Wednesday, October 07, 2009

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

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