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.
All materials on this website are Copyright 2001, Mark T.
VandeWettering. Permission is granted to reproduce and distribute
these files for non-profit, personal use.