Building A Really Great Homemade Reflector Telescope: Part 2 - The Newtonian Telescope

Overview

In part one we covered how to build John Dobson's low-cost, non-equatorial plywood telescope mount. Dobson's clever invention makes it possible for astronomers on a budget to build some pretty spectacular homemade reflector telescopes. Instead of spending as much or more on the mount, you can put your money into the optics of your scope. A fancy clock-driven equatorial mount is all well and good, but the cost of one can force you to buy a scope that's smaller than you really wanted - and who doesn't want a giant instrument?  

Ahem....... 

Dobson's invention lets us spend our cash on stuff for the biggest, light-sucking telescope you ever saw. To go with the mount in part 1 of this series, we will give measurements for a 10-inch telescope of the relatively simple Newtonian design. 

Materials

Sonotubes painted black

• 12 inch Sonotube 8 feet long (available at a concrete supply store)
• 10 inch primary telescope mirror
• 10 inch mirror mounting cell
• 10 inch Spider mount for the secondary mirror
• Secondary mirror
• 2 inch focuser
• Eyepiece

Tools:

• Saber Saw
• Drill and bits
• Hole saw
• Screwdrivers
• 1-inch screws, 1 box
• Wood glue

Building the Scope:

Mounted cell and eyepiece

Step 1

First we mount the mirror onto the mirror mounting cell. It should come with instructions unless you've cannibalized a mirror from an old telescope and then, it's probably already mounted on the cell.  Follow the instructions that come with the mounting cell.  Test fit the mirror mounting cell in the lower end of the sonotube and mark the tube where the screw holes go for the mount. Drill the holes, fit the mounted mirror into the end of the tube with the mirror facing inward.

                                              

Mount the telescope mirror on the mirror-mounting cell following instructions that came with the cell.  Drill holes in the lower end of the Sonotube to match the mounting screws and screw the mount into the end of the tube. Use flat washers to protect the integrity of the tube around the holes.

Step 2

Focuser

Check your mirror specifications (comes with the mirror). Subtract 6 inches plus the length of the focuser from the focal length of the mirror.  Measure from the center of the mirror and mark the side of the Sonotube at that distance.  The six inches is for the radius of the tube. Added to the length of the fully partially retracted focuser (right) it shows you where the secondary mirror needs to be in order to put the focal point of the mirror within the eyepiece when it's mounted in the focuser.  When you've marked the spot, then  measure an additional 4 inches and cut off the rest of the Sonotube at that point.  Be careful to hold the sonotube upside down with the mirror on top while cutting. That way you don't get dust on the primary mirror. 

Step 3

Drill a 2-inch hole in the side of the tube with the hole saw. the center of the hole will be where marked the adjusted focal length. Don't forget to hold the sonotube upside down to carry off the dust. When you are finished wipe the inside of the tube with a damp cloth to get up any extra dust particles. Dust is the enemy of the astronomer.

Step 4

Next you'll mount the secondary mirror in the spider mount (right shown assembled). I bought my already assembled. You can hand build them, but that's a whole other blog.  Mount the spider across the open upper end of the tube so that the secondary mirror is directly below the 2-inch hole with its center exactly at the adjusted focal length (focal length minus 6 inches plus half the length of the focuser half extended. Once I figure out where the legs of the secondary spider will attach to the side of the sonotube, I drill holes for the screws. Then I cut a bit above and below the holes in line with the tube so it makes a slot so that I can adjust the legs of the secondary slightly. You'll need that later he you collimate the mirror, secondary and eyepiece. Also, this spider doesn't show it, but I use flat washers to protect the integrity of the tube around the drilled and enlarged holes.

Step 5

Screw the focuser assembly into the tube directly over the 2 inch hole. You'll need to collimate or align the three main optical elements - the primary mirror, the secondary mirror and the eyepiece. Again, that's a whole other blog, but when the three elements are mounted, it will look like this:

Alignment of the primary, secondary and eyepiece.

Step 6

Mirror cell mount and adjusting screws.

The body of the focuser will align the eyepiece. When you remove the eyepiece and look down through the focuser, you should be able to see an image of the primary mirror in the secondary mirror. If you don't see the primary and the shape of the open end of the scope centered there, adjust the secondary's mounting screws up or down to align the image. Once it looks right in the focuser hole, the image will center in the eyepiece. If you did your measurements correctly, the focus point should be findable in your eyepiece by adjusting the focuser up or down. If the primary is out of whack, you'll have to twiddle with the collimating screws on the bottom of the mirror cell mount. Don't force the screws. Let off very gently on the screws on the side you need to tilt the image toward and very very gently tighten the screws on the opposite side. Do this in very small increments. If you tighten a screw too tightly, you can crack your very expensive mirror.

Step 7

Handy right angle
finder scope
shown without mount.

Mount the finder scope. You want to put it about a quarter of the way around the tube from the focuser. Far enough that you don't bump into it with your head, but no so far that looking through it requires you to circumnavigate the telescope every time you move the scope to reacquire the image. With Dobs, you do have to move the scope by hand, so you want to be able to switch easily between the finder scope and the eyepiece.

Step 8

Calibrate the finder scope. It may have instructions, but if not, here's how I do it. Note, I'm giving you this before you mount the scope on the Dobson mount. Really it's kind of pointless to adjust the finder till the telescope is mounted, but it's kind of a logical step in setting up the optics so here goes.

In daylight, point the telescope at a distant building or object (Mt. Ranier works up here in my neck of the woods because you can see it from everywhere).  Adjust the telescope so the object is in the center. Look through the finder scope and see where the object is. There are adjustment screws on the side of the finder scope mount that can be tightened or loosened. Carefully so as not to move the telescope off target, adjust the mount screws on the finder scope till the object is centered on the cross-hairs of the finder. The telescope image should be centered on the exact spot. That way, when you are looking for something at night, you can push the tube around till the finder scope is lined up on what you see and then the same thing will be in the eyepiece of the telescope.

Summary:
That's pretty much it. It's really not hard except for getting your measurements right.  Having that focal length right is the key.  If for some reason you don't have the focal length for your primary, try the method for calculating the focal length described at this link.

Reference:

Mother Earth News: A Homemade Telescope 

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

Larry Brown: Homemade Astronomy

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

Scopemaking:  Plans for a Homemade Dobsonian Telescope

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

Howdy Ya Dewit:  A Homemade Telescope: A Quick Run-Through

http://howdyyadewit.blogspot.com/2009/10/home-made-telescope-quick-run-through.html

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