In addition the tube is used to exclude heated air from passing thru the optical path from outside sources. Most of this heated air is the direct result in the temperature drop after the sun goes down. Heat is stored in stone, concrete, water, glass, earth, metals and so forth., as well as air. A flow of warm air entering the optical path will cause optical distortions if the entering air flow is warmer or cooler. As a rule, it is best to have the air temperature inside the tube the same as the outside air. But in the case of closed tube telescopes (refractors, Schmidt-Cassegrain, Maksutov types) the temperature can be quite different and cause no problems because the air is kept from mixing. It is the mixing of air with large temperature differences that causes optical distortions. In large research telescopes fans are used to mix the air to keep the temperature differences as low as possible. With large open framework tubes it is not possible to physically exclude warm air from entering the optical path so temperature of the parts is adjusted so that all parts are at very nearly the same temperature to avoid air currents of unequal temperature.
Solids and liquids usually hold much more heat than gasses. Metals can change temperature faster than stone or concrete. As the air temperature drops in the evening rather quickly, heat is released from metals rapidly while heat is released from concrete and glass rather slowly. If you set up your telescope on a concrete patio the stored up heat will be slowly released to cause optical distortion most of the night. The same is true if the observing path is over buildings, parking lots, or highways. On the other hand, if the telescope is set up in an area covered by grass and trees no large amounts of heat are released during observations making the viewing better.
The area the telescope is used in will determine the ground air current problem. But the tubing also affects currents close to the telescope. The metal telescope mount hold a lot of hat and gives it off as the air temperature drops. Body heat of the observer can be a problem if the heat can rise up and enter the tube. And although you may feel cold your body gives off heat continuously because it doesn't cool down much. Concrete or rock close to the tube will give off heat that may enter the lower end of the tube and rise up0 thru it. In some cases you may find it better to put a dust cover over the lower end of the tube to avoid this. If you find you have a tube current problem it is very easy to try out on several nights to see if there is some imp0rovement. By the way, in some cases a dust cover over the lower end of the tube will reduce formation of dew on the optics inside the tube. A wood covered walk area around the telescope can also help. Covering the lower end of the tube will still allow warm air to rise up and out the upper end of the tube.
Some sources of tube currents are parts inside of the telescope. If you use uninsulated metal tubing about half the heat given off by the metal will be inside the telescope tube. Years ago this problem was solved by adding a thin layer of cork to the inside of the tube for insulation. Most of the heat is then released outside of the tube. Nowadays you can use flock paper or black plastic foam sheet which also reduces internal reflections as well as tube currents. By using non-metallic tubing (fiberglass, cardboard, wood, plastic, etc. ) you can avoid insulating the inside of the tube as usually recommend with metal tubing but non-metals are generally less stiff or rigid so are less able to support weight. For example, Young's modulus of elasticity for aluminum is 10,300,000 lbs./sq. in. and a stiffness of 107,290,000 in. Young's modulus of elasticity for fiberglass is 500,000 to 1,000,000 lbs./sq. In. (Handbook of Chemistry and Physics-Properties of Commercial Plastics) and stiffness of 7,650,000 to 23,070,000 in. Thus aluminum is 4.6 to 13.9 times more stiff than fiberglass. The uncertainty in the values come about because fiberglass tubing is a mixture of glass fibers and plastic. While most aluminum tubing is 98 percent aluminum the composition of fiberglass tubing can vary widely between manufacturers, specify formulas and individual batches.
Most non-metallic tubing is made with thicker walls than metal tubing to help make up for the lower thickness. Doubling the thickness also doubles the stiffness. And you should consider how much stiffness is needed by the telescope. Most seamless aluminum tubing has thicker walls than needed because it is difficult to make large diameter aluminum tubing with very thin walls. For 8 inch aperture telescopes or smaller the wall thickness of seamless aluminum tubing actually needed is only about 0.050 inches. Also remember that telescopes most popular today use quite short tubes compared to the long refractors. A shorter tube means you need less stiffness. In addition, non-metallic tubing dampens vibrations better.
While most commercial non-metallic tubing is made of fiberglass, other materials can be used. Spiral paper tubes are available from some telescope suppliers and from concrete contractor suppliers. Wood makes a good telescope tube but does require more skill to make. A simple square tube is easiest to make. Six, eight or more sided tubes are more difficult. Round wood tubes can be made by forming thin plywood (about 1/8 inch thick) in curved sections then overlapping sections in top layers. Or you can make a 2 sided (or more) tube then turning the outside down on a lathe. Needless to say, making a round tube is quite difficult without the tools and skills required. But a round tube is refereed to allow the tube to rotate. Square and other multi-sided tubes can be made to rotate by adding curved segments to the outside of the tube to form a circular profile.
Some amateurs choose to make their own fiberglass tubing. A fabrication method is described in "Standard Handbook of Telescope Making" by Howard. Since fiberglass tubing is fairly difficult to make most amateurs compromise and fiberglass a spiral paper tubing. Spiral wound fiberboard tubing is available locally from concrete contractor suppliers and sometimes from concrete contractors. they are generally known under the trade name of "Sonotube" but there are other brands. Since they are designed to be used as concrete column forms it is important to take care in selecting the tube. The inside or the outside is often coated with wax or sheet plastic to reduce moisture assorbsion from wet concrete. For telescope use this coating is difficult to paint or to recoat with fiberglass. A layer of fiberglass will fill in the spiral joints. increase stiffness, and reduce moisture assorbsion. But it only coated tubes are available the wax or plastic coating usually must be removed before painting or fiberglassing. Most often the coating on the paper tube is wax. Removing the wax usually requires the coating to be heated with a heat gun or set in the sun on a warm day and the wax scraped off. If the tube is to the fiberglassed the surface can then be roughed with coarse sandpaper for better surface grip. If too much wax remains on the surface, the sandpaper will cake up with wax very quickly. Go back to scraping the wax off before continuing to sand. If the tube is plastic coated, very often the plastic is a type that can't be painted on or fiberglassed. The plastic can be removed with coarse sandpaper. If the plastic is inside the tube ii is usually too difficult to remove. You can line the inside of the tube with adhesive backed flock paper (Edmund No. 79,621) which sticks quite well. Since it is rather time consuming to remove either a wax or plastic coating it is better to start with uncoated tubing if you want to paint or fiberglass the use. But the uncoated tube is more difficult to get in some areas.
Metal tubing can be made in any diameter from one to five feet and in most common telescope lengths from large sheets of metal rolled into shape. Many smaller tube sizes can also be rolled depending on metal thickness and length. However since seamless aluminum tubing is available in sizes 10 inches and smaller most often a tube is rolled from sheet metal to get a special wall thickness or diameter. Tube rolling requires equipment of a large sheet metal fabricating firm. Smaller firms that do gutter and downspout work or heating and air conditioning duct work usually don't have the rolling equipment in the right size and capacity to handle the job and often don't have aluminum sheet metal most rolled tubes are made of. The sheet metal thickness to use for rolled tubes in up to the telescope builder to select. As a rule the wall thickness is usually between a half percent and one percent of the tube diameter. The wall thickness depends on tube length, weight and personal opinion of the builder. Most 8 inch telescopes don't have too much weight to handle so a well made aluminum tube is more likely to use a wall thickness of one half percent of the tube dimeter. On the other hand, a 20 inch telescope will have a long tube even if it is a rich field telescope and the optics and optical support system will be quite heavy so is more likely to use a wall thickness of one percent of the tube diameter. if you are not sure of yourself in selecting the wall thickness to use it is worthwhile to note that most aluminum tubing used for 8 inch to 20 inch apertures use a wall thickness of three quarter percent of the tube diameter or something fairly close with generally very good results. Professional quality telescopes ($10,000 or more) in 16inch aperture or larger are usually made of rolled aluminum tube with internal ring welded in place to give better stiffness with less weight. This allows thinner wall thickness but cutting and welding rings inside the tube is difficult and/or expensive for most amateurs.
After the tube is rolled from sheet metal a joint must b made the length of the tube. The joint can be welded and ground smooth on the outside. If that is too expensive you can add a strip of aluminum on the inside of the tube bolted or riveted in place. By countersinking the screws or rivets you can cover the heads and the outside seam with auto body putty. Sanding leaves the outside with a smooth " seamless" finish after painting. In some cases if the metal is thin enough the rolled tube can be joined with a lock seam. It will leave a hump of folded metal usually on the outside of the tube. Steel sheet can be used to make rolled tubing but be sure it is thick enough to support the optics. A 24 gauge galvanized steel hot air duct may be easily available but in most cases is not adequate without considerable reinforcing. The cost of having tubing made of rolled sheet metal varies widely among firms that can do the work.
Tube reinforcing rings are often used on telescopes. Normally fiberglass tubing is available with metal reinforcing rings. Unless the fiberglass is quite thick these rings should not be considered an optional accessory merely for decoration. The metal rings are important to give adequate support to the optical system while keeping the weight and the cost of the fiberglass tubing reasonable. With tubing of any material assembled into a telescope you may find additional support is needed to reduce vibrations and stabilize components. Reinforcing rings are most likely to im0rove performance without major rebuilding. First it should be determined, if possible, what part of the tube needs reinforcing.
The three most common places additional support might be needed is at the mirror mount, at the cradle and the spider. A fourth common source of vibration is in the telescope mount because of a shaky base but this does not involve the tube directly. If the tubing is made of a material that lacks sufficient stiffness a problem may exist at the tube cradle. For good performance with most non-metallic tubing the tube cradle should be 25 to 50 percent of the length of the tubing. The long cradle reduces the flexure of the tube under normal loads or vibration. Since most amateurs have little control over cradle length you may find it is desirable to add reinforcing rings near the cradle either inside or outside the tube. Another method of adding support is to have heavy sheet metal rolled to fit the outside diameter of the tube for about a quarter of the circumference and of suitable length. This usually can be added between the existing cradle and the tube with little trouble. Felt or similar material should be used to protect the tubing.
The mirror mount end of the tube may need additional reinforcement but usually the most common place the tube needs extra support is at the spider end of the tube. Low secondary vibration depends on good spider and a good support for the spider. If a sturdy spider must be used to reinforce a weak tube then low vibration will be achieved at the cost of high diffraction in what you observe thru the telescope. Low diffraction spiders require a firm stiff support to hold the diagonal with minimum vibration. If you find your telescope tube could use reinforcing rings you will probably have to make them fit our telescope. In some cases you might be able to adapt the commercial reinforcing rings made for fiberglass tubing to fit the inside of another size tubing by cutting the rings and squeezing them down. The amount these rings can be reduced is limited. Cast aluminum rings can be machined to fit the outside of larger tubes but can't be cut down for smaller sizes. Other rings can be reduced in diameter by a quarter to half an inch. In making your own reinforcing rings you are necessarily limited to the material and equipment available. With a hand saw you can cut rings from plywood to fit any tube size. Metal rings can be made of flat strip steel or strip aluminum from some hardware stores. It is fairly difficult to bent thick wide strips but even a couple of 1/16 X 1/2 inch strip rings can help a spider considerably. If 1/8 inch thick aluminum is available it can be bent if it is not too wide. Bending metal rings should be done over a form about ten percent smaller in diameter than the finished ring. Plywood cut on a band saw is good but a round post or pole will do. It is difficult to bend the ends of the strip so don't try. Use a long strip and bond it to form a spiral. Then trim off the straight ends to fit the ring to the inside diameter of the tube. In some locations the hardware stores don't have metal strip. You can substitute threaded rod of about a quarter inch in diameter. If more support is needed you can use serval rings next to each other. Carefully fit the rings so the ends butt against each other then glue in place. Of course, the easiest thing to do is buy good telescope tubing to begin with and avoid the need to patch it up later if you discover a problem. Often for telescopes 16 inches or larger but sometimes in smaller sizes the telescope tube is made in the open style skeleton type. Early this century large professional telescopes were made in the Rectangular form skeleton tube. Construction is basically a series of large diameter rings connected by rows of small diameter rods or tube which run parallel to the optical axis. The rings are also joined by diagonal bracing between the parallel rods or tubes. The design is important ot amateurs because it allows you to make any size telescope tube to suit your needs using commonly available equipment. A power band saw is preferred to cut the rings with but you can use a saber saw or even by hand. The rings are then drilled and bolted together using threaded rod with electrical conduit, pipe or tubing for spacers between rings. Sometimes no spacer tubes are used. The space between rings is maintained by nuts on both sides of the ring. Spacing between rings is generally one to two times the ring diameter.
While a large Rectangular skeleton tube is often easier to make than a fiberglass tube many amateurs have had trouble getting the excellent support a well designed tube can give. Thus skeleton tubes in general have been unjustly discredited. The problem has been in the amateurs adoption of the design, not the design itself. The most common error is to leave out the diagonal bracing between rings. It is the diagonal bracing that gives the tube very good stiffness with low weight. Without diagonal bracing the rings and the tubes or rods must be large and heavy to give adequate support. The second most common problem is overbuilding. Sometimes amateurs get carried away with that pet designs or out of fear not building heavy enough. Skeleton tubes have been observed that were well machined and very craftsman in appearance but with overbuilding apparent in design. Rings one inch thick by two inches wide in cross section has a nice "massive" appearance but only a 1/4 inch by one inch cross section was needed. A dozen parallel tubes were used when only six were needed. The telescope mount was homemade out well designed and machined. The problem was it was not adequate to support such an overweight tube so a great deal of effort produced a shaky telescope that was nice to look at in daylight but close to embarrassing to show off when observing at night. The third problem is open tubes allow warm air currents from the ground, telescope mount, observer, etc. to pass freely thru the optical path. Stray light also can pass into the telescope. Both unwanted air currents and light can be excluded by adding a thin metal heating duct or cardboard tube set inside the skeleton framework.
In most recent years large professional telescopes have adopted an improved open telescope tube called the Serrurier Truss. It gives exceptional stiffness with low weight using less parts. The design recognizes the great importance in the diagonal members and completely eliminates all the parallel rods or tubes while reducing the number of rings to only four. The rings are connected by eight diagonal members, usually thin steel or aluminum tubing. Two rings are fairly close together at the cradle for whatever is used to connect the tube to the telescope mount. Sometimes these two rings are replaced by a single box-like structure. The Serrurier Truss has been used by a number of amateurs for large telescopes with better results than the older Rectangular form. The Serrurier design forces you to use diagonal supports while in the Rectangular form the diagonal bracing was easily and often omitted.
Both the Serrurier Truss and Rectangular form of open tube can be built by amateur but the Serrurier Truss is more stiff and takes less parts which results in less weight and better performance. Both require cutting rings from sheet material. Usually aluminum is used but wood and steel can also be used. Cutting the outside of a ring is easy on a power band saw but cutting the inside diameter can be a problem. You can drill a pilot or starting hole and use a saber saw. Or you want to use a band saw you can drill a pilot hole, cut the band saw blade, insert it into the hole then weld the blade back together. Commercial band saws have built-in blade welders to handle such problems. For most amateurs it is not convent to cut and reweld band saw blades. You can split the ring by cutting thru it to cut the inside diameter then welding the split after cutting (glue and screw in the case of wood.
Bob's Note: For working with wood, the router is probably the best for making rings as you can make up an arm that the router is attached to and a pin in the center of the circle you wish to cut and then just run the router around the circumference about a half inch of depth at a time and thus cut a nice round cut in a few passes. Inside and outside cuts are equally easy to do this way. Aluminum can also be cut but it is a lot more difficult as it is a lot more grabby and small cutters are best used if you attempt this type of a cut.
In addition, one thing to note is that the design of truss tubes is the same problem as that of making bridges, something that the amateur often forgets about.. End of Bob's note.
In the Rectangular form the rings are connected with small tubes or rods. This form of open tube works best when rods are placed inside the small tubes. The rods are tightened to apply a strong compressive force on the tubes. In effect the tube and rod because a prestressed structural unit for greater resistance to bending. In a Serrurier Truss only thin tubes are needed. Usually a plug is made for the ends of each tube and the plugs are then bolted to the ring.
Depending on where you live, you may be able to get plastic tubing made for other purposes and adapt it to telescope uses. Plastic tubing can be divided into two groups - reinforced and not reinforced. Non-reinforced plastic tubing such as polyvinyl chloride and polyethylene is used for home plumbing, sewer pipe, ducts for corrosive gases, etc. It is available in many sizes with about 3/16 inch thick walls. The problem with nonreinforced plastic tubing is that it is often not stable enough for good telescope performance. It can sag under its own weight and sag worse under the load of mirrors and accessories this is not a bad property for sewer pipe but is bad for optical alignment in a telescope. In some cases it can not stand prolonged exposure to sunlight without cracking and ultimately failing apart.
Reinforced plastic tubing is usually glass fiber reinforced polyester or epoxy resin. In most cases additives are mixed with the plastic to prevent deterioration by exposure to sunlight. The glass fiber can be applied by cutting in short lengths and spraying it on with the resin, fiberglass cloth fabric applied between layers of resin, or a continuous glass filament wound on a form and coated with resin. Filament wound tubing is quite expensive but has the greatest strength. The glass fiber reinforcing increases strength over plain plastic tubes but most important it increases stiffness to reduce sagging. Reinforced plastic tubing is available in 10, 12, 14, 16 inch inside diameter size (among many other sizes including some larger sizes) with a wall thickness of 3/16 inch to 1/4 inch or more. A number of firms specialize in making industrial fiberglass reinforced tubing and other firms handle it. Joseph Ryerson and Son, Inc. with offices in several dozen major cities handles both reinforced and non-reinforced plastic tubing. The cost can be very high for some types of plastic tubes.
Thin wall aluminum tubing (3/32 inch thick or less) is made for irrigation systems. If irrigation suppliers are within driving distance you can get at 5, 7 and 10 inch OD tubing in lengths to 20 feet long at fairly low cost. Getting the tube shipped adds to the difficulty and cost. If you want to look for tubing locally start first in the yellow pages. If you live in a smaller city or town contact the telephone office for getting a telephone book of a nearby large city.
Bob's Note: Hastings Irrigation (on the web) has a special page of their tubing just for the telescope maker and will cut and even roll the edges of a tube to your size and in the9r available diameters and in several wall thicknesses. The cost is not that large for these services so they are another source of good aluminum tubing. End of Bob's Note.
Bob's Note: Serrurier Truss and Rectangular type telescope tubes are basically designed the same way that bridges are designed - all compression and tension members with compression members being the tubes or other forms that can support a log of pressure on their ends and tension members where the forces on the member is all trying to pull the member apart. Every effort is made to insure that there are no side forces on any of the members of the structure, even to the point of making the joints flexible with pins rather than welding or riveting the structure together. Serrurier Truss design goes another step and makes the two ends of the structure as a whole, when it does flex, flex equally well and in such a fashion so that the alignment of the optics are not affected by this flexing. Remember that EVERYTHING flexes to some degree or another, it is just a matter of getting that amount of flexing to a point where it can be handled. The Serrurier Truss design accepts that there will be flexing so it makes the flexing work for the end purpose rather than fully trying to eliminate all of it. Please also remember that you're working in three dimensions so the structural rigidity needs to be the same in both the vertical and horizontal of the tube design. Failing in one direction is going to make a bad design and we don't want that! End of Bob's Note.