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

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