riv-nuts arranged in a triangle pattern with ¼” hex cap screws threaded through them, which press against the lower plate. Both plates were cut using a hole-saw from galvanized steel electrical-box covers.
Both plates are secured to the threaded rod with a pair of loc-nuts and a small spring, which keeps the lower plate pressed firmly against the tips of the three adjustment screws. A small section of PVC drain pipe, cut at a 45° bevel, is bonded to the lower plate using JB-weld epoxy. The secondary mirror is secured using clear RTV silicone, and includes a “safety leash” made from mason twine to prevent the secondary from falling onto the primary mirror in the event that the RTV silicone bond should fail.
The mirror holder is secured to the spider with a pair of loc-nuts. Adjustment of mirror position along the length of the tube is done by adjusting the position of this pair of loc-nuts. Rotational adjustment is facilitated by a small flat machined into the end of the threaded rod, which allows it to be gripped securely with a small wrench; loosening the top nut slightly allows it to be rotated and held in the desired position while the nut is re-tightened. In application, the secondary mirror has only needed to be adjusted once during initial setup and has not moved or required re-adjustment since then.
Step 4: Base and Bearings
The base supports the rocker box and enables the telescope to pivot, as well as providing a mechanism for positioning the scope accurately with respect to azimuth. It is constructed from the same ¾” plywood as the rocker box.
Both the altitude and azimuth bearings are made from FRP panel and teflon pads. The upper (moving) portion is secured to the lower (stationary) portion with a simple elevator bolt through a bronze bushing and a loc-nut. The lower portion of the base has three rubber feet constructed from hockey pucks, secured to the plywood base with wood screws and washers in counter-bored holes in the pucks; these serve to provide three-point support to the base which prevents wobbling, and also isolate the plywood base from the ground to avoid exposure to dirt and moisture.
The lower portion of the base incorporates a plastic ring that rides in a small channel routed into the base. This ring was cut from the same FRP panel as the bearings, and includes angular markings in single-degree increments. The ring can be rotated by sliding in its channel; this allows it to be “zeroed out” by first aiming the scope to a target with a known azimuth coordinate (such as Polaris) and then sliding the ring until the reference mark on the upper portion of the base aligns with the corresponding coordinate on the ring. This enables the scope to be placed on the ground arbitrarily (i.e. without aligning to any particular reference) and then zeroed quickly. Because the ring is fairly large (approximately 24” in diameter), the space between single-degree increments is fairly large (around ¼”), making it is relatively easy to aim the scope in the azimuth direction accurately.
One future improvement for the design will be the addition of a vernier scale instead of a single tick-mark reference on the moving portion of the base. This will make accurate azimuth positioning down to 0.1° possible without need for electronics or other aids.
Step 5: Accessories
The rocker box includes a steel carrying handle positioned around the CG of the scope to allow it to be picked up easily. Because the tube assembly is fairly large and also heavy, a pair of steel anchor loops and a nylon should straps are affixed as well, which in practice make moving the tube assembly significantly easier.
The tailgate supporting the primary mirror cell is held in the closed position with a pair of rubber latches, typically seen on picnic coolers. These keep the tailgate closed and under tension to prevent vibration or movement of the mirror during use, while still allowing the tailgate to be opened easily, which is useful in acclimating the mirror to the ambient outdoor temperature. The tailgate includes a support made from parachute cord to limit the opening of the tailgate and prevent it from “falling open” against a hard stop.
The rocker box also includes a pair of accessory tie-down rings for affixing beanbag-style counterweights. This is necessary to compensate for the variation in mass and balance when using different eyepieces; the altitude bearings are of sufficiently low friction that without the removeable counterweights a heavy eyepiece will tend to tip the scope down, while a small eyepiece will allow it to rise.
At the other end of the scope, a plywood accessory platform is clamped to the exterior of the tube with a pair of plywood rings. This platform supports a Telrad reflex sight for aiming the scope visually, as well as a manual inclinometer which indicates the altitude orientation of the scope directly. When used in conjunction with the azimuth ring, this allows the scope to be accurately aimed in alt-az coordinates. As a future upgrade, replacing the analog inclinometer with a digital version will allow for more precise altitude positioning, as the analog indicator is only graduated in single-degree increments while the digital indicator can accurately measure to 0.1°.
Step 6: Conclusion
Overall, the scope works well. Motion on the bearings is smooth and easily controlled, and the scope is free from vibration during use. The low center-of-gravity and wide base make the scope very stable, and position the eyepiece low enough that children can look through it without need for a step-ladder. The mirror cell tends to hold collimation very well, and the optical performance of the entire assembly is surprisingly good.
If I were to do the project over, the only things I would change would be routing out areas of the plywood base and rocker box to remove weight, as the finished scope is rather heavy, and positioning the altitude bearing farther forward on the tube to reduce the need for counterweights on the back end. Considering that this scope delivers a relatively large aperture, accurate positioning capability, along with great stability and usability, these seem like fairly small problems for a project that can be realistically executed for under $500.
I hope my experiences in this project prove valuable to other builders!