Introduction: Wooden Telescope Part 1: the Mirror
This is the first of a 2 part Instructable explaining how I built a wooden telescope from scratch. In this part, I will show you how you can build the key element of a telescope: The primary mirror.
A good mirror will show you details on the moon, planets and deep sky objects whereas a poor quality mirror will only give you blurry views of the skies.
A telescope mirror requires an extreme surface precision. However, you will see that this quality is most of the time best achieved by hand rather than machine polishing. This is part of the reason why some people prefer building their own mirror instead of buying a cheap instrument. The other reason being the knowledge you acquire by slowly creating a high quality optical instrument.
Here's a list of website I used during the construction of the telescope:
More detailed article about my mirror: http://www.thomasjacquin.com/build-your-own-telesc...
Step 1: Materials
Here's a list of items that you will need to build your mirror:
• A glass blank made from a material with a low expansion coefficient (Pyrex, Borosilicate, Duran 50, Zerodur, etc)
• Silicon carbide in different grit size (example: 60, 80, 120, 220, 320)
• Aluminium Oxide (25, 15, 9 and 5 microns)
• Cerium Oxide
• Sharpening stone
• Waterproof plaster (dental plaster)
• Ceramic tiles
• Epoxy glue
I bought a kit from First Hand Discovery than contained all these items for $155 (8" mirror kit).
Step 2: Prepare the Glass Blank
Glass blanks often come with marks on the surface. The circular mark was left by the bottom of the kiln while the top marks are coming from the temperature difference when the glass was cooled down.
We'll start by bevelling the edge of the glass to limit the risk of chips. A sharpening stone is a good tool. Remember to keep both mirror and stone wet since glass dust is very bad for your lungs.
The bottom of the mirror needs to be as flat as possible before we start working on it. To even out the surface, we are using coarse carborundum (Silicon Carbide #60). Spread the powder (and water) on a flat piece of glass and rub the glass blank onto it. After a few seconds, you will see a gray paste. Wash it of and add more wet grit. Continue until the surface gets free from holes and bumps.
More info here:
Step 3: Build a Plaster Tool
This tool will be used to create a concave surface on the glass blank.
Cover the glass with a plastic film. Make a cardboard cylinder around the glass and pour the plaster inside. Let it dry and remove the cardboard. Carefully separate the glass and file off the burs on the edge.
Step 4: Cover the Tool With Ceramic Tiles
We need a strong surface to grind glass. That's why we need to cover the tool with ceramic tiles.
Glue the tiles on the plaster tool using epoxy. I used a quick gel epoxy but I would not recommend it since it was slowly eroding over time.
Note that you should avoid placing a tile or a hole in the center of the tool. Instead, slightly offset a tile to avoid any central defect in your mirror surface.
Step 5: Start Grinding
You can now start the real work. Put some wet coarse grit on the tool and start rubbing the glass on it.
The stroke here is the chordal stroke.
After a few strokes, turn the mirror in your hands and walk around your polishing station the other way. This ensures a good randomization of the courses and prevents cyclic errors.
Step 6: Measure the Sagitta
Continue grinding until you get the desired curvature. To estimate the curvature, we will use a dial indicator to measure the sagitta.
If you want to build a telescope for planets and moon observation, you will need a longer focal ratio (F/8 or higher). This means a shallow mirror.
On the other hand, if you want to view galaxies and nebulae, you will need a fast aperture (F/4 for example). This means a deeper mirror.
My mirror is F/4.75. The sagitta on my 8" mirror is 0.1".
Sagitta calculator: http://stellafane.org/tm/atm/general/scope-calc.ht...
Step 7: Smooth Out the Surface Using Finer Grit
Once the sagitta is reached, we have to smooth out the surface while keeping the same curvature.
By switching mirror on top / tool on top, we keep the shape of the mirror. At this stage, we can use finer grit. The courses are now reduced to 1/3 center over center. Mark the larger holes with a Sharpie and continue grinding until those are gone. That's a sign that means you can switch for a finer grit.
Go down to Silicon Carbide #320. Once you've reached this step, you should start to see some reflection when viewed from the side.
More info: http://stellafane.org/tm/atm/grind/fine.html
Step 8: Make a Pitch Lap
Now is the time to get a shiny surface. We need another tool with a smoother surface. You can make one with plaster or thick plywood. We will cover it with a soft material: pitch.
Pitch comes from conifers. It is sticky and hard to clean. I recommend that you use it in a safe place and wear old clothes.
Make another cylinder around the new tool. Melt a good amount of pitch and pour it on the tool. Let it cool down and remove the cardboard siding before it is too hard. Using your hands, you can start shaping the surface to make it a bit convex. Making channels will also help you when pressing the glass on it.
Step 9: Polishing
Put some wet cerium powder on the pitch lap and press the mirror against it. This will make the cerium penetrate the surface of the pitch. Use some soap the enhance lubrification if you need to.
The courses are still 1/3 diameter center over center. When the aspect of the surface has changed on the whole surface of the mirror you can switch for a finer powder.
Remember to keep groves on the pitch for a good abrasive circulation.
Step 10: Build a Foucault Tester
A Foucault Tester is an instrument designed to analyze the surface of parabolic mirrors. It has a light source that shines on the mirror. When the light comes back, it focuses in a different area depending if it came from the edge or the center of the mirror.
The tester uses this principle to let you visually see errors in the range of 1 millionth of an inch. Adding a Ronchi Screen to the tester will save some time as you will get an idea of the surface without making any measurements.
To make your life easier, make a mirror stand. A screw in the back lets you adjust the tilt.
Mirror Stand: http://stellafane.org/tm/atm/test/tester-stand.htm...
Step 11: Make a Paraboloid
The polishing stage left us with a fully polished mirror with a nice spherical surface. However, a sphere won't work for astronomical purposes. We need a paraboloid.
The difference between a sphere and a paraboloid is small (order of 1 micron). To achieve this paraboloid, we will use the Foucault tester. Since we know what the reflection should look like, we will make special courses with cerium oxide until the reflection on our mirror matches the theoretical one.
The general course is a "W" with several push/pull moves. The amplitude should be 4/5 diameter laterally and longitudinally.
There is also a full list of different courses to correct specific surface errors.
Step 12: Control the Surface With the Foucault
This is what the reflection looks like in a foucault tester equipped with a Ronchi grid.
Depending if the grid cuts the light before the radius of curvature or after, you can interpret the lines and deduce the shape of the surface.
Step 13: Finished Mirror
Once the surface reaches certain criteria, you can consider your mirror finished.
Lord Rayleigh proved that a mirror having a wavefront error less than 1/4 of green light wavelength (520 nm, also referred to as Lambda) is indistinguishable from a perfect mirror. As a mirror amplifies errors by a factor 2, you will need a mirror smoothness of 1/8 of green light wavelength minimum which means a peak to valley distance inferior to 65nm.
On the surface error analysis above, you will see that this distance is 28 nm, which equals to 1/20 Lambda on the glass and 1/10 Lambda on the wavefront.
The Strehl ratio is also an important rating since it will be used to determine the overall quality of the final instrument.
The Couder mask is used to take measurements with the foucault tester. The column "Average knife reading" on the analysis software corresponds to the reading on the dial indicator of the foucault. The "Zones" are the cut-out parts on the mask with 1 being the center and 7 the outermost one.
More info to build your own mask: http://stellafane.org/tm/atm/test/mask.html
Step 14: Aluminization
Now that the mirror is finished, we need to send it for aluminization. Currently our mirror only reflects 4% of the light. An aluminium deposit upon its surface will increase this percentage to more than 90%.
An optional additional SiO2 coating can protect the metal from any source of oxidization.
Most companies will add a true center mark upon request. This helps during collimation and does not impact the quality of your mirror since the center does not participate in the image you see in the eyepiece.
As I live in Canada, I looked for a canadian company and got a great service with Normand Fullum Telescopes