Wooden Telescope Part 1: the Mirror

78,731

468

52

About: Most of the things I build usually relate to either astronomy, physics or woodworking in general.

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:

Stellafane

Mel Bartels' Amateur Telescope Making

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
• Pitch
• 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:

http://stellafane.org/tm/atm/grind/side.html

http://stellafane.org/tm/atm/grind/bevel.html

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.

More info: http://stellafane.org/tm/atm/grind/plaster-disk.ht...

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.

More info: http://stellafane.org/tm/atm/grind/tile-tool.html

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.

More info: http://stellafane.org/tm/atm/grind/rough.html

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.

More info: http://stellafane.org/tm/atm/polish/pour-lap.html

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.

More info: http://stellafane.org/tm/atm/polish/polish.html

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.

More info: http://stellafane.org/tm/atm/test/tester-main.html

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.

More info: http://www.bbastrodesigns.com/JoyOfMirrorMaking/Pa...

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.


More info: http://stellafane.org/tm/atm/test/ronchigrams.html

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

More info: http://stellafane.org/tm/atm/coatings/atm_coatings...

Share

    Recommendations

    • First Time Author

      First Time Author
    • Toys Contest

      Toys Contest
    • Big and Small Contest

      Big and Small Contest

    52 Discussions

    0
    None
    anelexus

    2 years ago

    Truly amazing,Thanks for sharing!!!

    0
    None
    Kweek

    3 years ago on Step 5

    You might want to mention somewhere that it is simply the stroke that makes the concave surface. It took me a little while to figure that out. I went back to see if you had curved the grinding tool somehow. Rubbing two flat surfaces together and getting a curve doesn't make a lot of sense.

    0
    None
    ThomasJ1Sock Puppet

    Reply 3 years ago on Introduction

    You're right, any liquid in rotation will produce a parabolic shape. Some people use this property to make liquid mirrors. A container is carved to a circular shape and a small quantity of liquid metal (gallium alloy usually) is poured inside. Once you start spinning the container, the metal spreads onto the surface creating a parabola. However, you need to isolate that system from any kind of vibrations and you can only look at the zenith.

    More info here: Liquid mirror telescope

    0
    None
    rschneblin

    3 years ago on Introduction

    Though I love astronomy, my big passion is Special Effects. One of the effects I am looking in to requires a large, concave mirror. These can be expensive, so the knowledge I glean from this Instructable will come in very handy. Thanks!!

    2 replies
    0
    None
    a-morpheusrschneblin

    Reply 3 years ago on Introduction

    Maybe this technique using a piece of mylar distorted by vacuum would work for you also if your "concave" isn't too specific:

    https://www.youtube.com/watch?v=_8sd9UgjXLE

    0
    None
    ThomasJ1a-morpheus

    Reply 3 years ago on Introduction

    If you don't need an extreme precision on the surface mirror, this is indeed a a quick and cheap way to get a large concave mirror. You might have to pump some air out once in a while as I don't think it would hold its vacuum over a long period of time.

    0
    None
    3967

    3 years ago on Introduction

    I wonder if anybody ever tried to build a home made cnc (+rotary) grinding / polishing machine for parabolic mirrors. Using computerised machines (arduino or rasperry for example) could reduce the errors and production time. just an idea maybe someone can pick it up :)

    2 replies
    0
    None
    pnosko3967

    Reply 3 years ago on Introduction

    My late uncle was a genius and built a mirror grinder to make a telescope from scratch in the late 60s when our space program got underway. I was too young at the time and once I got old enough to get interested, he wasn't around long enough to teach his methods to me. He had his mirror coated by Edmund Optics and they were so impressed with his 6" mirror, they asked him where he got it. They could not believe he made it in his garage using a flywheel from a tractor engine as his turntable. It took him several weeks to grind it. He used two blanks; one became the tool for the other. I still have some 12" blanks he started but never finished. Check out these photos of his hand-built telescope. He claimed to have been able to clearly see the Apollo 10 spacecraft orbiting the moon. The saddest thing about this story is that he didn't take the time to fiberglass the tube and it eventually deteriorated from weathering in his driveway.

    telescope.png
    0
    None
    ThomasJ13967

    Reply 3 years ago on Introduction

    CNC machining is already the standard way of grinding and polishing telescope mirrors at an industrial level. This method usually produces consistent and good results. Some amateurs actually use grinding machines to cut most of the grinding and polishing time.

    Figuring is actually the most sensitive part. Machines can achieve a good result for longer focal ratio (f/6, f/8, etc) but hand figuring is probably still the best option, especially for faster mirrors.

    0
    None
    mosmond

    3 years ago

    Wow - absolutely incredible! This is one of the best instructibles I've seen. It looks like a very enjoyable project. It must be so satisfying to use something you've made with your own hands in such a painstaking way. This is true craftsmanship. If I get the time to devote to it, I'll definitely be doing this. Thanks so much for sharing.

    0
    None
    TomW22

    3 years ago on Step 14

    It looks very interesting. How many hours did it take from start to finish to complete the process?

    2 replies
    0
    None
    ThomasJ1TomW22

    Reply 3 years ago on Introduction

    I won't lie, this takes a long time. I did not work on it every day but here's a rough schedule. I started in May and got it polished by the end June. July and August where spend on the figuring part (going from a sphere to a parabole).

    If you work full time on it, the rough carving, smooth grinding and polishing can all be done within a month. The time consuming part comes from the figuring process. I probably hit all the possible surface errors from turned down edge to central hole and oblong spheroid. 5 minutes of the wrong stroke can add 2 weeks of work.

    Between each figuring session, you need to clean, dry the mirror, let it cool down and take measurements with the Foucault. This multiplies the effective working time by 2 or 3.

    0
    None
    GrfxGawdThomasJ1

    Reply 3 years ago on Introduction

    I've been hand lapping a 4X4X1 hardened steel blank. Because of cooling mine is concave on both sides and the surface is already a very rough parabola - which I'm trying hard to eliminate. I desire ultimately a flat surface true to about .001 in. For machining I don't expect to need greater accuracy as most frequently accuracy into .01 of an inch are required for my tasks. But we're discussing machines, not optics. It is indeed challenging work. And at my chronological advancement, proving very painful. :(

    0
    None
    BhaskerR

    3 years ago

    Thomas,
    Compliments for the detailed instructions with photos and illustrations which will help for beginners.
    I built an 8 inch telescope about 6 years. The grinding of the mirror was done at home. It took more than a month to finish the mirror grinding and getting it silvered.
    The mount was also built and enjoyed viewing the craters of the moon and especially Saturn rings.
    Used the links you have provided.
    A. S, Bhasker Raj
    Bangalore
    India

    2 replies
    0
    None
    ThomasJ1BhaskerR

    Reply 3 years ago on Introduction

    Thank you Bhasker,

    I'm glad that successfully built your own mirror. It always seems like a huge amount of work at first but the view of Saturn rings through the eyepiece is worth all of it.