We start with the current state of the art dobsonian design and see where we can reduce. We can reduce the upper cage to a single ring, we can minimize the mirror box, perhaps even replacing it with a single ring behind the mirror. We can lower the height of the rocker by using large altitude bearings, and, by placing the feet under the azimuth teflon support pads, reduce the ground board to a 'Y' shaped affair. A good goal is to reduce the tube assembly weight to that of the mirror.
In my 20 inch, the upper end is a single ring of ½" plywood with a reinforcing sector of ½" plywood bringing the thickness to 1" next to the focuser. The diagonal is heated: sensors turn on the resistors when the diagonal temperature drops too close to the air temperature. The spider is a commercial Novak spider, but with a twist: it is split in half down the diagonal holder bolt hole. Very exposed large spiders and secondaries have a tendency to quiver rotationally in gusty wind, the result being elliptical star images at high power. Splitting the diagonal into two 'L' shapes, per Texereau's suggestion in his book, How to Make a Telescope, anchors the diagonal holder so that it can no longer rotate. I also removed large sections in the middle of the spider vanes to reduce weight and to avoid the thermal problems of metal radiating to the night sky where a thin layer of colder air forms around the vanes, leading to increased diffraction.
Not only because the upper ring assembly is light, but also because the wind loading is very low, I am able to reduce my truss tube diameter to thin walled 3/4 inch aluminum of 82 and 68 inches length. Total weight of the truss tubes comes to 6 pounds. Wooden plugs fill the ends to prevent their collapse. The upper ends of pairs of truss tubes are tied together to a block of wood so that I slide the upper ring onto four bolts extending from these blocks, the truss tubes folding parallel for transport.
Chris Westlund's composite stainless steel 0.5 meter scope uses only 6 truss arms.
And now a word about truss tubes and their function. Truss tubes hold the upper ring by compression. The goal is to pick a truss tube diameter so that the compressive forces do not buckle the truss tube. A perfect truss tube design would taper at each each, with the fattest part in the middle. Several schemes such as filling the tubes with expansive epoxy and pre-stressing them with wire have been dreamt up to strengthen their resistance to buckling. Since we are aiming for the simplest, most minimalist design, these interesting ideas don't fit our scheme. Long thin truss arms do vibrate when struck (actually all truss tubes vibrate some). The key is to make sure that the frequency of the vibration is not a resonant frequency of the overall telescope.
And now a word about vibration, amplitude, frequency, and dampening. Vibration is the to and fro motion of an object in the eyepiece. Amplitude is the amount of motion, a convenient unit of measure is arc minutes, or even 'Jupiter' diameters. Frequency is the speed of the vibration, and dampening is the time it takes for the vibration to die out. The goal to aim for is an amplitude of less than a Jupiter diameter under 1 second dampening time. Large amplitude is disconcerting if your scope is sensitive to the wind. A higher frequency is preferred as 'the pants will shake the ants out quicker'. A completely hands off approach, with motorized tracking and focusing, and a single upper ring with split diagonal for wind resistance, gives the smoothest views. A good test for vibration is to knock on the upper end of the scope as you would knock on your neighbor's door. Rapping your knuckles causes a displacement of the upper end. The telescope tube swings back at a characteristic frequency. The mount and ground also 'feel' this vibration, and will reflect or absorb it. A wide stance of the ground feet will absorb the vibration because the natural frequency of a wide stance is lower. Since the rest of the scope has a higher natural frequency and will not vibrate at this lowered frequency, the tube's vibrations are transformed into lower frequency vibrations which are dampened or absorbed by the ground. The result is that a scope that dampens quickly at the eyepiece. Hard rubber discs under narrower ground feet directly absorb the vibration. As long as the truss tubes naturally vibrate at a different frequency than the rest of the telescope, their vibration will not be seen at the eyepiece. The worse case is when the truss tubes do vibrate at the telescope's natural frequency and you have a small ground board: you can actually see and feel the vibration traveling down to the base of the mount, then being reflected back to the eyepiece.
When the wind blows, it adds energy that shows up as vibration and that is eventually absorbed by the ground. By the way, an interesting idea is to dangle metal chains from the upper end, the chains' clanking against each other absorbing vibrational energy. A single upper ring is more resistant to the wind, so exhibits less vibration. The traditional spider is a wonderful storage vehicle of vibrational energy, particularly when the diagonal mass is large. At high power when the wind is blowing, stars appear elliptical thanks to the diagonal's rotational vibration. Tightening the spider vanes only succeeds in increasing the frequency. Splitting apart the spider into two separate 'V's as mentioned above completely does away with this problem.
Bruce Sayer's 20" is the most minimal mirror end possible - a single aluminum ring that holds the mirror mount, the truss tubes, and the aluminum altitude bearings. The rest of us are experimenting with wooden mirror boxes, heavily cut away to reduce weight. Important is to round off the bottom of the mirror box, as the traditional square bottom eats up extra rocker height.
Gary Wolanski is going a step further than most of us, basing his ultra light 16 inch on a super light weight mirror. His 16 inch weighs 40 pounds!
My 20 inch is built from ½" and ¼" apple plywood, the upper ring and truss arms weigh 13 pounds and the mirror box weighs 20 pounds. The mirror mount is made from 1/4" thick aluminum. The mirror is 50 pounds. So, total tube assembly weight is 83 pounds. The auxiliary 6" f/4 that sits on top of the mirror box weighs 9 pounds, and requires a 9 pound counterweight underneath the mirror box. The rocker weighs 23 pounds and the base weighs 20 pounds. Because of the Byers drive gears that are attached to hubs that sit under the altitude bearing and under the rocker, the rocker and base are heavier than a pure hand tracked scope.
Greg Babcock's hand pushed 18 inch weighs 40 lbs (tube, rocker, and base) without the mirror. Contrast this with our local club's first dobsonian, a 12" f/6 entombed in a solid tube of ¾" plywood with doubled and tripled plywood for the rocker and base. It's weight was 300 pounds!
Proper baffling ensures that no unwanted light enters the focuser. There is a baffle just below the focuser and a baffle opposite the diagonal. The focuser baffle is particularly important. Both are covered with Edmund Scientific black felt. The primary is also baffled, just in front of the glass, and totally enclosed in ultra flat black. It is an impressive demonstration to shine a powerful flashlight on any part of the scope, and discover that your observing buddy looking through the eyepiece with his eye cupped cannot tell you when you have the light on or off. The goal in baffling a minimalist ultra light is to block every ray of light not coming from the primary mirror. Extra baffling to absorb secondary reflections from the baffles is used in high performance refractors, but this is a luxury we can forego since the flashlight test is quite convincing when using light trapping felt or velvet.
Some traditionalists are upset by the absence of a shroud. With a heated diagonal, and proper baffling, the shroud is superfluous. Dewing of the primary is a concern, that's why many of us use a fold up mirror box cover plate or a separate lightweight piece that sits on top of the truss arms extending above and outward from the mirror box. A fan will help keep the primary at air temperature and thus avoid moisture condensation. Mirrors that sit in a totally enclosed lower end rarely dew, but think a moment about what this really means - namely that the primary will take a long time to cool down and give razor sharp images. One of the unmentioned reasons for off axis masks is exactly this - not lack of optical quality or bad atmospheric seeing, but instead, optics that are forever cooling and never giving those wonderful refractor like planetary images. Finally, ah, to be a little unscientific, shrouds look, well, ugly, you know, here come the Conastoga Wagons. Of course, you can throw on a shroud in the dark if it comes to that - the shroud issue is really not at the core of minimalist ultra light designs. I will say that I have never used a shrould in my dobsonian career, and I have been building and using large aperture dobsonians since 1980 (that's when my amateur astronomy life was turned upside down when I met John Dobson at Crater Lake National Park, and got to use his 24").
The goal of ultra light and minimalist dobsonians is to get more aperture into the hands of amateurs. By incorporating these ideas, and hopefully improving upon them, you will be able to shuttle more aperture and enjoy it better.
Speaking of adding aperture, my experience since 1990 with tracking dobsonians leads me to make the following conclusion: one can see as much with a well made high quality tracking as a scope 50% bigger. For instance, I can see about as much with my 20" tracking scope as with a standard 30" dob. Click here for more on computerized dobsonians.
Click here for CAD graphic of the 20 inch - overall
view
Click here for CAD graphic of the 20 inch -
upper ring
Click here for CAD graphic of the 20 inch
- mirror mount
Click here for CAD graphic of the 20 inch
- rocker side view
Click here for CAD graphic of the 20 inch - base
top view
Click here for CAD graphic of the 20 inch - azimuth
drive detail
an email from Tom:
Subject: Sixteen inch f/6 - second light at Mt Magazine Star Party
Despite intermittent high winds and high cirrus, I got in about six
hours of
observing over two nights at the Mt Magazine Star Party last weekend
with
the sixteen inch f/6 - call it second light. Here are the major
findings:
Upper ring (instead of a secondary cage/cylinder). It's frequently
breezy
(or worse!) on the mountaintops, and the upper ring design is fantastic
for
this! Even in light breezes the 36 inch nearby was moving and
bobbing and
required hands-on steering to counteract the breeze. . .but my scope
was
barely moving (about 1/4 of Jupiter's diameter in light breezes. .
.at least
less than the planet's diameter in most breezes). In higher breezes.
.
.when many big scopes (especially those with shrouds) were having trouble
staying put - the sixteen inch could still be used at lower powers.
In
really high winds the secondary vanes (or maybe the truss tubes?) start
fluttering/buzzing and the image moves so rapidly that it's blurred
into a
useless mess. The flutter problem in high winds is the limiting
factor
here, not the upper ring/low wind profile design. Great concept!
Thanks to
Mel, Bruce, and others for this idea!
Focuser and upper ring baffling: Works well! After the sun
dropped below
the horizon I observed the four day old moon. . .I didn't side by side
compare with a solid tube newt, but the contrast appeared good.
By pulling
my eye back about four inches from the eyepiece (use a long focal length
eyepiece for this test) I could see the secondary and primary mirror
in
focus. . .and baffling material. . .and nothing else. . .which is what
you
want. By moving my head to the side (to examine the edge of the
field of
view) I could see if the baffling was the only thing visible at other
angles. It was. The baffling layout works well if you don't
try to use a
low profile focuser setup. In case I observe in a street light
infested
area I've made my focuser bottom baffle aperture removeable.
I can insert a
smaller baffle that allows the eyepiece to "see" only the secondary
mirror.
. .and very little else. It will limit the size of the unvignetted
field a
good bit, but may help improve contrast in tough street light situations.
For observing in daytime I'd probably also rig a sun shade (or short/half
shroud near the mirror box) to make sure no sunlight falls on the mirror
and
inside of mirror box.)
Mirror box baffling. I've noticed that some large dobs with open/tailgate
mirror cells don't baffle the back of the mirror box. I've temporarily
taped some strips of flocking paper across the back of the mirror box
so
that you can't see the ground/grass from the eyepiece. I didn't
get a
chance to compare this to other non-baffled scopes, but it was easy
to do
and certainly can't hurt. Still plenty of room in back for air
flow.
Filter holder. I mounted my helical focuser to the upper ring
by making a
short wood box to provide mounting points for the focuser base and
baffle
just below the focuser. In between them I set up a system for
quick change
filter capability by cutting two grooves in the wood box. The
two inch
filters (colored glass for planets and nebula filters too) are mounted
on
squares that slide in/out. This may not be as fast/easy as a
long slider
bar with several filters, but that can get rather large with four or
more
two inch filters. I think it works pretty well. . .especially
when
"blinking" the O-III filter to find planetary nebulae or showing how
much
more visible the Veil nebula is with/withouth the O-III
Tape measure/protractor setting circles. Work well! After
aligning on the
crescent moon I dialed in Vega and Altair in bright evening twilight.
Also
found Uranus in darker twilight. At one point in the night I
was observing
with a 170x eyepiece and four objects (in a row!) I dialed in on the
setting
circles were in the eyepiece field of view! Who needs a low power
eyepiece?
;-)
Star testing the optics. Seeing was not very good, but this mirror
looks
better than my tench inch f/7. . .especially in the outer zones where
the
most mirror area lies. (My ten inch tests as a bit better than
1/6 wave and
less than 1/8 wave. The sixteen inch's outer zones star test
as
clearly/obviously better in the outer zones) The very inner zones
of the
sixteen inch (only out to a radius of about 2 - 2.25 inches, which
is a
small portion of the entire mirror's area) are a bit overcorrected
but
beyond that area the star test is very good. I still need steadier
skies
for a more definitive result. This is a good mirror! I
may not star test
again until spring/summer or whenever I get much better seeing.
This mirror
is plenty good enough to enjoy right now and at this point additional
star
testing may only be good for the ego and bragging rights around the
water
cooler ;-)
Collimation. Using only a cheesy, home made sight tube and peep
hole this
f/6 system only may need one minor tweak of colimation adjustment with
the
star test. . .if the seeing is good enough. A laser collimator
is not
needed.
Spring counterweights. Continue to work well! A lucky break:
Since I use
two springs (one on each side of the rocker box) I've found that when
I
unhook one of the springs and put a light/1.25 inch eyepiece in the
focuser.
. .it's almost perfectly counterbalanced!
Observing ladder. I use a six foot ladder. I added about
three additional
rungs between the bottom three rungs, so now there's a step every six
inches
or so. Once I got over the initial strangeness of a ladder with
such close
spacing it was very comfortable to use! I don't remember any
object/observeing height where I could not find a rung to step on that
gave
a comfortable observing position. There was no neck craning or
stooping
needed! Also, a six foot ladder leaves enough ladder above me
that when
doing high mag observing I can lean/rest my torso/shoulder on the upper
parts of the ladder for steadier viewing. A good ladder helps
for
longer/comfortable observing.
(Off topic, but this is why I build em. . . .) Observing highlights:
Comet
Giacobini-Zinner sporting a tail about 1/4 degree, maybe a bit longer.
M-33
and its brightest H-alpha region (NGC 604?). Veil nebula.
Horsehead.
Rosette. Biggest surprise that I didn't expect: color differences
on
Jupiter and Saturn are so much more enhanced in this scope compared
to my
ten inch, and the cloud banding on Saturn with medium blue filter.
Ray D.
also helped point out to me spokes on Saturn's ring system. . .my first
time
seeing them. Best long-term payoff: My son wants to take
another camping
trip to dark skies in three weeks!
Capt Tom Krajci
B-52 Intelligence Officer
PS. It's looking like I'll soon get stationed in Clovis, New Mexico!
Compared to NW Lousiana I'll spend far more time observing and far
less time
building scopes. Hmmmm, what astronomy clubs/observatories are
active out
there?
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