Building a 10 Inch Yolo Telescope

This is my 10 inch F15 Yolo telescope. The light path is unobstructed which results in high contrast, high resolution views of the heavens.

I ordered ceramic tiles for making grinding tools, grit and pitch from Tom Moulton (GotGrit.com)

Dental stone plaster from Amazon.

Art Leonard came up with the Yolo telescope in the 1960’s. It is quite similar to Anton Kutter’s Schiefspiegler. The main difference is Art Leonard used a concave secondary mirror instead of a convex mirror as found in the Schiefspiegler design. It appears from Art Leonard’s document “The Yolo Reflector”, that Art Leonard suggests an unobstructed telescope could be made by grinding and polishing two identical spherical mirrors of long radius of curvature, then warping the secondary mirror with a tensioning harness. The secondary mirror would need to be stopped down a bit in aperture since the contact points of the warping harness creates “spiking” to appear in a focused star image. Spherical mirrors are the easiest mirrors to make and since the mirrors are the same, one could use the same grinding tool and pitch lap on both mirrors.

The diagram above is from Art Leonard’s “The Yolo Reflector".

The photo is of Art Leonard with his 8 inch Yolo telescope at the Western Amateur Astronomer's convention in Reno, Nevada in 1965.

(photo provided by Jack Borde)

My Yolo telescope and most others that have been built since have a secondary mirror with a polished-in toric surface to correct the astigmatism. This eliminates the spiking and stability problems that occur when using a mechanical warping harness. The toric shape reminds me of the side of a football. The curvature is stronger along one axis (sagital) and weaker along the other axis (tangential).

Another change that I made was to make secondary mirror with a longer radius of curvatures. This requires an increase in the secondary mirror tilt to correct coma. This increased secondary tilt has the advantage of eliminating the need for the “Muffle baffles” at the top of the telescope. There is no direct light path from the sky to the eyepiece and the overall length of the telescope is a bit shorter.

The focal ratio is F15 so that the primary mirror can be left spherical and still would result in diffraction limited performance. Having a high F-ratio does not bother me since this type of telescope allows me to keep both feet on the ground when observing. That's more important as you get older.

Making the back focal length about 15 inches longer than the mirror spacing, leaves enough room for the primary mirror cell, focuser, star diagonal, bino-viewer and eyepieces. When the star diagonal is not used, an extension tube is added in front of the eyepiece.

I used and Excel spreadsheet with the necessary Yolo formulae and Dave Stevick’s raytrace program “Winspot” for the design.

The course of action to follow, is to come up with the design. Then, make the mirrors as close as you can to the design radius of curvatures (ROCs). When the mirrors are finished, put the actual ROCs back in Winspot and slightly adjust the mirror tilt values to end up with a final design with coma and astigmatism nulled out. Then finish building the rest of the telescope.

At my age (71), I’m not going to be pushing glass by hand any more, except near the end of polishing to smooth out the figure. Most of the work is be done by machine. Besides, it’s great fun tinkering with motors and pulleys. I found a motor at a garage sale for $5. The rotating platform is a 24 inch table top from Menards. The main drive belt and idler wheel, I got on Amazon for $20. It’s the same drive belt you find rotating the drum inside your electric clothes dryer. I rigged up a water drip bag like those used to water plants and a little Arduino running basically a grit salt-shaker mounted on an RC servo. The grit salt-shaker is made from an old pill container and two screw-on caps. The two caps form a temporary chamber to hold a small amount of grit. Each cap has a small hole for the grit to pass through. The holes are offset to prevent a continuous flow of grit. The time between grit shakes is adjustable, typically 30 to 60 seconds between shakes. I would let the machine run this way unattended for about an hour. Then I would check the progress and add grit or water.

Grinding the mirrors continued using common techniques known to amateur telescope makers. The final grit size used was 5 micron. Along the way, the radius of curvature was measured using home-built spherometers.

The secondary mirror had an additional grinding step added at the end using the 5 micron grit. Using long straight strokes, in one direction only, working by hand and checking the toric gage frequently, the required toric shape was ground in the mirror in about 12 minutes total. The toric gage, that I came up with, rests on the glass at three points around the periphery. The depth gage touches the glass on the opposite side. This design actually amplifies the gage reading 4 times the sag difference. The gage is first zeroed out in orientation of the grinding strokes, then the gage is lifted off and rotated 90 degrees. Then set back down for the reading. In my case, the gage read 0.0007 inches. That’s quite small, but workable. The photo shows that actual reading obtained. Great care was used when setting the gages on and off the glass so as not to damage the surface.

An improved Foucault test is a must! It is a “slit-less” Foucault type, using a straight edge as the knife-edge. There is a variac transformer to adjust the lamp brightness. When using tester, the 10 inch primary mirror is almost 38 feet from the knife edge. The 8 inch secondary mirror is roughly 46 feet away from the knife edge! I added a single-aperture 8X binocular viewer to the Foucault tester. You need help like this to get a good look at the mirror’s figure. Using both eyes is a real benefit. A motorized mirror stand was made to facilitate alignment of the tester. The mirror can tilt vertically, rotate horizontally and rotate on the mirror axis for checking the toric ROCs, all while seated behind the knife edge! The DC gearmotors are connected to a switch box using a 50 foot cable. The switches used are momentary double pole double throw type. Having it all motorized was very handy since the tester is so far away from the glass.

After about 30 hours of machine polishing the 10 inch primary glass, the surface roughness is good enough to pass the laser test. The laser used is a low power red laser similar to ones that you might play with your cat. The laser power is less than 1 milliwatt. By aiming the laser at the front surface of the mirror, a rough polish will allow you to see a fuzzy light scattering spot where the laser beam enters the glass. A good fully polished surface will not scatter light and the spot where the laser beam enters the glass cannot be seen. The arrow indicates where the laser beam hits the front surface. The bright spot to the left is where the beam is shining through and hitting the plywood support.

Polishing the primary mirror continued using common techniques known to amateur telescope makers. My Yolomatic machine worked similar to the well-known “Mirror-O-Matic” machines. Checking the ROC occasionally and then working mirror-on-top or tool-on-top (MOT or TOT) to maintain the ROC. The pitch lap is on top. The pill container is dispensing polishing slurry, a few drops at a time. For dispensing liquids, there is only one small hole in the pill container cap.

While polishing the primary mirror, I could not obtain a nice sphere using the machine, so final smoothing was done by hand. What really worked nice, at smoothing the surface, was long straight strokes (1/3 to ½ length) center over center, mirror on top, walk around the barrel for strokes in all directions, very slow rotation of the mirror relative to the full sized lap. The following photos show the smoothing progress. Each smoothing session was about an hour. Note: the bright dot in the middle is simply a reflection from the flat back side of the mirror.

Polishing the secondary was done using a parallelogram linkage system to maintain the toric shape.I believe that grinding in the toric shape is much faster than polishing it in later. (12 minutes vs. many, many hours)

https://www.youtube.com/watch?v=zbaz4PCu6TA

The secondary mirror surface developed a rough grid pattern due to the fact that the parallelogram linkage system prevented any relative rotation between the mirror and lap. Final smoothing of the secondary was done by hand. The same smoothing strokes were used as on the primary. However, the secondary mirror and lap relative orientation, was maintained. A parallelogram linkage was not used. The toric axes were marked on the glass and an axis line drawn on either side of the lap. While polishing by hand, I was able to keep the relative orientation between the two. During a polishing session, I would stop and slightly lift the mirror, only to reduce pressure and not lift it off the lap, then give the mirror a 180 degree rotation, then continue. I did this every few minutes to help average out lap imperfections. I arrived at a very smooth surface, but with a very slight turned edge. I decided to quit since the outer zone of the mirror is stopped down with an aperture and won’t be seen.

Both mirrors ended up with slightly larger ROCs. The initial design angles needed to be changed slightly, to optimize the performance.

First, the ROC values were inserted into my Winspot design. At this point you can click Focus! and Plot to see the raytrace plot.

Next, click Display! and change the one of the mirror tilt values in order to reduce the aberrations as seen in the Trace! window.

Note: Adjust the primary tilt value to minimize the AST' value

Note: Adjust the secondary tilt value to minimize the Coma'T value

Once you get hang of it, the value adjusting process goes fairly quickly. It may require repeating the process again and again since the aberration results interact somewhat. Make smaller changes as you approach the ideal angles.

The structure of the telescope is composed of three sections with trusses made out of oak. The location of the center section was determined by placing all three sections, with mirror glass and eyepiece, on a long balance beam, setting the center section at the fulcrum and adjusting the mirror sections position until a balance was found and the mirror to mirror spacing was correct. Then the trusses could be made. All eight primary truss rods are identical and all secondary truss rods are identical. That way, assembly can go quickly since I don’t have to keep track of which truss rod goes where.

Covering the entire structure are two shrouds made of black cloth. Elastic bands are sewn in at the ends. This covering and 5 additional internal baffles eliminate stray light. The cloth covering also helps to minimize weight and "tube currents".

Since the secondary mirror is facing down, it is supported at three points near the edge of the mirror. Using the program “Plop”, it appears that the 8 inch secondary mirror would bend about 1/10 wave, due to gravity, when observing near the zenith. Using Plop again with the primary mirror in mind, I discovered that I could also set the primary mirror on three points, strategically located, so that the primary mirror would have roughly the same bending as the secondary. I am hoping this will minimize the effects of bending due to gravity.

The mirrors were then sent to OstahowskiOptics, Inc. for multi-layer enhanced aluminum mirror coatings. The cost was very reasonable. I got the mirrors back, arriving at my home in Minnesota, in only two weeks. The surfaces look flawless. No scratches, no fingerprints, just beautiful bright coatings. I highly recommend them for coating needs.

Additional items on the telescope:

Laser dot aligner on the top

8x50mm Bausch and Lomb finder scope with cross-hairs ($5 find at thrift store)

Nexus digital setting circles

Five additional internal baffles inside the structure