It is easy to underestimate the importance of telescope tubing. The simple cylinder shape has no moving parts. Yet the telescope tube and its cradle form the backbone to support the alignment of the optical system. Amateurs often question the cost of good telescope tubing. The true value of telescope tubing is not fully understood until you are stuck with a good telescope mounted in a poor tube. This is especially true in a Cassegrain type telescope.
A Newtonian diagonal has a physical center but no unique optical axis. This is what allows the diagonal flat to be tilted to a 45 degree angle. Refractor lenses and Cass secondary have both an optical center and a physical center. Most good lenses are accurately adjusted to put the physical center in the same place as the optical center. In aspheric optics (most primary mirrors and Cass secondary mirrors in popular optical designs) there is a unique optical axis. Thus in all Cass telescopes the optical alignment requires that
All this gets back to the telescope tube because after the adjustments are made the tube must be able to hold the optical alignment. The primary and secondary of a Cass should be centered to each other within 0.005 inches or less. Suppose that the tubing is not very stiff so over its length is sags 0.010, 0.020 or even 0.030 inches. From the outsized of the tube this sag can not be seen and is difficult to measure. In Newtonian telescopes the effect on optical performance is quite small and is usually unnoticed. In a Cass this sag can cause the loss of optical centering and thus degrade optical performance. Tube sag is least when the telescope is pointed at the zenith and greatest when the telescope is pointed at the horizon. It is difficult to this fact to test for tube sag because atmospheric conditions are also best at the zenith and worst at the horizon.
This discussion is not intended to discourage beginners nor to scare present Cass owners. It is to point out that little extra care is needed and reason for it. Now here are some pointers so you know what to watch out for to avoid tube sag and resulting optical problems. First, don't try to save a few dollars by getting a cheap tube with poor stiffness. Stiffness is the most important consideration in tubing for a Cass telescope. Second, if the tube is not made of metal or the metal is very thin you should use end rings. Most non-metallic tubing is available with end rings as an optional accessory so use them. Some industrial fiberglass tubing is available with 1/4 inch thick or more walls which may not require end rings. This tubing gives good support to the optics of a Cass telescope but is not often used because it is heavy and quite expensive. Third, if the tubing is non-metallic use a tube cradle that is 30 to 60 percent of the length of the tube. If necessary, get a cradle extension made as described in the previous section. Forth, be careful in selecting and using large guide telescopes. In a Cass telescope most of the weight is at the lower end of the tube so the cradle is positioned closer to the lower end. This leaves the upper end of the tube to overhang the cradle. If a large guide telescope is mounted with a ring mount near the upper end of the tube the extra weight can cause the upper end of the main tube to sag. Because a Cass telescope is so short and the guide telescopes quite long for their respective apertures the guide telescope can easily be as long or longer than the main telescope tube. This is particularly true with long focus refractors. The operation of refractors is usually the heaviest. Unless the main telescope tube is very rigid, keep the necessary load off the upper end of the tube.
If you discover a problem with tubing stiffness after the telescope is built, you can usually correct the problem. Much depends on the optical quality you expect the telescope to give. Sensitive telescope users will often "fine tune" a telescope to make sure all the optical adjustments are carefully made and that performance problems in the tube, cradle, mounting, etc. are corrected as each is found. A tube cradle extension can be added as can reinforcing rings at the cradle and at the spider. Sometimes a fiberglass tube has the ends built up with additional layers of fiberglass. If the only tubing you can get is too thin for your needs you can add additional layers of fiberglass at the ends of the tube much easier than trying to make a fiberglass tube from scratch. You can use fiberglass boat repair kits from Sears or other suppliers either in local stores or by mail order.
If it is necessary to glue or add fiberglass to a finished telescope remember to remove any paint and roughen the surface to insure good bonding. Before painting clean the surface with a pad soaked in alcohol. Polyurethane or epoxy base paint should be used on fiberglass or phenolic tubing with polyurethane paint being slightly preferred for better durability. However epoxy base paint is more readily available in spray cans.
For larger telescopes you may have difficulty getting tubing sufficiently rigid, reasonably low cost, and fairly light weight. Exceeding the load limit of a telescope mount will cause serious deterioration in stability. Heavy tubes can be used only if the telescope mount can carry them. High strength industrial fiberglass tubing with 16 inch OD and about a quarter of an inch thick walls is available but the weight is about 10.7 lb./foot in 18 inch OD and about a quarter inch thick walls the weight is about 11.5 lib./foot. A telescope mount designed to carry an eight foot fiberglass tube with eighth inch thick walls should have no trouble carrying a four foot tube with quarter inch think walls. The tube weight is about the same but you gain stability with a shorter tube.
A wood tube made by curving plywood is easier in larger sizes of 15 inches or more, in larger cities Birch plywood is available in 7/64 inch and 1/8 inch thickness with three plys in each sheet. By gluing one sheet on top of another with the joints overlapped a smooth, round tube can be formed. Some skill in handling wood is needed. If steam or water is needed to make bending the plywood easier, check to be sure the plywood glue is suitable.
The Serrurier Truss tube is quite suitable for larger amateur telescopes. While the original design was for large research telescopes the design can be adapted to amateur telescopes. See Sky and Telescope magazine on page 227 of the April 1971 issue and page 211 of the October 1971 issue for photos of some amateur designs. The Serrurier Truss is seen more often on larger fork mounted telescopes. Fork mounted telescopes require a more rigid tube. The Serrurier design is very rigid and is easy to adapt to a fork mount. However the Serrurier Truss tube can also be used on a German mount. It is rigid enough not to require a long cradle to reinforce the tube. The two center rings (for the center ring assembly) is a good cradle itself.