By Rick Fienberg
There are several ways to observe the sun safely with a telescope. One is to use a special-purpose aperture filter – that is, a filter that fits over the instrument’s aperture, or front opening – to block all but a minuscule fraction of the sun’s light and thereby create a comfortably bright image in the telescope’s eyepiece. Most solar filters are made from metal-coated glass or Mylar-type film.
Sometimes, though, it can be hard to convince people to look through a filtered telescope; despite your most sincere reassurances, they’re afraid that they’ll hurt their eyes. In any case, only one person at a time can view the sun in the eyepiece.
One way around this is to project an image of the sun onto a card or screen. Now nobody has to look through the eyepiece (which means no refocusing and no bumping), and many people can view the solar image at the same time. But there’s a risk that someone might accidently look into the bright beam of sunlight emerging from the eyepiece.
The solution, and even safer way to use a telescope to observe the sun, is rear-screen projection. That’s the technique used in the Sun Funnel. This simple and inexpensive device makes it easy for many people to observe the Sun simultaneously. Gene Zajac and Chuck Bueter adapted an existing design for a 2003 Great Lakes Planetarium Association Conference workshop. The rear-screen projection material, a key ingredient, is an idea borrowed from Bruce Hegerberg’s Sun Gun.
Supplies and tools
- Blitz Super Funnel #05034, 17.75-inch ×5-inch ×5-inch (round top), $2 to $5 at your local hardware or auto-parts store.
- Large hose clamp, 2.5-inch ×5.5-inch, e.g., Breeze #62080; $1 to $2 at your local hardware store.
- Small hose clamp, 13/16-inch ×1.5-inch, e.g., Breeze #62016; $0.50 to $1 at your local hardware store.
- Da-Lite High-Contrast Da-Tex rear-surface projection screen #95774 (or here), 8-inch ×8-inch; ~$10 for 1 square foot (usually the minimum order size).
- Inexpensive (e.g., Huygens, Kellner, Plössl) telescope eyepiece, 1.25-inch barrel, focal length ~5 to ~25 mm (tips on choosing the optimum focal length are on a later page). Use one that you already have lying around. If you don’t have one, they’re available from numerous manufacturers and dealers, including Celestron, Sky Instruments, Meade, Orion, and Edmund Scientific. The cheapest option is Surplus Shed (www.surplusshed.com).
- Flat-head screwdriver
- Small hacksaw
- Medium-to fine-grit sandpaper
- 12-inch ruler
Match the eyepiece to your telescope
What type of telescope should you use? A refractor! Using a Newtonian reflector or a catadioptric (mirror-lens) telescope is strongly discouraged, as concentrated sunlight can destroy such instruments’ secondary-mirror holders. That said, you can use a reflector if you stop down the aperture by placing a piece of cardboard over the front end with a small (1 to 2 inch) hole cut off to one side to let in only a little bit of sunlight.
A refractor’s focal length is usually indicated on the barrel of the objective (front) lens. Or, you can calculate the focal length of your telescope from its diameter (D) and focal ratio (f/ratio or f/number):
FLtelescope = Dtelescope × f/ratio
The best eyepiece for a full-disk (~100 mm diameter) solar image is:
FLeyepiece (mm) ≈ FLtelescope (mm) ÷ 43
An eyepiece with a shorter focal length will produce a larger Sun image; an eyepiece with a longer focal length will produce a smaller Sun image. Don’t worry about getting the eyepiece focal length exactly right. Depending on what you have available or what you can buy cheaply, aim to get an eyepiece whose focal length is within ±10% of the ‘ideal’ value of FLtelescope ÷ 43. For the mathematical backgrounds see this pdf.
The funnel has a sharp little piece of plastic protruding from the side about halfway down its length. Using the sandpaper, grind it smooth so it doesn’t scratch your hands.
Using the hacksaw, cut the little flat tab off the wide end of the funnel. It works best to cut halfway through from one side, then halfway through from the other. Sand the rough edge smooth now or later in step 5.
Using the hacksaw, cut about 7 inches off the narrow end of the funnel so that what’s left measures about 10 inches long (use the ruler). Try to make the cut perpendicular to the axis of the funnel, but don’t panic if it ends up slightly tilted. Rotate the funnel as needed to complete the cut.
Stand the funnel on its wide end. Using the hacksaw, cut straight down across the middle of the narrow opening, making your cut about 1 to 2 inches deep. The narrow end of the funnel will now have two semicircles of plastic rather than a solid circle.
Using the sandpaper, smooth all the cut surfaces on both ends of the funnel.
If your eyepiece has a rubber eyecup and/or rubber grip, remove it/them. Note that if you have a yellow thread-in eyepiece filter, you can screw it into the eyepiece barrel to produce a yellow Sun. But note, too, that the true color of the Sun is… white!
Insert the eyepiece into the narrow end of the funnel: lens in, chrome barrel out. You may need to pry apart the two semicircular halves of the funnel’s opening. If the eyepiece still won’t go in, cut away a little more of the funnel to widen the opening, then try again. Aim to get at least a half inch of the length of the eyepiece into the funnel.
Place the small hose clamp over the narrow end of the funnel and, using the screwdriver, tighten it around the funnel to securely hold the eyepiece.
Turn the funnel wide end up (you might find it easiest to sit down and hold the funnel between your knees). Place the Da-Tex screen over the wide opening; it doesn’t matter which side faces down. Lower the large hose clamp over the wide end of the funnel and, using the screwdriver, tighten it around the funnel to securely hold the screen; as the clamp begins to get purchase on the funnel and screen, gently pull down all around the loose edge of the material so that the screen ends up flat and taut over the funnel’s wide opening. This is an iterative process; you’ll need to pull down on the material after each turn of the screw to keep it taut.
Congratulations! You have successfully built a Sun Funnel!
Insert the eyepiece barrel into your telescope’s 1.25-inch eyepiece holder, secure it with the thumbscrew(s), aim your telescope at the Sun (first taking care to cover your finder scope, if any), focus, and enjoy group viewing with your Sun Funnel!
Warning! Always supervise the use of the Sun Funnel. Never point an unfiltered telescope at the Sun. Severe eye damage may occur!
How to aim a telescope at the sun
How do you aim a telescope at the Sun when you’re not supposed to look through it, and when you’re supposed to remove or cover your finder so that you don’t look through that either (and so that bright sunlight doesn’t melt its crosshairs)?! One solution is to watch your telescope’s shadow on the ground and adjust the aim until the tube’s shadow is as small and as round as you can get it. Another solution is to add a special-purpose Sun finder that projects a shadow or a spot of sunlight onto a target. There are several commercial units available, including these:
Yet another solution is to make something yourself based on the design of one of these products.
Observing the sun safely
Whenever you observe the sun, with any technique, you must put safety first. The key to successful solar projection, including use of the Sun Funnel, is to use the right kind of telescope – one that can tolerate having full-strength sunlight pass through the optical train – and to use an eyepiece that doesn’t have any plastic in it. We’ll say it again: we recommend using a refractor (a telescope with a front lens) – not a reflector (unless you stop it down to a 1 or 2 inch aperture), and never a mirror-lens telescope – and a decent-quality commercial eyepiece. Also, remove any finder scopes.
Always take the utmost care when passing unfiltered sunlight through any optics, as you’ll do when using the SUn Funnel or any other projection device. If you see or smell smoke, your equipment is unsuitable or set up incorrectly – get it out of the sun immediately!
Do your solar observing in short stints. Don’t leave the telescope pointed at the sun – or even sitting out in the sun pointed elsewhere – for hours at a time.
Most importantly, never leave a solar-observing setup unattended.