Getting Started with Building Your First Telescope

So, you think you want to build a telescope. Good for you! I think you will enjoy it. I've prepared this list of things you can do and buy to help make grinding and polishing your first telescope mirror simple, straightforward, and sucessful. It took me seventeen years to finish my first telescope (I'm a bit faster now) and to keep you from doing the same, here are some tips.

Caveat Reader: This isn't meant to be a complete list of everything you need to know to grind and polish a mirror, although I hope someday to flesh this out to that point.

Get a Mentor

Building a telescope isn't difficult, but it is much easier if you have the help of someone whose made a few before. If you don't know of anyone who builds telescopes, try a local astronomy club. Many astronomy clubs have at least a couple of members who have built telescopes, and in my experience most people are eager to help out beginners.

If you are lucky enough to be in the Oakland/San Francisco area, you might try attending the Chabot Telescope Maker's Workshop. We are a free workshop which meets weekly to work on mirror making, and provide people with as much expertise as we have or you can take.

Read All You Can

There are several excellent books out there that can help answer many questions. Take advantage of them, the authors have worked hard to answer many questions you might have.

Take your time

Telescope making can be a slow process, but in my experience too much eagerness makes for mistakes, and mistakes are even slower than slow progress.

Don't Panic!

Basically, if your mirror blank didn't break in half, it is probably fixable. So don't get panicky if someone doesn't appear to be working out right. Do the best job you can on each step of the process, but if you end up with a scratch on your mirror, or chip the edge, or if you screw up your pitch lap by leaving it in the car on a 100+ degree day (all of which I've done by the way), you can probably recover.

Work Carefully

Work neatly. Every mess you avoid making is one less that you have to clean up later. Spread lots of newspaper around to catch loose grit. Keep a log describing what you did, and for how long. This can be useful in later stages of figuring when you might work for just a few minutes at a time. It is easy to forget what you are doing and what you tried.

Basic Materials

If you buy a mirror kit from a commercial supplier like Newport Glass it will come with the blank, tool, and the abrasives and polishing compounds you need. There are some other bits and pieces you'll want to have along the way though, so reviewing the following lists are probably a good idea.

Grinding

To grind a mirror, you'll need the following materials:

A mirror blank: A mirror blank is just a thick disk of glass. Typically these are made from Pyrex, although good mirrors have been made from thick plate glass as well.
A grinding tool: In the past, amateurs would use another piece of glass as a tool, but since glass is more expensive these days, it is typical now to make a tool using plaster and tile.
Abrasives: Abrasives are used to shape and smooth the surface of your mirror. Grits are typically labelled with either grains per inch (typical grit sizes are #80 for very coarse, progressing through #120, #220, #320, #400, and #600). Finer grits are often labelled by particle size in microns, with typical fine grits being 17micron, 9micron and 5micron. Grit is normally silicon carbide for coarse abrasives, and aluminum oxide for finer work.
Spray bottle: Grinding is done wet, so you'll want to be able to lay down a fairly thin layer of water, and a spray bottle is just the thing.
Paper towels: Grit can be messy, and you'll need to clean up. You could use regular towels, but you want to be careful to avoid accidently getting coarse grit on your mirror when you are fine grinding. The easiest way is to clean up with paper towels, and throw them away.
A grinding stand or board: You'll need some kind of stand or board to hold your mirror steady while grinding. The old (and preferable) way to do this is to get something like a large 55 gallon oil drum, fill it with bags of sand to stabilize it, and mount a board with three cleats on it to hold the mirror. In our workshop, we don't have the space for drums like this, so we make due with grinding boards, which are usually just a board with three cleats that we can hold onto the table with clamps. When adjusting your cleats, you want the mirror and tool to be held in without play, but without pinching the mirror.

Polishing

When you get to polishing, you'll need some more materials:

Optical pitch: Polishing begins with making a pitch lap. Pitch is really nothing more than refined tree sap. At room temperature it is normally solid, but becomes soft and liquid when heated. A layer of pitch is poured onto the surface of the tool, and the mirror pressed into it so that the lap conforms precisely to the surface of the lap. When the pitch cools, channels are cut into it to allow the pitch to flow slightly, and to allow polishing compound to flow easily and smoothly over the mirror.
Polishing compound: In years past, this was rouge, a red polishing compound that stained EVERYTHING it came into contact with. Because it is so messy, it is more uncommon, although people still do use it because it is slow and leaves a very smooth surface. More common today is cerium oxide, a rare earth that is considerably faster than polishing with rouge.
Tub to hold both the mirror and the tool: You will want to be able to soak both your mirror and tool in hot water, so you need a tub big enough to hold both of them.
Weights for pressing: You need some heavy weights to help press the mirror and tool together to maintain good contact. If you have an old weight set lying around, you are set. Small mirrors require less weight than larger mirrors. Ten pounds is probably fine for an 8" mirror, maybe 20lbs for a 12.5" mirror.

Testing

For testing, you'll need a couple of more items:

A mirror test stand: You could just balance the mirror on edge somewhere, but it will be much easier and reliable to make a test stand that will hold the mirror and allow adjustments up and down.
A tester: More on this later.
Software for analyzing the mirror: Okay, you could do the analysis by hand. But software makes the job much easier. Virtually any computer will work: cheap DOS machines are fine. If you don't have one, you can probably dig around and find one cheap. I use a 486 based laptop for most of my analysis.

The Tube, Mounting and Other Bits

I'll presume that you want to make a Dobsonian style telescope. A Dobsonian is a simple mounting that nevertheless works great even when your carpentry skills aren't all they are cracked up to be (like mine). Check my links page for some examples of various kinds of mounts, in particular, the plans for the 4 inch Dobsonian listed there are very well done, and worth scanning. Richard Berry's Build Your Own Telescope is also excellent. Just to flesh out this section, the main list of things you will need are:

A tube: Telescope tubes are used to hold all the optical bits in alignment. It needs to be long enough to hold all the components, a good rule of thumb is that it should be a little longer (3" or so) than the focal length of your telescope. It should be at least an inch bigger in diameter than your primary mirror, I use seven inch or eight inch tubing for six inch scopes. The most common material for tubing (at least for home made scopes) is cardboard. If your telecope is small, then the cores of carpet rolls might work. If the cardboard is thin, you may wish to reinforce the ends of it to keep it from being crushed or dented. Other possibilities are aluminum (nice, very sturdy), phenolic or fiberglass, and PVC.
A mirror cell: A mirror cell is just a gadget that holds the mirror in the back of the tube, and allows it to be adjusted to tip and tilt it so that it reflects squarely back down the tube. A number of outfits still make these, but for small scopes they are pretty simple to make out of plywood and hardware you can get at your local hardware store. I should be writing this up shortly.
A diagonal mirror: Newtonians have small flat mirrors called diagonals which reflect the light out at a right angle through the side of the tube. These mirrors are small and difficult to manufacture for the average amateur, so they are normally just bought from suppliers.
A spider and diagonal mirror mount: To hold the diagonal in place, you need a holder and a spider. A spider is normally just a set of three or four metal vanes which meet in the center of the tube and hold the diagonal holder, which must be equipped with tip/tilt adjustments like the primary holder. I've mostly bought or salvaged these in the past, although Texereaux's book as a nice design for one if you want build one from scratch.
A focuser: A focuser is really just a holder for an eyepiece, with a focus adjustment that moves the eyepiece in and out. If you are totally strapped for cash, you can make one of these out of hardware store PVC parts, but adjusting focus will be tricker. I prefer to actually spend money on these and get really nice, accurate smooth focusers, such as the ones from Jim's Mobile Inc.
Eyepieces: Eyepieces are typically also bought. I've made inexpensive telescopes that use eyepieces salvaged from pairs of broken binoculars, but they are typically only reasoanble at low power. A number of outfits sell reasonable Plossl eyepieces in the $50 range. I normally recommand a low power eyepiece (such as a 25mm) first, and a second higher power eyepiece (say a 10mm) for a second buy.

Suppliers

Mirror blanks: For most people, I recommend ordering good Pyrex blanks for telescope making.
Abrasives, Polishing Compounds and Pitch: For these, there is in my opinion no better supplier than Salem Distributing Company. They sell high quality products to amateurs with a minimum of fuss. They even have a webpage.

Mark T. VandeWettering <markv@telescopemaking.org>