David Steinhauer's Telescope Making Site

  Home     Telescopes     Mirror Making     Contact     Site Map   

Home --> Mirror making

Telescope Mirror Making



Making your own telescope is a rewarding hobby. There is a real learning curve, though. I'm no expert, but I have learned a few important lessons on the 1 1/2 mirrors I've completed so far (I'm polishing my second mirror).

See my resources page.

What Kind of Tool Should I Use?

In the old days everyone used a glass or tile blank for all stages of making a mirror - hogging out, rough and fine grinding, and even for the pitch lap base. These days there are many other options that people use. Here are some points to consider.

It is beneficial to use different tools for grinding and polishing. That way, if you scratch the mirror and have to go back to grinding, you don't have to scrape off all that pitch and make a new pitch lap when you are ready for polishing again.

Others have claimed that hogging out with a smaller tool, such as a galvanized pipe end cap, is faster than using your full-diameter tool for hogging out. From my experience I'm inclined to believe this is true.

So what are the pros and cons of the various types of tools?

Cleaning Between Grits

On my first mirror I used lots of paper towels and dish soap to clean everything over and over again in my garage utility sink. It always seemed like there were a few pieces of grit remaining. Then with my second mirror it suddenly hit me - why not just take everything outside and blast it with a hose? That's what I do now. I bring the mirror, tool, their storage containers, bucket, and grinding stand out to the lawn. I squirt some dish soap on the item I'm cleaning, spread it around with my hand (while wearing latex gloves), and then spray it with the hose. I do this two or three times with every item except the bucket, which takes a bit more work.

Grinding Stand

I use a free-standing grinding stand in the garage. The one shown in to the right is a lot heavier than it needed to be. I could have cut the 2x4 legs in half lengthwise and it would have been sturdy enough.

Grinding and Polishing Turntable

The rotating table set up for a 6" mirror. The central pin is a 1/4x20 bolt. Two of the furniture glides are shown. For a 12" mirror, I just used smaller blocks.

I use a rotating table for fine grinding and polishing. Rubber feet on the bottom of the base are all it needs to keep it in place on the kitchen counter. No amount of pulling or pushing will cause it to slide. Don't use ball-bearing lazy suzan hardware or it will rotate way too easily. For the rotation bearing I used furniture glides on one surface, and bare (painted) plywood on the other surface. This provides an adequate amount of friction so it doesn't rotate too easily.

Where Should I Grind and Polish?

I like to do my rough grinding in the garage; it's way too messy to do indoors. Once I reach about 25 micron grit, I move indoors to the kitchen. This is a safer place with a lot less dust. I also prefer to grind during the warmer months (when I don't freeze using the hose to clean up outside). The cooler half of the year is better for polishing and figuring here in the Seattle area. This is because temperatures are much more controlled in the house in the winter. From about May to September we just leave the windows open in the house (no one around here has air conditioning) so the temperature during those months is highly variable. Of course, if you live in a place that uses air conditioning during the warmer months, this doesn't apply to you.

Why Grind the Back of the Mirror?

We're told to grind the back of the mirror, but why? This is important for thin mirrors so they can be supported evenly by the mirror cell, but what about full thickness mirrors? Here are a few good reasons that I discovered:

I ground the back of my 12" mirror down to #220 grit, which was sufficient. I don't see any reason to go through the grits (#80, 120, 220) on the back before starting on the front; it worked fine for me to grind the back with each new grit that I'm using on the front. This works well for me since I hate the cleanup between grits, mainly because I'm paranoid that I won't clean well enough.

How Many Grits Should I Use?

This is a question that is answered differently depending on who you ask. The 12" mirror grit kit I bought from www.gotgrit.com contained eight grits: #80, #120, #220, #320, 25 micron (#500), 15 micron, 9 micron, and 5 micron. I hate the clean-up between grits, so I was happy to read that Mel Bartels doesn't use so many. So I was willing to accept the risk, and skipped #320, #15, and #5 with my 12" mirror. So the sequence I used was: #80, #120, #220, 25 micron, and 9 micron. The table below shows how much time I spent with each grit on a 12" mirror. Keep in mind that this will be different for every ATM. I also was being very cautious, especially in the cases where I was skipping a grit; I probably could have spent less time on most grits, but I just wanted to be sure.

#80 (Hogging)8.5 hours
#80 (Tile tool)8 hours
#1204 hours
#2203.5 hours
25 micron5.5 hours
9 micron6 hours
Polish (CeO)9 hours

Testing the Polish

There are many ways to determine whether you are done polishing. I prefer two methods, both of which use a 60x-100x illuminated magnifier I bought for about $10 from Radio Shack.

The first method is to turn out the lights in the room and use the illuminated magnifier at its lowest magnification, 60x. At 60x I can easily see the pits, if they are present.

The second method is to shine a bright light on the surface, in a dark room. Some people use laser pointers. I just use the light from the same magnifier, inspecting the mirror with the naked eye rather than at 60x. When polishing is incomplete, I can see a haze on the surface. Once polishing is complete, the haze disappears.

Making Pitch Laps

I'm no expert at making pitch laps. But the following are a few things I've learned.

With pitch laps, it's not too hard to correct mistakes. If the pitch doesn't flow all the way to the edge of the tool, don't panic - just give it some help with your CeO/water covered hands. This isn't ideal, but it gets the job done. If the center is too low, you can pour some more pitch on top. I've had to do both of these things.

But to avoid the above problems, heat up the tool! If it's not water proof, like my wood tool was before the top surface was covered with pitch, warm it a bit in the oven. I neglected to do so, and yes indeed, the pitch cooled too fast and I needed to help it along as I describe above.

On my first pitch lap I tried using a piece of metal to press channels into the tool. It didn't work too well - after pressing the channels had pretty much closed up, so I had to cut the channels in the end anyway. On later laps, I didn't bother. I personally get better results making a flat lap initially; once it cools some I cut the channels with an exacto knife.

Speaking of an exacto knife, how do you cut the pitch without breaking off big chunks? Place the tool in the bottom of a bin to minimize the mess (you'll make a mess anyway, but at least it won't be as bad). Put on latex gloves since you'll get pitch fragments all over your hands. Then scrape away. The key word is scrape. Don't try to cut away pitch, or you'll have little control over how much pitch breaks off. Just scrape it away a little at a time. But if you do chip away more than you intended to, don't stress about it. If the damage does stress you out, just warm up the pitch fragment in hot water, and press it into the damaged spot.

I tried using a soldering iron to channel a pitch lap based on John Upton's detailed instructions. I really did try, but it was a total failure for me. My soldering iron quickly started overheating (emitting smoke) before I could cut through much pitch, and it kept bending the soldering tip, even though it's a pretty rugged one. But if you're feeling brave, that could be an option.

Foucault/Ronchi Tester

Front view of the mirror tester. The knife edge or Ronchi grating are inside a plastic slide holder. The front of a flashlight is rotated down to allow rough alignment with the mirror. A three-way switch allows me to turn on the flashlight bulb or the LED. My mirror stand for a 12" mirror. Another view of the mirror stand.

My tester is a modified version of the Stellafane tester . It sits several inches above the table because the surface I put it on (the kitchen table) is shorter than the place I put the mirror (on the kitchen counter).

We have a skylight in our dining room, so the room can't be darkened during the day. However, using a bright LED and painting the piece of wood behind the mirror black allows me to use it during the day.

Turned Down Edge (TDE)

I worked for more than eight months to get rid of a nasty TDE with my 6" mirror. I tried every technique I heard of, including different kinds of accented pressure. I even tried using a local polishing tool. None of these techniques worked. In the end I realized that the problem all along had been poor contact, and eliminated the TDE within two weeks. It turned out that my pitch lap was extremely hard, which makes it impossible to achieve good contact without hot pressing. Mike Lockwood has an excellent way to tell how hard your lap is based on how long it takes the channels to close up: 1.5 to 4 hours is ideal.

Again I'm no expert, but from what I've learned (and read) there are two main causes of TDE:

  Home     Telescopes     Mirror Making     Contact     Site Map