Some of the members of the SPAC mirror lab with their finished telescopes in 2004.
This is a potpourri of information from my journey of building telescopes. This page is pretty long so the small hamburger menu at top left is a quick way to locate the major topics or go back to the top or home page.
Some of the members of the SPAC mirror lab with their finished telescopes in 2004.
Everything here is my opinion and it has worked for me. There's at least a dozen ways to do everything. I have requested input from other members of the Mirror Lab and their suggestions have been added to this page. Do your research and find the option that is best for your abilities and goals.
There are usually reasons why telescopes of the same type have similar designs. They have been proven to work well and been time tested. I suggest that you understand the basic designs before changing them radically. Innovative amateur telescope makers (ATMers) have come up with amazing designs.
We're going to look at a truss pole Newtonian with a descriptive pointers added where appropriate. This is a friend's 16" Dob. The photo below shows the primary components nicely. So that everyone is on the same page if you hover your mouse over the photo below or tap if it's a tablet names of the major parts will be displayed. Go ahead and try it, I'll wait.
There is an order to building a classic Newtonian telescope on a Dobsonian mount that will give you an optical tube that doesn't need counter weights and has the option for the lowest rocker box. I'll be discussing the parts below in more detail further down.
☑️ Have a finished primary mirror. It doesn't need to have its aluminum coating yet.
☑️ Have your primary mirror cell.
☑️ Have your secondary mirror and spider (secondary holder).
☑️ Have your focuser. You'll need to know how far it can travel in and out.
1. Build the Optical Tube (solid or truss) without the altitude bearings.
2. With the Optical Tube fully loaded (mirrors, focuser, telrad, shroud, dew heaters, etc.) find the center of balance with a heavy and a light eyepiece and mark the mid-point. This will be your center of balance and the center of where the altitude bearings should be located and it may be partially out over the truss poles.
3. Design the altitude bearings so that the truss poles do not cut through the bearing surface (if the truss poles are on the outside of the mirror box). Make sure that it can attach adequately to the mirror box.
4. Knowing the size of the altitude bearing and how much clearance the bottom of the mirror box or tube needs to clear the rocker box will determine the height of the rocker box. Add extra of clearance, usually about an inch, if encoders will be added for digital setting circles.
You will need to know your primary mirror's focal length. This the point where parallel light bounces off the surface and focuses to a point. If you have a commercial mirror or one that has had a Foucault or Interferometer test the focal length likely came with the mirror. With enough light, like the Sun, this could be a very hot point capable of starting fires so be careful, especially if the mirror is coated with aluminum or silver.
If you don't know the focal length a distant light source could be focused by bouncing that light off of the mirror to a brick or block. You will need a measuring device, such as a tape measure.
⚠️ Even an uncoated mirror can start a fire if the Sun is used for a light source so be careful.
Using your light source move the mirror closer or further away from the block until it makes the smallest spot possible. Interestingly, the spot will elongate horizontally or vertically depending if you are too far or near. When it is in focus use the measuring device to measure from the front of the mirror to the focal point (the in-focus dot). Once you have the focal length value do a sanity check and divide the mirror's focal length by its diameter to get the focal ratio, or F-ratio.
This should be obvious, but NASA crashed a lander for this: Make sure that you are using same measurement system (inches to inches or millimeters to millimeters, etc.) You should end up with number between 2 and 20 and more likely between 4 and 8. A 12.5" diameter with a 58.75" focal length will have a (58.75/12.5) focal ratio of 4.7, or more commonly expressed as f/4.7.
Consider writing the mirror's diameter, and f/ratio on the side of the mirror with a Sharpie.
The Mirror Cell may be may be purchased or made. The design will depend on your mirror's size and thickness. A small to medium sized (6-12.5") mirror that's 1.5"+ thick can usually get by with a fixed cell. Fixed cell meaning that there are not any floating triangles. It will still need the ability to move freely against the cell and not be glued to it.
The cell below is for an 8" mirror. Two pieces of plywood were used separated by three carriage bolts with springs and a washer on each end of the spring. All metal parts are stainless steel or, in the case of the knob inserts, brass. The heads of the carriage bolts were recessed to be slightly lower then the top if the plywood. A felt pad was attached over each of the bolt heads. The side 'clamps' do not press on the mirror and the tip bends slightly above the mirror. This allows the mirror to move slightly as it cools or moves with the telescope's tilt. In this example the side bolts must be long enough to stick out of the sides of the scope tube by about 1/4" to allow the lock nuts enough threads to stay attached.
ATM made 8" mirror cell.
8" mirror cell Bottom with 12v fan using a 9v battery.
8" mirror cell top before the felt pads were added. Later switched to six mounting bolts.
A larger or thinner mirror will need a floatation cell. This usually requires metal cutting and welding either steel, aluminum or both. Like most ATM parts a floatation mirror cell may be purchased from a out of the garage shop vendor, contracted through a local welder or made yourself. For my 18" 1-5/8" thick Pyrex mirror I lucked out and located someone selling a mirror cell on Astromart. This cell uses the classic seat-belt sling to support the side of the mirror with an 18 point floatation cell.
18" mirror cell with the uncoated mirror on it.
18" mirror cell Bottom.
18" classic sling mirror cell.
My one issue with the classic 'seatbelt' sling was that it kept sliding off of the side of the mirror during transport. This was rectified with the help of the thick mirror. Since the mirror was thicker than the seat belt I cut six thin strips of the 'hook' Velcro and stuck them to the side of the mirror about 4" apart leaving enough between each pair for the belt to fit in. Three 1" wide strips of the 'loop' velcro were attached to to each of the hook pairs with the belt between them and the mirror. This allows the belt to slide freely under the loops (blue) and narrow hook part (red) to act as guardrails and not allow the sling to slide off of the mirror.
This makes for a captive, yet fluid, mirror sling.
When I was working on the 16" diameter 1" thick plate glass mirror I decided to dig deeper into mirror support. Ideally I wanted to get rid of the sling. Fortunately there was quite a bit of activity testing Whiffletree side supports. Further digging introduced me to the web site of JP Astrocraft. This turned out to be a perfect match for my mirror. He was able to make the cell to fit my slightly smaller than standard mirror box size. This is a beautifully made mirror cell.
16" Whiffletree mirror cell.
16" Whiffletree mirror cell bottom with variable speed fan.
Historically I have overbuilt many of the parts. On the 16" I bucked the trend and mostly used 1/2" Baltic Birch. Some of the older books suggest using two 3/4" sheets of plywood glued together and pressed overnight by driving your car on them for weight. Maybe this was needed in a time when quality plywood wasn't an option. Baltic Birch is stiff, heavy and strong. I have no doubt that the 18" scope could have been made out of 1/2" instead of the 3/4" that that I used. With Titebond III glue and the wood sealed with urethane it holds up well even when swamped from rain. I have not seen the need for Finish Birch, which is basically Baltic Birch with waterproof glue bonding the layers together for marine use. It's also heavier than Baltic Birch.
My preferred method of joining the corners of a box together is to use box joints. These are also called finger joints. They are square or rectangular cuts in the edge of the boards that allow them to snugly fit together similarly to you interleaving your fingers. The advantage over a dovetail joint is that it can be made on a table saw with a dado blade. The box joints allow for more glue surface area making a stronger corner. See the section on the box joints and the Lynn Jig further down.
The big questions with a mirror box is how big to make it. I made mine about as tight to the mirror as I could.
My 16" mirror has 1" of clearance on all sides. I have have learned that 1.5" is considered normal, at least by the people that make mirror cells.
The height can be figured a few ways. There are some complicated web calculators for doing this. My method was to go to a star party, look for a similar
diameter and thickness mirror and measure a few mirror boxed and average out the values. If anything I wanted the box to be even with the center of balance or slightly shorter.
That goal was achieved when I was able to assemble the Optical Tube Assembly (OTA) and find the balance point. It was almost 2" above the top of the mirror box. That worked nicely and meant that I didn't overbuild the height of the mirror box. The trunnions were designed larger than usual to be centered at the balance point and their larger radius made for a very smooth motion moving the scope up and down even when the eyepiece weight changed significantly.
Upside down mirror box where the curve will allow the mirror box to be lower in the rocker box. The mirror cell will be screwed to the mirror box and the corner braces will support a 1/8" plywood baffle and that will support a 1/8" circular cover. This is very important to prevent the sun from hitting the mirror.
The inside of the box is painted flat black and I'm testing where the trunnions will be located. Their center point, about 2" above the mirror box, and the placement of the pole clamps required that they be trunnions of unusual size. The scope's motion ended up being very smooth and the balance was perfect.
Cutting box joints requires a jig of some form so I made a couple of Lynn Jigs. Traditional box joints use a pin jig that each new cut is placed over to space it out for the next cut. The problem with that is that if the pin is off at all the error accumulates and soon two boards will not align. The Jig uses a screw offset each cut by counting the number of turns before making the next cut. A 16 Threads Per Inch (TPI) screw will always average out to 1" of motion after 16 full turns even if one threads slightly closer or farther from another.
The jig attaches to the table saw's miter gauge. Mine has two slots in the top that the bolts with wingnuts attach to. This gives you a firm sliding path. The miter gauge must be locked down at 90 degrees to the blade. On my 4' Lynn Jig (shown in the last photo) I let the threaded rod extend out the end opposite of the knob so that I can attach a drill to it and reset the jig back to its initial position. The diameter of the threaded rod isn't too important but the thread count is. The normal size and the one that I recommend is 3/8" x 16 tpi. The length needs to be as long as your jig. I recommend making one that's 4' long for telescope work.
The part of the jig that moves, the sled, is half the length of the jig and the longest box that you can cut. The side of the sled nearest to the knob sticks out and becomes the fence that the boards align against. The opposite side of the sled is flush with the front of the sled board. The fix ends of the jig only need a snug fit to the rod or an unthreaded brass insert to act as a bearing. The sled needs either threaded inserts, T-Inserts or something that has threads to make it move when the threaded rod turns. I used a brad hole T-Nut attached at each end.
Notice the writing on the jig and the shape of the handle with the yellow dot on it. For each 16 HALF turns of the handle the jig moves 1/2". For every 24 half turns it moves 3/4" The yellow dot helps remind me if it's a full or half turn. I generally would want the cut outs to be square. In the case of 1/2" plywood I would make a cut at the end of the board, flip the board vertically, make 16 half turns and then make another cut. Flip and repeat. You would want the boards to be an even multiple of the cut width.
The 16 threads per inch (tpi) rod is important here. 16tpi = 32 half turns per inch. That's 16 half turns to make a 1/2" wide move or 24 half turns to make a 3/4" move. A different thread count would require a different turn count that may be partial half turns. Also, do not get distracted while making the cuts. Your count may get off ruining your box.
Also notice the arrow next to the knob. Yep, that's the way to turn it when moving the sled. Do you really need to ask why that's there?
This is the 4' jig cutting the mirror box for the 16" scope. This jig is made of Poplar and has held up for many years. The bottom of the sled will be mostly cut out after the first use. This is normal. The fixed fide of the jig will only have a cut as wide as the widest dado that has passed through it.
It is important that you accurately get the width of the dado blade set up. Run a test with a couple of pieces of scrap wood and make sure that they fit together as expected. When you cut the plywood the wood grain should run vertically on the saw.
In this example I clamped all four of the box sides together and added a hardboard backing board (visible) to prevent tear out. I recommend this method. The backing board was slightly wider than the wood sides so there is a small extra 'pin' on the top left. Notice that the cuts alternate between the top and bottom of the boards. Once the boards are unclamped they should all fit together and make the sides of a sturdy box. If the saw depth is adjusted correct expect the fingers (called 'pins') to stick out very slightly when the box is assembled. these will need to be sanded off to match the side of the box exactly. This also removes any saw marks from when the board was cut.
Here is the box being glued up and the finished corner after urethane. Band clamps are helpful. If there are any gaps around the joints, and there will be, try this: After the box is glued up coat it with a mix of 80% urethane and 20% mineral spirits. This will allow the urethane to penetrate everywhere and make the color even and start waterproofing the wood. (Coat your jig with this same mixture) Now get some clear epoxy - I like 30 min. One side of the box at a time use a toothpick to insert the epoxy into any gaps. It may take a few passes to fill in a larger gap. Wait for the epoxy to start to gel and pack it into a larger gap to make a dam for the next pass.
If you don't urethane first the epoxied areas will be a different color than the rest of the wood. After the epoxy thoroughly hardens sand it smooth and add more epoxy if needed. Once the epoxy is finished and smooth recoat with urethane and sand lightly for a few coats. For the last coat I use spray urethane of the same brand and do not sand after applying. For a mirror box I coat the inside with at least two coats of urethane for moisture proofing before painting it flat black.
Truss pole telescopes need something at the top to support the secondary holder, focuser and keep the two in alignment with the primary mirror. I used the diameter of the mirror and added 1" to it for the inside diameter of the cage rings. These were cut out of Baltic Birch using a home-made circle cutter on the router with a straight 1/4" bit. The length of the aluminum tubes was enough to allow a telrad between the rings. They could have been made an inch or so shorter.
The Kydex is a pain to cut and get the hole positions correct. I resolved that by using some translucent plastic from Michael's and cut a couple of strips then glued them together into a strip. That is placed inside the tubes where the Kydex will go and a Sharpie was used to mark the length with half circle cut-out for the focuser and mark where the holes will be for the #4 screws that go into the aluminum poles an into the end plugs. This information is transferred to the Kydex for a perfect fit.
The Delrin end plugs for the aluminum cage poles were purchased from Moonlite. Unfortunately they are no longer sold. See the section below on Truss Poles & Clamps for links to 3D printable tube plug STL files. The top and bottom use stainless bolts and finish washers. A 3D printer pole plug option is below.
A 3D printed pole plug to attach the pole to the secondary cage ring.
The plug should use a #4 screw on the inside of the cage (not shown) to go into the pole and into the side of the plug.
These have #8 x 32 bolts in them and need to be tapped.
Cage Tube Plug (STL)
An entire book could be written about the best angle to place the focuser. Some like the focuser knob to be aiming down while I like mine to be angled to the right. I built it so I win this time. As you may have guessed, I am right handed. There are focusers that rotate at their base to make everyone happy. This also means that the scope goes lower to my left as I'm at the eyepiece. Placing the Telrad to the right of the focuser keeps it from ending upside down when the scope aimed near the horizon.
Test fit of the cage circles and the aluminum tubes.
Translucent plastic was used to simulate the Kydex and locate the holes in the tubes and the focuser's location.
Here is the almost finished 16" cage next to the finished 18" cage. On the 16" I used 1/4" angle aluminum on the back to attach the focuser and telrad boards to the rings. This prevented the #6 x 3/8" screws from being seen on the outside.
The finished cage for the 16" Mini Maxx.
Like most of the parts you may buy one or make your own. This is my method of making a secondary mirror holder. A section of PVC is cut that is about 1/4" longer than the metal vanes. The vanes are stainless strips purchased from the local ACE hardware. The PVC is slotted to match your vane count (usually 3 or 4) and cut into the PVC the height of the metal strip. If you want a secondary dew heater drill a small hole in the end of two of the strips. These will connect to wires that either pass through the bottom of the PVC or connect to tiny bolts (#4 or #6) that pass through the bottom (see below).
The top part of the holder is filled with 2 hour epoxy and then drilled and tapped for a center flexible bolt and three cap head bolts. Do not use quick curing (5 Min) epoxy here because it stays flexible and does not hold tapped threads well.
The center bolt is made from a nylon bolt with the head being inside the lower portion of the holder (upside-down). Nylon was chosen to offer a little flexibility when the adjustment screws align (collimate) the lower portion of the holder. The top end of the bolt was cut with a slot to allow fine tuning with the up and down position with a flat blade screw driver.
The cap head or knurled finger bolts adjustment bolts press against a fender washer on the top of the lower portion od the holder. these are used for collimation once the secondary mirror is centered in the eyepiece hole.
Underside of the top part of a three vane secondary holder. The power bolts are sticking up. This will be filled with epoxy
Adjustment and power bolts between the top and bottom of a four vane secondary holder.
The lower part that the mirror attaches to three 470ohm 1/2 watt resistors wired in parallel that will be the dew heater. I carefully placed the mirror face down on its shipping paper and used GE Silicon II (aka RTV) to attach the resistors to the center back of the mirror leaving enough space around the edge to attach the mirror to the PVC that is cut at a 45 degree angle. If you have a miter saw (or a friend with one) this is a good use for it to make the 45 degree cut. After gluing the resistors to the back of the secondary mirror the wires will be fed through the holes and attached to the power bolts on the upper part of the holder. With resistors positive and negative don't matter as long as the 12v DC comes in one end and goes out the other.
A test fit of the dew heater resistors on the 45 degree cut PVC that is the (here upside-down) bottom of the secondary holder.
The 45 degree bucket secondary holder that I used to glue on the mirror. Three beads of GE Silicon II were used and pennies were spacers between the PVC and the mirror's back. The heater wires were fed through the holes as the mirror was lowered onto the PVC.
Attaching the vanes to the Telescope's cage or tube used a slotted bolt. My first attempt was using black nylon bolts that I cut the heads off of and then drilled and slotted. See the black bolt below. This worked for a few years and finally broke apart. The design worked but the materials needed an upgrade. I remade the bolts using 1/4x20 stainless. A washer and lock nut was used on the outside of the tube to hold the molts and center the mirror.
Like the previous nylon bolts the heads were cut off, holes were drilled and slots were made with a Dremel (slow going) with a grinding wheel. A matching hole was made near the end of each vane and that was placed in the slot. With the holes aligned a #6 bolt with two washers is used to hold them together. Before the nut goes on a dab of blue Loctite is placed on the #6 threads to keep the nut on. This should outlive me.
The three new bolts (later painted black) and the old torn out nylon bolt. These new metal bolts are going into the 8" wood tube Fritz telescope that tore out a nylon bolt while I was collimating it. I need to make the same modification to the 12.5" Griffin scope...
In mid 2024 I finally got around to making the four new secondary vein bolts for the 12.5" Griffin scope. Starting with four stainless steel 1/4" x 6" long (what I had handy) bolts They were cut to the proper length (two cuts for each bolt), drilled and slotted. I did the cutting and slotting using a Ryobi PSBCS02 cut-off tool. I don't usually do metal work so my cut-off bench was the kayak trailer with a metal bar and a couple of locking pliers. Then the holes were drilled using in a small drill press with a vice. After that I placed each piece in the drill press and used a file to smooth and round over the cut off end of each bolt. Sandpaper was used used in the slot to smooth it a bit. The threads were cleaned up to make sure that a but would go on smoothly, Finally, the threads were taped and the bolts were painted a flat black. It's important that the final bolt length is appropriate for your set-up since each may vary a bit. The new bolts were installed 19-1/2 years after the scope's first light. The old nylon bolts were a bit stretched but held up right to the end.
Cutting the bolts with the hot cut-offs drop into a bucket with water. The threaded end was already cut shorter here.
Drilling the holes in the 1/4" bolt for the #4 bolts that attach it to the secondary veins. Use a center punch or nail to make a indent to keep the drill hole centered.
The finished bolts. Not pretty but they are stronger than plastic and will look really good in the dark.
🤔
If I were doing this again I would likely add a slightly recessed dim LED to the top of the secondary holder to show that it is getting power. Likely a medium brightness red LED and a 10K resistor would do the trick, like the one used on the DEW heater (Red LED 655nm, 3mm, Jameco #2186603). Unlike the resistors LEDs must have the positive voltage attached to the correct (usually longer) lead. If you do this don't be surprised if the LED blinks on and off (called Pulse Width Modulation, or PWM). The variable 'on' time controls how much heat the resistors generate while saving power during the 'off' time.
For both my 16" and 18" Dobs I used 1" outside diameter black anodized aluminum poles from Brunner Enterprises. The truss lower clamps and top ball & sockets as well as the cage tube plugs were from Moonlite, but they are no longer offered. After some experimenting I came up with some similar 3D printed parts that work nicely. I printed them using ABS plastic to that they could tolerate heat better. They are available for download at the bottom of this section. I am not printing or selling parts. Look for your local Maker Space or library for access to a 3D printer.
Starting from the top. The secondary cage tube plugs are intended to be tapped for a #8x32 thread. I used a #8x32 stainless oval head bolt with a stainless finish washer (any marine parts store). While the plug presses in the tube you will also need a #4 screw (I used a 3/4" pan head) to go in the tube and into the plastic plug. Do this from the inside of the cage and use the screw to also hold the Kydex in place. A black Sharpie will darken up the screw head nicely. You will need two plugs for each aluminum tube in your cage.
Next down is the ball and socket clamps. This combo was used to allow the tubes to come in from different angles, depending on your mirror size and f/ratio. The clamp uses a captive stainless steel 1/4 x 20 nut against the bottom of the cage rings that the clamping bolt goes into.
Clamp that is mounted on the cage and clamps the truss pole ball. This is the updated clamping knob with the plug in place.
These clamps are attached to bottom of the secondary cage and clamp to the truss pole ball.
Truss pole ball with carriage bolt hole.
Truss pole ball assembled.
The balls, like the cage plugs, need a #4 screw to keep them from coming out of the poles. The neck of the balls were the weak link in this.
The first attempt (v1) was just a 80% solid ABS build that fractured at the neck along the plastic's layering lines.
The second attempt (v2) added a Stainless Steel #8 x 2" bolt from the bottom with a countersunk head ('V' shaped head) or oval head that was epoxyed in each balls using 24 hour epoxy (5 minute epoxy was too soft). Two hour epoxy should have worked well also. This worked as long as the side torque pole tension wasn't too great. Better than the first attempt but the neck still fractured some times.
The third attempt (v3) flattened the top of the ball and added a square indent and had a 1/4" hole through the entire structure. This matched up with a 1/4 x 2" stainless carriage bolt's curved head to make the top of the ball. A washer and lock nut were used on the bottom. An important note here is that you must thread the lock nut on a bolt before attaching it to the bolt in the ball. The 3D plastic isn't strong enough to handle the torque of the initial threads being created in the lock nut.
Truss mirror box clamp showing the stop screw and washer and the clamping knob and spacer.
Truss ball, sample pole and the mirror box clamp.
These are attached to the mirror box and allow the truss poles to slide in and then be clamped. I have only used them on the outside of the box. There is a 1/4x20 nut held captive on the back side and clamping knob with an aluminum bushing to get the handle to clear the tube. The #6 x 3/4" pan head screw and washer need to go into the threaded hole at the bottom of each clamp. This acts as a stop for the pole and may be removed during building to get the exact tube length to allow the truss poles to extend below the bottom of the clamp. I recommend only attaching the mirror box clamps using one screw until you get the exact angle when the truss poles are attached to the cage and focus is confirmed. Then add the other three screws. As always I recommend using stainless steel screws.
Below are the parts or links to them. You will need access to a 3D printer and its slicing program to make the STL files.
Lower Clamp Aluminum Bushings 3/8" OD x 1/4" ID x 3/4" Length
Lower Clamp Knobs 1/4 x 20 x 1-1/4"
Cage Tube Plug (STL)
Truss Tube Ball v3 (STL)
Socket Truss Pole Clamp (STL)
Socket Clamp Knob (STL)
Mirror Box Truss Pole Clamp (STL)
I'm a big fan of using templates to make wood parts, especially duplicate parts. If all goes well the final piece will be an exact duplicate of the template so it's worth taking the time to make a really accurate, smooth template. I start with a paper plan. Often these are larger than a standard sheet of paper so I add some very fine lines to help realignment and printed tiled. The pieces are taped together (trunnion photo 1), cut out. I knew how much higher the center of the circle needed to be to be the center of balance for this scope by balancing the fully 'loaded' scope (mirror, shroud, eyepiece, Telrad, etc.) with the truss tubes briefly riding on a dowel. That cut out plan is then traced on some 1/4" hardboard. In this case I then drilled a center hole and used a router with a circle guide to make a smooth inner and outer radius (trunnion photo 2). The straight lined were cut with a jigsaw against a straight edge and the curved parts freehand.
After the hardboard template was sanded I taped it to the side of the scope to see if the clamps cleared and where the truss poles would end up (trunnion photo 3). I also did this with the paper template earlier. Once I that looked good I traced the template on some 3/4" Baltic Birch. The Birch was cut out with the jigsaw about 1/4" larger than the finished part. Using carpet tape on the template (never larger than the template) I taped the template to the roughly cut Birch pressing it down firmly. A router in a table with a tracing bit finished the job. This is a router bit that has a bearing on the bottom and matching straight bit above it the same width as the bearing. The bearing rides against the template making the Birch match its shape (trunnion photo 4). It's pretty unforgiving so any bumps in the template will be reproduced on the final part. I flip the template over between each trunnion to make sure that they are symmetrical.
Since I wanted a 1/4" center hole on the trunnion opposite the eyepiece I used the template's hole used to make the semi-circle as a drill guide to make the center hole in the trunnion, but not all the way through. This would later be used to align the mount for digital setting circle encoders.
Each trunnion is carefully centered on the mirror box and its vertical position is matched from the line on the original plan. The inside of each trunnion is marked against the top and side edge of the mirror box. Two 1/4" pieces of Baltic Birch are cut and match to the template and then some of it is cut off to align with the visible parts of the inside of each trunnion. The 3/4" trunnion may now have a slot cut in each for where the truss pole will go through them. The 1/4" pieces are glued to the inside of their respective trunnions and should exactly align with where the trunnion rest against the top and side edges of the mirror box. Trunnion photo 6 shows the backing board resting on the top of the mirror box. before it was rounded over 1/4" (not the formica surface) and urathaned. The backing boards helps support the trunnions then the scope is at low altitudes and, if everything alines, appears as a nice transition from the bearings to the mirror box.
In the end I glued the last of my old, bumpy Ebony Star 'Formica' to them and screwed them to the mirror box from inside the mirror box so the stainless steel screws could not be seen. I drilled five holes in the template to align where the screws would go into the mirror box.
The template method has a few advantages over a pair of one-offs: It's reproducible. It may be mirrored by flipping the template. The center pivot hole is not needed to cut the semi-circle with the router. It's easier to make complicated shapes using the hardboard due to the lack of grain. The template may be kept for a future build. I recommend spraying them with urethane to moisture protect them before storage. The template may be used for positioning the screw holes, as mentioned earlier. In the case of this scope the rocker box side cut-out template was turned 90 degrees and used to make the cat eyes in the front by added a pupil semi-circle.
The Visio Paper Plan taped together but not cut out yet. You can see the vertical line going through the center hole towards the lower right of the photo.
Hardboard template partially cut out using a router and circle guide that pivots using a pin in the center hole. The outer semi-circle where the formica goes and the center hole are critical, the rest is cosmetic but I still want it to look good.
Template trial fit. You can see the five alignment holes for where the screws will go to hold the trunnion to the mirror box.
Pattern cutting Baltic Birch from the template. The Template is on the bottom and it's carpet taped to the Birch. Note the bearing on the bottom of the tracing router bit.
Outside (top) and inside of the trunnions. The top trunnion has a 1/4" hole about 1/2" deep on the other side for the encoder bracket alignment. The bottom trunnion shows the backing birch that is outside of the mirror box and the five screw holes that will hold it on to the mirror box.
The final test of the finished focuser side trunnion. The screw holes and screw heads inside the mirror box were later painted flat black.
The rocker box supports the weight of the telescope and allows it to move up and down and side to side. In most cases you will want a rocker box as low as possible. The ground board is the part of the rocker box that comes in contact with the ground.
I usually like to make the rocker box width 1/2" larger inner dimensions than the outer dimensions of the mirror box. This allows the mirror box side bearings to rest on the rocker with small gap and it allows the mirror box to be rotated 90 degrees and sit inside of the rocker box for storage and transit. Usually the rocker box is not built to lift the weight of the mirror box in the transit mode (against the bottom) but when the telescope is resting on the bearings it should be okay.
The glued up and sanded rocker box for the 16" Mini Maxx.
Notice that the corners of the rocker box extend over the circular bottom board to give a small drainage hole on the inside of each corner.
I already determined where the balance point is on the 16". It's a couple of inches above the mirror box. My trunnion needed to be designed so that the center matches up with the balance point of the scope and the size and design doesn't hit a pole clamp. That part was done with the mirror box design. The template that I made for the trunnion will be used here, double sided taped to the mirror box, to check the clearance of the rocker box.
I'm using a paper template of the rocker box candidate will clear the mirror box and leave room for the encoder.
Similar to the side bearings the rocker box side hardboard templates are from the paper template and later this is used to make the plywood parts. Note the home-made circle cutter. I've been using it for 20+ years.
With a successful paper template test I made a hardboard template that will be used to cut out both sides. This is a test of the scope as if it were pointing at zenith.
Another test with the scope at 45 degrees. This is where the curved cut on the mirror box really helps to keep the rocker box low.
After cutting the sides of the rocker box and adding the box joints a dry fit checks the size. Perfect.
The template is used to draw the pattern, jig-sawed out and then double sided taped and finished with the router (see the previous section on making trunnions)
The bottom board is cut so that the corners of the rocker box allow about a 1/4" gap after rounding it over at each corner to allow any rain or dew to drain. Biscuits are used to attach the bottom board to the rocker box sides. You would need a Biscuit cutter for this.
The ground board is made reasonably light weight and normally barely seen. I used hockey pucks for feet - strong and waterproof. They are each held in place by two stainless screws to prevent turning and coming loose. Two of the three are already installed here.
The Ebony Star (Formica) is glued to the bottom of the rocker box and the Teflon pads are placed on the ground board so that any dirt will fall out and not ride on the Ebony Star or Formica. The transfer of weight is important. Each Teflon pad is placed directly over the hockey puck foot so that the weight of the scope is supported.
We kept having an issue where the center pivot bolt, which connects the ground board to the rocker box and the digital setting circle encoder attached to, slip. The bolt should stay stationary with the ground board when the rocker box turns. The solution was to get some carriage bolts (yes, stainless) and weld a torque washer (also stainless) to it. McFeely's doesn't carry the 3/8" stainless torque washers any longer although they are available in galvanized steel on Amazon. Torque washers have barbs that dig into the bottom of the ground board and prevent the carriage bolt from turning much. If the carriage bolt doesn't precisely fit into the square opening of the torque washer it may move back and forth slightly. This is where the weld comes in. Once the torque washer and the carriage bolt are welded together there isn't any motion. You will still need a union at the top that threads to the end of the carriage bolt (I use blue Loctite) and also has a 1/4" hole in the top center with a side nylon thumbscrew to hold the optical encoder's shaft. All of this is so that the encoder's shaft does not move when the telescope turns. The encoder's body will, but not the shaft. It equally valid to design a system when the encoder's body remains stationary and the shaft turns, but that is an unusual configuration.
I've been using these handles on my 18" Dob for a few years now. It makes moving it to the back yard really easy. I bought the handles at an Ace Hardware during a sale and the airless wheels from Harbor Freight. The wheel axle bolts and all other hardware was stainless steel except for the brass threaded inserts.
After drilling the holes seal them with urethane before inserting the brass inserts. In the lower foreground that's a Q-Tip with urethane on it to coat the inside of the new holes.
The brass threaded inserts are installed. I used one of the handles as a guide to drill the other handle and both sides of the rocker box. Either handle may be used on either side.
The felt pads do a nice job protecting the handles from rubbing against the rocker box. Four eye bolts (5/16-18) and a fender washer are used to attach the handles to the rocker box. After striking out at the big box hardware stores Ace Hardware had the brass inserts.
The handles clamped in place when I drilled the holes in the rocker box. Only move the scope with the wheelbarrow handles if the mirror box is supported by the trunnions like when it is being used. Never lift when the mirror box is resting on the bottom of the rocker box. The wheels are about an inch above ground level when installed to prevent hitting the ground until the handles are lifted.
Don't start this unless you have glued up the parts and sanded off any excess glue.
If you talk to ten ATMers you'll get 15 different preferred methods to finish your telescope. Like everything else here this is one ATMer's opinion.
I use Helmsman Spar Urethane and generally apply it with a foam brush. Many foam brushes actually. I also use Mineral Spirits.
I start with a smallish cheap Hefty plastic container mixing about 80% urethane and 20% mineral spirits.
Slather that all over the wood. Get in all of the cracks, nooks and crannies. This is your last line defense against water damage. This will also darken up most plywood a bit.
Do this in a well ventilated place or you'll get a mineral spirit buzz. Wait for it to dry. The wood fibers will become stiff and feel slightly rough in the surface.
Next I use 220 sandpaper on a 1/4 sheet palm sander. This takes off pretty much all of the roughness. Expect to change the sandpaper on it a few times during each round of sanding.
Repeat this using 90% urethane. I find that when the thicker 100% runs it's too thick to brush smooth. There will be runs. Sand them off and coat it again.
You may be able to reuse a brush once or twice if you put it in a plastic bag and rubber band the opening closed.
For the last coat I use the spray version of the same brand, usually in a satin finish. This sometimes gets two coats a day apart. Do not sand after your final coat. The parts will outgas for a few days so in the house might get the natives restless. Urethane takes many days to fully harden so let the parts sit for a week or more if possible.
Whatever finish the last coat is will be the final look.
