How to Build a Copyscope

Brian Reynolds

What is a Copyscope?

A Copyscope is a widefield refractor built from a surplus photocopier lens. These lenses are short focal length and have very flat fields. I assume that they are at least achromatic, especially if the lens is from a color copier. (Unless you pull the lens from the copier yourself you don't really know what type of copier it came from.) Photocopier lenses tend to be mounted in aluminum cells. Telescopes similar to the ones I describe here have previously been described in Astronomy and Sky & Telescope magazines. I drew heavily from these magazines for my inspiration in building my Copyscopes.

List of Materials

Where to get a photocopier lens

I have two photocopier lenses that I've built into Copyscopes. I picked one up at a local camera store for about $9. It is a Fujinon-Xerox with a focal length of 240mm and a focal ratio of f/4.5. The other I saw in The Starry Messenger for about $10. It is unlabeled, and I haven't bothered to measure it.

Unfortunately the camera store I got my first lens at went out of business years ago, and I haven't seen another ad in TSM for photocopy lenses in some time. I have seen another Fujinon-Xerox at Lens & Repo (a large format specialty camera store in NYC), but it is in poor condition (coated in dust) and they wanted $30 for it. There is good news though. I recently found out that the surplus supplier in the original copyscope article (Astronomy, May 1986) is still in business (although they've changed their name). The address is:

American Science & Surplus
3605 Howard Street
Skokie, IL 60076
(708) 982-0870

Be sure to ask for their optics catalog as well as the main surplus catalog. Note that since they deal in surplus only, specific items might not be in stock (or in the catalog) at any particular time.

The Focuser

The next step is to choose the focuser. You can either buy or build a focuser. I built a helical focuser. A helical focuser works like the focusing mechanism in the lens on a SLR camera. A set of threads converts rotary motion (twisting the focuser) into linear motion (moving the eyepiece and objective in relation to each other). This focuser is easy to build, smooth, accurate, and accepts 2 inch accesories. Unfortunately helical focusers are somewhat annoying to use with a diagonal in a situation where you need to refocus often (e.g., at a star party). When you focus the telescope the eyepiece and any accesories (e.g., a diagonal) will rotate with the focuser. You can buy helical focusers that do not rotate accessories, but they are somewhat expensive for this type of telescope.

The other common options are rack and pinion focusers (which can be subject to gear slop and sticking) and Crayford focusers. Ads for rack and pinion and Crayford focusers can be found in Astronomy and Sky & Telescope magazines. The construction of Crayford focusers has been described in Sky & Telescope magazine. Which ever type of focuser you choose, make sure it is a low profile focuser.

I made my helical focuser from a plastic toilet tank flapper valve assembly that you can get at a hardware store. The nut (which is hexagonal) is mounted to the endplate of the Copyscope. Take a hacksaw and cut off the threaded part of the flapper valve. This part will hold the eyepiece, or diagonal. If the fit between the nut and the threaded section is loose you can use a narrow strip of plastic packing tape across the threaded section to adjust the tension.

Now you have to decide whether or not to use a diagonal. A diagonal will add to the cost of the Copyscope (it doesn't make much sense to build one), but it makes observing more convenient. Without the diagonal the telescope will have to be mounted higher to use, and you will be subject to neck strain. Unfortunately, as mentioned above, diagonals and helical focusers can be annoying to use together.

Optical Tube Assembly

The original Astronomy and Sky & Telescope ("The Tuneable Finderscope", October 1990) articles used PVC plumbing pipe and fittings to construct the optical tube assembly. Unless you find a pipe that fits your lens cell without modification, I wouldn't bother with PVC. PVC is hard to work, and heavy.

I use a cardboard tube meant for artists to use to carry around drawings as the optical tube. A single artist's tube can be used for several telescopes. The end caps for the tube should be saved as lens caps. My tube has an inside diameter of 4 inches. Depending on the size of your lens, a mailing tube might also be a good choice.

Paint the inside of the tube flat black. Krylon super flat black works nicely. You can also use flock paper available from art supply stores, or Edmund Scientific.

I used strips of bass wood (1/8" thick strips worked perfectly for the unlabeled lens and artist's tube) to shim the lens cell inside the cardboard tube. Measure the outer diameter of the lens cell and the inner diameter of the tube and divide by 2 in order to figure out how thick to make the shims. A sheet of 4" wide by 1/4" thick basswood works well as a mounting surface for the focuser. (You can get the basswood at a hobby shop.) Cut a 4" diameter disk with a 2-1/8" hole in the center (adjust the size of this disk and hole to fit your tube and the focuser you choose). I mount the disk flush to the end of the tube with eyelet wood screws. You might want to drill pilot holes for the wood screws in order to prevent splitting the wood. The eyelet in the screws can be used as a zero power finder.

At this point you need to figure out how long to make the tube. Start off by attaching one end of a yard stick to the lens cell with a few rubber bands. Take this assembly outside with an eyepiece (and the diagonal if you are going to use one). Point the lens at something far away. A building or TV/radio antenna on the horizon works nicely. Move the eyepiece along the yard stick until the object on the horizon is in focus. Measure the distance between the back of the lens cell and the shoulder of the eyepiece (or diagonal). This is the back focal length (the focal length measured from the back of the lens) of your photocopier lens.

Now take half of the travel of your focuser (the distance between the fully racked in and fully extended positions) and subtract that from the back focal length. This is the distance between the back edge of the lens cell and the edge of the optical tube. Add this amount to the length of the lens cell. This is the minimum length of the optical tube, however in order to avoid dew and glare you should add at least one lens cell diameter to the length of the tube. Cut the tube to this length, press fit the lens cell and shims into the tube, push the lens cell back until it's rear edge is one back focal length minus half the focus travel distance from the back edge of the tube. Mount the focuser to the endplate, and then mount the endplate to the back end of the optical tube.

Finishing Touches

To finish off the Copyscope you have to devise a method of mounting. You could handhold the Copyscope, but you would find that you would quickly grow tired and not be able to hold it steady enough to look through. A quick and dirty mount can be made with a camera tripod, a scrap of wood at least as wide as the optical tube, a couple of pipe straps, and a 1/4"x20 tee nut. You can get the pipe straps and tee nut from the hardware store. Drill a hole in the center of the piece of wood and hammer the tee nut into it. It helps to countersink hole a bit. Place the tube on the wood and tighten the pipe straps around it and the wood. Make sure that the tube and the tee nut are on the same side of the wood. This will allow the mounting stud on the tripod to pull the tee nut into the wood, rather than pull it out of the wood.

Finally, you need at least one eyepiece to use with the telescope. You should not skimp on the eyepiece. I would recommend a 26mm plossl to start off with for wide field views, and probably a 10mm or 13mm eyepiece as a second eyepiece for high (relatively) power views. Now you can take the Copyscope and tripod outside and go observing.

Clear skies.

Brian Reynolds <reynolds@panix.com>
Last modified: Tue Mar 4 08:55:06 2003