What Kind of Tiles to Use for Telescope Mirror Grinding Lap

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Telescope Mirror Making

Contents:

• Introduction
• What Kind of Tool Should I Utilize?
• Cleaning Between Grits
• Grinding Stand
• Where Should I Grind and Polish?
• Why Grind the Back of the Mirror?
• How Many Grits Should I Employ?
• Testing the Shine
• Making Pitch Laps
• Foucault/Ronchi Tester
• Turned Downwards Edge (TDE)

Introduction

Making your own telescope is a rewarding hobby. At that place is a real learning curve, though. I'm no expert, but I accept learned a few important lessons on the one 1/two mirrors I've completed so far (I'yard polishing my second mirror).

See my resources page.

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 at that place are many other options that people utilize. Here are some points to consider.

Information technology is beneficial to utilise different tools for grinding and polishing. That style, if you scratch the mirror and have to become back to grinding, you don't have to scrape off all that pitch and make a new pitch lap when you are gear up for polishing once more.

Others accept 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?

• Glass or pyrex tools: I tin't retrieve of any real advantages for using a glass or Pyrex blank as a tool, except for the obvious "no assembly required." These tools are relatively expensive.
• Dental stone: At that place are many advantages to dental rock. For 1, it cures fast - within an hr you tin be grinding with it. It is water proof - you don't have to coat it with anything to make information technology water resistant. It is easy to embed tiles in the plaster without whatsoever agglutinative, if you keep the tiles together in a tile mat. On the downside, it is more expensive than plaster (I paid $25 for 25 lbs.) and can be difficult to find (await for dental suppliers in your area). I also have had trouble with bubbles between the tiles, which leave deep crevices for grit to sit in. Attempts to make full them with more than dental stone mostly failed - my repairs for the most part didn't stay in place during use. I ended up filling the holes with epoxy, which worked well. I'thou sure with improved technique I will be able to reduce the bubble problem in the time to come.
• Plaster (eastward.g. Plaster of Paris): This costs about half as much as dental stone. However, it requires much patience and planning ahead, since it takes a week to cure. Once it has cured, you must glaze it with something to make information technology waterproof. Some people utilise epoxy for this task, but epoxy is expensive enough that it probably negates plaster's one reward over dental rock: its lower toll. I'm sure coating it with polyurethane finish would exist just as effective, and less expensive.
• Wood: I don't hear a lot about people using wood, although I know Mel Bartels uses information technology for his polishing tools. For grinding the back of my 12" mirror, I used an 8" deejay of 3/4" particle lath, glued tiles to the front end with epoxy, and coated the back and sides with polyurethane. It worked great. I made my polishing tool for my 12" mirror out of iii layers of 3/4" particle lath glued together. I used a chisel and a belt sander to put a curve on the front approximately matching the bend of the mirror. This was a swell low-toll solution. I'm not sure how I'd utilize wood for a grinding tool, though, since I wouldn't exist able to form it to the mirror's shape, which would mean a lot of grinding to achieve contact betwixt the glued-on tiles and the mirror.

Cleaning Between Grits

On my first mirror I used lots of paper towels and dish soap to make clean everything over and over again in my garage utility sink. It always seemed like in that location were a few pieces of grit remaining. So with my second mirror it suddenly striking me - why not but take everything exterior and nail it with a hose? That's what I practise now. I bring the mirror, tool, their storage containers, bucket, and grinding stand out to the lawn. I squirt some dish soap on the detail I'one thousand cleaning, spread information technology effectually with my hand (while wearing latex gloves), and so spray it with the hose. I do this 2 or three times with every item except the bucket, which takes a chip more work.

Grinding Stand up

I utilize a free-standing grinding stand in the garage. The 1 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

I use a rotating table for fine grinding and polishing. Rubber feet on the bottom of the base of operations 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 brawl-bearing lazy suzan hardware or it will rotate mode too hands. For the rotation begetting I used furniture glides on one surface, and blank (painted) plywood on the other surface. This provides an adequate amount of friction so information technology doesn't rotate too easily.

Where Should I Grind and Shine?

I like to do my rough grinding in the garage; it's way too messy to do indoors. One time I reach about 25 micron grit, I motion indoors to the kitchen. This is a safer place with a lot less dust. I also adopt to grind during the warmer months (when I don't freeze using the hose to clean up outside). The cooler half of the yr is improve for polishing and figuring hither in the Seattle area. This is because temperatures are much more controlled in the firm in the wintertime. From well-nigh May to September we just leave the windows open up in the house (no one around here has ac) so the temperature during those months is highly variable. Of course, if you live in a place that uses ac 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, simply why? This is important for thin mirrors and then they tin can be supported evenly by the mirror cell, but what about full thickness mirrors? Hither are a few expert reasons that I discovered:

• It allows you make a expert decision regarding which side of the mirror will exist the forepart. I got lucky with my 12" mirror - I didn't make up one's mind to grind the back until I'd started #120 dust. Once I did grind the back I discovered a few tiny merely relatively deep surface bubbles that would have been a real headache to grind out had I chosen the back to be the forepart!
• It allows you to see whether the lap is in contact during polishing.
• If y'all practice your mirror testing in a room that isn't very dark (similar me) information technology reduces noise in your measurements from the back side of the mirror.
• It reduces the hazard that grit volition get stuck in rough spots in the mirror, coming loose and causing scratches later on.

I ground the back of my 12" mirror down to #220 grit, which was sufficient. I don't encounter whatsoever reason to become 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 dust 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 plenty.

How Many Grits Should I Apply?

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 viii grits: #lxxx, #120, #220, #320, 25 micron (#500), xv micron, 9 micron, and 5 micron. I hate the clean-upward between grits, so I was happy to read that Mel Bartels doesn't utilize so many. And so I was willing to accept the risk, and skipped #320, #15, and #5 with my 12" mirror. Then the sequence I used was: #fourscore, #120, #220, 25 micron, and 9 micron. The table below shows how much fourth dimension I spent with each grit on a 12" mirror. Proceed in mind that this will be different for every ATM. I also was beingness very cautious, especially in the cases where I was skipping a dust; I probably could have spent less time on almost grits, but I just wanted to exist sure.

Grit	Time
#eighty (Hogging)	8.5 hours
#80 (Tile tool)	8 hours
#120	4 hours
#220	3.5 hours
25 micron	v.5 hours
9 micron	6 hours
Smoothen (CeO)	ix hours

Testing the Shine

At that place are many ways to decide whether you are done polishing. I prefer two methods, both of which utilise a 60x-100x illuminated magnifier I bought for well-nigh $10 from Radio Shack.

The commencement method is to turn out the lights in the room and employ the illuminated magnifier at its lowest magnification, 60x. At 60x I can easily run into the pits, if they are nowadays.

The 2d method is to smooth a brilliant light on the surface, in a night room. Some people use laser pointers. I just use the low-cal 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 consummate, the haze disappears.

Making Pitch Laps

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

With pitch laps, it'due south non too hard to right mistakes. If the pitch doesn't menses all the manner to the edge of the tool, don't panic - but requite information technology some help with your CeO/h2o covered hands. This isn't ideal, but it gets the job done. If the center is also low, you can pour some more pitch on top. I've had to practise both of these things.

But to avoid the above problems, oestrus up the tool! If information technology'south non water proof, like my woods tool was before the pinnacle surface was covered with pitch, warm it a fleck in the oven. I neglected to do then, and yeah indeed, the pitch cooled likewise fast and I needed to help information technology along as I describe in a higher place.

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

Speaking of an exacto pocketknife, how practise yous cutting the pitch without breaking off big chunks? Place the tool in the bottom of a bin to minimize the mess (y'all'll brand a mess anyhow, merely at to the lowest degree it won't be every bit bad). Put on latex gloves since you lot'll go pitch fragments all over your easily. So scrape away. The key word is scrape. Don't try to cut abroad pitch, or yous'll accept little control over how much pitch breaks off. Only scrape it away a little at a time. But if you practise chip away more you intended to, don't stress about it. If the damage does stress yous out, just warm up the pitch fragment in hot water, and press it into the damaged spot.

I tried using a soldering fe to channel a pitch lap based on John Upton's detailed instructions. I actually did try, only it was a total failure for me. My soldering iron quickly started overheating (emitting smoke) before I could cutting through much pitch, and it kept angle the soldering tip, fifty-fifty though information technology'south a pretty rugged ane. Only if you're feeling brave, that could exist an selection.

Foucault/Ronchi Tester

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 identify I put the mirror (on the kitchen counter).

We have a skylight in our dining room, and then the room can't be darkened during the twenty-four hours. However, using a bright LED and painting the slice of wood behind the mirror black allows me to utilize it during the day.

Turned Down Border (TDE)

I worked for more than than eight months to become 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. Information technology turned out that my pitch lap was extremely hard, which makes information technology impossible to achieve proficient contact without hot pressing. Mike Lockwood has an fantabulous way to tell how hard your lap is based on how long information technology takes the channels to close up: ane.v to iv hours is ideal.

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

• Poor contact. If the mirror skips and slips while polishing, contact is poor. When you are in good contact, it takes a uniform, sizeable strength to move the mirror. You can oft see poor contact through the dorsum of the mirror.
• Oblateness. Supposedly once you get rid of the oblate effigy, the TDE should get away on its own.

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