October 1998 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - October 1998 Issue

The Telescope Nut
by Jeff Baldwin

Mirror Testing

Our goal today is to test the mirror to see if it is a sphere that is ready to be parabolized. Let's start with Ronchi testing.

Ronchi testing involves the use of a Ronchi grating, a plastic or glass plate with parallel lines etched or engraved on it. These lines are very small and perfectly spaced and straight. I use 85 lines per inch (lpi) gratings for most of my work, but also use 135 lpi or more for special problems. The Ronchi test shows us if there are severe deviations from the mirror being spherical, especially at its edge.

I place a small red light emitting diode (LED) against the grating and mount it on a firm stand, such as a Foucault test stand, something that will be discussed later. The light doesn't have to be a red LED, it could be any light source. I use red LEDs because they are bright, monochromatic and cool. For Ronchi testing, the stand doesn't need to be complicated, just able to be moved and adjusted. The mirror is set on a stand so that it is facing horizontally. The Ronchi grating and LED on their stand are place so that their reflection can be seen in the mirror. Once this is done, then they are moved towards the mirror or away from the mirror so that their reflected light fills the mirror. This would place the grating at or near the center of curvature of the mirror, or twice the focal length. Once this position is established, the grating is pulled back farther from the mirror by small amounts until you can see lines in the reflection. You can also see lines in the reflection if you go forward towards the mirror, but I am going to establish a method that is used when the gratings are farther from the mirror than the center of curvature. Move it back from the center of curvature until you can see about 6 to 10 lines on the mirror.

Notice that the farther you get from the center of curvature the more lines appear. When you are at the center of curvature you see no lines, but rather a messy, confused circle. Also notice that you can move so far away from the center of curvature that the mirror is filled with zillions of lines. This is important: the farther the grating is from the center of curvature, the more lines that are seen.

If the mirror is spherical, then these lines will be straight and parallel, no exceptions. Anything else implies a non-spherical mirror.

Let's do a conceptual error analysis. Image an area on the mirror where the lines are spread out more than other areas. If this were the case, it would imply that the grating is not as far away from that region's center of curvature as other regions, indicating that this region is closer to the grating, or in other words, is a high zone. The mirror is too high in that region, and therefore needs lowering, or correcting. Just the opposite, let's assume that there is a region on the mirror where the lines are closer together than the other areas on the mirror. That would imply that the grating is farther from this region, indicating that the mirror has a low spot at that location, or is over corrected.

The most important part of Ronchi testing is to check the edge of the mirror for Turned Down Edge (TDE). TDE is a mirror killer. A mirror that is perfect with the exception of TDE really does have problems. Here's how to detect TDE in a Ronchi test. If the lines are parallel, straight, smooth and look great except that they hook outwardly at the edge of the mirror, then you have TDE. The reason is that a turned edge has an extraordinarily long radius of curvature, even though it is only at the edge. This pushes the edge's radius of curvature so far out that is very much closer to the grating, and therefore the lines appear farther apart right at the edge. You don't want to progress with the mirror without destroying the TDE. Parabolizing a mirror with TDE is not a compromise, but a failure. I'm guilty of that myself, but I strive to not let it happen with my mirrors these days. The TDE must either be licked before parabolizing, or the TDE needs to be physically eliminated at the completion of the project.

To polish TDE out, there are a few methods. Texereau says to use short strokes with your lap's edge running right at the edge of the mirror. Bartel's approach is to use a rectangular lap (sub-diameter) on the edge. Kestner's approach (the one I use) is to use a very small sub-diameter star lap. I use a 2" pentagram lap. This is run in straight strokes (never in curves concentric to the mirror's center) so that the vertices of the star run on the edge of the mirror on odd strokes and the legs of the star run on the edge of the mirror on even strokes. The star polishes (removes) a "v" groove, while round laps and rectangular laps remove "u" grooves. One would immediately think that the last thing you would want to do to a TDE is to remove more material there. Wouldn't that make the TDE worse?

No. Think of the TDE as a low area at the edge, while only a fraction of an inch farther in the mirror is a higher spherical surface. If the outer edge is planed gradually with the next zone in, then they simply make that region appear as a sphere with a longer radius of curvature. When the Ronchi grating is put back the lines are terribly zigzaggy, but overall are straight. When the mirror is then polished with standard W strokes, the zigzags are blended smooth again, and the lines are straight.

Usually there is a hairline of TDE on any finished mirror. There are two ways out. One is to paint the edge of the mirror with black paint or felt tipped marker. This disallows that edge to be a reflecting surface. The other way is to remove the edge of the mirror physically. I do this by placing the finished mirror onto flat plate glass with 5 or 12 micron grit (tiny) and rub the mirror in little circles. Since the mirror is concave the only ground portion will be the edge, nothing else. This grinds the edge away leaving no TDE. Be sure to CAREFULLY wash the mirror of this nasty grit. To be sure of not scratching the glass, I paint the mirror with a few coats of enamel beforehand, and when finished, remove it with lacquer thinner.

Now that the mirror has straight Ronchi lines, even at the edge, with absolutely no variation, it is ready for parabolizing. Whee!

Clear Glass...Jeff Baldwin
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