Building A Really Great Homemade Reflector Telescope: Part 1 - The Dobson Mount

Amateur astronomer John Lowry Dobson, co-founder of the San Francisco Sidewalk Astronomers group is the inventor of a simple telescope mount design that is easy, inexpensive and effective. The "Dob" as it has been affectionately known by amateur astronomers on a budget for over 50 years, is made up out of plywood, old phonograph records and an assortment of nuts and bolts and felt strips.  Because you save so much money on the mount (which tends to be rather expensive if you buy a professionally made one) you get to spend your money on bigger mirrors and more powerful lenses.  We're going to assume a 10-inch Newtonian reflector telescope.  A 10-inch reflector has a 10-inch mirror. The actual tube will be more like 12 inches in diameter to accommodate the mirror mount.  We'll give you the general instructions for building the scope itself in Part 2 of this series, but for now we'll start with the mount itself.  If you already have bought the tube, you can use the tube to get the dimensions of the mount right.

We're going to assume some rudimentary carpentry skills. Be sure and double check measurements against the tube you will be using to insure it will fit in the cradle properly.    

You'll need a bit more than a 4x4 sheet of plywood to build a 10 inch scope.  This cutsheet diagram fails to account for the extra inch in the 18x13 front piece, so I just get a 4x8 sheet of plywood and cut the 18x13 piece out of the extra wood. You could use a 4x4 sheet of plywood for anything smaller than a ten inch scope - an 8 inch for instance would only need an 18x9 inch front. It's pretty easy to pair this design down or expand it to fit other size scopes.

This is what you'll need in the way of materials:

Materials

• Ten inch Newtonian reflector telescope (12 inch outer diameter tube)
• 4 x 8 foot sheet of half inch plywood
• Two unwanted phonograph records
• Felt
• Glue that will stick to vinyl and wood
• Saw
• Drill and bits
• Screws - 1 and 2 inch
• Wrench
• ½ inch by 3 inch carriage bolt, washer and nut (wing nut optional).

Building the Dobsonian Mount

There is a matching "ear" on the other side of the tilt box

Step 1

To make the tilt box, cut out a pair of 12x12 inch plywood squares.  Set these aside

Step 2

Cut out  and two 12x13 inch plywood squares. 

Step 3

Fit the plywood squares together to form an open-ended box. Lightly nail it together to form a 12x12 inside diameter.  Try fitting the tube of your telescope through the box.  It should fit snugly.

Step 4

The 12x13 squares will overlap the ends of the 12x12 squares so that you have an inside diameter of 12x12 inches.  Don't try to miter the ends.  Drill small holes in the overlap, glue and screw the 12x13 squares to the 12x12 squares so you have a sturdy open ended box.  

Step 5

Cut two 6 inch diameter plywood circles for the “ears” of the mount. Sand the edges smooth.

Step 6
Glue and screw the flat disks to the center of opposite sides of the mounting box. Do not put screws all the way through the sides of the tilt box and leave the center of each ear clear for a mounting screw to be added later.

Side view of the rotating stage

Step 7

Cut two 12 inch diameter circles out of plywood. These will form the rotating "stage" on which you will mount the upright support box.

Step 8

Drill a half inch hole in the center of both disks. One will be the lower stage. You will need to drill a shallow ¾ inch recess hole in the hole in the lower stage to allow a slight recess for the carriage bolt that will hold the lower stage to the base. The carriage bolt head will be fully recessed. The picture shows the head sticking up a bit to make the assembly easier to understand, but must be recessed fully or the stage will wobble when set on the ground. 

Step 9

Insert the carriage bolt through the lower stage so that the head is recessed.  Turn the stage over so the 3-inch carriage bolt sticks straight up.  

Step 10

VERY CAREFULLY Drill half inch holes the centers of the phonograph records.  

Step 11
Glue the bottom of one record, slide it over the lower stage bolt and press firmly against the lower stage and let it dry.

Step 12
Match up the hole in the second record with the hole in the upper stage and glue it in place and let it set firmly.

Step 13
Flip the upper stage over so the record side is down and slide it over the bolt and down onto the lower stage so that the records rub together.  

Step 14
Bolt the two stages together.  I like to use a wing nut so I can adjust the tension in the field or disassemble the whole thing when transporting it. Adjust the tension on the nut so that the upper stage turns smoothly, but has some friction to keep the stage positioned once you have it where you want it.

This is what the mount looks like
complete and attached to the stage

Step 15

Cut two 26x12 inch rectangles out of your plywood. These two pieces will form the sides of the mounting pillar.  

Step 16

Now cut an 18x13 inch rectangle to make the front of the mounting pillar.  The pillar will have three sides and be open on one side so the tube can be positioned to point directly overhead.  

Step 17

At this point you are going to make the pillar. Use both glue and small screws to make the pillar for added strength.  Predrill the screw holes so the wood doesn't split when you drive the screws in.  Position the side pieces on their long edges and lay the front piece so that it's bottom edge is even with the bottom edge of the sides.  Where the top will be, the sides will stick up 8 inches above the front piece.  This allows the telescope to be pointed horizontally and even to be depressed slightly below 90° to catch near horizon objects if you're viewing from a height.

Step 18 

Cut a semi-circle arc out of the top edge of the sides of the pillar mount as shown.  This will form the cradle for the ears of the mounting box.  A 6-1/8 inch circle should work nicely.  

Step 19

Sand the cradle and glue a piece of felt to the inside edge of both circular cradles.

Step 20

Glue the pillar mounting box to the top of the upper stage of the base. Drill screws through the edges of the pillar mount int the stage.  Be careful not to screw all the way through the upper stage or it will prevent the stage from rotating properly.

This version with the tilt
box in place adds holes in the
sides and front to reduce weight

Step 21

The mounting box is turned with the ears at the side.  Glue felt around the edges of both ears. This provides the friction needed to hold the telescope in place. 

Step 22

Set the ears of the mounting box into the curved cradles in the upper sides of the pillar mount. 

Step 23

You can paint or sand and varnish the mount as you like. You're pretty much done for now.

When the telescope is mounted in the Dob mount, it will be able to rotate and elevate from below the horizon to straight overhead. The Dob mount is a steady platform. When you have the telescope mounted, you'll need to spend some time adjusting the tension on the stage bolt and the friction between the cradle and the mounting box ears.  If the telescope is balanced properly in the mounting box, adjusting the elevation should be a matter of a soft push to make is slide smoothly to the next position you desire.  The Dob is all about balance.

Our next step will be to complete our telescope and mount it in the Dobsonian mount you've just constructed.  Stay tuned for part 2.

References:

Sidewalk Astronomers: Building a Dobsonian Telescope

http://pages.sbcglobal.net/raycash/manualv2.pdf

Homemade Astronomy: Larry Brown

http://home.fuse.net/astronomy/

Plans For A Homemade Dobsonian Telescope

http://www.scopemaking.net/dobson/dobson.htm

Scopemaking: Homemade Telescopes

http://www.scopemaking.net/

The Pickle Bucket Telescope

http://www.utoledo.edu/as/rpbo/pdfs/howtobuild.pdf

A Homemade Telescope

http://www.motherearthnews.com/Do-it-Yourself/1985-05-01/A-Homemade-Telescope.aspx

© 2013 by Tom King

A Home Made Telescope - A Quick Run-Through

  by Tom King

Introduction
Galileos first telescope was two lenses and a leather tube. My first was pretty much the same thing only with cardboard instead of leather.  I paid $10 for it at Wal-Mart. For much of the history of astronomy, advances in telescope technology has been driven by the efforts and genius of gifted amateurs. Most serious amateurs at some point in their lives will try building a home-made scope. Here's a simple reflector telescope I'm currently working on. Spacing the optics can be very complicated as I'm discovering, but there are plenty of amateur astronomy sites that give you very detailed directions on lining up your lenses and mirrors. I've included a couple of links to some good astronomy sites at the end.

Materials

• Sonotube with a diameter 2 inches larger than the mirror (For my scope it's an 8 inch tube for a 6 inch mirror). Got this one at the hardware store.
• Telescope primary mirror (This one's a 6 inch gimme mirror from a friend)
• Mirror cell (I was lucky and this one came already mounted on the mirror)
• Diagonal mirror mounted on spider the size of the sonotube (eBay - $20)
• Focuser (eBay - $35)
• Eyepiece to fit the focuser (home-made from Lens Surplus kit I bought on-line - got 5 really good lenses out of it)
• Sheet of plywood I like 1/2" for sturdiness and sheer heft.
• Two old vinyl phonograph records (Perry Como Christmas and "The Blodger Family Sings Todays Hits")
  

The Reflector Telescope


The Optics

Start out by drawing up a diagram of the telescope you are building. Get help from one of the astronomy sites below to help you figure out the spacing of the three primary optical elements of the telescope - the mirror, the diagonal and the eyepiece. For this example we wil use a 42" focal length, six inch mirror. The radius of the 8" tube is 4 inches. Take the 42" focal length, subtract the 4" radius and you have 38". That means that the surface of the primary mirror needs to be 38 inches from the center of the diagonal mirror. Measured from the surface of the primary to the outside of the eyepiece hole, should give you 42" (the focal length of the mirror). Your eyepiece, inserted into the focuser should be able to bring the image into focus at that distance.


The Tube

Measure from the base of the cell, add the 38" to the diagonal, add enough for the spider mount plus a couple of inches and cut off the Sonotube there. You can get Sonotubes at the lumber yard or concrete supplies. Concrete contractors use them to make concrete pillars. Spray the inside of the tube with flat, non-reflective black paint. I used a flat black over textured paint. To break up any reflections on the inside of the tube.


The Cell

The cell on which the mirror is mounted has screws on the side for mounting to the telescope tube. If your cell is separate from the mirror, mount the mirror as the cell's instructions tell you to. Then screw the cell into the tube by drilling holes in the base of the tube to match the mounting screws. Install the mounting screws and tighten to center the cell. The mount is adjustable for lining the mirror, but wait to do fine adjustments till you have the spyder, diagonal and eyepiece focuser installed.


The Spyder

Mount the spider in the opposite opening of the scope (the one that points at the sky). The diagonal mounts on the spider. If it didn't come mounted, follow the instructions that came with the spider to mount it to reflect the image precisely 90 degrees.

To position the spider figure the distance (in this case 38 inches) from the mirror to the center of the diagonal. To figure out where to drill holes for the legs of the spider, measure the cell thickness to the center of the primary mirror, add the 38" we figured earlier to the center of the diagonal, then add the distance from the center of the diagonal mirror straight back to as close as you can figure to where the screws in the four legs of the spider line up. Cut a slot shaped hole for each spider leg so that they are spaced at 90 degree intervals round the top end of the tube. The slot allows the spider screws to slide forward and back so you can center the diagonal and align it with the eyepiece later.


The Focuser

Find the spot on the outside of the sonotube that is directly perpendicular to the center of the diagonal mirror. Double check the spot by measuring to the center of hole. Add the distance from mirror to diagonal to the depth of the cell that you figured before (in this case 1 3/4" + 38" = 39 3/4"). Drill a hole the width of the eyepiece mounting tube to allow the light to pass through to the eyepiece. In this case I've installing a 2" eyepiece focuser, so I'm drilling a 2" hole. Then all I have to do is screw the base of the focuser (which is conveniently curved to fit the tube) into the sonotube over the hole.


The Eyepiece
To adjust the mirror and diagonal, simply look down through the empty focuser hole and lining up the diagonal and primary so that you see a centered image of the sonotube opening and the spyder in the eyepiece hole. This could take a considerable amount of fiddling, but isn't that difficult to do, especially if you have a little help.

When everything is lined up, place the eyepiece in the focuser and test it by focusing on a nice bright land target. The finderscope is ready to mount.

The Finder

Screw the finder scope mount to the tube somewhere so it's handy to where you will be standing to look through the eyepiece, but so it doesn't interfere with your view or is obstructed by the mount or eyepiece. It must be lined up parallel with the tube. Focus on a large, easy to find target during the day and then clamp down the tube. Next use the alignment screws on the focuser to align it so that what you see in the finderscope is exactly what you see when you look in the eyepiece (only in the finderscope it will be smaller).

Now we're ready to build a mount for the very fine Newtonian reflector we just made.


The Dobsonian Mount

Cutting the Pieces
Mark your plywood sheets as indicated in the diagram and cut them out and assemble them as shown in the diagrams.


The Tube Mounting Box

Assemble the tube mounting box so that it fits around the tube snugly. Cut the circular ears out and mount them to the sides of the tube mount box. Hold the box by the "ears" and slide the tube up and down in the mount to balance it.

The Base

Assemble the support pieces so the ears of the tube box will fit in the cradle. Glue felt along the cradle arc to provide friction with the "ears". If the ears still move too freely, add a lining around the edges of the ears.

The Stage
Drill a hole in the center of the wooden circular stage piece and mount the upright support on the stage. Glue a phonograph record underneath the stage centered on the drilled hole. Glue a second phonograph record on the second stage piece and drill through it. Set the top stage on top of the lower stage so the records sit face to face. Bolt the stages together. This allows the stage to rotate smoothly with slight friction.

Set the tube and tube box atop the mount and the telescope is ready to collimate (line the optics up).

Collimating
Cut the bottom off a film canister or large pill bottle the diameter of an eyepiece and drill a quarter inch hole in the center of the lid. This is a simple collimater and forces you to line up the scope with your eye in the exact center of the eyepiece. Now adjust the primary mirror so it appears centered in the secondary mirror (the one on the spider)

Adjust the secondary mirror (the diagonal) so the reflection of the diagonal in the primary mirror is in the exact center of both the primary mirror when viewed from the center of the open end and should center underneath the focuser hole.

Focus the telescope on a distant object, leave it pointed, then align the finder on the same object so the finder points at the same thing the telescope does.



Additional Information Resources


A Homemade Telescope: Mother Earth News


Homemade Astronomy: Larry Brown

Plans for a homemade Dobsonian telescope


 (c) 2009 by Tom King
All images (c) 2009 by Tom King