Notes from John Dobson's Telescope Making Class

If you would like to grind your own telescope mirror, or build a telescope the Dobson way, these notes should be very useful.

These notes were taken during John Dobson's Telescope Making class in San Francisco, Fall 1996.

You can use these notes as an accompaniment to the actual class, to the online plans for building a Dobsonian telescope, or to the Telescope Building with John Dobson video. You can purchase the video online from the Sidewalk Astronomers national website.

Week 1 (class introduction, view video) October 10

• Ideal sizes for telescopes
  • City observers who wish to see planets - build 10-inch, f/7 scope (6 foot focal length).
  • Observers who wish to see galaxies and star clusters - build 16-inch scope with 8-foot focal length.
• Overall process for grinding the mirror
  • The mirror is ground using a "tool". The tool is a round glass blank slightly smaller and thinner than the mirror blank itself.
  • First, rough grind the mirror blank with coarse abrasive to get the basic curve.
  • Do a basic check for the mirror's focal length.
  • Then, fine grind the mirror blank with a succession of finer abrasives to get a smoother, more precise curve, and to get the mirror into the shape of a paraboloid.
  • Use a pitch lap, together with a fine polishing agent (cerium oxide) to polish and smooth the surface in preparation for an aluminum reflective coat.
  • The polishing stage is the time to finely adjust the curve, or "figure" of the mirror, through polishing and testing.
  • The mirror is then aluminized. A thin layer of aluminum atoms is applied to the mirror surface in a vacuum tank, and then coated with a layer of silicon dioxide to enhance the reflectivity of the aluminum coating.
• Rough grinding notes
  • Don't touch edge of mirror to tool! The mirror will chip if you do this.
  • If you grind focal length of mirror too short, reverse mirror and tool.
  • Carborundum coarse grit will clog drains - do not pour grit-filled waste water down any drain you are responsible for unclogging.
• Fine grinding in this class uses a smaller progression of abrasives than normal
  • 100, 240, 400, 1000.
  • 8 wets each. A "wet" is the process of putting water and abrasive on the mirror and tool, then grinding the mirror against the tool until the abrasive is worn down.
  • Use warm water for wets 400 and afterwards.
• Pouring pitch lap
  • The pitch lap is a hardened layer of melted pitch, poured onto the surface of the grinding tool.
  • Best time to pour lap is when it reaches "Mars Bar" consistency (as seen in TV commercials for Mars Bar).
  • Don't dent the pitch lap after it has been poured.
  • Be careful placing the pitch lap against mirror glass. Place it flat against glass.
  • Add a small amount cerium oxide (one quarter teaspoon) to the suface of the mirror blank, then add warm water and smear. Warm water is important.
  • Don't add too much water. Water should not puddle on mirror or tool.
• Eyepiece for scope
  • Scavenge pawn shops for an eyepiece from a pair of 7x50 coated binoculars. This is a very user-friendly (wide field of view), inexpensive eyepiece.
  • To remove gunk (lubricant) from eyepiece, use a rag coated with some utility oil and use it to wipe the lubricant off.

Week 2 (rough grinding) October 17

• Rough grinding is done to create the curve in the mirror. Fine grinding is used to subsequently smooth the curve.
• One side of the mirror blank will be rougher than the other. Grind the rougher edge of the mirror blank. The smooth edge is kept for ease when mounting the mirror.
• If the tool has been used previously, use the side of the tool that has already been used. One side of the tool must always be flat, in order for it to sit effectively in the polishing stand.
• When performing rough grinding, hold the mirror on both sides and place pressure on a line across the center of the mirror blank.
• Grind the center of mirror across the edge of the tool.
• Grind a few strokes on the left side of the tool, then a few strokes on the right side, then rotate the mirror.
• Turn the mirror continually - use the same amount of turning for the same amount of grinding (left, right, turn, repeat).
• Turn the tool itself every once in a while.
• To place the mirror on the tool, the center of the mirror should first contact the edge of the tool. Do not touch the edge of the mirror to the tool ever, otherwise you will chip the mirror.
• When grinding each side of the mirror, do not go too far over one side, or not far enough. Go a little bit, but not a lot off center.
• If you grind too much curvature, reverse the tool and the blank and grind the tool against the mirror.
• For the ten-inch mirror, you will typically grind away a penny plus one-half a dime's worth of thickness in the middle. This produces a 6-foot focal length.
• When you rotate the mirror, always rotate it to the right. When you rotate the tool, always rotate the tool to the left.
• When you reverse the tool and the blank, continue to turn the mirror often, not the tool.
• Don't put on too much grit for a wet. If you do, the wet will not work effectively. If you put on too little grit, you will waste time because the wets will end quickly and you will clean up more frequently.
• Keep a bucket of water nearby. When the carborundum breaks down for each wet, rinse the tool and the blank in the bucket, then put on grit for next wet.
• If you chip the tool, that will not affect the grinding of the mirror itself. There is no cause for alarm.
• To hold the tool against the grinding board, use three furring nails. When you must swap the mirror and the tool, remove one of the nails and reposition to hold the mirror blank against the board.
• When sitting on the board, sit on a towel to avoid wearing out your pants (carborundum will grind through fabric). Do not reuse towels between grits.

Week 3 (Fine Grinding) October 24

• Use a straightedge to check the thickness of the mirror. Place the straightedge against the mirror, across the center. Then, use the pennies and dimes between the mirror and straightedge to check thickness.
• Do not grind too far! It is almost as much work to correct an over ground mirror (say, 4-foot focal length) as it is to start from scratch.
• If you get the curve deep enough, but the curve is not all the way out to the edge of the mirror, then turn the mirror/tool over and continue grinding. Grind the center of the tool against the edge of the blank.
• If you turn the mirror/tool over to correct an over ground curve, then be careful not to grind two curves into the glass.
  • One easy way to see if you have two curves: check the focal length using the bright sun test. Move the cardboard back and forth through a good range of distance. If you get more than one focus at different distances from the mirror, then you have two curves.
  • If you do have two curves, the old, over ground curve should be in the middle of the mirror (because you have been grinding center of tool over edge of blank). Do another half-hour or so of wets grinding center of tool closer to center of mirror to grind out the "shoulder" between the two curves and even them out into one curve.
  • Check focal length often.
  • To completely finish grinding out the shoulder, you may need to grind center over center for a few wets.
  • The tool should be on top of the mirror through this corrective procedure.
• If the mirror is at proper focal length before the grind makes it to the edge of the glass, then grind with tool on top.
• Fine grinding uses six successive grades of grit, from coarsest to finest: 80, 120, 220, 320, 500 and finally 12 micron.
  • The grade numbers are a designation of the type of mesh screen the grits will pass through. 80 grade abrasive will pass through a mesh screen containing 80 lines per inch.
  • 12 micron, is of course a different type of designation than the other grades.
• In fine grinding, each successive grade of grit goes through the pits left by the previous grade.
• Generally, eight wets with each grit will work to get out the pits of the previous grade. If you are dubious, do a few more wets with the current grade of grit and then go on.
• Clean the mirror, dry it, then check it in the light to make sure that all pits from the previous grade of grit are removed.
  • Do sparkles of previous grit show? Hold the mirror up to a light bulb and let the light shine through to see the sparkles. Remember what the sparkles for one grit look like before you move on to the next grade.
  • If there are still sparkles at the middle or the edge of the mirror, then keep grinding until all the pits are gone.
  • If you see a big pit, put a piece of tape on the back of the glass to mark it. Do a few more wets, then check the spot you marked with the tape to see if the pit came out.
• From grade 320 onward use warm water between the faces of the two mirrors. The warm water causes the faces to bend away from each other at the edges, preventing turned-on edge.
  • Water should be the temperature of hot bath water.
  • Add water, grind the tool against the blank, repeat 3 times. Squish out excess water.
  • Add the grit, then jiggle to make the grit settle against the two faces.
  • Finally, grind.
  • Neither the water nor the grit should be put on in piles.
• Use center over center strokes for the fine grind. Use short, fast strokes.
• Sometimes center over center strokes form vacuum pressure and the two faces of glass will stick together. If this happens, then soak them in bathtub with warm water until they separate.
• Much less grit is needed the finer the grade. The number of particles per teaspoon is much larger with finer grains of grit.
• After 500 grit, you should not see any pits.
• After 12 micron, you should see smoothness.
• On the very last wet of the very last grit, grind with the mirror on top. This will help parabolize the mirror later, during polishing.

Week 4 (Pitch Lap pouring, polishing) - October 31

• When polishing with the pitch lap, wet press is when the mirror and the pitch lap are pressed together with water and cerium oxide between them. In dry press, the pitch lap and mirror are fully dried separately, then pressed together.
• Don't leave mirrors wet pressed together for more than one hour. Otherwise, the mirror and pitch lap can stick together, and you can etch a pattern of the pitch lap into the surface of the mirror.
• To dry press:
  • Separate lap from mirror.
  • Wipe face of mirror.
  • Let the lap dry by itself.
  • Squeeze dust off lap with hand.
  • Set lap on mirror face to face.
• In wetpress state, keep small amount of cerium oxide and water between the plates.
• To get from drypress to a working (polishing) state:
  • Place small amount cerium oxide in middle of mirror. Use a small amount of cerium oxide at the start, so extra friction will help the pitch warm up.
  • Add warm water, smear evenly.
  • Put lap to glass and work slowly for first few minutes, until surfaces heat and pitch warms up.
• Do not carry around mirror plus lap in dry press state - this will cause pitch to crumble!
• If broken pitch gets on mirror face, do not put faces together! If you do, you can cause one facet of the lap to catch the broken pitch, and get minutely higher than the rest of the lap. This will gouge the mirror.
• To remove pitch stuck on mirror face:
  • On a wet surface, remove with fingernail.
  • If that fails, use turpentine, then clean face of mirror with soap and water.
• We will test the curvature of the mirror in the fully assembled telescope. We figure the curve of the mirror with the pitch lap by polishing.
• After polishing for about an hour, you can check the focal length of the mirror, and then cut the focus hole in the main telescope tube. Do not order the telescope tube until you are certain of the focal length. If the scope is set up, you can begin correcting the curve.
• If you think the mirror is fully polished, then get it tested soon. If you touch the lap to the mirror again, it will need to be polished again.
• The mirror is polished adequately when you cannot see shadows of your fingertips on the mirror in bright sun.
• The mirror and lap should remain together always, until you are certain the curve is correct and will perform no more polishing/figuring before the aluminizer.
• The cerium oxide embedded in the lap actually does the polishing. The pitch embeds the polishing agent and planes the curve.
• Do not let the mirror/pitch lap get so dry that they squeak.
• Do not wash the mirror, do not perform "wets" as in rough and fine grinding. When the mirror becomes dry, just add more warm water and cerium oxide.
• You should be pushing hard. If it doesn't push hard, push down harder.
• Polish each side mid-way out near edge. Don't go past edge of mirror with the lap, but put pressure on edges.
• The curve should be proper at the edge - the amount of polishing is less important. It is better to have slightly hazy edge with the correct curve than to have a bad curve.
• When you remove the pitch lap, the mirror should not be too dry. This will help avoid scratches.
• Minor scratches in mirror surface are not deadly to the mirror.
  • Dobson: "You could get everyone in this class to sign their name on the surface of your mirror and it wouldn't make any difference at the eyepiece"
• Cleaning mirror for aluminizing
  • "Slosh wash" with detergent, and a light cloth (such as cheesecloth). Tilt mirror to drip dry.
  • "Slosh dry" with clean dry towel.

Week 5: Construction notes (November 7)

• Do not paint the main telescope tube white! White paint contains titanium dioxide, which will radiate infrared heat well into the night and cause air currents in the telescope tube. The air currents will obscure images through the telescope.
• Scavenge a pair of 7x35 binoculars for the optics you will need for the eyepiece.
  • Make sure the binoculars are fully coated. This means that both the objective lenses and the oculars have special coatings to enhance light transmission.
  • To check to see if the lenses are coated, look into them. Every reflection, on all glass surfaces, should be colored. White reflections indicated uncoated surfaces someplace in the optics.
  • Pawn shops, flea markets, and garage sales are good places to search for used binoculars.
  • To remove the oculars:
    • Remove any rubber eyepiece cups around the ends.
    • Search around the circumference of the eyepiece barrel, near the end, for one or two small set screws.
    • Using a jeweler's screwdriver, undo the setscrew(s).
    • Pull the eyepiece out of the binocular barrel.
    • To remove the lubricant, use rags and some 3-in-1 oil or other solvent. Be careful not to get the lubricant on the optical surfaces.
  • Fit the oculars into the kitchen drain pipe. Shim with some bent cardboard in order to get the ocular to fit snugly inside the kitchen drain pipe.
  • A 7x35 binocular will typically get you an ocular with a 20 mm focal length. This is a good starting magnification. You can also use 7x50 binoculars for a 30 mm focal length, which will get you a lower magnification, wider field of view. But Dobson recommends 7x35.
• For the spider, buy three cedar shim shingles.
  • Cedar-shingled roofs are no longer legal in California. Shingles may not be easy to find.
• To compute placement of focuser hole:
  • Measure down toward open end of tube to focal length of mirror.
  • Back off from focal length by the radius of the tube minus one inch. This is where the center of the focuser hole should go.
  • This figure is for low-profile focusers, such as the cardboard tube assembly in the Sidewalk Astronomer plans.
  • The more of the focal point falls outside the tube, the flatter and more larger the secondary mirror must be. This also increases the cost of the secondary mirror.
  • For the 10-inch F7 scope, use the 1.83 inch (minor axis) secondary mirror.
• Notes about placement of teflon for bearings:
  • Tube bearings: higher teflon is above cradle boards, the more friction you will get.
  • Bottom board: farther teflon is from pivot hole, the more friction you will get.
  • There should be enough friction to be sure that telescope tube stops when you let go, without drifting about.
  • There should not be too much friction. It should be easy to finely push the telescope in a particulary direction, without "stickiness" or "backlash" in the teflon bearings.

Week 6: Mirror Testing and Figuring (November 14)

• To read the accuracy of your mirror's figure, you place the unaluminized mirror in your completed scope assembly, and look through the eyepiece. You adjust the eyepiece so that it is out of focus by being too far inwards, as well as too far outwards.
• You can interpret the out-of-focus ringed patterns to determine which parts of the mirror are properly figured and which ones require more polishing to reach the proper curvature.
• In, center of focus disk is bright, Out center is dim
  • Center of mirror is too deep.
• Out, center is bright, In center is dim
  • Center of mirror is too high.
• Out, edge is bright, In edge is hairy
  • Turned down edge
• Turned down edge and high center are most usual for amateur mirror makers.
• Flatness at edge can be cured as follows:
  • Polish and put pressure at edge. This digs out the "hill" near the edge of the mirror.
  • However, polish without going over the edge of the mirror. If you go too far over edge when polishing, you will get a turned down edge.
• The brighter the defect is, the more surface area of the mirror has that defect.
• If defects show at extreme ends of focus, then the defect is a gross defect. If defects show just outside of focus, the defect is very minor.

Weeks 7 and beyond: More Mirror Testing and Figuring (November 21)

• Most books on telescope making have you grind to a sphere and then parabolize the mirror because their testing method has a null test for a sphere. In this class our test (star testing) has a null test for a paraboloid, so we grind directly to that.
• Light sources for testing:
  • Use Christmas lights for test light source.
    • Especially easy to point scope at string of Christmas lights.
  • Another option: the glint from the electric insulator on a power pole
    • Point scope at pole, then move up and down to find the glint
  • Best is the glint from Christmas tree ornament.
  • Mag lite at least one half block away
    • Shine flashlight, point scope at flashlight, then remove flashlight and replace with Mag-Lite without the lens.
  • Regardless of which light source you choose, it should be at least a half-block away, even farther if possible. The idea is to have a pinpoint light source when viewed through the telescope.

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