The construction of an 8.25" Newtonion reflector
What makes a man spend hundreds of hours of his spare time in the making of
a (to amateur standards) reasonably large sized telescope ?
Ever since my youth, I have always shown great interest in the skies. As a schoolboy I could stand up for hours, gazing upward into the sky and wondering what it was all about the stars and planets. At my thirteenth or fourtheenth birthday my parents gave me the well known booklet "Which star is that ?" bij Walter Widmann, which explained all these questions in detail. Quite soon after that the desire rose to possess a telescope of my own. After some reading it became quite obvious that purchasing would be to expensive. A 'make-it-yourself' project was the only feasible thing. So I made a small telescope from a cardboard tube and a spectacle-glass. The eyepiece consisted of a small magnifier, mounted in a hollow, cylindrical chunk of wood.. The windowsill served as a mounting, brought to hight with the help of some books. Although it allowed me to make some nice observations, I got disappointed after some time; mainly due to the brilliant photographs which were published in books and magazines, with which I could not compete.
So, I needed a better telescope. As I could not afford a bigger lense,
the only alternative was to build a reflector and grind the mirror
myself. In rather a short time I put into practice a 50 and a 110 mm reflector.
Indeed, even without a reflective coating (plating the mirrors myself turned
out to be too cumbersome) these telescopes gave better images than the
previous one.
Of course, in the meantime, a mounting was constructed from some small
timbers. The next problem turned out to be the inherent instability of the
whole contraption. At the slightest touch the stars in view moved to all sides.
I learned that even a good telescope proves worthless without a stable
mounting.
However, before I managed to purchase or construct a proper one, my study got
priority. Thereafter I had to join the military. The hobby got a low profile.
Later I picked it up again but the construction of a real good and stable
mounting proved to be too difficult for the time being.
Some three years ago our daughter came home with some questions about the stars and planets. She turned rather surprised in learning that I ever made a telescope myself. Finally, all this resulted in the plan for a large instrument, but mindfull of past experiences, I decided to start with the construction of a good, sturdy mounting.
September '95:
The planning starts. What kind of telescope, what should its capabilities be, how large, what do I make myself, what will I buy....how much will it cost and most important, what budget will be available.
In the end this resulted in the design of a Newton type reflector of some 8" and an opening of F6 to F8. Reasonable lightgrasp power and yet a sufficient long focal distance for planetary observations. In the light of the many hours which I had to put in the construction of the mounting, I decided to purchase the mirror rather than grinding and polishing it myself. At last, it had to be completed some day.....
Indirectly I found someone able and willing to do the necessary turning and milling labour.
December '95:
The construction plans for the mounting are finished. Based upon the
so-called 'STAKU' mounting from the book
Fernrohrmontierungen und Ihre Schutzbauten (Telescope mountings and housings)
. It's gonna be an equatorial pillarmounting; about 70
centimeters high and with 35 millimeter shafts rotating in ball bearings.
The shaft housings will consist of 2.5" gas pipes, fixed in chuncks of massive
hardwood. Because there is no ideal permanent observation position in our
backyard, the whole thing is supposed to be transportable. I should have
known better....
March '96:
The construction of the mounting has made a lot of progress. I ordered
a 22 cm F7.3 mirror. The telescope tube (found with a merchant
in tubes) consists of a fibre, wound around a mould to form a pipe and soaked
in a synthetic resin. Strong, easy to work, but rather heavy (about 12 kg).
The declination axis has a fine adjustment, consisting of a tangential
grip. It works just fine. For the polar axis I wanted a 20 centimeter
diameter wormgear with some 250...300 teeth. It turns out to
be difficult to obtain. At last I found a standard gear (from a specialized
company, dealing in components for mechanical drives). Actually too small
(only 12 cm diameter and 150 teeth) but good enough to start with and not
too expensive.
The counterweight consists of a piece of lead of about 5 kg,
melted down on the furnace on a sunday afternoon while my wife was off (she has
found out now of course !!!) ) and poured out into an empty can.
April '96:
The mirror has been delivered and is put in its housing. This is a sturdy piece
of plywood with a raised edge in which the mirror loosely fits.
Three bolts M8, surrounded with valve springs from the engine of an old MINI
serve as fine adjustments. As the telescope itself and the mounting are about
finished, I can see the first light that same evening. Immediately, the first
problem rises: the mounting does not fit through the door !!! I have to cut
about 5 centimeters from the sideward extending legs.
That night the weather is good and there is an allmost full moon.
The first impressions of the image are good. But the mounting is not really
stable. There are a number of reasons for that:
In the first place the large moment of the tube, next the (too) small wormgear
and last but not least the pillar easily starts vibrating. The latter is solved
quickly bij welding a few small pipes between the top of the pillar and the
ends of the legs. After a couple of days I decided to replace the heavy tube
by a light plywood construction. A quick calculation showed that the weight
would drop down to some 6 or 7 kg.
Also, both the declination and polar axis extended a few centimeters more
than required above their respective housings. That was easily corrected.
In the meantime the search for a larger wormgear continued.
May thru december'96 :
Work continues on the details.
Both shafts are provided with calibrated scales and apart from
the 10 x 60 finderscope I made a unit-power finder
(see my article in the March 1997 issue of 'Zenit', the Dutch magazine for
amateurastronomy). I also designed and build a variable frequency generator
to control the (future) synchronous motor on the polaraxis.
The calibrated scales are heavy laminated computer generated drawings and
are illuminated by a few green LED's. The diagonal mirror mounting which
consisted of three spokes from the wheel of a motorcycle was replaced by a
construction of four thin steel sheets. More sturdy and you only get 4 spikes
rather then 6. An adapter near the counterweight allows for the future mounting
of a camera. A flexible shaft of about 70 cm with a handwheel connects to one
side of the wormgear; long.....but for some observation positions not long
enough.
January '97 thru March '98:
The heavy fibre tube is being replaced by a lightweight eight sided
plywood casing. Its construction is a separate story.
First 6 heavy octagonal plywood boards were cut. Each got a circular hole
with increasing diameter. These served both as mountings for the sides
of the tube and as baffles.
Next the eight skins were cut to the proper size. To make them fit
seemless to one other, their sides had to be planed smooth under an angle.
A small hobby sanding machine properly performed that job.
The skins were screwed and glued on the mountingplates. The small seems
which nevertheless remained were filled with epoxy and the whole inside
was painted black twice.
Rather then the almost traditional white paint on the exterior of the tube I
preferred a number of transparent layers of varnish. This shows the original
woodgrain and gives the tube a nice look. An additional advantage of this
construction are the flat sides, to which all kinds of accessories easily
are fitted. The counterweight had to be melted again, adapted to the lower
weight of the tube.
In September '97 I finally got hold of a nice bronze wormgear, about 22
centimeters diameter and 300 teeth. Exactly the right size. Along with it came
a synchronous motor with nominal 1 rpm on the shaft. Combined with the 1:300
ratio of the wormgear this brings along 1 rotation every 5 hours.
So, approximate an additional 1:5 gear is still needed to obtain the
necessary one revolution in 24 hours.
April 1998:
A timing belt with a transmission ratio of 1:5 was purchased.
After connecting it to the wormgear the motor proves to have sufficient power
to drive the telescope. This was something I had some worries about on before
hand. The whole mechanical drive is now a seperate unit which can be easily
clamped to the polar axis. The variable frequency generator allows smooth
speed control. The controls are mounted in a small hand-held unit
which connects to the generator by means of a thin 9-core cable.
May thru July 1998:
What was already bound to happen for a long time finally occurred.
I am getting more and more reluctant to drag the telescope out of
the garage and to put it up somewhere in the garden.
The whole mounting is simply far too heavy and clumsy to be called
transportable, despite the fact that tube and counterweight can be
removed from the mounting itself.
After some thought my wife and I decide to sacrifice a flowerbed to my
hobby. At the south-side of the garage we find a suitable spot to erect
a more permanent mount.
I purchased a heavy steel tube of 125 millimeters diameter on top
of which an adjustable head is welded. This head serves as the base for a kind
of fork which in its turn will hold the polarshaft. In this way
there are sufficient adjustment possibilities to allow a carefull alignment
of the polaraxis with the celestial pole. A hole in the top of the stand
allows future acces to the interior to fill it up with sand or concrete,
might such deem to be necessary in the future. At the bottom of the stand
a few small 'feet' are welded to let it stand upright and to fix it tight
in the concrete which will be poured around it later.
A hole of 40 x 40 x 40 cm is dug in which the stand is carefully put down in
vertical position in such a way as to let the headcenter point North.
Middle of August 1998:
About 200 kg of concrete has been poured into the hole. After setting, the
stand turns out to be free of any kind of vibration. The mounting is screwed
on the stand and the alignment procedure is started. After a couple of
nights that has been done sufficient accurate to allow visual observation.
Also the electronic drive circuitry functions properly. At magnifications of
up to 200x the visual drift after 5 minutes is neglectable.
End of August 1998:
A 15 centimeters thick layer of sand surrounds the pillar.
It serves as the base for a tiled floor. Ample spacing between
concrete and floor prevents vibrations, caused by walking, to
reach the mounting and thus influencing any observation in progress.
The spacing is filled with gravel.
Early September 1998:
The telescope is about ready now. At the bottom of the pillar a metal box is
attached which houses all electronic circuitry. It also contains an outlet
for the connection of an enclosure heater and low voltage wiring to supply
the illumination of the scales, the finder scope and the reticle eyepiece.
To protect the mounting against rain a housing is constructed from
acryl-glass in an aluminum frame. Easy to remove and a real eyecatcher !
The wooden scopetube is removable and is stored in my garage when not
in use.
Overall, I look backward with satisfaction to the job. It has been a longlasting and demanding task which took me many hundreds of hours to complete, but it has been worth while. Especially as the instrument (besides functionality) also shows some artistic value. At least, that's my personal opinion.
Of cours I have additional requirements. The declinationshaft will need
a motordrive in the future. Further I am thinking of controlling the scope
by a PC. Last but not least the connection of a webcam to the eyepiece
looks like fun.
Currently the synchronousmotor has been replaced by a
steppermotor, controlled by a micorprocessorunit. This unit will soon have
remote control possibilities via a serial connection running to my study.
The webcam has been mounted already (see the section
Astrofotgraphy with a webcam.
For further particulars please send an email. Make sure the reply address is right. I had a few emails returned to me because of errors in the replyaddress.
email: Leo Bolier
Rev. April, 21, 2002
Some nice links