Baffling Reflections

Jack Kramer

One of the observing bugaboos is having light rays randomly bouncing around the inside of your telescope. This causes the field of view to have a bright background. It's especially annoying for deep sky observing where you want the best contrast of faint objects against a black sky. This extra light may come from three sources - ground lights, a bright background sky, or reflections from a bright object you're observing. One way to improve the esthetics of an image is to have your telescope well baffled. The source of stray light can be reflections off any part of your telescope, so "baffling" in the widest sense simply refers to all the various methods that prevent light from being reflected off the inside of the telescope. This includes not only the telescope tube itself, but also the inside of the focuser, star diagonal and eyepieces.

One reason why refractors are noted for giving high-contrast images is that the better-designed ones are well baffled. The baffles are graduated in size because the light cone in a refractor tapers from the primary lens down to the eyepiece. The web site shows how a typical refractor would employ baffles positioned so that looking from the tailpiece (eyepiece end) the observer would not be able to see the inside of the tube.

One thing that compromises the images in poorer quality equipment is that in order to cut costs, baffling is given short shrift. Instead, the manufacturers rely on the insides being painted a flat black. But as you've probably noticed, flat black paint on a smooth surface still reflects some light. However, there are ways to baffle a telescope without using rings of graduated size along the inside of the telescope tube. For example, Tele Vue and TMB refractors have successfully employed a system of flocking the interior of the tubes so that light reflections are at an absolute minimum. Another method is to use "micro baffles", which basically look like screw threads running the length of a tube. Due to cost considerations, micro baffles are seldom used on telescope tubes, but are typically found inside eyepiece barrels and in the better star diagonals and focusers. In fact, some are not simply concentric rings milled into the tube but are actually threads that allow filters to be screwed into the end of an eyepiece or star diagonal. In addition, the better star diagonals, focusers, and eyepieces will often have the traditional baffle rings somewhere along the inside to further block extraneous light. "Ghost images" usually originate with an eyepiece that isn't well baffled or that doesn't have the lens edges blackened. (Upon checking, I found that all my Tele Vue, Pentax, TMB, University Optics, Vixen, and Orion equipment employ one or more of these methods. The only components that have no baffling are my Meade Super Plossl and Ultra Wide Angle eyepieces ... and it shows)

This brings us to reflectors. Since the light path travels along the length of the tube walls, any baffling needs to take a different tack. The following illustrates how that baffling might look in a solid tube Newtonian.

In a truss tube Newt, the framework itself provides a baffling effect, plus a black shroud over the open trusses helps keep out stray light. Seldom do you see a Newtonian with multiple ring baffles of the type shown in the above illustration. If this type is used, it's generally only a single baffle placed about where the one closest to the primary mirror is shown. A simple method of making baffle rings is to place strips of self-adhesive sponge rubber weather-stripping material around the inside circumference of the tube. More common is flocking the interior of the tube, generally with black, self-adhesive felt or flocked paper found in craft stores. At the very least, a large piece of this material placed opposite the focuser makes a big difference. That's what I did on my Newtonian. Under the night sky I can barely see the inside of the tube, but the part with the felt is like looking into a black hole - totally dark. If your focuser or star diagonal is not well baffled, you might find that flocking material is a welcome addition there too.

The Newtonian is a screen print from the freeware program Newt for Windows, which has a lot of very useful information for Newtonian users - you simply enter the specifics for your equipment and it calculates all the different parameters, including eyepiece field of view and magnification...and where to place any baffles.

Published in the August 2008 issue of the NightTimes