A 27 pound travel scope
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Before
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After
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This telescope was designed around
a 13.1" f/4.5 mirror and the desire to get it inside a suitcase that meets
the requirements for carry on luggage of most airlines so that no part
of the scope would have to be checked. There is a lot of variation
from one airline to another so I decided on 14" x 9" x 22". My weight
goal was to keep it under 40 pounds. The weight, as pictured above
right, ended up at 27 pounds. The 22" diameter of the altitude trunions
and the very low rocker box height necessitated a very low center of gravity
which I could only achieve with the use of a 32 pound counter weight.
This is the configuration I use at home. When traveling I use (actually
I should say I will use, if I ever fly anywhere) spring virtual counter
weights to balance the tube assembly on the rocker box and 2 six packs
of soda to balance the rocker box on the ground board. With the 32
pound counter weight the total weight comes to about 60 pounds. With
the springs and soda the weight comes to just under 40 pounds, but the
scope can be transported without haveing to lug around 2 six packs of soda
resulting in a 30 pound package. The truss tubes will fit in a long
garment bag which I believe can also be carried on to most airlines and
hung on a rack between seating sections. At any rate, I've lost the
desire to segment them so they'll remain single piece for now.
Tools
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Hacksaw, jigsaw, drill, dremmel
tool, screw drivers,
clamps, SAFETY
GLASSES ... |
Materials
43 feet of 3/4" aluminum square
tubing with a wall thickness of 1/16". Cost: $32.25 @ $0.75/foot
from a local metal supplier. Much thanks to the employee at the hardware
store who advised me I could do a whole lot better than $2.25/foot!
12" x 24" x 1/16" aluminum sheet
for secondary support. Good thing I bought enough for 2 because I
screwed up the first one.
Cost: $7.00
2 feet of 1/8" x 4" aluminum sheet
for corner braces. Cost: $8.20
24" square 1/8" aluminum sheet for
altitude truniuns. Cost: $24.54
24" square 1/16" aluminum sheet
for mirror cover. Cost: $13.50
I used 3/16" screws and nuts with
nylon inserts throughout except for the altitude bearings and attachment
of the counter weight. I also used quite a few t-nuts that were attached
by 56-20 screws. Cost: $26.83
20" x 10' aluminum flashing for
secondary baffle. Cost: $9.49
Four 1.25" rollers (used on sliding
doors) to support altitude truniuns. Cost: $7.98
Springs for virtual counter weight.
Cost: $6.31
32 pound counter weight, 2" x 4"
x 13.5". Cost: $51.14 (ouch)
Total cost of materials: $187.24.
If I had investigated the possibility of spring counter weights first,
I probably would have skipped the steel counter weight and saved myself
$51.14.
The telescope
(a larger version of most images
can be seen by right clicking
on the desired image and then left
clicking on "view image".)
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These two images show the scope pointed straight up from the front
and from the side. I made the scope with 5 trusses and a very simple
upper tube assembly to keep the weight down. Since this photo was
taken I have added a secondary baffle and finder scope. |
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The scope was not yet finished when these photos were taken but they
are of better quality and help to show the configuration of the truss tubes. |
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This view shows the scope from the top. |
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These views show the secondary baffle and the finder. The inside
of the baffle has since been painted black. I was amazed at the size
of the baffle required by this scope. It is about 36" in diameter.
The focusser baffle and secondary baffle calculations can be done at Mel
Bartels site. A secondary baffle is a must from a light polluted
site and also helps more than one might think from a dark site. But
for casual observing from a relatively good site the secondary baffle is
not necessary. |
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The upper tube assembly, including telrad, weighs 2.75 pounds.
I used an astrosystems focusser. The quartz secondary has a minor
axis of 2.1" and is .5" thick. It was made by Bill Marriott and performs
very well. |
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Here's another view prior to painting and installation of the telrad. |
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The secondary holder is 1/8" aluminum bent to about 45 degrees.
The secondary is attached by a 3/4" blob of silicone that is 1/8" thick.
If I had it to do over again I would use a thinner blob.
The secondary support is 1/16" inch aluminum. |
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My original plans called for bracing of the altitude truniuns.
They are about 2" wide, 22" in diameter and are 1/8" thick. Each
truniun is attached by 2 screws and turned out to be very stable with no
additional support. The 32 pound counter weight can be seen attached
to the back of the mirror box. |
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The primary is a 13.1" f/4.5 that is 1" thick and weighs about 10 pounds
if I recall correctly. I was fortunate to have John Hall refigure
this mirror for me a few years ago. |
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The primary is attached to three 1/8" thick aluminum triangles that
are attached to an inverted "T". The upper leg of the T is fixed
and is not adjustable. Collimation is done by turning the screws
that attach the horizontal component of the T to the mirror box. |
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Collimation of the primary is done by adjusting a screw on either side
of the primary. These screws can be turned while the laser spot is
observed returning to the focusser tube. |
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Without the clamp the whole scope would tip over. When horizontal,
the center of gravity is well ahead of the forward bearings as well as
the feet of the ground board. For more on virtual counter weights
read this
from Tom Krajci. I also recommend Toms website. |
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Two six packs of Mountain Dew weigh 10.5 pounds. When located
13" behind the rocker box they perfectly counter balance the scope when
it is horizontal. When the scope is vertical the counter weight is
not needed and the scope tips over backwards. To solve this problem
I will attach a simple linkage between the mirror box and the sliding tray
to move the 6 packs closer to the scope when it is vertical. Why
go through all this trouble? The 32 pound counter weight makes the
scope to heavy to carry on to an airplane. The springs, tray, and
tray support/guide are all very small and light, and I can buy 2 six packs
to act as counter weights just about anywhere I go. As you can see
in this photo the counter weights can also be used to quench your thirst
after a long night of observing. (The 32 pound counter weight
would normally not be in place when the springs are in use. I just
didn't want to remove it to take this photo.)
It has also occurred to me that if a curved counter weight support/guide
were used and correctly linked to the mirror box, no springs would be needed.
I may investigate this further in the future if I get the time. |
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A view of the non virtual counter weight. |
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The truss tubes attach by means of t-nuts that are located inside the
square tubing of the mirror box. The t-nuts are attached to the square
tubing by means of very small flat head maching screws and nut. The heads
are counter sunk into the tubing. |
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The camera was held up to the focusser for this photo. The vanes
of the secondary support are not as thin as most 3 or 4 vane supports,
but are much thinner than the original equipment coulter support I had
been using before I built this scope. A little bit of the focusser
baffle can be seen on the left. |
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This photo is a close up of the center spot on the primary with the
laser collimator in place and turned on. The scope had just been
collimated while at an altitude of about 45 degrees. When the scope
was lowered to a horizontal position the laser spot moved down to the position
you see here. Similarly, when the scope was raised to a vertical
position the laser spot moved up an equal amount. I will eventually
try and track down the source of this movement. I think it is almost
certainly in the trusses. But it could also be in the secondary holder
and/or support. (The laser spot is not actually a spot. I use
a very cheap collimator that projects a short line instead of a spot.) |
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The cover for the primary has been cut out and is waiting to be bent.
The secondary support was so easy to bend I anticipated no problems with
this cover. I couldn't have been more wrong. The cover is much
wider than the secondary support and, though only 1/16" thick, was extremely
hard to bend. |
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The components of the mirror box being assembled. |
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The secondary support after being cut. |
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Bending the secondary support. I had no problem with this but
will bring all such jobs to a machine shop in the future if they deal with
larger pieces of metal. |
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I haven't had the whole scope in the carry on case yet but I have had
individual components in the case to see if they fit. This is the
ground board and rocker box. The case measures 22" x 14" x 9".
I am confident the scope, except for the trusses, will fit. One of
these days I'll get around to segmenting the trusses so they'll fit in
the case as well. |
This scope did not turn out quite as well as I would have liked but
I'm generally happy with it, and it was a lot of fun to build. The
split ring equatorial weighed about 120 pounds resulting which resulted
in the scope not being used on some nights. I mainly use this scope
for planetary observation. The best seeing in central Missouri comes
at about 3:30 in the morning and the thought of carrying 120 pounds out
to the driveway often caused me to stay in bed on good nights. Now
when I start my Jupiter observations the first week in September I won't
miss any nights due to a heavy telescope. I also hope to complete
a cylindrical bearing equatorial platform by then. Much thanks to
Jerry Kelly for suggesting I use aluminum instead of wood. I have
never worked with metal before, but found it to be not much harder than
wood to work with. The idea for the basic design came from several
scopes built some time ago by Thane Bopp. Thanks to Clive
Milney and Bruce
Sayre for making their expertise available to the ATM community.
The idea for the cell
came from an unkown member of the S.T.A.R. astronomy club.
email me
