Optical Tube Assembly (OTA) Page
Telescope Design Issues Optical Tube Assembly Alt-Az Mount Homemade Gears
Truss vs Solid Tube
I preferred to use a solid tube for my telescope,
since this is
simpler and blocks more stray light, but couldn't find anything locally
available. Out of necessity, I was forced to build a truss tube optical
tube assembly (OTA).
A truss tube telescope just replaces the telescope
tube with
a series of tubes, forming a cage assembly that holds the
primary
and secondary
mirrors.
I
drilled three holes into the primary mirror box to accept the threaded
rods on the original primary mirror cell. The secondary mirror
is
epoxyed to a piece of scrap pine (cut at a 45 degree angle). The
prototype secondary
mirror cell attached to a piece of iron
rod that was
epoxyed into the secondary mirror box. I used a hole cutter to cut out
12 truss
tube connectors (discs of laminate flooring) and
attached them to the mirror boxes with wood screws. The
8 truss tubes screw into these disks to connect the primary
and
secondary
mirror cells.
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The secondary mirror cell and support ring (below left and right, respectively). The secondary cell will be suspended from the three small (3 mm) holes in the support ring. The larger support ring holes (6.5 mm) are for the collimating screws.
The below photo shows the secondary mirror cell side and back views (left and right, respectively). In the event that the adjustment screws come loose, the central bolt with the large washer will prevent the secondary from falling into the primary mirror.
The finished secondary cell with collimation screws and the prototype secondary holder and diagonal (below left and right, respectively):
Since the secondary mirror box is too narrow to accommodate the new secondary cell, I had to retrofit an anchoring system. I inserted 3 threaded M6 rods into the secondary box (120 degrees separation). The secondary holder was initially suspended on bicycle brake cable, which was narrower and stronger than my first design idea, bicycle spokes. In this test fit (below photos), the brake cable is clamped between M6 washers and lock nuts. Unfortunately the brake cable flexed too much and the secondary mirror vibrated as the OTA was elevated; I removed the brake cable and fabricated a more traditional secondary spider.
I had several scraps of 2 mm aluminum that I cut and attached to the secondary cell with M3 bolts (below left photo). I replaced the M6 threaded rods on the secondary mirror box with M8 threaded rods; these larger rods will flex less when the spider is tensioned. The spider is bolted onto 5 cm aluminum right angles (homemade) with M5 bolts. The slit in the spider vane allows it to slide around the M5 bolt for adjustment (below right photo). The right angle bends can also be raised, lowered, or rotated on the M8 bolts. The angle between the spider vanes can also be adjusted, allowing the secondary to be rotated. All of these adjustable features give the options to raise, lower, rotate, and adjust the secondary to center with minimal effort. Once adjusted, the bolts are tightened and the mirrors are collimated with the three wing nuts.
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microwave
tower, inserted a low power eyepiece about mid position in the eyepiece
draw tube, and
slid the secondary box along the pine board until the
system was in focus. The proper distance between both mirror boxes was
measured and I repeated this process several times to check measurement
accuracy. Note: be sure to focus on an object in the far
distance
since the focal
plane moves as objects become nearer. Using a
close object will give too long a measurement and you will be out of
focus on celestial objects (you will need to reduce truss tube
length). Back
To Top
e truss tube
connectors.
Back To Top
I routed out the truss tubes on the OTA top and bottom (the sides without sectors) and installed sections of aluminum channel bar (from an old curtain holder) into the truss tubes. I filed down several metric M6 bolts to fit inside the aluminum square bar. These bolts can slide inside the channel bar, but can't be rotated. This allows them to be slid to a desired position and be locked by tightening a nut.
I constructed a sector box around
the OTA from the same square dowel used for the truss tubes.
Stacking together the two sector boxes,
with a spacer between them, created a channel around the M6 bolts
protruding from the aluminum channel bar. As the sector box slides
along the OTA, these bolts move outward or inward relative to the
center of the sector box, due to the angle of the truss tubes. A router
with a circle cutting attachment cut the 22.3 cm diameter sectors,
which are connected
to the sector box by two scraps of old bed posts.
Primary and Secondary Mirror Boxes
The primary mirror cell was purchased in 1981 for my Dobsonian construction and was reused in this project. I built two mirror boxes from 2.5 cm x 6.5 cm scrap pine (recycled from my son's bunk bed) with simple miter joints held together with woodworking glue and countersunk screws.I
drilled three holes into the primary mirror box to accept the threaded
rods on the original primary mirror cell. The secondary mirror
is
epoxyed to a piece of scrap pine (cut at a 45 degree angle). The
prototype secondary
mirror cell attached to a piece of iron
rod that was
epoxyed into the secondary mirror box. I used a hole cutter to cut out
12 truss
tube connectors (discs of laminate flooring) and
attached them to the mirror boxes with wood screws. The
8 truss tubes screw into these disks to connect the primary
and
secondary
mirror cells. Back To Top
Secondary Mirror Cell
The prototype secondary mirror cell was just a small wood block, cut at a 45 degree angle. I eventually fabricated a combination tube to improve my collimation; this showed that the prototype secondary mirror cell was too primitive, giving rather poor collimation. The final secondary mirror cell was designed with adjustable features, allowing the secondary to be raised, lowered, rotated, and collimated. The total fabrication cost was $1.25 for a large washer plus some left over parts from other projects (bolts and aluminum scraps).The secondary mirror cell and support ring (below left and right, respectively). The secondary cell will be suspended from the three small (3 mm) holes in the support ring. The larger support ring holes (6.5 mm) are for the collimating screws.
The below photo shows the secondary mirror cell side and back views (left and right, respectively). In the event that the adjustment screws come loose, the central bolt with the large washer will prevent the secondary from falling into the primary mirror.
The finished secondary cell with collimation screws and the prototype secondary holder and diagonal (below left and right, respectively):
Since the secondary mirror box is too narrow to accommodate the new secondary cell, I had to retrofit an anchoring system. I inserted 3 threaded M6 rods into the secondary box (120 degrees separation). The secondary holder was initially suspended on bicycle brake cable, which was narrower and stronger than my first design idea, bicycle spokes. In this test fit (below photos), the brake cable is clamped between M6 washers and lock nuts. Unfortunately the brake cable flexed too much and the secondary mirror vibrated as the OTA was elevated; I removed the brake cable and fabricated a more traditional secondary spider.
I had several scraps of 2 mm aluminum that I cut and attached to the secondary cell with M3 bolts (below left photo). I replaced the M6 threaded rods on the secondary mirror box with M8 threaded rods; these larger rods will flex less when the spider is tensioned. The spider is bolted onto 5 cm aluminum right angles (homemade) with M5 bolts. The slit in the spider vane allows it to slide around the M5 bolt for adjustment (below right photo). The right angle bends can also be raised, lowered, or rotated on the M8 bolts. The angle between the spider vanes can also be adjusted, allowing the secondary to be rotated. All of these adjustable features give the options to raise, lower, rotate, and adjust the secondary to center with minimal effort. Once adjusted, the bolts are tightened and the mirrors are collimated with the three wing nuts.
Back To Top
Truss Tube Length
The primary mirror focal length determines how far the eyepiece needs to be from the primary mirror. It's better to cut the truss tubes too long, since they can always be shortened. I decided to do a quick experiment to directly measure the correct distance between the mirror boxes rather than calculating it theoretically. I installed the optics in both mirror boxes and clamped the primary mirror box to a pine board on my portable workbench. I pointed the OTA at amicrowave
tower, inserted a low power eyepiece about mid position in the eyepiece
draw tube, and
slid the secondary box along the pine board until the
system was in focus. The proper distance between both mirror boxes was
measured and I repeated this process several times to check measurement
accuracy. Note: be sure to focus on an object in the far
distance
since the focal
plane moves as objects become nearer. Using a
close object will give too long a measurement and you will be out of
focus on celestial objects (you will need to reduce truss tube
length). Back
To Top
Truss Tube Jig
After I determined the proper distance between the mirror boxes, I made a jig to hold the mirror boxes at the proper separation. The jig was just a long piece of scrap flooring with wood blocks screwed on to hold the mirror boxes at the proper separation distance. Once the mirror boxes were properly orientated in the jig, I dry fit the truss tubes (2 cm x 2 cm wood dowel), cut them to length, and screwed them onto the truss tube
connectors.Back To Top
Sector Box
I decided to build some added flexibility into the OTA by making the center of gravity adjustable. This means that the OTA can be slid forwards or backwards to adjust the center of gravity if I place a heavy object on the end of the telescope (piggyback camera or guide scope, etc.). The solution was to mount the sectors on a sector box that fit around the OTA and could be slid to adjust the center of gravity. This eliminated the need for adjustable counterweights.I routed out the truss tubes on the OTA top and bottom (the sides without sectors) and installed sections of aluminum channel bar (from an old curtain holder) into the truss tubes. I filed down several metric M6 bolts to fit inside the aluminum square bar. These bolts can slide inside the channel bar, but can't be rotated. This allows them to be slid to a desired position and be locked by tightening a nut.
I constructed a sector box around
the OTA from the same square dowel used for the truss tubes.
Stacking together the two sector boxes,
with a spacer between them, created a channel around the M6 bolts
protruding from the aluminum channel bar. As the sector box slides
along the OTA, these bolts move outward or inward relative to the
center of the sector box, due to the angle of the truss tubes. A router
with a circle cutting attachment cut the 22.3 cm diameter sectors,
which are connected
to the sector box by two scraps of old bed posts.
The Finished OTA