The Upper Tube
Assembly (UTA) for the 12.5-inch
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The UTA is made almost exactly like Dave Kreige describes for the
Obsession in his book.
Basically, 15 3/4-inch (o.d) rings 7/8-inch wide were cut from
1/2-inch thick American birch ply with a router
(see Method of Cutting Circles). The rings
were joined using 7-inch long 3/4-inch aluminum tubing (two can
be seen here) using threaded inserts that pound into the ends of
the tubing. These inserts were obtained
from Superior Components. The
spacing of the aluminum tubing around the rings was not even and was dictated
by the spider purchased from
Gary Wolanski
which is really excellent.
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The uta also carries the focusing board (left), a board for the
Rigel Quickfinder (middle)
and a board for the 8x50 finder (right). These boards are 1/4" Baltic
birch reinforced top and bottom with another thickness of 1/4" Baltic birch
as can be seen. Thus, the tops and bottoms of the boards are 1/2" thick
and allow for screwing into them. A hole was cut in the focusing
board to accommodate the focuser
(Astrosystems Phase 4)
which can be seen in the picture at the bottom of this page.
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The light shield at the top of the UTA is Kydex. The
velcro closures can be seen above the focuser. Also, even though
the shield fits tightly in the UTA and rests on the
spider vanes, it is also velcroed on in 3 places around
the UTA (one piece of velcro can be seen above and to the right of
the focuser). |
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To attach the struts to the UTA, four 3 1/2" pieces
of 3/4" aluminum angle were bolted to the bottom of the UTA using brass
threaded inserts as described in the rocker box section.
Two holes were drilled into each of the aluminum angle
to accommodate the #10 -32 stainless bolts shown here. The
stainless wing nuts shown are tightly backed onto the bolts which
allows screwing and unscrewing the bolts into the nuts (see below)
without using a screwdriver (i.e. they are just hand
tightened). |
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The inside of the aluminum angle shows that the #10 stainless
nuts are epoxied onto the aluminum angle with PC-7 epoxy paste, available
in hardware stores. This stuff is incredibly strong
and it seems that it will last forever (I hope). The insides of the
truss tubes were covered with black electrical tape; a low-tech approach
to decreasing reflections. This system is very cumbersome and has many small parts. If you can think of a better approach that still fits inside the mirror box, let me know. Nevertheless, when assembled, the scope is very stable. |
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The UTA fits inside of the mirror box as shown.
The mirror box fits inside of the rocker box and the altitude
bearings, removed from the mirror box also fit inside the rocker box. Note that the focuser needs to be removed from the UTA so that it fits. The secondary mirror is also removed for shipping. The UTA sits on the baffle 9" below the top of the mirror box. The height of the UTA is 8" plus 3/4" for the aluminum angle used to attach the poles. |
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