How to make a good telescope

Advice for the man who would do it himself.

–

From a Local.

–

Part One.

There are few pleasures available to mankind that match stargazing!  Even the naked eye is an instrument of sufficient power to resolve many of the universe’s most majestic features.  But how much more powerful is the experience of observing those celestial objects through a machine designed to collect much more light than the human eye can hope to gather.  For many it is madly habit-forming and the search for ever-larger and more powerful means to view the stars and planets will drive many to the obvious end of fashioning his own device.  For the man who is so moved, he may follow this neat, economical menu.

First get a galvanized steel pipe, 11 inches in diameter and 90 inches long.  You can get this for 95¢ at Harriman’s Feed and Supply in Burdett.

Next, order a 10-inch glass blank and its corresponding glass grinding tool for $8.00 from Gaertner Scientific Corporation, 1201 Wrightwood Avenue, Chicago.    Postage is extra.

If Harriman does not have it, from Bausch & Lomb Optical in Rochester, NY, order one box of No. 900 carborundum for rough grinding, one box of No. 902 for smoothing, one box of No. 904-E for fine grinding, and one box of No. 906-E for extra-fine grinding.  These all cost 26¢ a pound.  You will also want a can of dry Rouge.  This comes in 5-pound cans which cost $1.50 each.  Postage is additional with all these items.  From American Optical Company in Southbridge, Mass, order one box of Wellsworth Emery in each of the two finest grades, M301 and M303½.  These boxes are each one pound and cost 32c each, plus postage.

Harriman’s stocks Archangel, Wilmington, and Los Angeles Pitch, so you can acquire those any time.  He is also able to get Snow’s grafting wax, which is very clean and has a good fine texture.  It costs 30¢ a pound.  He often has apiarist’s foundation comb, which costs 10¢ a sheet.

From J. I. Morris in Southbridge, Mass., you can get Morris Polishing Pads for $2.35 for a box of 10, postage extra.

As for the eyepiece, write to H. E. Dall, 186 Dunstable Road, Luton, England.  They make Tolles eyepieces from Chance dense barium crown.  The light transmission is of high quality and the planetary images are sharp.  A good ocular will run you $5.00, duty free, plus postage.  You will have to wait a while but they are worth the wait.

Eventually you will need diffraction grating replicas to test your mirror, and you can get these from Central Scientific Supply, 460 E. Ohio St., Chicago, Ill.  Ask for Bulletin 22 for specifics.  If you need advice at any point the John M. Pierce Telescope Works of Springfield, Vt., is especially helpful.  They do not charge for their advice.  For blue prints of a complete observatory you can send $10.00 to A. F. Schroeder, 1075 Forest Cliff Drive, Cleveland, O.  It is just a little wooden dome and if you had to you could figure out how to build it yourself but it is a pretty good design and worth the money to save yourself the bother.  Mr. Schroeder does not charge extra for postage.

This is the end of the list of unusual supplies.

Now, to make the mirror you need a few things.  First you need two different places to work.  One to grind, one to polish.  If you have a barn you can do the grinding in the barn and the polishing in the yard.

For a grinding post you will need something that does not move.  One tried-and-true method is to find a twenty-gallon oil drum.  This item is about two feet high and about eighteen inches across.  You should empty the oil from it but you do not need to be fastidious about this.  When it is empty of oil, mount it to a comfortable height.  This height will depend on you, but, for example, you might build a small table and bolt it into the barn floor and then bolt the oil drum to the table so that the top of the drum comes to about your waist.  The idea is you want to put this oil drum on top of something so it does not move when you shove it, but so that it is at a comfortable height for working.  When you have it mounted, fill the drum to the top with water and hammer the lid down.  Make sure you have bolted the drum down in such a way that it remains water-tight.

This is your grinding post.  Like as not it will take a few days to build.  When you are done give it a good push.  It should not move.  If it moves, take steps to tighten it down, or you will regret it later.

–

While you wait for your glass blank and grinding tool to arrive, determine what shape you want the mirror to be.  Here is how to do this.  Mark out a length on the barn floor that is twice the eventual focal length of your telescope.  For a 90-inch-long telescope your focal length might be 84 inches, meaning you will need 168 inches of room.  This is fourteen feet.  A hallway will do all right if you do not have a barn.  Drive a nail into the floor at one end of this space and tie about fifteen feet of string around the nail.  You do not want this string to stretch so baling twine is good.  Now measure 168 inches from the nail and affix a 2′x2′ sheet of zinc to the barn or hallway floor, roughly centered on this spot.  If you do not have zinc Harriman sells it for 11¢ a square foot.  Measure 168 inches from the nail again and mark a spot on the sheet of zinc at exactly this point.  Now tie a sharp nail or an awl to the string so that its sharp point encounters the zinc exactly at the 168″ mark.  Now move the nail from side to side so that the sharp point inscribes a curved line into the zinc, in the way a pendulum would inscribe a line in the air.  This line is the exact curvature you want to impart to your mirror.  You will see that this curvature is quite small, only about two millimeters across a span of ten inches.

Now take up the sheet of zinc from the floor.  Cut carefully along the incised line with tin shears.  File it true.

This is your template and as you proceed you will need this to check your progress as you grind the glass disk.

–

Now find a good piece of oak, an inch thick at least, and cut out a disk from it about nine inches across.

Next choose a wooden handle, making sure that it is something comfortable to grasp.  A four-inch length of one-inch dowel is a good choice.  Drill a hole an inch from one end and choose a bolt that will fit it eventually.  You do not need the bolt now.  Then sand the handle smooth and screw this to the center of the oak disk.  Make sure the screw head is flush.

When the glass blank comes from Staetner, examine it for flaws.  Since it will eventually end up getting silvered it does not have to be flawless through-and-through.  Only you want it to be without cracks or chips.  When you are satisfied with the glass blank, find some plumber’s cement or any other heavy-duty fixative.  Spread this cement over the side of the oak disk that does not have the handle sticking up from it.  Then cement this wooden disk to the glass one.  It is good to have it centered but you can do this by eye as it does not need to be exact.  When the cement dries you should be able to grasp the dowel handle and lift both wooden and glass disk.  Set this aside.

What is called the grinding tool is also a glass disk.  Cement this second disk to the center of the top of the oil drum.  It ought to be firm and not move one bit.

–

Now, put on an apron and long gauntlets.  Take down your box of No. 900 carborundum.  Mix a teaspoon of the powder with about a quarter cup of distilled water, making a thin paste.  With a wooden tool, for example a kitchen spoon, spread this paste gently across the glass blank, holding it it facing upward.

Finally, take the glass blank and set it down on the grinding tool.  You will hear a little crunching sound.  You are now ready to begin grinding.

This is done according to the following pattern.  Make three clockwise, circular strokes to the right with your glass blank, then a single straight stroke that goes three-quarters of the way across the disk.  After each stroke, take a step leftward around the grinding post, so that you travel all the way around about every 25 strokes.  This will be tiring and it is also tedious.   It is good to have the grinding post somewhere with glad or changing scenery or you will soon tire of being on duty.

After 20 or 30 minutes with the No. 900, take your zinc template and lay it across the glass blank.  You will notice a central depression beginning to form in the glass.  It will not be deep enough yet but you will get a sense of the speed of your progress.  Continue for another ninety minutes with the No. 900, checking progress with the zinc template every thirty minutes.  When the curve in the zinc fits the curve in the glass to a reasonable degree, stop grinding.

It is now imperative that you sponge every surface absolutely free of No. 900 before continuing.  Not only the surface of the glass blank; but also the grinding tool; the grinding post; also the gloves, the apron, and any other material that is likely or even unlikely to come in contact with the glass!  One grain of No. 900 is sufficient to ruin your work and require a return to a coarser grind, which is discouraging.  Take it from one who knows!  Do not skimp on this step.

When all traces of the No. 900 have been banished, make a thin paste with No. 902 and repeat the process as above, washing it away when you have worked for about two hours.  Do the same with Nos. 904-E and 906-E.  When you have finished with the 906-E and everything is washed, put the carborundum boxes away and take down the box of M301 emery.

Wellsworth emery is used by the world’s leading lens-makers and you will not be disappointed by it if you use it correctly.  You may use it in two ways.  You may use it dry, by dusting it scantily across the surface of the grinding tool.  If you wish to use it in suspension, the procedure is as follows.  Procure a very clean glass jar.  Fill the jar halfway with distilled water.  Place a quantity of M301 into the jar and screw the lid down tight.  Shake the jar.  Set the jar aside and wait thirty minutes to allow the emery to settle to the bottom.  Gently remove the lid and use a clean dipper to remove a quarter cup of the water out of which the emery has settled.  The very finest particles of emery will still be in suspension and it is this water with these particles suspended within that you will now use as a grinding medium.  Pour this water across the grinding tool and proceed.  Repeating this process with the M303½ will produce a very brilliant shine.  When you have done with the M303½ wash it all away and take off your gloves and your apron.  You are now done with the grinding and ready to polish.

END PART ONE.

–

TO BE CONTINUED.