Stockton Astronomical Society
Valley Skies - December 2000 Issue
The Telescope Nut
by Jeff Baldwin
A Baffling Subject
Baffles. Hmmm, my telescope doesn't have baffles, what's the big deal?
Not too many people baffle their telescopes, but to optimize a telescope would require a baffling system. Some are elegant and simple, others complex and difficult. Is it that important? Probably.
When you look through your telescope there is a level of contrast that can either be acceptable or unacceptable, hopefully excellent. To obtain the utmost details of scrutiny, the contrast must be maximized.
Contrast can be thought of much like light pollution. If you are at a very dark site and are looking at a faint galaxy, the dark sky background allows that galaxy to be seen against it. If you move to the city where the background sky brightness is at the same amount of lumens per unit area of sky as the galaxy, the galaxy isn't going to be seen. A telescope can actually affect the amount of light scattering that can be viewed as this illusion, that of light pollution.
For starters, the optics must be very clean. If there is dust on the mirror, the light that hits the mirror and is reflected back to be focused as an image is not the only light going into the eyepiece. Light can hit the dust and scatter into non-imaged background light. This can also happen if the mirror is under-polished.
If the telescope has a tube (most do, but not all), light can reflect off the inside wall of that tube and enter the eye as unfocused, unwanted light, most likely illuminating the background darkness and making the sky appear not as black. This obviously decreases contrast. A good baffle system in the telescope can block these unwanted reflections. The baffles don't need to be black, but most are, and it wouldn't hurt. Most importantly, they need to be properly placed.
The trick to proper baffling is to set the baffles in the tube so that no light ray can reflect off the walls of the telescope to strike the mirror and reflect back to the eyepiece. This can be done with a minimal amount of baffles. If you have more baffles, that's ok, but you might as well minimize the number.
This article is going to refer to Newtonian baffles, but check out Telescope Optics, Evaluation and Design, Rutten & van Venrooij, page 220-225, to find out how to baffle a refractor.
The baffle in the front end of the telescope is the entrance pupil, and it must be not only large enough for clear aperture of the optic, but a little more for the fact that the telescope will have a field of view that you do not want vignetted by the tube structure. The rule of thumb is that the entrance pupil must be the aperture plus the focal length divided by 100. So, if you have a 10" f/5.6, then the focal length is 56" and the entrance pupil must be at least 10 + 56/100 = 10.56".
From here out you have three ways of doing this. You can do it by math, or you can do it by scale drawing, or you can do it by full size layout. Eric Reichenbach did the full size layout and the results are unmatched. His telescope is nearly perfect in optics and baffling, and if you haven't checked his scope out, do so.
I'm going to go through a scale layout since this paper is only a foot tall and a full scale layout would be difficult to mail to you in this forum. Start by drawing a scale drawing of the tube, primary mirror and secondary mirror. Add to this drawing the entrance pupil baffle. Now draw two lines each of which connects the edge of the entrance pupil baffle to the edge of the primary mirror. These two lines are the inside diameters of the baffles. Let's call these the I.D. Lines.
The next baffle is the back end baffle, or Baffle A. It is at the back end of the tube and protrudes far enough so that the inside diameter is smaller than the diameter of the primary mirror. This keeps light from sneaking in the back end. Some people paint the back of the mirror black as well.
Draw two lines, Line 1 and Line 2, such that Line 1 goes from the edge of the primary mirror and passes the edge of the secondary mirror that is closest to the front end of the tube, and Line 2 goes from the other edge of the primary mirror and passes through the edge of the secondary mirror that is closest to the primary mirror. Draw a fake field of view line. This is a small line segment that is as far from the center of the secondary mirror as the focus, but is in front of the telescope rather than near the focuser. Its length is between the intersections of this line segment and Line 1 and Line 2. Line 1 intersects this segment at P and Line 2 intersects at Q.
Draw a line from point Q to the point where Baffle A touches the tube wall. Where this line intersects the I.D. line will be the placement and inside diameter of Baffle B.
Draw a line from where Baffle B touches the tube wall to point Q. Where this line intersects the I.D. line is the placement for Baffle C, and the intersection itself is the inside diameter of Baffle C.
Continue this for each successive baffle until the baffles are too close to deal with or until you are at the entrance pupil. You will have to cut them to avoid the focuser.
Sometimes the secondary mirror is small enough that light can reflect off the tube wall opposite the secondary mirror as seen by the focuser. For this reason black velvet or a concoction of flat black paint mixed with Elmer's glue and chopped walnuts is placed on the wall.
The larger the tube the less baffles this geometry will create. You can minimize the baffles by making the tube wider, but that causes the secondary mirror to enlarge. Once the secondary mirror is 18% the diameter of the primary you need to begin being concerned about diffraction caused by the secondary mirror. You can design your telescope to have a balance between the size of your secondary mirror and the number of baffles.
Another baffle that is often used is a baffle at the end of the focuser itself. If the telescope is ray traced you can see that the outer edge of the end of the focus tube has no needed light path, and an annular baffle can be placed there. Different eyepieces may require a different focuser baffle. This may seem like more work than is needed, but if you are going to have the best mirror, the best eyepiece, the best filter, why not continue the optimization process with baffles.
Another important contrast item is the color of your telescope. Have you seen deep sky observers with white tubed telescopes? Yikes! Any color other than white will keep your vision and contrast optimized. Dan Chaffee of Kansas City told me that he uses a black conic light guard at his eyepiece to make his observing area near the eyepiece itself a totally dark place. His whole head is surrounded by black. Not a bad idea.
Good luck, I hope this wasn't too strange, I think it's an important topic.
Clear Glass...Jeff Baldwin
Copyright © 2000 by Jeff Baldwin
For more information on Telescope making jump to the ATM page.
Lasted Updated: 12/10/2000
Clear Glass...Jeff Baldwin
Copyright © 2000 by Jeff Baldwin