Expert: Paul Wagner - 12/8/2011

Question
I'm in a bind here.  My son and I started a project based on internet and books from the library for my son's science fair project.  It was to build a reflector telescope.  Seemed pretty simple and based on recommendations, we bought a tube (a 4-inch diam pvc pipe), a 4" 10X concave shaving mirror, a 1" mirror and some Meade eyepieces (a 9mm, a 25mm and a barlow 2X).

I just can't figure out where to put everything.  I'm very handy at construction but just can't figure out the distance between the two mirrors and the secondary mirror and the eyepieces.  Everything seems to look like a blob of light the way I have it right now.  Which is:  the secondary mirror is about 18 inches (I don't know why, probably estimating from what I've seen) from the primary and the eyepiece is movable in a tube between 5 and 10 inches from the secondary mirror.

Please help as I can't seem to focus on ANYTHING no matter where I place the eyepiece or secondary mirror.

Your assistance is appreciated.

Answer
HI Behjat

You've come to the right place.  The challenge here is to determine the focal length of that shaving mirror.  If you can, take it out tonight and point it at the moon.  Then try to get an image of the moon to form on a small slip of paper (1 inch wide) that you hold in front of the mirror.  I don't know what the focal length will be...but this is how to determine it.

Once you  have that information, the rest is easy, but it helps if you have a drawing.  Draw the shaving mirror...and draw how the light from both edges comes to a focus at a point exactly as far away from the surface of the mirror as the focal lengtt you measured with the moon.  It should look like a long skinny triangle.  Draw a third line from this point directly down to the center of your shaving mirror.  

Now imagine that you have to insert your smaller mirror at a point that will reflect this light 90 degrees out the side of your tube.  And since you need to clear the tube and give yourself a little room for focus, maybe three inches is good.

So mark your focal length line 3 inches from the apex of the triangle.  draw the smaller mirror there, at a 45 degree angle so that it will reflect the light out the side of the tube.  

That should give you an optical assembly plan.  

Now mount your mirror at the bottom of the plastic pipe tube. Once you have that down, measure from the front of that shaving mirror up the side of the tube to where the diagonal is going to intersect the cone/triagle of focusing light.  That's where you cut your hole for the eyepiece.  (this is NOT from the bottom of the tube--it's from the front of the mirror!)

Put the smaller diagonal mirror in place, and adjust both mirrors so they are centered.  

Now point the scope at the moon, and play with your eyepieces to get an image in focus.  It shouldn't be hard.

Remember that focusing on an object that is much nearer, light a light bulb in your house, will not work---because your telescope is designed to focus on objects that are at infinity--not closer objects.  And the image will be upside down....because that's the way this kind of scope works.

Hope that helps.

Paul wagner