July 1999 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - July 1999 Issue

The Telescope Nut
by Jeff Baldwin

Turned Down Edge

"Turned Down Edge". Telescope makers consider those three little words very seriously. Eric and I have just beat down another method of killing turned down edge, so I thought that it would be a good topic for this month's TN.

First of all, what is Turned Down Edge (T.D.E.) and how is it created?

There are a couple of theories about the creation of T.D.E. When a lap is working back and forth across a mirror, the edge of it moves off the mirror and then back onto it. Since pitch and pads are not rigid they tend to expand a hair when they move off the mirror, and then contract when they move back on. It is the mirror that is supplying the structure in which the lap is pushed back, and so that force pushing at the edge of the mirror over and over again may be what wears the edge off. Another theory is that T.D.E. is created during grinding and only becomes apparent when the mirror is reflective and examined. One other theory is that warm fingers on the edge of the glass heat up the glass, expand it, and then the glass is worn away in its expanded state. When the glass cools off again and contracts, it falls low due to that removed glass. I buy the first theory.

Here are exaggerated pictures of what T.D.E. does optically. In the top image you can see that the mirror is concave, but right at the edge the radius of curvature rolls over dramatically. In the lower image you can see that the light rays reflecting from the mirror all come to the same focus except for the rays from the T.D.E., which focus too far from the mirror. This light will be out-of-focus at the focus of the other rays. Since the outer area of the mirror has the largest surface area, this is not a trivial problem, but actually quite serious.

The star test shows this for T.D.E.

Inside of focus the outer rays are too far out, and not regular. They appear as hair because T.D.E. is not uniform in RoC around the glass. Outside of focus the star doesn't seem so bad, there's no hair, but most importantly, it is different from that of inside of focus. (Remember, the star test must be similar inside and outside of focus.) Sometimes you will see hair inside of focus that is dynamic. This would be turbulence. If it is stationary, it's T.D.E.

This leads us to two things: prevention and correction. T.D.E. can be prevented, but if it exists, it can be corrected. Prevention would include using hard pitch rather than soft pitch. You can buy pitch in varying hardness, and those hardnesses are measured by the temperature at which the pitch melts. Hard pitch is not always desirable for the TN, so you can also prevent T.D.E. by controlling your strokes. If you work at pushing the edge over by heavy outward strokes, you'll probably get it. Working on very hot days will soften the pitch up and accelerate the rate at which the edge rolls. Using pads instead of pitch accelerates T.D.E. as reported by some TNs, and makes no difference as reported by others.

Correction of T.D.E. has been a mystery for TNs for a century. Here are a couple of methods to kill T.D.E. They all work on the theory that a T.D.E. is really a lump just inside the edge of the mirror. That is based on the knife-edge positioning that shows a crest. Jean Texereau claimed that doing center over center strokes with a full diameter lap with the weight on the inner edge will plane off the hill. He would do short back and forth strokes with the weight at the edge, and slowly shorten the strokes while moving inward toward the center of the mirror. I have never found this to work for me. Kestner uses a 1.75" star lap at the edge of the mirror. This digs a V-shaped trench just inside the edge, and then the mirror would be blended until that zonal damage is gone. It doesn't remove the T.D.E., but it pushes it out further toward the edge of the mirror where it can be ground off. Mel Bartels uses a small rectangular lap much like Kestner. Mat Hutchings has used this method and said that it worked, as long as you have enough time in your schedule to spend blending out the zonal damage. A method we just recently used on Eric Reichenbach's 20" f/5 was long, tangential strokes. Using a 12" (60%) lap we made long tangential strokes with about 4" of lap hanging off the edge. As much as this sounds like it would create T.D.E., it actually planes off the hill and kills the T.D.E. I like this method for a couple of reasons. It doesn't require you to use a tiny tool, which really makes your fingers hurt after going around the mirror a few times. It doesn't create zonal damage requiring blending repair. It doesn't require making a new lap. It works really well and quickly. Here are before and after Ronchigram of Eric's mirror after about an hour of tangential strokes. Notice that even though there is still a little T.D.E. to be repaired, most of it died.

When the telescope mirror is finished, most people send it in to be aluminized. I like to remove the edge of the mirror first. Even though you can't see a T.D.E. in the Ronchigram, you will probably still see some in the star test. Even if it is only 1/100" into the edge of the mirror, it still produces an enormous amount of energy outside the Airy disk. Instead of packing the mirror up to be coated, I like to first place the mirror down onto a flat plate glass sheet with a little 5 micron grit and rub little circles. Since the mirror is concave it will only work the edge leaving the optic clean. Any T.D.E. is now goners. Then I have the mirror coated. Upon return from the coaters, I like to paint the little frost ring made by this edge grind and paint it black for a nice finish. T.D.E. is now a non-issue.

In future issues I will start discussing the 40" project I'm about to begin. It may take two years, but probably less. I'll discuss, from beginning to end, the manufacturing of a 40" f/3.6 Kriege style Dobsonian telescope. I plan on starting it around November, but possibly sooner. Keep watching this column.

Clear Glass...Jeff Baldwin
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