Ed Grinds A Mirror
And Makes a Telescope:
A Serial Article (Conclusion)
By Ed Ting
Latest Installment: 11/13/00
Chapter Nine: Ed Builds a Telescope
I have decided to make a Dobsonian mount around my mirror. Gathering
the parts took a couple of weeks. I decided to splurge a bit and ordered a
5' X 5' sheet of Baltic Birch plywood instead of settling with the commercial-
Here, I was more in my element, as I knew exactly what kind of mount I
wanted. I was also able to draw upon my experience from having tested
so many telescopes over the years. My priorities were: smooth motions
on the axes, light weight, and a low rocker box for use while seated.
As time went on, aesthetics began to rise in importance as well.
Copying plans from Richard Berry's Build Your Own Telescope and
Starmaster's excellent 7" Oak Classic, I came up with a design. A 1.25"
focuser, aluminium mirror cell, 1.52" secondary/spider, and several
Sonotubes were purchased. The azimuth axis is teflon on formica,
while the altitude axis is teflon on velcro, like on the Starmaster ELs
(see, I do learn something once in a while!)
What I did can be duplicated using a good router, table saw, and electric
drill. The altitude bearings are oversized at 8", and I used an 8" diameter
Sonotube. Those #$!@#% Sontubes caused me all sorts of heartache -
they come with an annoying waxy coating on the exterior, don't drill
cleanly enough, and flex a bit in dewy weather.
Otherwise, assembly went smoothly, if a bit more slowly than I wanted. I
had to wait between the four coats of stain I applied, and then again with
the three coats of varnish. Anyone who has ever started a woodworking
project can sympathize with how many times I wound up going back (and
back, and back) to the hardware store to get that "last" item I needed.
An early mock-up, put together
to test collimation and balance
Preparing the tube is a demoralizing activity. Once you get all the
holes drilled in the right locations (not easy) and get the blasted thing
collimated (ditto) you have to take everything apart again in order to
paint the tube. More waiting. I used two cans of spray paint and
one of spray varnish before the tube looked presentable.
Eventually, though, all of the items on the construction "to do" list
got checked off, and on the night I finished everything (Nov 7th, 2000)
it was clear, and I went observing.
They say evaluating your own mirror is a bit like a parent evaluating their
son's performance in a school play - there is no way an objective opinion
can emerge. In this vein, I can only plead guilty. I am thrilled every
time I look through my mirror. The figure is undercorrected by perhaps
1/4 wave, and the hill in the middle can still be seen in the star test.
This suggests that my parabolization was incomplete. Luckily, the little
zone we were so worried about is invisible.
In focus, however, none of this matters too much. The double-double is
an easy split at 100X, and Saturn and Jupiter show impressive detail.
Cassini's Division is blown wide open and I can see the "C" ring under
steady conditions. I caught a shadow transit on Jupiter. On another
night, a friend came over with a C102HD for about three hours. My 6"
was clearer, sharper, and brighter; we both agreed it was the better
of the two scopes.
For comparisons, I dragged out the Traveler and the 10" Starmaster EL.
The 6" is nice, but the two premium scopes are a little sharper and
contrastier. The Zambuto mirror in the Starmaster, in particular, rather
puts my mirror to shame. This doesn't bother me too much; I set out
to complete a mirror of "good" quality, and I think I succeeded.
So Carl Zambuto, Rick Singmaster, Dave Kriege, and others shouldn't
be too worried at this point. I didn't care; the scope turned out much
better than I ever expected. The motions on the axes, in particular,
are velvety-smooth. I have no trouble tracking at 150X. I intend to
make good use out of this telescope; it may be coming soon to a star
party near you.
Sidebar 1: What did all this cost?
Many of you have been asking how much this telescope project cost. I
kept track of my expenses and it breaks down something like this:
Mirror blanks, pitch, grits, $49
Mirror cell, secondary, focuser, $172
One 5' X 5' sheet of Baltic Birch plywood, $29
Misc hardware, Sonotubes, stain/varnish, etc, $93
If you want, add in another $37 or so for stuff I bought but never used,
stuff I bought twice due to drilling/cutting errors, etc. $343-$380 plus
six month's labor doesn't make for a very cost efficient project, but
cost was not the reason for doing this in the first place.
Sidebar 2: The Grinding Guru Revealed
Ed Dougherty has been making telescopes longer than many of us have
been alive. His steady, guiding hand and dry humor kept us on our toes
throughout the past six months. During this time, he drove nearly 60
miles to make these meetings every other Sunday. We're indebted to him.
Doing what comes naturally:
Ed D at the Foucault Tester
Many older astronomers sadly lament that mirror grinding is a dying art.
Why spend hours of time over several months on a monotonous task when
you can order an entire telescope through the mail for about $350? What's
more, machines now manufacture these mirrors with little or no effort on
the part of a human operator at all, and at minimal cost.
As the months progressed on this project, I can admit I felt the same way
many times. This was a boring task. Yet hundreds of you from all around
the world wrote in during the past six months, offering words of advice or
encouragement. Many of you wrote to tell me how "exciting" all of this
sounded, which was a pleasant surprise to me, since "exciting" is not a
word I would use to describe such a lengthy, repetitive task.
In fact, there was a time, around the 120 grit, that I seriously considered
abandoning the project altogether. Between working a full-time job, being
president of my astronomy club, and running the rest of Scopereviews,
this mirror project was one I began to regret starting. At times, only
your kind and enthusiastic letters kept me going. I would have quit more
than once were it not for your overwhelmingly positive response to this
article these past few months. So, in a way, this is really our telescope -
yours and mine.
The completed telescope, Nov 11, 2000
And maybe that's part of the answer to that question, "Why grind your own
mirror?" The journey is as important as the destination. Two moments
during the past six months - the time we watched the clouds pass in front
of the sun on a piece of paper, and that cold morning when I looked at
the moon with the uncoated mirror at 4:00 AM - were ones I will carry
with me forever. During those moments, time seemed to stand still. I
was filled with the breathless wonder of a small child seeing something
magical for the first time, which in a sense I was. You cannot buy this
from any store.
So my attitude towards this has changed yet again. Initial enthusiasm
gave way to boredom, but now has come full-circle again. I am already
talking about possibly griding a larger mirror, perhaps one as large
as 15". This would also be a good excuse to try my luck at building
a truss-pole Dobsonian. So you see, it never ends.
I'll take a break from all this for a while. Afterwards, I may yet embark
on that new journey.
To Be Continued...Someday.
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