Collimation

Pinhole Artificial Star It would be nice to be able to collimate during daylight hours indoors or at least in a sunny backyard. To do so, however, requires something to fill in for a star. The time honored method is observing the glint of the Sun off a distant power pole insulator. What if none is in view or the day is cloudy? Make a star. A simple artificial star can be made using aluminum foil and a high

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DOI: 10.1007/978-0-387-09772-5_12, © Springer Science + Business Media, LLC 2009

intensity light source. The "star" is formed by making a very small hole in the foil, and that is the only marginally difficult part of artificial star construction.

To work effectively at reasonably close distances, the hole must be tiny. You can make one using a small sewing needle, but an even better-sized hole can be made with a .12mm acupuncture needle. Light source? Any bright light will do, and you can get good results with a PAR floodlight. Even better for the electronically inclined would be a high intensity ("super bright") LED. Don't have the soldering wherewithal to make an LED illuminator (most LEDs need to be connected in series with a resistor for use with batteries)? Bright LED flashlights are easy to come by at discount and sporting goods houses.

A very neat LED artificial star can be made from a rubber or plastic bolt/nut cover from the hardware store. Look for one that fits snugly over a the high intensity LED that's to be used as the light source, and make an appropriately small hole in it with a needle or a very small drill bit.

In a Pinch Artificial Star Need a simple artificial star right now? A miniature Christmas tree bulb will do yeoman's duty for collimation. The small bulbs found in almost all modern Christmas light strings and decorations produce a decent diffraction pattern without the need for a pinhole of any kind. Just focus the scope (or defocus, actually) on the brightly illuminated filament of a bulb. The diffraction pattern bull's-eye may not look quite as good as one produced by a fancier artificial star, but is definitely good enough to get CAT collimation "in the neighborhood."

Slide Projector Star One day while rummaging around in the garage for a light fixture of some kind to use with an artificial star, my eyes came to rest on an old 35mm slide (transparency) projector. "Hmm," thought I, "bright light source, AC powered, convenient on-off switch. I wonder if it will work?" Indeed it did. I figured the bright projection bulb would be good, but I was not sure about the lens: leave it on or remove it? In the end, I just mounted my pinhole-in-foil star on the end of the lens. To my surprise, the addition of the optics seemed to make the diffraction rings look a little better than I'd achieved by placing a bare light source behind the pinhole. Discarded but working slide projectors are plentiful at flea markets and yard sales in this day of digital photography.

Fiber Optics Star Want the ultimate artificial star? Some enterprising amateurs are experimenting with fiber optics. "Fiber optics," as you probably know, uses strands of glass fiber material to conduct laser light for communications and other purposes. Don't use a laser to collimate, of course. One is temptingly bright, but even a low power laser pointer carries the potential for eye damage. One's not needed anyhow. Fiber optics conductors also pass normal visible light—if not as efficiently. Attach a fiber strand to a light source; mount the other end so the tip is surrounded by a dark square of cardboard or other material, and the artificial star blues will be over. Nothing produces a smaller and sharper star than fiber optics.

What kind of fiber optics? There's multi-mode and single-mode optical fibers. Single mode fibers are smaller than multi-mode, but multi-mode is better at conducting white light. In reality, it probably doesn't make much difference. Where do you get the stuff? Surplus retailers and outfits like Edmund Scientifics have short lengths of fiber available for small prices—or be creative. Holiday decorations that use optical fibers are available at flea markets and similar venues. For optical fiber to be effective as an artificial star, a high intensity light source will be required. A high-intensity LED will do the trick.

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