Voice your opinion about this topic in the forums

Two scope designs using three parallel struts Albert Highe Overview This article describes two different telescopes. The telescope in Part I was featured in the June 2005 issue of Sky and Telescope. I decided to include it with this article for a couple of reasons. First, the article contains material and images that were not included in S&T. Second, I think pairing the two scopes demonstrates how design solutions can vary when confronted with constraints to similar goals. Part I: Mark IV Lightweight 12-1/2" f/5 Dobsonian Introduction I enjoy designing and building telescopes. I spend a significant portion of my free time exploring new ideas and like creating unique structures. Minimizing weight is a common theme. Consequently, when I discovered some new (to me) lightweight structural composite panels, I immediately began thinking how I would use them. At the very least, panels can provide significant weight savings just by substituting them for solid plywood in existing designs. However, new materials often offer new design possibilities in addition to design challenges. In the back of my mind I thought how nice it would be to take a 12-1/2" telescope on an observing vacation to Australia. If the telescope were light enough, i.e. total weight with packaging is less than 50 lbs, it easily could be taken as checked baggage. This article is about how I used composite panels to construct a 12-1/2" f/5 portable Dobsonian incorporating some new features and weighing only 40 lbs. Summary of goals Weigh significantly less than the Mark III ultralight design, which weighs 50+ lbs. Easier to build than the Mark III (i.e. does not require clamp blocks). Replace solid plywood with lightweight Tricel Honeycomb panels. Must perform well, (e.g. have excellent views, move smoothly, damp vibrations quickly, provide comfortable eyepiece positions, etc.) Easy to assemble (fast assembly of minimum number of parts, requiring one or no tools). Include fans to minimize the stagnant thermal layer at the front of the primary mirror. Figure 1 shows the fully assembled telescope that met all of the above goals. Figure 1 Portable 12.5" f/5 telescope made with lightweight composite panels has a total weight of only 40 lbs. Materials Composite panels Composite panels have been used as lightweight structural members for a long time. Aluminum sheets laminated to an aluminum honeycomb core probably are the most well known lightweight composite. Aluminum honeycomb is commonly used in the construction of airplanes because of its very high stiffness to weight ratio. I've avoided these materials because of their high cost and construction challenges. A friend, Alan Adler, recently introduced me to Tricel Honeycomb panels. Searching the Internet turned up similar products available from other manufacturers. Used extensively in boat building, lightweight structural panels can be purchased with a variety of skins and core materials. Lightweight cores include end-grain balsa wood, closed cell PVC foam, and honeycombs of polypropylene and impregnated paper. Skins include plywood, plastic laminates, and fiber reinforced resins. Figure 2 shows a representative sample of available materials. Figure 2 (lower right) Fiber reinforced polyester and fiber reinforced wood veneer skins over Balsa wood cores. DecoLite Composite Panels available from Alcan/Baltek Corporation. (upper center) Fiber reinforced polyester skins over foamed PVC cores from Baltek (Airlite DecoLite Panels) and DIAB (ProLamina Sandwich Panels). (lower center) fiberglass and plywood skins over polypropylene honeycomb from Nida-Core. (left and sheet along bottom) thin plywood skins over impregnated paper honeycomb from Tricel Corporation. Composite panels have high stiffness to weight ratios because bending forces are concentrated near the surface. Material farther from the surface contributes significantly less to stiffness. Consequently, a large fraction of dense core material can be replaced by less dense, weaker materials without sacrificing much stiffness. The core only needs to resist shear between skins on opposite sides. For example, a composite panel of given thickness may weigh 25% of one made of solid plywood while retaining 70% of its stiffness. A composite panel slightly thicker than solid plywood can be as stiff yet weigh only 30% as much. Tricel Honeycomb Based on cost, weight, availability, and ease-of-use, I decided to use materials available from Tricel Corporation. All of their composite panels use a moisture-resistant impregnated paper honeycomb core (Figure 3). Tricel honeycomb panels are available in 4' X 8' sheets with skins made with different types and thickness of plywood (Figure 4). I used the 1/2" thick panels with mahogany plywood skins, known as Tripanel Marine, in constructing the mirror box. 3/4" and 1" thick panels with lauan plywood skins were used for the sides and bottom, respectively, of the rocker box. Figure 3 The structure of Tricel's impregnated paper honeycomb is not the familiar hexagonal pattern. Cells are roughly triangular. Figure 4 Representative samples available from Tricel Honeycomb. (left) 3/4" and 1" thick panels with lauan plywood skins. (right) 1/2", 3/4", and 1" thick panels with thin mahogany plywood skins. Special Construction Techniques Most composite panels can be cut with standard woodworking tools. In fact, I found them a joy to work with. For example, a 4' X 8' sheet of 1" thick Tripanel Marine weighs 20 lbs, and is easy to lift. In comparison, a solid plywood sheet 1" thick can weigh 75-100 lbs and is a challenge to drag across the floor. Because a saw blade passes through a couple of thin veneers, a little paper, and mostly air, cutting panels requires little effort. On the other hand, structural composites have some drawbacks that require special, time-consuming construction techniques to overcome. Thin veneer skins are easily damaged along panel edges, provide a narrow glue line to hold pieces together, don't provide enough material for screws to hold onto, and can't support the forces of a bolt tightened across the panel. Rocker box All areas where a screw or through bolt are used must be reinforced. One approach is to drill out the weak core material and insert a solid plug. Figure 5 shows how this was done for the rocker box pivot bolt. A 1-3/4" diameter Forstner bit was used to drill through the bottom plywood skin in the center of the rocker box. The drill removes the paper core down to the interior surface of the top plywood skin, which remains intact. A disk of solid wood was cut from a 1-3/4" diameter hardwood dowel. It is slightly thicker than the depth of the hole it fills. I glued it in place with epoxy. After the epoxy cured, I sanded the disk flush with the surface and then glued Ebony Star laminate to the bottom. Afterwards, the hole for the center pivot bolt hardware was drilled through the solid wood. Figure 5 A 1-3/4" diameter Forstner bit was used to drill through one plywood skin and remove the paper core. The remaining plywood skin was left intact. A disk of solid wood, cut from a 1-3/4" diameter wood dowel will plug the hole and provide solid support for the pivot bolt mounting hardware. To prevent moisture ingress and protect the fragile veneer, solid wood should replace the paper core around the panel edges. I start by cutting the 4' X 8' sheet into smaller panels the approximate size to be used. A table saw works well to remove the core quickly and cleanly (see Figure 6). Set the blade height to the desired depth. With the panel on its edge and its face held firmly against the rip fence set to the proper position, make repeated passes to cut out the core. Be sure to use push blocks and keep your fingers away from the blade. The finished depth of solid wood around the perimeter should be at least the thickness of the panel. For the 3/4" thick vertical rocker box panels, I typically used strips 1" wide and cut and/or sanded to width to fit the space between the veneer. Figure 6 A table saw was used to remove the paper core around the edge, in preparation for gluing wood strips around the perimeter. The type of wood used to reinforce the edges depends on individual goals and tastes. I glued Finnish Birch plywood around the perimeter because it is strong, I like the look, and I happen to have a lot of scrap from prior projects. In addition to sealing the edges, the solid wood edging allows the use of traditional joining techniques. Figures 7 and 8 show the tongue and groove shapes cut into the edges of the rocker box panels. When glued together, the joint will be as strong as if the entire panel were solid wood. Figure 7 After gluing in wood strips, a tongue is cut into the edges of the rocker box front and rear panels. Figure 8 The rocker box side panels have corresponding grooves to receive the tongues. 3/4" edge around the perimeter of the rocker box bottom panel is removed, leaving the bottom skin. Figure 8 also shows how the bottom of the rocker box was cut to mate with the front and side panels. A 3/4" wide section of top plywood skin and core material was removed. The side panels were glued to the bottom skin and the side of the top skin. One particularly tricky construction challenge was creating solid wood along the entire length of the cut outs for the altitude bearings. I couldn't think of a good way to remove the paper core along the edge if I cut the arcs first. Instead, I removed material along the top edge down to a depth of 3-1/2" and glued in a wide strip of plywood. Working with an exposed blade 3-1/2" high is quite dangerous. If you try it, be very careful. With solid wood glued along the top 3-1/2", I was able to rout out the curved section, leaving a solid surface along the arc. Note that this construction method leaves a wide portion of solid wood in the upper corners of the rocker box. Although the amount of weight in the corners is small, I decided to drill a 1-3/4" diameter hole in each corner to remove a few more ounces (Figure 9). More importantly, the holes serve as convenient carrying handles. Figure 9 Telescope packed up for local transport. Note the holes in the upper corners of the rocker box. This is the only location with a substantial width of solid material. Holes were drilled to remove a small amount of weight. More importantly, they serve as convenient lifting handles. Lifting is easy since this configuration weighs only 35 lbs. Mirror box Although these materials offer the promise of lighter weight telescopes, that isn't necessarily their only advantage. New materials often offer new design possibilities. For example, thin mahogany plywood is one of the few woods that can be bent into a tight radius. The composite panel, by design, is too stiff to permit bending. However, if the inside skin is removed or separated into short, disconnected segments, the panel becomes flexible (Figures 10 and 11). This is one of the reasons mahogany is found on luxury yachts. Curved sections of the dark wood are very attractive. I wanted to incorporate this feature into the mirror box. Figure 10 The lower panel with nine cuts spaced 3/4" apart will allow the panel to be bent 90º, closing up the gaps. Figure 11 The lower panel with nine cuts spaced 3/4" apart allows the panel to be bent 90º, closing up the gaps. It is straightforward to determine the number and spacing of cuts necessary to make a given radius knowing the panel thickness and blade width. For the 1/2" thick mahogany panel, I needed nine cuts spaced 3/4" apart (lower panel in Figure 10). When that panel was bent 90º, the gaps closed up (Figure 11). When glued together, the curved structure once again is very stiff. However, the outside surface of the resulting panel was not a smooth curve. The individual sections were long enough so that the outside surface was a series of corresponding facets. In order to achieve the smooth curved surface I wanted, I found I had to space cuts no more than ½" apart (top panel in Figure 10). The larger number of closer-spaced cuts allows the panel to bend in a much tighter curve. However, when bent 90º, the gaps do not close completely. Wood glue normally does not bridge gaps well, resulting in weak structures. Epoxy can fill a gap and produce a very strong structure. However, the viscosity of typical epoxy formulations is too low. Epoxy would just run out of the gaps and into the cells of the core. To prevent the epoxy from running, I added a thickening agent - Cabosil. Adding a few percent by weight of Cabosil makes the epoxy the consistency of peanut butter. It stays where it is put until it cures. Alternatively, the interior surface can be fiber glassed, surface tension working to hold the epoxy in place while it cures. The panel for the mirror box wraps around the sides, making a closed loop. However, the top and bottom edges have to be sealed. Figure 12 shows the last test piece I prepared before cutting the final pieces for the mirror box. Figure 12 Pattern of cuts on the inside of the mirror box. The finished mirror box is 11" high. As can be seen in Figure 12, a 1/4" wide groove, 1/4" deep, was cut around the inside surface approximately 4" from the top. This groove accepts a 1/4" thick plywood bulkhead containing matching strut holes in the corners and a central opening (Figure 13). The primary purpose of this bulkhead is to prevent the struts from accidentally striking the primary mirror when they are inserted. In addition, the bulkhead stiffens the structure and provides a secondary baffle for the primary mirror. The inside veneer and core of the honeycomb panel were removed along the top 1/4" and lower 1/2", leaving the outer skin (can be seen in Figure 12). This allows the 1/4" thick plywood top and 1/2" thick plywood bottom pieces to drop in place, sealing the paper core. In Figure 14, the bottom 1/2" plywood panel can be seen recessed into the bottom of the mirror box. Figure 13 View of the interior of the mirror box without the primary installed. Interior 1/4" plywood bulkhead provides guide holes for the struts, stiffens the structure, and acts as secondary baffle. Figure 14 View of the mirror box bottom. The solid plywood supports the mirror cell and seals the lower edge of the honeycomb panel. Four legs prevent damage to the bottom and adjustment screws when the mirror box is placed on the ground. Power connection and switch for the fans are located on the left. The honeycomb panel on the sides of the mirror box is not strong enough to hold screws used to attach the altitude bearings. One approach is to install solid core plugs as was done for the rocker box pivot bolt. However, I decided to construct the mirror box in steps, creating top and bottom "U" shaped sections that are subsequently joined together with flat side panels. Two of the vertical seams are visible in Figure 15. The core along the adjoining edges was removed and a solid wood spline (Figure 16) was glued into place. The spline creates a strong joint and provides solid material to support the bearing attachment screws. 1/4"-20 screws pass through the mirror box and screw into T-nuts inserted on the inside. Figure 15 Each altitude bearing is attached via two 1/4"-20 screws passing through the side of the mirror box and into T-nuts inserted on the inside. Note the two seams aligned with the screw holes. Figure 16 Each rocker box seam is held together by a plywood spline glued between the panel plywood skins. The solid wood provides the support for the T-nut and attachment screws. Fan Placement I spent a great deal of time conducting experiments determining the optimal size, number, and placement of fans for blowing across the surface of moderate sized mirrors. The 1" thick mirror used in this scope cools quickly, especially in the low profile mirror box. However, I was determined to implement the results of my testing and decided to install fans in this structure. The interior skin and core were removed in the areas of the fans and exhaust holes. 1/4" thick plywood was glued in place to allow fan and exhaust holes to be drilled. I think fans sitting on the outside is unattractive, so I mounted them out of sight on the interior surface (Figures 13 and 17). Rather than make the mirror box larger than necessary, I used fans that were 15mm thick vs. the standard 25mm. Two 80mm diameter fans are mounted side-by-side on individual vibration damping gaskets and are directed slightly downward and across the face of the mirror. Figure 17 Two thin 80mm diameter fans are mounted on the inside of the mirror box and direct flow across the surface of the mirror. The fans are angled slightly downward. Upper Ring and Struts The rest of the scope is a minimalist design using a single upper ring instead of an upper cage assembly (Figure 18). The upper ring and mirror box can be joined together in a variety of means. The traditional approach is to use eight angled struts to form a truss. That would work well. However, I chose to continue the parallel strut theme I've used in prior scopes. The square mirror box suggests placing one in each of the four corners (Figure 19). However, symmetry isn't necessary. Three points define a plane. Three struts provide adequate alignment of the upper ring to the mirror box. Figure 18 A single upper ring supports the focuser, spider, and baffle and is supported by three parallel struts. Figure 19 The obvious location for the struts are the corners of the mirror box. On previous parallel strut scopes, I've used wooden clamps to attach the struts. The clamps are time-consuming to make. Consequently, I wanted to explore simpler attachment methods. The Mark IV uses the same size struts as the Mark III. Each has a diameter of 1-1/2" and wall thickness of 0.049". Threaded inserts are placed inside both ends of all three struts. 1/4"-20 screws protrude through the bottom of the mirror box (visible in Figure 13) and are permanently held in place with T-nuts on the inside. When assembling the scope, the struts are inserted into the corner holes and screwed into place. Likewise, 1/4"-20 screws attach the upper ring to the tops of the struts. The screws are held captive in the upper ring, but require a tool for installation. Substituting clamp knobs with a threaded stud would make assembly tool-less, but I prefer the look of cap head screws. Attaching the struts at only the ends creates a long unsupported length. This is the easiest arrangement to implement, but it allows the struts to flex the most. However, if the spacing of the struts on the upper ring is different than on the mirror box, the struts will be deflected inward (or outward) and force the struts against the walls of the holes on the top of the mirror box. It is almost as if the struts are clamped into place at the top and bottom of the mirror box and the upper ring. The effective length decreases by the height of the mirror box. Since stiffness is proportional to the cube of the inverse length, the stiffness increase can be substantial. There are basically two choices for the location of the focuser. It can be placed on the side where there is one strut or on the side where there are two struts (Figure 20). If placed on the side with only one strut, there are more options for focuser position. For example, on smaller scopes, it is often more comfortable to have the focuser located at an angle approaching 45º vs. horizontal. However, since the distance between the struts is great, the upper ring should be stiffer (i.e. thicker). If the focuser is placed on the side with two struts, the upper strut (at 45º) restricts where the focuser can be mounted. On the other hand, the upper ring does not need to be as stiff (i.e. thick) since the focuser attaches to a short span between struts. The finders also can be conveniently located on the top strut, near the focuser (Figure 21). Figure 20 The long arc between struts at the top right and bottom left offer lots of space to mount the focuser. The short arc between struts on the top right and bottom right offers little space to mount the focuser. Figure 21 The base of a JMI RCF mini 1 focuser was removed, permitting the focuser to be rotated 45º and mounted directly onto the mounting bracket, close to the strut which also carries the finders. On this 12-1/2" f/5 scope, I wanted the focuser mounted near 30º. The focus knobs of most focusers further limit how close the focuser can be mounted to the strut. Generally, rotating the focuser so that the focus knob is farther away doesn't work because the corner of the square focuser mounting plate gets in the way. However, JMI's RCF mini 1 focuser offered the opportunity to do exactly what I wanted. The entire RCF focusing mechanism attaches to its mounting plate with only two screws. I discarded the mounting plate and attached the focusing mechanism directly to my home built mounting bracket (figure 21). I was able to rotate the focuser 45º so that the focus knob cleared the strut, while placing the focuser at approximately 25º from vertical - providing a comfortable eyepiece viewing angle for most elevations. Eyepiece height at Zenith is 60". Some people might point out that I could have used a helical focuser or ask why I didn't use a 2" focuser. One of the subtle benefits of the 1-1/4" focuser used is its 3" long drawtube. A long, narrow drawtube allows a smaller cone of light to reach the focal plane than shorter and/or wider drawtubes. Consequently, the baffle on the other side of the ring can be smaller, lighter, and catch less wind. In addition, an f/5 scope does not sacrifice much field of view by using only 1-1/4" eyepieces. On the other hand, helical and 2" focusers are available with finer focus, which I would prefer. Summary and Conclusions When creating a new design there is always risk that it won't work as planned. In particular, there was considerable risk with this new scope since I implemented four new elements simultaneously: Construct mirror box and rocker box with paper honeycomb composite panels. Attach parallel struts at the ends and without any intermediate clamps. Use only three struts at the corners of the box. Install fans to blow across the face of the primary mirror. There was plenty of opportunity to fail. Well, OK, installing the fans didn't entail much risk. They were unlikely to jeopardize mechanical or optical performance. The thin primary mirror cools quickly, and has plenty of time to do so since I usually set up before dark. To see any obvious improvements from the fans, I will need excellent seeing conditions coupled with immediate use of the scope after moving it between significantly different thermal environments. However, I see no deleterious effect of the fans while viewing through the eyepiece at high magnification. The damping gasket and damping characteristics of the honeycomb panels effectively eliminate fan vibration being transmitted to the eyepiece. I am extremely pleased with the scope. I've received numerous compliments about its appearance and performance. In particular, the curved mahogany mirror box attracts a lot of attention. Because of its stiffness and plywood edging, most people assume the rocker box is made of solid wood. People are amazed that it weighs only 9.8 lbs and the entire scope, including finder, weighs 40 lbs. When observing near Zenith, vibrations damp out in about 2-3 seconds. At lower elevations, vibrations damp out within 1-2 seconds. I very much notice and appreciate the light weight when packing up. On the other hand, at star parties, I am now more concerned about the scope being tipped over during wind gusts. This isn't a problem when the scope is in use because the minimalist design presents a small profile to the wind. When other scopes are being pushed around like wind vanes, this scope is steady. My concern arises when the scope is covered during the day. The composite panels reduce overall weight, perform very well, and the mahogany skin is particularly attractive. There are downsides. Materials are available only through a handful of specialty distributors or directly from the manufacturer. Often minimum quantities must be purchased. Materials with the lauan skins cost no more than typical hardwood-faced plywood, but the mahogany panels cost 2X - 3X more. Unfortunately, the mahogany panels are the only ones that can be curved. The honeycomb panels were not the only key elements to making the lightweight scope. For example, it was necessary to minimize weight of the primary mirror (11 lbs) and six-point floatation mirror cell (1.4 lbs). Top and bottom views of the custom lightweight cell are shown in Figures 22 and 23. Figure 22 Top of lightweight six point floatation cell. Figure 23 Bottom of lightweight six point floatation cell. Is it possible to build a lighter scope? Yes, but I think the labor and materials costs soon reach a point of diminishing returns. Except for bragging rights, I see little need (even disadvantages) to building a lighter scope. This scope is now light enough to take as checked baggage to Australia - a trip I hope to be taking soon. ************************************************************************ Part II: Mark IV 13.1" f/4.5 Travel Scope Introduction I had planned to take to Australia the Mark IV 12-1/2" f/5 I built last year (Part I of this article). However, there are some drawbacks using it as a travel scope. The mahogany rocker box looks too nice to bang around. The long struts would have to be packed separately from scope and other luggage, requiring a third checked bag (incurring extra baggage charges). The fan and exhaust holes in the mirror box are difficult to seal, potentially allowing more dust and debris to enter while traveling. I wanted to keep the basic Mark IV design. The travel scope had to meet the following goals: Modify design of mirror box, substituting other lightweight materials for the paper honeycomb mahogany panels. The mirror box must contain and protect the mirror during transport. Most of the scope components must pack within a cube 18" on a side. These components will be checked as bag #1. The struts and altitude bearings must fit within my second checked bag carrying clothes, charts, etc. Lower eyepiece height so I can observe more of the sky while seated. The scope must be easy to assemble (fast assembly of minimum number of parts, requiring one or no tools). The assembled scope must perform well, (e.g. have excellent views, move smoothly, damp vibrations quickly, provide comfortable eyepiece positions, etc.) Figures 1 and 2 shows the fully assembled telescope that met all of the above goals. Figures 1 and 2 Mark IV 13" f/4.5 travel scope made with segmented struts. The struts are joined at the intermediate ring via 1/4"-20 threaded posts screwed into threaded inserts. The weight of the fully assembled scope, including finders, is 45 lbs. Each of the three segmented struts disassembles into two pieces (total of six) approximately 26-1/2" long. Struts have a diameter of 1-1/2" with a wall thickness of 0.049". Mirror "box" The mirror "box" is essentially a fiberglass tube with wooden end rings (Figure 5). It is very strong and stiff. A layer of fiberglass epoxied within an outer shell of Ebony Star™ laminate makes up the tube. I cut the top and bottom tube rings from 1/2" Finnish Birch plywood. The top ring has an opening large enough to insert the mirror through it. Because the lower ring supports the mirror cell, I cut a smaller opening in the bottom ring to permit airflow through the mirror box. On the inside surface of each ring, I used a router to cut a 3/16" circular groove about 3/16" deep with a diameter the same as the fiberglass tube. I then epoxied the end rings to the fiberglass tube (Figures 3 and 4). Note the four small feet on the bottom of the mirror box (Figures 6 and 7). These feet keep the mirror box from resting on the adjustment knobs while I assemble the telescope and while it is sitting in the rocker box during transport. The rubber feet slide over 5/8" diameter dowel rods that are glued into holes drilled approximately three-fourths of the way through the bottom of the mirror box. The four feet are longer than the adjustment screws, but short enough in position so they don’t touch the bottom of the rocker box when the telescope is in use. Tongues cut into top and bottom of the two side panels are glued into the slots along the sides of upper and lower rings. Each altitude bearing will be attached to the side panels using two 1/4"-20 "T" nuts. Figure 3 Details of mirror box construction, showing fiberglassed Ebony Star tube, wooden end rings, and side panels. Figure 4 The fiberglassed Ebony Star tube will be epoxied into the grooves of upper and lower rings. The tongues of the mirror box side panels will be glued into the side grooves. Figure 5 Dry fit of the mirror box parts. Figure 6 Bottom ring of mirror box with mirror cell installed to test fit. Figure 7 Close up of cell attached to the inside of the mirror box bottom. The end of each strut contains a threaded insert. Like the Mark IV 12-1/2" f/5, struts screw onto 1/4"-20 screws in three corners of the bottom of the mirror box. However, the struts would leave visible gouges where they contact the wood. So I installed large stainless steel washers to protect the finish (Figure 8). A pair of #8 stainless steel wood screws hold each large washer in place. I had to drill and countersink the holes so that the screw heads were below the surface of the washer. Figure 8 2" diameter stainless steel washer prevents tube from damaging wood when screwed onto the 1/4"-20 threaded stud. Figure 9 1-1/2" strut screwed into place. The completed empty mirror "box", without primary, mirror cell, lid, or altitude bearings, weighs 6 lbs. The completed mirror box with primary mirror, mirror cell, attached lid and altitude bearings, weighs 25 lbs. Mirror cell The mirror cell pads are considerably larger than those used on the 12-1/2" f/5 (Part I of this article). Although the mirror on the 12-1/2" is firmly attached, I was concerned about rough treatment by baggage handlers. The diameter of the pads on this cell was increased from 3/4" to 1-1/2", providing four times the contact area to the back of the mirror. Figure 10 Top view of mirror cell. Figure 11 Bottom view of mirror cell. The central triangular support is cast aluminum. Figures 12 and 13 show the parts used to make and attach the rocker bars. The rocker bars are cut from 1/2" x 3/8" aluminum bar stock. The ends are drilled and tapped to accept the weld studs used as support pads. The rocker bars pivot on 1/4" diameter stainless steel shoulder bolts and are separated from the main triangle by 1/4" washers. A stainless steel spring washer under the head of each shoulder bolt maintains the proper tension as well as allows for differential expansion and contraction as the temperature fluctuates. Figure 12 Detail of rocker bar hardware. Each rocker bar is supported by a 1/4" diameter stainless steel shoulder bolt. Washers position the bar away from support triangle. The curved disc spring allows the cell and mirror to expand and contract without stressing/distorting the mirror. Figure 13 The height of each 1-1/2" diameter stainless steel pad is adjusted by screwing its post into a threaded hole at each end of the rocker bar. A jam nut locks it into place. The rocker bars were cut from 3/8" X 1/2" aluminum stock. I enjoy viewing through excellent optics. The 13" f/4.5 mirror shown in figure 14 was refigured by Woden Optics and now has a reported Strehl Ratio of 0.988. It is 1-5/16" thick and weighs 13.2 lbs. I was concerned whether gluing the mirror cell with larger pads to the back of the mirror would affect its figure. I see no visible distortions when star-testing under very good seeing conditions. Star images in focus are nice and tight. Figure 14 The 13" f/4.5 primary mirror with attached cell used in the travel scope. Hinged lid I didn't know if I would have a clean and convenient place nearby to set the mirror box cover during observing sessions abroad. While tired and in unfamiliar settings, I also didn't want to risk leaving the mirror box cover behind. I decided to attach the mirror box cover with a set of custom made hinges (figure 15). I bought one larger aluminum hinge, cut it in half, and flipped one set of leaves on each new hinge. This allowed the hinge to attach to the mirror box and the lid whose heights are different. The lid flips out of the way in seconds (figure 16). It is easy to remember to cover the mirror box first before tearing down the scope. The clip that holds the mirror box up is a plastic tool holder that is normally attached to a wall. I couldn't find the exact size holder I wanted. Larger sizes wouldn't hold the lid securely. Smaller sizes required too much force to attach and remove the lid. I used a drum sander to increase the inside diameter of a smaller holder until the clamping force was just right. Figure 15 The attached hinged lid prevents it from coming off the rocker box during transport. Figure 16 In use, the black plastic clip holds the lid up. Joining struts at the middle ring After screwing the lower set of struts into the mirror box, I screw threaded rods into the threaded inserts on the tops of the struts and lock them in place with a wing nut (figures 17 and 18). The middle ring (1/2" thick) then just slips over the ends of the struts. I counterbored 1-1/2" diameter holes 1/8" deep on both sides to locate the struts. The through hole has a diameter of approximately 1-1/8" to allow the wing nut to nest within it. Next, I screw the top three struts onto the threaded rods. Finally, I attach the top ring. The resulting structure is surprisingly stiff. Without the intermediate ring, the struts tend to buckle at the joints. The intermediate ring prevents any lateral motion. Figure 17 A 1/4"-20 rod, screwed into the threaded insert, is locked in place with a wing nut. Figure 18 The intermediate ring slips over the lower set of struts. The hardware on top of the ring in the image is there to show the pieces in the joint. The rocker box and altitude bearings are the same as the ones built for the 12-1/2" f/5 Mark IV (Part I). Due to the lightweight honeycomb panels, the rocker box weighs less than 10 lbs. Packing for transport Although baggage restrictions vary by airline, typical international flights required checked baggage to be less than 62" (length + width + depth) and weigh less than 70lbs. Domestic flights further limit checked baggage to 50lbs. Larger and heavier items are possible, but with increased cost and hassle. My design goals were defined by the typical restrictions. In order to allow the scope to be as large as possible, I decided not to use a hard shelled case or shipping container. These add considerable weight, reducing the amount available for the scope. They also reduce the available volume and are expensive. Based on my experience shipping via carriers such as UPS, I decided that a well-designed pair of nested cardboard boxes would protect the scope well and allow the greatest latitude of scope size and weight. Cardboard boxes are readily available in a variety of sizes, shapes, and load ratings. I decided that I would nest an 18" cube box within a 20" cube box. The total outer dimensions of the 20" box were 21-1/2", nearly the maximum allowed. Figure 19 shows the scope as it is packed for transport. Note that the altitude bearings have been removed. The intermediate ring can't be seen, but it lies in the bottom of the rocker box. Although the secondary is shown here, at the last minute, I decided to remove it and pack it with the 8X50 finder inside the mirror box. No, the mirror and cell were not removed. I covered the mirror and then packed the inside of the mirror box with gray closed-cell foam. One 2" thick circular foam piece sits on top of the primary. A second 2" thick circular foam piece lies on top of it. This second piece has cut outs to accept the finder and secondary mirror with its holder. Finally, a third 2" thick circular foam piece fills the remaining space. Even if the mirror were to break free of its cell, it would be held securely within the mirror box, well-protected and unable to move. The packed up scope shown in figure 19 fits within an 18" cube. The combined weight is 37 lbs. Figure 20 shows the six segmented struts and the two altitude bearings. These easily fit in my second checked bag containing clothes, books, and charts. I carried my eyepieces on board in my camera bag. Figure 19 The mirror box, top ring, intermediate ring (under mirror box in the bottom of the rocker box), and rocker box, packed for transport. Figure 20 The six strut segments and altitude bearings (dimensions 10" X 26-1/2") will fit easily into my second piece of checked luggage (15" x 16" X 30"). Figures 21 shows the collapsed tube assembly alongside the nested cardboard boxes. Squares of 1" thick foam padding, 3" x 3", glued in each corner, suspend the 18" cube within the 20" cube. Figure 22 shows the rocker box already sitting in the bottom of the 18" cube. The intermediate ring sits on the bottom of the rocker box, underneath the collapsed tube assembly. Figure 21 I decided to remove the secondary mirror and holder. Along with the finder, they are held securely by foam padding within the mirror box. Figure 22 The intermediate ring sits on the bottom of the rocker box. Figure 23 shows the scope before I sealed up the boxes. Note that I also removed the ground board, wrapped it in bubble wrap, and inserted it between the upper ring and mirror box. This allowed me to add more foam padding under the rocker box. The total weight of the scope within the sealed boxes was 52lbs, well within the international weight limit. With a little effort, I believe I could shave off two pounds to meet the domestic baggage weight limit. Figure 23 Ready to seal the boxes containing the packed OTA and rocker box. Summary and Conclusions As mentioned at the end of Part I, creating a new design entails risk that it won't work as planned. The risks were associated with the following design elements: Use segmented parallel struts that fit within checked baggage. Glue the mirror to larger support pads. Transport the scope (including primary mirror) as checked luggage. Experiments I conducted on joining struts end-to-end indicated that they would be much too weak. The intermediate ring supporting the strut joints was very successful. The scope incorporating the intermediate ring and segmented struts is stiffer than when I substituted three long unbroken struts. The cell modeling program PLOP assumes point supports. In real life, support "points" of all cells have some physical size, although they tend to be small. Also, in most conventional cells, the mirror is free to move relative to the supports. In prior cells, I've glued mirrors to small (3/4" diameter) supports with compliant silicone adhesives. I was willing to sacrifice a little performance in order to have a mirror securely held by its cell. However, I don't know how large support "points" can be, or how firmly attached they can be, before they introduce visible distortions to the mirror surface. I was pleased that the larger (1-1/2") supports introduced no distortions that I could see. Furthermore, the primary mirror, along with the rest of the scope, survived the trip without any damage. Protected by the two nested boxes, the scope made it to Australia and back without damage, despite the fact that the boxes were opened and inspected on the way there. I am confident that this approach will work just as well on future trips. How was the trip? This past April 2005, two other amateur astronomers and I each spent 50 hours at the eyepiece over the eight nights available to us. Approximately 27 hours after leaving home in California, two of us arrived in Hawker, population 300, 365 km north of Adelaide . We spent two nights at a small cottage just north of town. Despite only a few hours sleep on the plane, we couldn't resist the dark skies. We assembled our telescopes and managed to observe three hours before collapsing from exhaustion. The third member of our expedition joined us in Hawker the second night. Conditions this night turned out to be the worst of the trip. Clouds moved in during the day, and took their time dissipating after sunset. Transparency suffered. Clouds returned in earnest and shut us down by 1:30AM. The skies were partly cloudy all day during our drive north to Wirrealpa Station, population 2, 550 km north of Adelaide . The homestead and our cabin were the only occupied structures on the 35 km by 45 km working sheep ranch. The nearest town, Blinman (population 25), is 40km to the West. When you look up "middle of no where" in the dictionary, you'll see a picture of where we were. The terrain looks similar to the Mojave or Arizona deserts, except for the dried up stream beds which are cluttered with large gum trees, hundreds of years old. As you might expect, the sky is as dark as it is going to get and light domes were non-existent. In fact, around 3-4AM each night we experienced our worst light pollution. At that time the sky was noticeably brighter and the ground and surroundings were easier to see. This is when Sagittarius was directly overhead and the Milky Way was a broad bright band of light extending east to west all the way to the horizon. I've read descriptions of the Milky Way casting shadows. Now I understand why. Consequently, I don't think the skies were any darker than mag 6.5. During the first of three nights at Wirrealpa, the clouds cleared around Midnight and we observed until dawn. The second and third nights were clear throughout. All three of us observed from dusk until dawn the second night. The third night one person managed a second marathon all-nighter. The other two had to call it quits at 3AM. Our accommodations were rustic but functional and comfortable. More importantly, when we called it quits, we walked less than 10 yards from our scopes to our beds. Our hosts, Warren and Barbara Fargher, are among the few remaining authentic Australian ranchers. Although they have a difficult life working the 5500 head of sheep and 1000 head of cattle in the semi-arid Outback, they are out-going, gracious, and considerate people who made us feel welcome. For example, Warren thoughtfully drove out one morning at 4AM with only his parking lights on. He couldn't tell if we were observing, but he surmised the lights would disturb us if we were. He also noticed a flat on David's car and fixed it for free. They invited us over to their house for a barbecue one night. We learned a lot about South Australia over grilled chicken, sausage, and steaks. For dessert, we ate homemade quandong pie, made from the fruit Barbara gathered from the wild quandong trees on their property. The evening was one of the highlights of the trip. We then drove 130 km over rough gravel roads north to Arkaroola (http://www.arkaroola.com.au/) where we spent our final three nights. Arkaroola is an oasis in the middle of no where. This wilderness sanctuary lies 700 km north of Adelaide . Our rooms were almost plush. The complex can hold as many as 200 guests. The owner, Doug Sprigg, understands astronomers' needs and was exceptionally accommodating. He has a C-14 set up in two different domed observatory buildings which he uses to conduct sky tours for the guests. Although we could have used these instruments for free, we opted to use our own telescopes - equipment familiar to us. He allowed us to set up our scopes in a large roll-top observatory sitting atop a hill 150 feet above, and overlooking, the complex, approximately ¼ mile away. When we were done observing for the night, we just closed the roof, locked the door, and drove to our rooms. Very convenient. Doug also is an accomplished pilot, conducting daily scenic flights over the surrounding mountains. We took one of his flights our last day there. Observing conditions were very pleasant. Temperatures were in the mid to low 70's from sunset until after midnight and dropping into the 60's by early the next morning. We typically observed in our shirtsleeves until 2AM. A light jacket was the most anyone wore. Humidity was typically 40-50%. There was little to no wind during most nights. One night at Wirrealpa the wind rose to10-12 mph for a couple of hours. The gusts were strong enough to occasionally move the scope. The ever-present outback flies were a nuisance during the day. However, after the sun went down, the night was completely bug-free. We never saw any of the legendary poisonous snakes. Emus and Euros (medium-sized kangaroos) were rather plentiful. We saw at least a dozen of each. Observing Stats My list of potential targets had grown to almost 1000. It included all the NGC objects between Right Ascension 5 - 20 hr and below Declination -40. It also included selected IC and ESO targets, a few Gum nebulas, and brighter showpieces between Declination -20 and -40 that would be more favorably placed from the southern hemisphere. Although the position of the Large Magellanic Cloud wasn't optimal, I placed heavy emphasis on observing objects within it. I hoped to observe a large fraction of the 300 LMC objects plotted on pages A24 and A25 in the new Uranometria - South. The new scope performed flawlessly. I ended up logging approximately 650 observations - 325 within the LMC. Although this number exceeds the number plotted in Uranometria, I didn't finish observing all of those objects. However, the LMC is so rich, I frequently encountered other bright targets not plotted in Uranometria. I saw more than 325 objects in the LMC, but only counted those objects I wrote descriptions for and was able to unambiguously identify. Simply put, every serious astronomer MUST spend at least several weeks observing from the Southern Hemisphere. Although only a small fraction of the sky is not visible from mid-northern latitudes, most of the sky we can't see contains the "best in class" of almost every object type. I can't wait to go back. Albert