How to make a Sun projector
by Neil Zimmerman and Michael Hepler
revised April 2015
It is wonderful to see the Sun’s surface up close and check in on the drifting parade of sunspots. Although there are plenty of finely-made solar telescopes manufactured for hobbyists, they can be costly. Here we show the do-it-yourself alternative: building a solar telescope at home with minimal costs and tools. With a little planning, it is easy and safe to project a sharp, magnified image of the Sun’s surface. Plus, you gain the added satisfaction of building your own instrument, along with a deeper understanding of how it works.
The first sketch shows how a cardboard box can support a small telescope, at the same time acting as a small projection chamber. Large (foam/cardboard) altitude bearings glued to the box give one axis of freedom for pointing.
Details of the telescope design are shown in the second illustration. For completeness, we include the mathematical formulas describing the optical projection, but we promise it is absolutely unnecessary to understand these to make a perfectly functional instrument.
- Optics. The heart of our sun projector is a simple refracting telescope, in principle identical to one you’d star gaze with. In fact, come nightfall, it may give you an excellent view of the Moon. The lenses and mirror that make up this telescope are the only specialized parts not “street” available.
You’ll want to order three components to build your telescope from:
- Achromatic objective – the lens for the end of the telescope pointed at the sky. For a solar telescope, bigger is not necessarily better: smaller sizes (less than 3 inches) are easier to mount, and will still gather more than enough sunlight. Focal length, the other key parameter, should be in the neighborhood of 150 mm (6 inches) to 500 mm (20 inches). Alignment is easiest when the lens is already mounted in some kind of cell. In our projector, we used, and can recommend this 27-mm diameter, 285 mm focal length objective from Surplus Shed: http://www.surplusshed.com/pages/item/l1403.html. More candidates, all mounted in cells, can be found browsing in the “Objective Lenses” category on Surplus Shed.
- Eyepiece – the lens on the projecting end of the telescope. This should have a focal length in the range of 10–20 mm. Barrel diameter comes in one of two standard sizes: 0.965″ and 1.25″. Whichever barrel size you choose, make sure it matches that of your diagonal, since they must mate. For our projector, we used, and can recommend this 12.5 mm-focal length eyepiece from Surplus Shed: http://www.surplusshed.com/pages/item/l2019.html (update 26 Apr 2015 – out of stock now. In its place, you could use a slightly more expensive eyepiece with the same focal length: http://www.surplusshed.com/pages/item/l3872.html)
- 90-degree star diagonal – not strictly necessary, but by folding the beam from the objective to a right angle, the support structure/projection box is smaller and easier to build. Just make sure the barrel size matches that of the eyepiece (in other words, a 0.965″-type diagonal needs a 0.965″-type eyepiece). Surplus shed has 1.25″ diagonals here: http://www.surplusshed.com/pages/item/l2177.html (update 26 Apr 2015 – out of stock now. In its place, try a similar part sold by Corvus Optics: http://corvus-optics.com/product/1-25-telescope-star-90-degree-diagonal/)
- Telescope housing
- Tube #1: 1 1/2″ diameter mailing tube (Staples, for example, sells them for $1)
- Tube #2: toilet paper tube
- 1 1/4″ x 1″ PVC pipe bushing – this allows you to join the objective lens to the toilet paper tube, if it is small enough. Our rather small objective lens (with 28 mm cell outer diameter) fit nicely, but other size pipe fittings/bushings might be better for your objective. See what your local hardware store has.
- Projection and support structure
- Cardboard box – sized roughly according to your optics; see telescope sketch. Think around the size of a small moving box.
- Extra cardboard or foamboard for altitude bearings
- Sheet of clean, white paper for projection screen
- Large aluminum foil serving dish for base
- masking tape
- duct tape
- glue gun
- utility knife or box cutter
Follow the steps in the photos below to put the telescope together. It will help to know the focal length of your objective lens. WARNING: At no point during this process should you look at the Sun through any of the optics.
The method we used to adapt the lenses to the cardboard tubes may have to be adjusted depending on the diameters of your optics. Keep in mind that there is no single correct way to do this, and feel free to improvise as necessary.
Projection and support structure
Now that the telescope ready, you need to find a cardboard box that will support and point the telescope, as well as act as the projection chamber. This depends on two factors: the ratio of the objective and eyepiece focal lengths (also known as the magnification, for a conventional telescope), and how large you want the image of the Sun to be. For the exact details of this calculation, look at the telescope design sketch. But if your lens focal lengths are roughly similar to ours, then a projection distance (that is, the length between the eyepiece and the projection screen) anywhere from 1 to 3 feet should work fine.