Astronomy for Children under
Adventures with John Dobson by Jim ShieldsWhen people talk about the Dobsonian revolution
(big telescopes, on alt-az mounts), I have to remind them that all the previous revolutions were fought with cannons on Dobsonian
mounts, with merry-go-round bearings at the bottom and cradle rockers at the top. What are called Dobsonian telescopes are
designed to entertain soft, warm eyes, rather than photographic plates, and to accomodate large enough apertures to allow
the stupid rod cells (which stay awake at night) to see the galaxies without the intervention of photography. - J.D. John Dobson was born and raised in Beijing, China, and came to San Francisco
with his parents in 1927. He earned a degree in chemistry from UC Berkeley, became a member of Carol Beals' dance troupe in
the late '30s, and joined a Vedanta monastery in 1944. Persuaded by his background in chemistry and physics that the universe
is made essentially of hydrogen falling together by gravity, he began making telescopes to see the thing. In 1967 he was asked
to leave the monastery because of his persistent habit of sneaking out at night to help kids build their own scopes and show
their neighbors the universe. He is still engaged in these activities, but on a somewhat larger scale.
Berenice's Hairclip (NGC 4565): A Sidewalk Favorite
How much do we take for granted?
Ray Cash's Sidewalk Astronomers Website Cosmology & the Art
of Mirror GrindingHow
can we see the universe for ourselves without a telescope? How do we know what we're seeing unless we study its nature? As
you can see, cosmology and the art of mirror grinding are closely related. John Dobson has been teaching classes in both subjects
for quite a few years now. What
is the universe made of?
What makes it run?This was our homework for the first week of cosmology class. During that same week we rough ground
our mirror blanks to a more-or-less spherical curve with water and grit. This is real caveman's work. Later we'll
be moving into the suburbs (with fine grinding) and then downtown (with polishing), as the work demands
greater and greater accuracy. For
an 8-inch f/6 mirror, you can tell you've got it about right when a penny just fits under a straight edge held across the
surface of the mirror. The next step is to measure the mirror's focal length by holding it up to the sun. All this to see
a bunch of hydrogen atoms falling together. Why do they fall? We call it gravity (whatever
that is.) After a month or so,
we had progressed to mirror polishing and had begun to assemble our telescope mounts from parts available in just about any
hardware store (or junkyard.) Our homework assignments in cosmology were: What is space?
What is time?
Why don't we fall through the floor?I'd try to tell you the answers, but you would
never believe me. Oh well, let's try the third question. The answer is: It's only our uncertainty! See, I
told you. I guess you'll just have to take the class for yourself. John was careful to explain that these were not his own personal crazy ideas (although
he has plenty of those, too), but those of Einstein and Heisenberg. They are the fundamentals of modern physics. For a novice
telescope-maker, the magic of creating a parabolic curve accurate to a millionth of an inch or so is almost as great a mystery
as the nature of the universe we'll be looking at. Two Months LaterAs it turned out, figuring a mirror isn't such a great mystery after all. All it really takes is a lot of
hard work and a knack for reading out-of-focus star images. (Actually, not star images but glints of sunlight off a telephone
pole transformer.) Depending upon whether the curve is too deep or too shallow, it bundles light (and thus appears brighter)
inside or outside of focus. At this stage most mirrors are too flat in the center and have turned-down edges, so you dress
the mirror and dig out the center for awhile and then read the mirror again. After a couple of hours of this, your arm
muscles ache, John blesses your mirror and you send it off to the aluminizer, hoping for the best. (Or in my case you say,
to hell with it, and go off hiking on Mount Tam with J.D.) In cosmology we jumped from the crazy ideas of Einstein and Heisenberg to those of
John himself. John insists on asking the very questions (mostly of the why variety) that modern science is not prepared
to answer. But I'll let him speak for himself.