|
|
Sidewalk Telescopes
SIDEWALK TELESCOPES BY JOHN DOBSON OF THE SAN FRANCISCO SIDEWALK ASTRONOMERS (draft for the wooden book??) In order to destroy the notion that excellent astronomical telescopes are difficult
to build, and too expensive, we have undertaken, at the request of
many, to write this little book. We write with the conviction that the only reason that millions of people in every large country are
not engaged in making their own telescopes is that the people at
large haven't the faintest idea that the ability, the know-how and the monetary wherewithall are within their easy reach. In order to remedy this situation we have undertaken, to set down in writing sufficient information to permit a person with reasonable ability, (down to grade
school level) and of very moderate means, to complete a sizable and
most useable, astronomical telescope. And the reader, whether he or she, whether 9 years old or ninety, can complete the job in but a few days or weeks. The materials we shall use are largely primitive, crude, and comparatively cheap;
the methods we shall employ are comparatively simple and easy; but
the telescopes we construct shall be the envy of many. We write with the further conviction that "the value of a book falls off exponentially
with its length". And we write in the hope that millions of
people, both young and old, both rich and poor, may be able to have telescopes, and in the further hope that when their telescopes are finished they will set them out on the sidewalks for the whole world
to use, "sur les routes du monde entier" (through the roads
of the entire world). THE SIDEWALK TELESCOPE What we have described as a sidewalk telescope is a simple, Newtonian telescope in
a sturdy, wooden, alt-azimuth mount, or rocker, easy to make, easy
to store, easy to move and easy to use. Essentially, it consists of a concave objective mirror mounted usually in the bottom of a tube with a small flat mirror mounted near the upper end to bring the image
formed by the objective out to the side of the tube where it may
easily be examined with an eyepiece. The whole tube is mounted in a three sided rocker box in such a way that it may easily be pointed anywhere above the horizon and so balanced that when so pointed
it will remain steady in that position. The San Francisco Sidewalk Astronomers
have telescopes of this sort, ranging in aperture from four to twentyfour inches, and ranging in focal length from three to thirteen feet. THINGS TO GET For the objective mirror we need some material which can accurately be shaped to our
desired curve, and which will hold that shape indefinately. It must
also take a high polish and a mirrored surface. Glass is very satisfactory and easily available. Almost any piece of thick, round glass may be made into a telescope mirror. We usually use porthole glass. We shall need two
pieces, one for the mirror blank and another for the tool, and we shall need some sand or carborundum for abrasive; so that we may generate our mirror curve by grinding one glass against the other with
water and abrasive in between. We usually start with 60 grit carborundum,
and for a big mirror, it will take several pounds. Much smaller amounts of finer grits, say 100, 220, 400 and 1000, will be needed to smooth the curve before polishing. For the polishing lap, we shall need
some rosin, perhaps a pound or two for a good sized mirror, and a
few tablespoons of turpentine. For the polishing agent we usually use cerium oxide. One or two tablespoons should do. Rouge
is slower, but gives a smoother finish. Eyepieces and small, flat, front-surface
mirrors for the upper end of the tube may be purchased ready made. We often use eyepieces from broken binoculars, and cut our diagonals from military salvage. Square tubes may easily be made from
plywood, or round ones, from cardboard or fiberglass, but the round, cardboard tubes made for pouring concrete columns, and available at construction supply houses, are more than satisfactory. The rocker
may be made of plywood or of window cutouts from solid core doors.
For apertures of 8" or under 3/4" plywood is thick enough, but for larger instruments, thicker material is required for steadiness. We will also need bolts, shingles, teflon, sheet metal screws, furring nails, galvanized
box nails, glue, and some simple tools such as hammers, saws, drills,
ice picks and screwdrivers, and, of course, a place to work. (One should not overlook the convenience of a table saw and a drill press if they are available for the woodworking.) Weather permitting, about the most convenient
place to do the mirror grinding is sitting astride a patio bench, or the like, on a lawn or a driveway. It may be done in the basement, on the street or in the park. We need a source of water, and
a place to wash away the mess. Polishing and carpentry may well be done
inside or out, but for the final correction of the curve of the objective mirror, we may count ourselves lucky if, looking out the garage door, or from some shady place by the house, we can see some
large, dark insulators sparkling in the sun at the top of a power
pole about half a block away. THE TELESCOPE TUBE For an 8", or 8 1/2", trirror we need a tube with a 10" inside diameter, such as the cardboard
tubes which are used for forms in the pouring of concrete columns.
Often they are waxed on the outside, and lined with plastic inside. The plastic lining peels out easily, but the waxed layer on the outside usually peels off with conspicuous difficulty. If the
tube is to be painted outside, it is better to peel off the waxed
layer. THE
SPIDER MOUNT The structure which holds the secondary mirror (the diagonal) in position near the front end of the main tube (and
at a 45° angle to the axis of the tube) to reflect the light
from the primary mirror (the objective) into the eyepiece tube is called the spider mount. It may be built from three shim shingles and a piece of dowel about 1 5 /8" in diameter and about three inches
long. (A piece of wooden hand-rail will do.) The dowel is cut at
a 45° angle at one end, and has three longitudinal (lengthwise) saw cuts into which the ends of the shingles may be fitted. One of the cuts should be made along the long side of the dowel. That will
make it easier to line up the diagonal in the finished telescope,
and will also make sure that one of the shingles (the spider bars or legs) does not run directly toward the eyepiece tube. In order to get the thin ends of the shingles cut off at a position where the thickness
is right to fit into the saw cuts, the saw cuts may be slid up the
edge of the shingles (from the thin end) till they will go no farther. The shingles may be cut off at that point with tin snips or scissors. They should be cut slightly concave, so that they will not
rock when pressed into the dowel, and they should fit tightly. With our three shingles
in place in the block (the dowel), we set our compass to the radius of the tube (in this case, 5"). Then, with the point of the compass set at the center of the square end of the block, we mark
the shingles and saw them off; so that the spider mount will just
fit into our tube (in this case, 10" i.d.). The corners should be cut off the outer ends of the spider bars (the shingles); so
that they will not split when they are pushed into position for the
alignment of the diagonal in the tube. Those portions of the spider mount which will face the eyepiece tube should be painted black. (A piece
of paper towel may be used as a paint brush.) The diagonal mirror must now be glued,
to the end of the block which has been cut at 45°. For this we need three small pieces of leather cut from a discarded belt or shoe. They may be cut square, and should be about 1/2" on a side. White glue may be
used to glue the leather pieces to the end of the block (widely spaced), and also for glueing the mirror to the leather. While the glue sets
up, the spider mount should be so positioned that the mirror surface is horizontal. THE EYEPIECE TUBE The eyepiece tube consists of a short section (1 1/2 or 2 inches) of fairly thick-walled cardboard tube
with an inside diameter of 1 1/2". It is firmly glued down to
a small piece of masonite (about 3"x 4") with a 1 1/2" hole through the center. (The cardboard tubes on which
the plastic bags at the grocery store are rolled are usually suitable.)
The inner edge of the outer end may be beveled with a sharp pocket knife; so that the cardboard may not be frayed when the eyepiece is inserted. For public use the eyepiece should have a focal
length close to one inch, such as the eyepieces from an old or broken
pair of 7x 50 binoculars. It may be pressed into a short piece of chrome-plated brass tubing from a wash basin drain line trap. The tube should have a 1 1/2" outside diameter; so it will fit
snugly into the cardboard eyepiece tube. They are usually available
from the scrap brass bins at the plumber's shop. (When the plumber installs a trap, he usually cuts off a short piece of tubing with his tube cutter which crimps the metal at the cut and makes it easy to fit
it into our cardboard eyepiece tube.) If our main tube is cut to the focal length of our objective mirror (say 5 feet),then the eyepiece hole
may be cut back from the front end of the tube by the radius of the
tube (for a 10" tube, that is 5"). The hole should be just large enough so that the cardboard eyepiece tube will fit snugly through it. The eyepiece tube is fitted through the hole from inside the main tube; so that the
masonite piece fits tightly against the inner surface of the main
tube. Eventually it will be glued in, but for now it may simply be fitted in place. The inner surface of the main tube, opposite the
eyepiece tube, should now be blackened. A piece of paper towel may again be used for a paint brush. When the paint is dry, the front end of the main tube is ready for the installation of the spider mount. INSTALLING THE SPIDER MOUNT Slip the spider mount into the front end of the main tube till the diagonal mirror
is in line with the eyepiece tube, and adjust its position so that,
looking down the axis of the eyepiece tube, you see the far end of the main tube reflected in the diagonal. If the spider fits
in too loosely, it may be tightened by slipping some folded cardboard under the end of the spider bar farthest from the eyepiece tube. When the alignment of the diagonal is satisfactory, the spider mount and the eyepiece
tube may be glued in place. However, there is no rush on this, and it may be postponed till the main mirror is finished, aluminized
and installed. At that time we put a half inch circular decal, or
a piece of tape, at the center of the objective mirror, and we line up our optics on that. The binocular eyepiece is fitted in one
end of the short brass tube, and the eye cup is removed. That makes it possible for the viewers to get close enough to the eyepiece with their eye-glasses on. If the eyepiece is loose in the brass
tube, one or two pieces of leather or cardboard may be used to make
it fit tightly. THE MIRROR SUPPORT "The Tailgate" For the mirror support, at the bottom of the telescope tube, we need a piece of i/4"
plywood, cut square and about two inches narrower than the diameter
of the tube. We will call this piece the tailgate, and for a ten inch tube we need a piece about eight inches square. We mark the center
of our tailgate at the intersection of the diagonals, set our compass at the radius of the tube (in this case 5"), and draw a circle around the center of the board. Only four small sections of the circle
will fall on the board, and they mark the positions of our saw cuts
to cut off the corners; so that the tailgate will just fit into the bottom of the tube. We now draw a smaller circle around the
center spot (A 3" radius will do for an 8 or 8 1/2" mirror.), and divide it into six equal segments, without changing the compass setting. On this circle, equally spaced, we will drill three
holes for the adjustment bolts. (In the finished telescope it is convenient to have one bolt close to the center of the top edge of the
tailgate, and to have the top and bottom edges horizontal. It is
also convenient, for astronomical observation, to have the eyepiece to the right and horizontal. For terrestrial use, we put two extra side bearings on the box around the tube and roll the tube 90°
in the rocker till the eyepiece is vertical. Then, looking down into
the eyepiece, with our backs to the object to be viewed, we have the image right side up.) We make the bolt holes in the tailgate
small enough so that when the bolts are threaded directly through the wood, they'll be tight and fairly hard to turn. (The bolts should not be more than one inch long on the threads; so that they cannot
push the mirror far forward of the tailgate, even when they are screwed
all the way in.) With the bolts driven through, till they protrude slightly on the inside of the tailgate, we cut a triangle from
flimsy cardboard, or very stiff paper, to cover them. It should be
large enough to cover the ends of the bolts easily. We glue the center of the triangle to the center of the tailgate in such a position that the three corners cover the ends of the three bolts. On the front side
of this triangle we glue three small pieces of masonite, leather or heavy cardboard (positioned over the bolts) to protect the mirror from direct contact with the bolts. (These pads are to prevent the shattering
of the mirror in case of accident.) A couple of thumb tacks may be
used instead of the glue (or in addition to it) to fasten the center of the triangle to the tailgate. For positioning the
tailgate in the tube, and for centering the mirror, we need four plywood blocks 3/4" thick, 1" wide and about 3
or 3 1/2" long. (These dimensions are for an 8 1/2" mirror
in a 10" tube.) At the forward end of each block we drive a furring nail. (When the telescope is in use, the furring nails must not touch the mirror. To prevent it we make sure that the
distance between the mirror and the furring nails is more than the
distance through which the bolts can push the mirror forward.) The nails are there simply to keep the mirror from rolling out the front end of the tube when the front end is lower than the rear. Near the middle of
each block, on the face opposite the furring nail we make a screw hole with an ice pick. (It is convenient to have it l i/2" from the end opposite the furring nail.) Then, with the tube in such
a position that the eyepiece tube, as seen from the rear, is on the
right side (and horizontal), we position our blocks inside the rear end of the tube. We put one on either side on the top,
and one on either side at the bottom; so that when the tailgate is
positioned in the end of the tube, with the top and bottom edges horizontal, the four corners will butt up against the ends of the four blocks. It should touch all four. For holding the blocks in
place it is convenient to drive the screws through ice pick holes
2 1/2" from the end of the tube. That will put the butt-end of the blocks 1" from the end of the tube, and leave room for our tailgate to fit in. The tailgate should butt snugly against all four blocks at once; so that it will not
rock when pushed on opposite corners. (If necessary, shift the position
of one of the blocks slightly, or glue a bit of leather or cardboard to the end of one of the blocks.) The tailgate should now be fastened firmly
in place with four sheet metal screws. An easy way to position the screws is to drive an ice pick through the cardboard of the main tube and into the corners of the tailgate when it is in its
proper position. When the objective mirror is positioned in the rear end of the telescope tube, it will rest on tha two lower blocks,
and it must just clear the upper ones. (It must not clear them by
much or it may roll down the tube when the tube is tipped forward.) It is convenient to use 3/4", #8 sheet metal screws for positioning both the
blocks and the tailgate. LINING UP THE OPTICS Lining up the optics
is best done in daylight, or in a well lighted room. Looking down the axis of the eyepiece tube, from six or eight inches out, one should see the diagonal and the spider bars silhouetted against the
sky, or against the brightness of the ceiling, as seen reflected in
the objective mirror. If the optics are properly aligned, the reflection of the objective in the diagonal will appear centered
on the axis of the eyepiece tube, and the silhouette of the diagonal
will appear centered in the objective. The reflection of the objective is centered by moving the spider bars, and that should be done first. Then the silhouette of the diagonal should be
centered by turning the adjustment bolts on the tailgate. READING THE MIRROR CURVE With our objective installed, and our optics aljgned, we are in position to read the
curve of our mirror. We shall read it with the mirror in the telescope,
and with our entire Optical train in tow. For a source of light, we may use a bright star, or the glint cf sunlight from a power pole insulator about half a block away. (The spot of reflected sunlight
from an insulator is usually less troubled by atmospheric turbulence,
and it won't drift out of the field of view by the 'spin of the Earth.) When we get the spot in focus at the eyepiece, we throw the eyepiece out of focus, first one way and then the other, and we compare
the distribution of light in these out-of-focus discs. It is from
the differences in.the distribution of light in the discs on either side of focus that we judge the defects of our mirror. Far out of focus,
the discs show us the shape of the objective mirror, much as we saw it through the eyepiece hole, with the silhouette of the spider across it, but this time the discs appear to be made up of concentric
rings of brightness with nearly parallel lines of brightness to mark
the spider bars. This is the diffraction pattern of a circular opening, with the diffraction pattern of the spider mount superimposed upon it. As we approach the focus, and continue to the other side, this
pattern merges to a bright spot (at focus), and spreads out to its
mirror image on the other side. If our mirror curve is fairly good (close to a parabola of revolution), the patterns on one
side of focus will nearly match those on the other, and the star,
or the spot of reflected sunlight, will appear very small in focus. If the patterns don't match, we can tell from their differences what is wrong with
our mirror. If, when we push the eyepiece too far in (too close to
the diagonal mirror), the central portion of the pattern is unduely bright, then the center of our mirror is too deep, and is focusing its light too soon. If, on the other hand, the center is too bright beyond
focus, our center is too flat. In short, any portion of the mirror
which bundles its light unduely in the pattern beyond the focus is too flat and needs to be polished more. If it's the region between the center and the edge, it will bundle its light into a ring in the pattern
beyond focus, and the corresponding region in the pattern inside
of focus will be too dark. A "turned down edge" is not a defect at the edge, but a flatness of the region near the
edge, and it will bundle its light in a bright ring at the edge of
the out-of-focus disc beyond focus. Severe defects in the mirror can be seen in the patterns far from focus, whereas minor defects can be
noticed only close to focus, or with a higher powered eyepiece. Defects
which involve a large portion of the light in the diffraction patterns involve a large surface of the glass. Shallow defects are more easily corrected, even though they may involve a large portion of the
glass. CORRECTING THE CURVE "Figuring" Figuring here does not mean calculating. It is used in the same sense as in the term
figure skater. It means reshaping our curve slightly; so that it
corresponds as closely as possible to our desired curve, which, in this case, is a paraboloid (a parabola of revolution). Compared to a spherical
mirror curve (a circle of revolution), a paraboloid will be more curved toward the center, and less curved toward the edge. A telescope mirror is said to be "under corrected" if its curve
falls somewhere between a sphere and a paraboloid. If the curve is
too deep toward the center, and too flat toward the edge, it is said to be "over corrected". Since polishing with short strokes, nearly
center over center, tends to produce a curve close to a sphere, an overcorrected mirror may be polished for a while in that way. Since polishing with long strokes, and with considerable lateral
overhang, tends to produce an overcorected curve, an undercorrected
mirror may be polished for a while in that way. To a first approximation, mirror curves may be improved in this way, but you will have to use your own inventive genious to develop strokes to
get your curve "right on". Rapid polishing tends to heat and expand the center of the mirror glass. This causes the center to polish
unduely, and become too deep. You can check for this during polishing
by pushing the overhanging edge of the mirror tangentially back and forth. If the mirror spins around the center, you're digging the center. If it spins around the far edge, you're digging the edge. If it's
polishing evenly, it should spin around a point on the lap between
the center of the mirror and the far edge. During figuring we sometimes use an excess of our polishing agent. This tends to float
the mirror over the lap and prevent much polishing except where the
pressure of the hands forces the excess out. This gives us a little more control in polishing the mirror where we want it polished without having it polish unduely somewhere else. WHEN DARKNESS FALLS When our telescope
is finished and mounted, with the mirror aluminized and installed, and our optics carefully aligned, we are in a position not unlike that of a child with his first bicyle, or that of an islander
with his first canoe. We have a world to explore, and the means at
hand to do it. Eagerly we watch the sun go down, waiting for the darkness to reveal that starry world most of which is hidden
by the light of day. What we see now, when darkness falls, depends
on the size of our telescope, on the depth of the darkness, and on the stillness of the sea of air above us. If fortune takes us to a mountain top, far from city lights, and several thousand feet above the sea,
and if the air above is free from clouds and very still, then, if
our telescope is large, the viewing will be fabulous indeed. Looking far out in space, and backward in time by many millions of years, we'll see the dust lanes in the spiral arms of distant galaxies.
We'll see galaxies in clusters, several at a time. We'll see star
fields and dust lanes in the spiral arms of our own galaxy, and beyond the spiral arms, those beautiful globular clusters, billions of years older than our sun. We'll see great, bright clouds of gas and dust
where new stars are forming in the disc of the Milky Way. We'll see
new stars still shrouded in nebulosity, and old stars with gaseous envelopes. We'll see the colors in the Great Nebula in Orion, and we'll see some old stars which have collapsed and blown their outer envelopes
away. If
fortune leaves us in the city streets, where we cannot so easily see those distant things, we'll turn our attention to the brighter things nearby, to the planets and the moon. The sharp detail on the quarter
moon may gather an exited crowd. So, too, the glass bead streamers
which were left when the craters were made, and which show so well when the moon is full. The bright rays from the crater Tycho would reach from San Francisco to Denver. Sometimes, in the sunset glow, we'll
see the bright crescent of Venus as she approaches us on the inside
track around the sun. Covered with white clouds of sulfuric acid, and closer to the sun than we, it is brighter than our snow fields at noon. Mars is difficult, except when it is close, but Jupiter is good for
several months each year. Although four times more distant than the
sun, it is eleven times the diameter of the Earth, and it spins in less than ten hours. Most of the angular momentum of the
solar system is in Jupiter; so the equatorial bulge can easily be
seen, and the cloud bands show detail. Finally, reigning supreme among public appeasers at the eyepiece of the telescope, is Saturn with those fabulous rings. Seeing them, the public can sense
the extra terrestrial gravitational field. Light from there takes
an hour and a half to reach us, and we tell the viewers, "If you had a very good Mercedes Diesel, and if you drove toward Saturn 60 mph, day and night without stopping, for one thousand
and two hundred years, you could see it like that with your bare
eyes." TURBULENCE Just as the good viewing of those dim and distant objects requires a dark, transparent sky, free from lights and
smog, the viewing of even these brighter objects requires a quiet
sky, free from turbulence (air masses of differing densities, mixing and moving across our field of view). As a bicycle is plagued by flats, and a canoe by leaks, our telescope will be plagued
by turbulence, and the larger the telescope, the more devastating
will be the effect. This is easily understood if we consider our larger objective to be made of several smaller ones. The effect of the turbulence on the image from a portion of our objective will
be two-fold; it will blur it slightly, and it will cause it to dance.
The increased difficulty with the full aperture is due to the fact that the images from various portions will dance out of
step. This
turbulence is the cause of "heat waves" in the daytime, and the "twinkling" of the stars at night, and
if we are to succeed in our exploration, we must learn to understand
it. It is a common thing, when walking in the sun, to see the shadows of this turbulence moving with the breeze across the ground, and, under the slit illumination of a solar eclipse, near the time
of totality, they become the famous "shadow bands". To protect our larger
telescopes, to some extent, from the effects of this turbulence, we use cardboard aperture stops that fit in the front end of the tube above the spider assembly. Typically, we cut a circular or elliptical
hole in the cardboard in such a position that the incoming beam misses
the diagonal, the spider bars and the edge of our objective. When the turbulence is slow, but troublesome, these stops can be very helpful on lunar and planetary detail, and the unobstructed aperture is
particularly adventageous on Venus since it gives clear, black sky
right up to the bright crescent. At night, the ground cools off by infrared radiation to the sky. Then the air cools off by contact with
the ground, and flows down hill like water. The advantage of getting
our telescopes to the mountain tops is two-fold. It gets us above the lower layers of air and smog, to where the sky is apt to be darker, and it gets us above the turbulence of these down-hill breezes
to where the air is more apt to be still. The bigger the telescope,
the more important are these advantages. For our sun telescopes, however, the mountain tops are against us. The sun warms the
ground, the ground warms the air, and the warm air climbs the slopes
in front of our telescopes all day long. Except in the morning, before the slopes are warm, sun telescopes behave much better in the valleys or on the flat, open desert. Probably the ideal place would be on an
island, in a lake, in the desert.
|
