MiataScope Plans

I have received a number of requests for the plans to my telescope, so here they are.

This page is a work in progress. I will try to add one or two drawings every day until it is done.

The MiataScope is a proud father. Flavio Vianna of Brazil has built a somewhat improved model based on the MiataScope.

Here are the parts of the MiataScope, starting from the outside in. I designed the telescope from the outside in because the rocker box needed to fit in the Miata trunk exactly. The Miata trunk determines the size of the rocker box. The rocker box determines the size of the mirror box, and so on.

If you're not designing a telescope with these limitations, it's best to start at the inside and work out. It's especially important to find the balance point of the telescope before you design the rocker box, so you'll know where to mount the bearings, make the cutout and so on.

Important: Balance the telescope before you build the rocker box. Flavio Vianna built an 11" telescope with a longer focal length than my Miatascope. Flavio's telescope has a different balance. If he sets the mirror box low enough on the bearings to to balance the scope, there isn't enough clearance in the rocker box. (He fixed the problem with balance springs.) Be prepared to modify the design of your rocker box. Even changing the focal length of the mirror can affect your scope's balance.

General

The entire telescope is made of ½" birch plywood. Baltic Birch or Apple ply is the best, but expensive and sometimes hard to find. Since I drive a Miata, I can't travel long distances with lumber, so I just used ordinary birch from the neighborhood lumber yard.

The edges of plywood look ugly, so I added edge banding to almost all of the pieces. See below for a discussion of edge banding. Edge banding pretty much requires a good table saw, but if you have one, it's well worth the effort. You can also buy pre-made edge banding for most common woods.

Rocker Box

Doubles as the carrying case. Circular cut-outs on the side are the altitude bearings. The back is mostly cut away, partly to allow the scope to swing out. In addition, it was necessary to cut away the front to clear the Miata trunk latch. It was the trunk latch that actually determined the height of the rocker box back. (See the Flavioscope)

This is the plan for the side (make two). Overall dimensions are 16" wide, 14" high. Circular cutout is cut to match the 6"-radius attitude bearings, below, plus a ¼" clearance for the Teflon bearings (not shown in drawing, but visible in photograph.) Note the fact that the back corners are cut out. This is to clear the lip of the trunk lid. I measured this by building a cardboard mock-up of the rocker box, putting it into the trunk of my Miata, and closing the lid. Any part of the cardboard box that got crushed got removed from the final design.

The bottom edge has been cut out slightly. This turns the four corners of the box into four feet. That makes the box slightly more stable, and helps resist mildew during storage. This cut-out is optional, but if you've got the tools for it, go for it.

This is a detail of the bottom of the side. A ¼"-deep groove is cut into the inside of all four sides to accommodate the box bottom.

This is the back panel.

This is the front panel.

This is the bottom. Dimensions are 15-7/16 x 15-7/16. There is a ½" hole drilled in the center to match a dowel attached to the ground board. The underside of the rocker box bears a layer of Ebony Star plastic laminate (photo). (See the Flavioscope)

Here is a view of the completed box. The hole in the center of the bottom is not visible in the drawing, but can be seen in the photograph. Sides are glued and screwed together. If you have the tools for it, you should counter-sink the screw holes and plug them with wood to match the plywood (see below). This will produce almost invisible joints, but you'll never be able to take the box apart again, so save the plugging until the telescope is finished.

The bottom rests inside the groove cut into the four sides. This is screwed into place, but not glued. This allows for expansion and contraction caused by changes in temperature and humidity. This may not be necessary for plywood, but it's a wood-worker's habit. It's possible that using glue would have made a more rigid box, which might be an advantage.

For completeness, add carrying handles to the side.

Mirror Box

Sized to fit snugly inside the rocker box. Shown on right with mirror and one altitude bearing installed.

The upper corners of each side contain brass threaded inserts. The truss tubes attach to these from the inside with ordinary 1/4-20 bolts.

NOTE: Test-fit the mirror box into the rocker box before final assembly. If it doesn't fit, you'll need to make some changes.

The crude drawing below shows the assembled mirror box. This box does not have a fixed bottom. Instead, its corners are braced with triangular pieces. These braces have holes drilled into them large enough to accept threaded inserts. Overall dimensions are 14-7/8 x 14-7/8 x 9.

The bottom of the mirror box is a square piece of plywood, with ¼" holes in the corners to match the corner braces. ¼" bolts go through the bottom and into the corner braces to hold the bottom in place. I did it this way to allow me to remove the bottom for access to the mirror cell.

Since I built the box, I discovered that I never need to remove the bottom in practice, so this whole design is over-kill. Just build any wooden box with these dimensions and you'll be fine.

There are three holes in the bottom equally spaced around the center to match the bolts in the mirror cell.

Below is one side of the mirror box showing its overall dimensions. Hidden lines show the corner braces and the box bottom.

Large holes near the upper corners hold ¼" threaded inserts which are the truss tube mounts. There are probably better ways to do this.

Note the three pairs of holes on each side. These are to mount the altitude bearings. The three pairs of holes allow the altitude bearings to be mounted in different positions to adjust the balance of the scope. One hole of each pair goes all the way through for the mounting bolt. The other only goes in a short distance and mates with a small peg on the bearing. That way, the bearing can be attached with a single bolt, but will not move.

Note: Drilling these holes should be one of the very last steps in the telescope construction. Install the mirror, upper tube assembly and everything else, then play with the scope's balance before committing yourself to drilling the holes. My telescope has a 2¼" thick 10" mirror and a 57" focal length. Changing any of these could affect the telescope balance.

(Note: these sides have mirror symmetry. On one side, the through-holes are on the left, and on the other they're on the right. If you mess this up, just drill them all through.)

Finally, here are the other two sides of the mirror box.

Here is a close-up photograph of the cut-outs made into the sides to clear the mounting hardware of the upper tube assembly. The metal parts rest in this cut-out, suspending the upper tube assembly inside the mirror box.

Bearings

Cut a 12"-diameter plywood disk (see router jig plans) and then cut the disk in half to make two bearings.

A piece of birch hardwood is glued to the long straight edge of the bearing. This piece extends slightly beyond the diameter of the disk and acts as stops to keep the telescope from rotating too far.

A ¼" hole is drilled through for the mounting bolt, and another ¼" hole is drilled ¼" deep into the disk and fitted with a small peg. This arrangement mounts the bearing to the side of the mirror box.

Finally, the outer edge of the disk is covered with Ebony Star laminate. (See photo).

Mirror Cell

A 10" x 10" square of plywood (to match mirror dimensions) is made. It is fitted with four posts made of ½" dowels which hold the mirror in place. The mirror should rest loosely between the dowels.

I attached the dowels by drilling ½" holes through the plywood and then gluing the dowels into place. The holes must be exactly perpendicular to the plywood base. I used a drill press for this, but maybe if you're really good, you can do it with a hand drill.

Three holes are drilled through the base for the mounting bolts. The holes are counter-sunk to make room for the bolt heads. Rubber furniture pads are glued to the base to support the mirror.

The height of the posts is determined by the thickness of the mirror plus the thickness of the rubber pads, plus a small safety margin, perhaps 1/8".

Holes are drilled into the ends of the posts to accommodate small threaded inserts (I just love threaded inserts). Bolts and washers attach to the posts to keep the mirror in place. The undersides of the washers are lined with something soft (I used leather) to prevent scratching the mirror. The washers should not normally touch the mirror except to prevent the mirror from falling out of the cell. The third photo shows the mirror in the cell and one bolt/washer combination in place.

The drawing shows the relationship between the collimation bolts, rubber pads, and support posts.

Finally, the corners of the base are cut off so that it will clear the upper tube assembly.

Upper Tube Assembly

Goal here was light weight. Two plywood rings are held by four plywood spacers. A fifth removable spacer holds the focuser.

This UTA is sized to fit upside-down inside the mirror box. The mirror cell just clears the hole in the upper ring.

My original UTA was three inches shorter and rested above the mirror cell. I rebuilt it because I had a bright idea for a sliding-board focuser that didn't pan out. I now wish I had kept the old UTA, as weight is critical.

NOTE: Test-fit the UTA into the mirror box before final assembly. If it doesn't fit, you'll need to make some changes.

The secondary holder is completely primitive -- taken straight from Dobson's sidewalk telescope plans. The holder is a short section of 1½" closet rod with four slits cut down it's length with a table saw. Four wooden vanes are glued into the slits and the whole rig is painted black. The spider is not attached to the upper tube assembly by anything but friction. So far, it hasn't budged a millimeter, but I will probably add a safety wire to it, just in case.

Second picture on the right is Flavio's upper tube assembly. Way more sophisticated than mine. Holes cut down on weight and it looks like a professional secondary holder to me. The workmanship is beautiful.

Here is a closeup of the top of a pair of truss tubes, showing the mounting hardware.

The ends of the tubes are simply hammered flat, and ¼" holes drilled through. A 2" x 1½" x ½" wooden block has a slit cut into it wide enough to accommodate the flattened truss tubes. ¼" holes are drilled, and pieces of ¼" dowel are forced through and cut off flush.

Important: Don't cut the truss tubes to length until you've found the focal point of your mirror. Once cut, you don't have much room for adjustment. What I did was use woodworker's clamps to clamp the truss tubes to the upper tube assembly and play with it until I had everything in focus. Then I marked the tubes and cut them to length. If you botch this step, you may need to buy more aluminum tubes, and they're expensive.

There is a small dowel peg in the block that mates with a hole on the underside of the UTA to align them. An ordinary draw hasp from the hardware store holds it all together.

In addition, the part of the draw hasp attached to the UTA acts as a hanger when the UTA is stowed in the mirror box.

Focuser

Here are two views (outside and inside) of the focuser. Extremely primitive. The white part that didn't photograph too well is a section of 1¼" diameter PVC sink drain extension with male threads. There is a hole drilled through the plywood just large enough for the threads of the sink drain extension to slip through.

The grey part on the inside is a piece of 1¼" PVC electrical conduit with female threads. It just so happens that the threads of these two parts are a perfect low-friction fit. I got the idea from Russ Orr's web page World's Cheapest Low-Profile Focuser. On his page, you can see that he painted all the parts black. Also, while the electrical conduit piece on mine is simply epoxied onto the backside of the focuser board, his protrudes all the way through to the front of his.

The focuser mounts to the UTA with two small dowel pegs on the bottom, and a single screw at the top.

(See the Flavioscope)

The only change I'm likely to make at this time will be to install a Crayford focuser instead.

Accessories Box & Ground Board

Shown here is the small accessories box and the ground board. The ground board is sized to fit exactly on top of the mirror box, acting as a lid. The accessories box is sized to take up the space from the top of the ground board to the top of the rocker box.

On the sides of the accessories box are two bits of wood that match the bearing cut-outs on the rocker box. These hold the latches which hold the accessories box in place when everything is stowed.

The underside of the accessories box has enough space to accommodate the feet of the ground board and the two rocker bearings.

The biggest mistake I made here was making the accessories box too shallow to hold eyepieces. In retrospect, I should have made the mirror box about 1" shorter so that I could have made the accessories box 1" taller. The dimensions I give for the mirror box above reflect this change.

Stowing

First picture: Side bearings are removed from mirror box. Mirror box goes into rocker box. Upper tube assembly goes into the mirror box.

Second picture: Ground board is flipped over and covers the mirror box. Side bearings sit on top of the ground board.

Third picture: Accessories box goes on top and is latched into place.

Finishing

All plywood raw edges (first photo) are finished with birch veneer which is made by slicing thin strips off of a solid birch board (samples: second photo). This is pretty easy if you have a table saw, and well worth it in the final result (third photo). If you don't have a table saw, wood veneer can be purchased commercially.

All screw holes are plugged with birch plugs. This greatly improves the final result, but makes it so you can never take it apart again. Plugs can be made with a drill press and special plug-cutting tool, or they can be purchased.