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How to Make an Eight Inch Reflector
Part 7: The Secondary Mirror

In this part of the tutorial we make and install the secondary mirror assembly. This is the little mirror that directs the light out of the tube and into the eyepiece.

part 1 of this tutorial (the beginning) is here



The secondary mirror installed


  • Parts list For the diagonal mirror assembly :
  • 18 inches of threaded rod - 8-32
  • nine 8-32 nuts for the threaded rod
  • 1 piece of one inch diameter dowel that is 2 inches long
  • 1 diagonal mirror

How the mirror lines up

We want that diagonal mirror to be centered in the tube. And we also want it to be placed so it is in line with the focuser like you see in this picture.


Cut at a 45 degree angle

Cut your wooden dowel to a length of two inches. Then cut one end of it at a forty-five degree angle.


drill three holes

Drill three holes in that wooden dowel piece. They are around the circumference and evenly spaced.


the three rods

They are like this. And you will be placing a threaded rod into each of them. But not yet!

apply epoxy glue

Apply an epoxy to the parts and attach them together.


glue the mirror

Center that wooden dowel on the mirror. It is centered both top to bottom and left to right.


line up with the telescope

Cut three pieces of your threaded rod so that they are longer than half the tube diameter. In my case I cut them each to six inches long. Now place them like this with the mirror mount.

Place that assembly on your tube like this. Align it so the 45 degree angle will direct the light directly to your eyepiece.

This picture doesn't show the secondary mirror yet mounted on the wooden dowel. But it should be on yours.


Now we need to figure out how far down to drill the holes so the mirror will be centered on the eyepiece focuser.

measure the distance

Measure the distance from the drilled holes to the center of the secondary mirror. Then subtract that from four inches.


distance down from end

That number is how far down you drill for the threaded rods. This measurement will place the center of the secondary mirror at the center of the eyepiece focuser.


How they will line up


So it will look like this.


install the mirror

Go ahead and drill those holes then install the rods and install the secondary mirror assembly.

Do not glue anything. Each threaded rod gets three nuts and with these nuts you can adjust the position of the secondary mirror so it is exactly in the center of the tube and lined up perfectly with the eyepiece focuser.


Installation complete

There you go. It looks good.


NextLet's continue and finish the telescope tube assembly




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