Instructables

How to make your own telescope mirror

[Fig 1] This is a large piece of glass a woman off Craigslist gave me. It was 40"x40" and 0.75" thick. Its what I decided to use to make the mirror. Cutting it into smaller sections can be difficult but scoring it with a glass cutter and tapping the line with a hammer will generally give you a good enough cut to make the piece small enough. You can also skip this step and buy glass blanks cut already to size for a reasonable price. Plate glass and Pyrex are the most common options you have. Plate glass Is cheaper but is easier to work with. However it also is more prone to temperature expansion than pyrex and is a little more difficult to figure in the end. Tempered glass will not work!

[Fig 2] After I cut the table into squares I needed to make them round. I rigged up a diamond cutter out of some parts and attached it all to a drill press. Here I used a "fly cutter" I swapped the cutters out with small sections of a diamond blade the same width as the cutter and attached them in the same place.

[Fig 3]I attached a round piece of wood to the base of a fishing boat chair. This allows me to rotate the glass so the diamond blade will make a spherical shape into the glass.

[Fig 3]Here is a picture of the turntable attached to my drill press along with the diamond blade in the chuck. This photo doesnt show the piece of wood. I recommend the wood if you need to go very deep or else the blade will bottom out. The round piece of wood allows you to bend the blade so it forms a sort of cup so only the edges that have diamonds on it will hit the glass.

[Fig 4]Here is the piece of wood you would attach the diamond blade to. I also used this on the drill press to start shaping the mirror. Set at the right angle it will start to make a concave curve in the glass.The angle determines how deep you want the center of the mirror to be. This is called the "sagitta".

[Fig 5]After you've ground the glass to the desired depth the mirrors surface will be opaque like this. The surface will look similar to this until its polished out. [Fig 5a]Above the glass you will see a small round wooden disk. This is a sphereomter and its used to measure consistency of the curvature of the glass. I made mine out of the tips of ink pens, a piece of hardwood, and a dial indicator.

Now you move to fine grinding. Here you will start with as rough as 80 grit, then move to 220, then to 30 micron then 9 micron.
[Fig 5b] I made a tool out of Dental plaster with pennies epoxied on it. I used pennies to carry the grit across the mirror. You will need to spend time with progressively finer grits until your to at least 9 micron. If some pennies fall off that's ok. As long as too many don't you'll be fine. You can see this tool at the very top of the picture with the spherometer.

While fine grinding make sure you don't move to the next grit until your sure you've gotten all the pits of the previous rougher grit removed.

After your finished with fine grinding the mirror you will need to make yet another tool.
This tool is called a pitch lap. It is made the same way as the penny tool you made to do the fine grinding except instead of pennies you will use "Pitch".

[Fig 6] Here is a photo that shows the tool being pressed against the warmed pitch. It is resting on my mirror with aluminum foil between the tool and mirror so it doesnt stick.

After you make the tool you will need to cut some grooves in it. I used a saw blade to cut the grooves but there are several ways of going about it. What matters that you leave grooves for the cerium oxide to flow through.

[Fig 7] I attached a picture that shows what my setup was for polishing. I used the drill press to turn my table because the drill press has adjustable belts and I can adjust the rpm. You can do this without a drill press or turntable but youll need to make sure and always move around the table just as you would while fine grinding. When making a mirror remember RANDOM RANDOM RANDOM. Random is the golden rule that will make this mirror better than any machine can.

If you made it this far your doing very good. Take a deep breath polishing out a mirror is a lot of work!

Your mirror should be very clear with no visible pits! Do not attempt to go on further until your 10000% sure your fully polished out.

[Fig 8]Here is a picture of a mirror polished out.


On to figuring.

Here things become a bit more difficult. You will need to make a tester. Search the internet there are different types but I recommend learning how to do Ronchi, and Foucault tests.

[Fig 9] Here is a picture of the tester I made to do Ronchi and Foucault tests. http://stellafane.org/ Has a lot of information. Plans on how to make this tester is also there.

[Fig 10]Here is an example of a "Ronchigram" from the tester I made. This will reveal to me if the surface is symmetrical or not. This is symmetrical however it is a far cry from a near perfect parabola. Understanding what these lines mean at different focal lengths is a test of patience!

Once your Ronchigrams start appearing as they should you will move to doing Foucault tests.

A lot of time goes into figuring a mirror if you have not done it a lot, but if you stick with it you will learn what does what when you do certain strokes.

[Fig 10]You will need to make a Couder mask. Here is a picture of mine for my mirror. Each hole represents a different zone of the mirror.
This is  "Couder" mask I made to test the slope of each zone of the mirror. A smart man named Leon Foucault figured out how to use areas of a mirror and a Knife edge with some fairly complex math to determine the exact curvature of the mirror surface.

I used a program thats a free download called FigureXP to enter my Foucault readings. Many hours went into getting it just right but if you can get it to 1/5 wave error you have a very nice mirror.

[Fig 11] Here is a picture of a finished mirror according to FigureXP.











oblique12 days ago

It's been two years. How did it all turn out?

oblique12 days ago

This is great! Thank you for this.

You are mad! Absolutely mad! I love it! You really skipped through your process so quickly I can barely follow. Why do don't you use a second plate of glass as a "tool" like others do? Is there an advantage or disadvantage to the way you do it? You use a diamond cutoff wheel or something? Not quite sure what that is in your drill press.
nightstang (author)  Eternal_Tristan2 years ago
Haha it does take a little bit of insanity to do but smaller mirrors with longer focal ratios are easier this one is a 12.5" F4.7 . I didnt use another piece of glass because of a few reasons. 1. Cost 2. A diamond blade is MUCH faster. 3. Dental plaster will contour to the form after the shape is formed. Yes I used a diamond cutoff wheel to "hog" out the initial shape.
The 1st picture of the drill press shows a "fly cutter" it has 2 pieces of a diamond cutoff wheel attached at each end to cut the glass into a circle.
The 2nd picture of the drill press shows the diamond cutoff wheel hogging out the initial curve.
The cutoff wheels can be bought at any hardware store they are used for concrete mine was 7" diameter. As long as its at least half the diameter of your mirror it will work.


rimar20002 years ago
Now it is clearer, thanks.

How do you aluminize it?
nightstang (author)  rimar20002 years ago
Aluminizing is done in a vacuum and small pieces of aluminum has electricity run through it and is vaporized. Your better off letting someone else do it unless your going to make lots and lots of mirrors yourself. My 12.5" will cost around 75-100 dollars.

nightstang (author) 2 years ago
I can see what you mean. I think I will just do some numbering so its easier to fit the picture with what I am talking about.
rimar20002 years ago
Interesting, but a bit confused. Maybe if you put image notes to photos, in order to correlate them with text...

If you do it, please warn me.
Pro

Get More Out of Instructables

Already have an Account?

close

PDF Downloads
As a Pro member, you will gain access to download any Instructable in the PDF format. You also have the ability to customize your PDF download.

Upgrade to Pro today!