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Building Your Second Homemade Telescope - Part 1


I am a freelance writer, author of 5 books, grant-writer, poet, river rat, amateur astronomer, and nonprofit consultant from Texas.

The Basic Optics Layout

The Basic Optics Layout

Building a Reflector

Most of us start out building a simple refractor scope. You know the one Galileo saw the moons of Jupiter. All you really need for that is a toilet paper tube(s) and two of the right lenses. Reflectors are a bit more intense.

The drawing above shows the layout of a Newtonian reflector. It’s different from a refractor in how the lenses are laid out. A refractor sends the image from the objective straight through to the eyepiece. A reflector takes light in through the opening in one end, bounces it off a curved mirror back up the tube to a mirror suspended in the middle of the which bounces the magnified image up through the eyepiece at the high end of the tube.

The astronomy sites in the Reference section below offer more detailed information to help you figure out how to space the three primary elements of the telescope – the eyepiece, the diagonal mirror, and the main mirror. For instance, In the drawing above we show a 42" focal length six-inch mirror. Now for the arithmetic.

The tube has a diameter of 8" with a 4 inch radius. The math is pretty easy. Take the 42" focal length, subtract the 4" radius and you are left with 38". Thirty-eight inches is the distance between the surfaces of the primary mirror and the center of the diagonal mirror. If you measure from the surface of the primary mirror to the end of the tube, it should be 42". That’s the focal length of the mirror. If you set the spyder mount so the diagonal is at 38” and mount the eyepiece rack and pinion directly above the diagonal, (you have to cut a hole there), you’ll have the focuser at the right distance for your eyepiece for it to focus on a clear image.

The Telescope Tube

A sonotube used to pour concrete pillars can be had at most lumber yards.

A sonotube used to pour concrete pillars can be had at most lumber yards.

Measuring Placement of Optics In the Sonotube

Once you get the mirror mount (the cell) mounted, You will measure from the base of the cell, 38" to where the surface of the primary mirror will be. It’s probably easier to get that measurement by eyeballing it before you mount the cell and mirror. If you’re a little off, the cell allows you to do some adjusting of the mirror. If the mirror surface is say 3 inches above the base of the cell, then add to that the 42 inch focal length of this particular mirror* and cut off the Sonotube there. You’ll mount the spider at 38 inches plus the 3 inches (or so) from the mirror surface to the base of the cell. More on that in a bit.

Sonotubes can be had at either your local lumber yard or a concrete supply outlet. Concrete contractors make round concrete pillars using sonotubes as frames. Fill them with wet concrete and some rebar and you’ve got a pillar. Don’t put concrete in your sonotube, though. Instead spray the inside of the tube with flat black, non-reflective paint. I like to use a textured paint. If you can’t find it in black, you can spray flat black over the textured paint. This helps scatter any stray reflections from moon or starlight bouncing off the inside of the tube away from the mirror.

Mirrors, even ones of the same diameter, often have different focal lengths. Check the references below for more on how to figure focal length of the mirror if it didn’t come with that information.

The Mirror Mounting Cell

For this you need a commercial mirror mounting cell.

For this you need a commercial mirror mounting cell.

Mounting the Mirror Mounting Cell

The mirror-mounting cell is pretty much something you’ll have to buy. I got mine on eBay. You can make one, but getting the mirror placed right is a beast. The commercial mirror mount has screws on the sides for mounting to the telescope tube and adjustment screws to help you get it pointed correctly at the diagonal.

If your cell is separate from the mirror (some aren’t), mount the mirror according to the directions that come with the mount. It’s easy to screw the mounting cell into the tube if you measured the holes you drilled in the base of the tube.Mount the primary mirror in place on the cell according to the instructions that came with the mount. Once the mirror is in place**, install the mount by running the screws through the holes in the tube into the mount. Then tighten the screws carefully one at a time to center the cell. The mount has screws at the base for lining up the mirror once you have the spyder, diagonal and eyepiece focuser installed.

** You might want to wait to do the final mounting of the mirror until you get the spider drilled and mounted, and the eyepiece focuser hole cut and the focuser mounted before you mount the primary mirror if you're not confident of your measurements. It's easier to adjust the one primary vs the spyder and focuser.

Mounting the Spyder

The Spyder I bought is a bit bulkier than some of the newer ones and is a prism reflector rather than a mirror..

The Spyder I bought is a bit bulkier than some of the newer ones and is a prism reflector rather than a mirror..

Mounting the Spyder and Diagonal

The spyder mounts at the opposite end of the scope (the end that aims skyward). The diagonal mirror (or in this case prism) mounts at the center of the spider. If the diagonal mirror didn't come mounted, you’ll have and extra step setting it to precisely reflect the image from the primary, 90 degrees up into the eyepiece.

Position the spider mount so the surface of the diagonal is your calculated distance from the main mirror (in this case 38 inches). Make sure to get the diagonal as close to the correct distance as you can. Measure from the center of the diagonal mirror down to the spyder legs. You’ll need to drill holes for the spyder legs so the mirror will be at the right distance from the center of the primary mirror. In this example, add the 38" we figured earlier from the center of the primary to the center of the diagonal. Mark the outside of the tube, then add the distance from diagonal mirror face to the legs.

Cut a slot shaped hole lengthwise to the tube for each spider leg. Space the holes at 90 degree intervals round the top end of the tube. Make sure the slots are placed to point the diagonal in the right direction***. It won't matter much at this point since you can rotate the tube to set in the mount. By cutting a slot rather than a hole, you can slide the screws forward and back till you get the diagonal pointed straight at the eyepiece once it’s mounted.

*** Don't cut the eyepiece hole just yet till you are sure where the diagonal is aimed once it's mounted..

The Focuser

Rack and pinion eyepiece mount/focuser.

Rack and pinion eyepiece mount/focuser.

Positioning the Eyepiece Mount

Next you need to find the spot on the outside of the sonotube directly above the diagonal mirror. An easy way to center the hole is to measure to the spot that is the focal length of the mirror minus the radius of the tube. Use the marks you made earlier in positioning the spyder (42 inches minus 4 inches from the mirror face in our example). Drill a hole the width of the eyepiece mounting tube to get the full image through to the eyepiece. I installed a focuser for a 2" eyepiece. So I drilled a 2" hole. Then all I hade to do was screw the base of the focuser into the sonotube directly over the hole.

Aligning the Focuser and Eyepiece

Next you need to adjust the mirror and diagonal. If you haven’t mounted and adjusted the primary mirror, now’s the time to do it. Look down through the empty focuser. You will have to line up the diagonal and primary so that you see a centered image of the sonotube opening and the spyder in the eyepiece hole. It may take you a while to adjust the image correctly, but it’s not hard to do, just time consuming. Here you might enlist a little help – one adjusting screws while the other looks through the focuser.

When everything is lined up, install the eyepiece in the focuser mount. Then you can test it by focusing the scope on some nice bright terrestrial object.

Spotting Scope

Spotting Scope I got from eBay.

Spotting Scope I got from eBay.

Attaching the Finder Scope

Figure out a handy place to mount your the finder scope so you can look through it without having to move the telescope. The finder has to be lined up parallel with the telescope tube. For now just mount the finder scope. You’ll need to align it after you get the telescope mounted properly.

Now we're ready to build a mount for the very fine Newtonian reflector we just made.

Homemade Dob DIY 150mm Newtonian telescope. Built over 8 months in 2009/2010. DIY 150mm Newtonian telescope. Built over 8 months in 2009/2010.

Additional Information Resources

A Homemade Telescope: Mother Earth News
Homemade Astronomy: Larry Brown
Plans for a homemade Dobsonian telescope

© 2021 twayneking


Oscar Jones from Monroeville, Alabama on May 07, 2021:

Wayne, this takes a lot of notes. Its much like learning music. If you get enough steps or notes linked together, well, you might end up sitting at the edge of the galaxy playing "Chopin". anyway thanks. This is pretty well collated and catalogued, waiting for some adventuresome enthusiast to join and take it to the next level!

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