6" f/5 Airline Transportable Dobsonian

Albert Highe

Most of my observing is with moderately large aperture reflectors (12.5" and 17.5"). Yet, I own and often use a 6" reflector at public star parties and for quick observing sessions. Over the years, Ive discovered that 6" is the minimum aperture Im content to observe with. And in dark skies, a scope of this size is capable of delivering satisfying views of hundreds of objects. So when I considered building a travel scope, I quickly decided that a 6" would be an excellent compromise between capability and portability.

Since I already owned the 6" optical tube assembly, I investigated whether I could modify it to create a scope that would meet the following goals.

  1. During transit, the major components of the disassembled telescope must be protected and fit within airline restrictions for carry on luggage.
  2. Pieces carried in checked baggage must take up minimal space.
  3. In use, the telescope must not sacrifice stability, performance, or comfort.
  4. The telescope must be easy to assemble (minimize the number of parts and require a single tool).
  5. The telescope must be lightweight (weigh less than 20 lbs.)

Figure 1 shows the fully assembled telescope that met all of the above goals. Similar to large portable dobsonian reflectors, the telescope consists of upper and lower assemblies held together with removable struts.

9fullcm.jpg (42380 bytes) Figure 1 The completed travel scope, using two C-channels to join the upper and lower optical tube assemblies.

Novel Features

C-channel struts

The upper and lower optical tube assemblies (OTAs) are held together with two parallel pieces of C-channel. In small telescopes, the conventional eight-member truss is unnecessarily strong. It was an obvious place to simplify the design and reduce the number of parts.

Ive found several benefits of using parallel struts, and especially C-channels. Fabrication is simple because they are very easy to cut and drill. In use, they facilitate proper alignment of the upper and lower OTAs. During transport, they lie flat, taking up little space. Also, altitude bearings attached to the parallel struts allow rapid and accurate adjustment of the balance point, eliminating the need for counterweights or springs.

In this particular scope, the struts are made from "X1-3/4" C-channel with 1/8" wall thickness and are 29-3/8" long.

Attaching the struts

I considered various methods of attaching the C-channels including wing nuts and thumbscrews. Although these fasteners require no tools to assemble the telescope, each would likely be a loose part and take some time to screw and unscrew. Instead, I use a combination of "-20 screws in the OTAs and keyhole slots in the struts (Figure 2). I only need to loosen the screws to slide off the struts. There are no small screws to lose in the dark and assembly and disassembly are very fast. In addition, the spacing of the keyhole slots is different at each end of the struts. This ensures that I place the struts right side up.

 

BearingsDriver.jpg (31502 bytes) Figure 2 The struts are attached via keyhole slots in the C-channels. The ball point hex key driver is as comfortable to use as a screwdriver, but engages with the screw head better and is more tolerant of misalignment.

I like button head socket head cap screws for their low-profile, attractive appearance and because a hex key engages better than a screwdriver. In practice, I use a ball point hex key driver because, if dropped in the dark, it is easier to find than a hex key. In addition, the ball point head is tolerant of misalignment.

Each "-20 screw is held in place by a T-nut that has been inserted into wooden blocks epoxied to the inside of each tube (figure 3). The size of the blocks is not critical. Mine are made from " plywood and are approximately 1-1/4"tall X 1-5/8"wide. One surface of each block has been shaped so that it conforms to the inner radius of the fiberglass tube.

TopDetail2.jpg (28356 bytes) Figure 3 Wooden blocks epoxied to the inside of the OTAs each contains a T-nut to hold the "-20 screws captive.

 

 

The altitude bearings

The altitude bearings consist of different diameter " and " plywood disks laminated together. The " disk provides the bearing surface and is sized to fit the ID of the OTAs. I drilled four holes in each to reduce weight. The " plywood disk provides a lip that rides against the sides of the Teflon pads, keeping the scope centered in the rocker box. And it is sized to match the OD of the OTAs, forming a lid for the OTAs during transport (figure 4). 

 

 

Bearings.jpg (25246 bytes)Figure 4 The altitude bearings are attached via keyhole slots in the C-channels. If a heavier eyepieces are used, changing the balance of the scope, counterweights are not needed. The bearings can be easily repositioned along the slots cut into the C-channels. The raised lip on the inside keeps the scope centered in the rocker box.

One "-20 carriage bolt and clamping knob holds each of the bearings in place. Carriage bolts have a square shoulder that rides in a slot milled into the C-channels. The shoulder also prevents the bolt from turning. When I want to adjust the balance of the scope, I simply loosen the knobs, nudge the bearings to a new position, and then re-tighten them. Although I cut my slot 3-1/2" long, 2" would do.

The rocker box

During viewing, I wanted the rocker box to be tall enough so that the eyepiece would be at a convenient height (figures 5) when the observer is seated.

 

AlbertHeidiA.jpg (41789 bytes)Figure 5 Author on the left demonstrating comfortable eyepiece location to friend Heidi. 14rockercm.jpg (32919 bytes) Figure 6 The rocker box has a removable base that gets stored in checked baggage.

The taller rocker box required a wider base for stability. The combination made the rocker box larger than airline carry on restrictions. The solution was to make the base removable (figure 6). For airline travel, the bottom of the rocker box is removed by unscrewing four clamping knobs. Together with its attached ground board, the bottom is quite flat and takes up little space in checked luggage.

 

Performance

 

Assembly, disassembly, and transport

As already mentioned, the scope is quickly disassembled by loosening the captured screws and removing the struts. To complete the carry on enclosure, I use two additional pieces that normally sit in the closet (figure 7).

 

TransportPcsA.jpg (25685 bytes)Figure 7 Normally stored in the closet, these two pieces become an integral part of the carry on enclosure. TubesHolderA.jpg (28173 bytes)Figure 8 The OTAs nest within the circular recesses, preventing them from sliding around 

The lower piece becomes the bottom of the carry on enclosure, and holds the upper and lower portions of the OTA within its recesses (figure 8). The upper piece in Figure 7 is inserted into the inside front of the rocker box. On one side is a convenient carrying handle mounted to a raised circular section that locks into the front of the rocker box, preventing it from sliding out. Attached to the other side is a piece of 3/4" thick soft foam centered over each OTA. When the four latches on the sides of the rocker box are closed, the foam is compressed, holding everything firmly in place (figure 9).

PackedUp.jpg (44477 bytes) Figure 9 The optical components are securely stored in the carry on enclosure on the left. The struts and removable rocker box bottom on the right take up little space in checked baggage.

The carry-on box enclosure with all of the optical components weighs 18 lbs, beating my goal of 20 lbs. Its dimensions are 9"HX9-3/4"WX19-1/2"L. I can assemble or disassemble the telescope in the dark in approximately 5 minutes using only the ball point hex driver.

The telescope in use

The rigidity of the scope is better than I expected for the number and type of struts. There is no displacement or sag of its components when I move the telescope around. I see little or no change in collimation at different altitudes. The scope also maintains collimation very well after re-assembly

When observing, vibrations are minimal and damp quickly. Its amazing what you can get away with when the struts are short.

Although I built this telescope for use when traveling, it has seen a lot of use locally. Normally, I leave the telescope is the assembled state. With a total weight of 18 lbs, approximately half the weight of commercially available 6" dobsonian reflectors, it is a pleasure to lift and carry. I find myself taking it outside often rather than setting up a larger scope. It also is my favorite scope to take to public star parties. It tends to draw attention as much for its looks as for its views. Its open structure makes it easy to describe how it works. It is especially a favorite among children who have difficulty reaching the eyepiece of larger scopes.

 

18packing2cm.jpg (71074 bytes)19packingcm.jpg (46645 bytes)22togocm.jpg (50181 bytes)

Disassembling the telescope for travel.

First the side bearings are removed. Then each of the struts is, in turn, removed. Each of the struts have keyhole slots cut into them. I only need to loosen the captured screws and slide the struts off.

 

 

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Copyright 2002-2003 by Albert Highe, unless otherwise noted. All rights reserved.
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