Introduction: Mini-Dob Telescope Mount
This is a pretty big project for an Instructable. I'll do my best to make it as clear as possible.
Astronomy enthusiasts are swapping equipment all the time. Sometimes you end up with scopes you really didn't need, but they came with something else, and so on. This was the case with this 8" f4 Newtonian. It had been sitting around for a while collecting dust. The thought occurred, why not build a small Dobsonian mount that was easy to carry around. A grab and go scope.
The base and frame are made from 1/2" MDF. The cradle for the scope is made from 1/2" plywood (stronger material). The project used a CNC Router and a Laser Engraver. If you don't have access to these. search around a bit and see if there are any makerspaces in your area. They can be very helpful. This mount was built using equipment at The MakerBarn, a non-profit makerspace near Houston.
Some special features are:
1. A cradle that allows easy movement of the scope to for balancing and removal.
2. A high precision setting circle for Azimuth.
3. A geared fine adjust for elevation, with a clutch for quick resetting.
Step 1: Some Introductory Photos
Here's a few photos of the build in progress. You may want to refer back to these as your developing your strategy for the build.
Step 2: Start With the Frame
Keep in mind that this was designed for a specific scope, you may have to change and redesign to fit your scope.
There are five components to the frame. A base, two sides, and two internal braces.
The frame is made from 1/2" MDF. The dado cuts are typically 1/4" deep. The dxf files should work well in the CAM program you use with your CNC Router. At The MakerBarn we use V-Carve for most CNC Router operations. Glue and clamp the pieces together. Tie-bond II is an excellent choice for glue.
Note the four Teflon Glides. They are made from Teflon bar stock 1/2" x 1/4" thick. This material was purchased from McMaster-Carr. The material was cut one inch long. Two holes were drilled and countersunk for #4 flat head wood screws. Make 7 of these, three are used on the base later on.
To make the bottom of the base very smooth, put a layer of smooth Formica on the bottom surface of the frame assembly. I used Super 77 spray adhesive on both the frame bottom and the Formica. A few minutes after spraying, join the pieces together. You get one chance, the glue sticks like crazy. Use a trim router to trim off the excess Formica both on the outside and inside hole..
Step 3: Cutout the Base
The Base is also made from 1/2" MDF. There is a through hole in the center of the base for a 1/4" x 2-1/2" carriage bolt. Around the bolt-hole is a donut shaped pocket for a bushing made from 1-1/2" PVC pipe. The pipe acts as the radial bearing for rotation of the frame.
Mount the three Teflon Glides to the base. Three rubber feet are mounted to the bottom of the base.
To make the azimuth bushing, place the frame on top of the base and carefully measure the distance from the bottom of the pocket in the base to the top of the frame base. Make the bushing slight longer than this measurement. The frame should spin easily on the base with the azimuth setting circle tightened.
Step 4: Make the Azimuth Setting Circle
Both the setting circle and the pointer are laser cut on the reverse side using 1/4" acrylic. Another small circle is cut to help reinforce the acrylic setting circle. It is glued to the setting circle using acrylic solvent. The etched lines on the circle and pointer are filled with black acrylic paint to make them more visible.
Assemble the frame to the base. Drop the setting circle other the 1/4" carriage bolt. Place a fender washer on top and secure with a knob or nut. Then locate the pointer ans secure it in place with two wood screws. The knob is loosened to set the circle to the desired azimuth, then tightened to hole it in place. The frame should still rotate freely, even though the frame is secured to the base.
Step 5: Build the Telescope Cradle
The Cradle is the assembly the holds the telescope. You may have to modify it to fit your scope.
The cradle consists of several parts. The front and back bulkheads as well as the side pieces are made from 1/2" plywood. The two brakes, and the crosspiece above are made from hardwood. Poplar or birch would would work well. There are 4 laser-cut pieces of 1/4" acrylic that make up the two elevation bearing disks.
First cut the front, back and sides of the cradle. Glue and clap making sure all is aligned.
Cut the two brakes and a piece for the brake support from 3/4" thick hardwood. The top support also acts as a handle to carry the scope. The top support has two holes drilled for 1/4" T-nuts installed from the bottom. Two 1/4 screws press down on the brakes to hold the tube in position. Glue and screw the top support in place. Make sure the arc cut in the brakes closely matches the tube so that the brakes will not distort the tube. A layer of vinyl tape of the brakes also help to securely hold the tube with little pressure.
The elevation disks are cut from 1/4" acrylic. Note there there are two disks for each side, an inner disk and an other disk. Countersink the four holes in each outer disk for the 1" #6 flat head wood screws that secure the disks to the sides. There are 1/4" holes cut in the center of the disks. These holes are used to align the disks and hold them in place while being screwed to the sides of the cradle. Insert a 1/4 bolt through each set of disks and the sides. Use the holes in the disks to drill pilot holes for the screws.
At this point you should be able to mount the scope and set the cradle in the frame. Both azimuth and elevation should work smoothly.
Step 6: Elevation Fine Adjust
I found the scope move too easily in elevation, and was hard to keep balanced, so this is my solution.
Using 1/4" acrylic, I made a large spur gear to fit on top of the elevation disk. Between the gear and the disk I cut a 4" diameter piece of cloth reinforced rubber gasket material to act as a friction clutch. Using the holes that were cut for alignment, I inserted a 1/4 carriage bolt through the side of the cradle sticking out through the large gear. Then I put on a fender washer and a large threaded knob to adjust the clutch tension.
The adjustment lever was also cut from 1/4 acrylic. It was laid on the side of the frame such that engaged the large gear. The pivot hole was then located and drilled. A carriage bolt, some washers, and a nylock nut were used to secure it. The nut was tighten until a comfortable amount of tension was achieved.
If you build one, let me know how it works out. I hope this instructable will at least give some ideas that can be used in the future.