By Dr. Frank Melsheimer, President, DFM Engineering, Inc.

The choice of what material to use for the structural components of a telescope needs to be based upon both performance and cost. The performance of the telescope is improved by reducing the weight  (and resulting moment of inertia) and increasing the stiffness of the moving parts. A high stiffness to weight material of adequate strength is the best choice for performance. The high stiffness requirement usually results in more than enough material to provide the needed load carrying ability. Material strength considerations are only of a secondary concern. Cost constraints are more difficult to evaluate because they are intimately tied into the manufacturing process which is a function of the material and how the material is used.

The stiffness to weight ratio of common materials used for telescope parts can be expressed as the ratio of the materials modulus of elasticity (Young's Modulus) divided by its density. If we then scale the resulting stiffness to weight ratio by dividing by the stiffness to weight ratio for steel, we find that most metals (with the exception of beryllium) have almost the same stiffness to weight ratio. Aluminum and titanium are 5% stiffer than steel per unit weight for example. If all of the structural elements of a telescope were made from aluminum, the telescope structure (for the same stiffness telescope) would weigh only 5% less.

The "Figure of Merit" for the material including cost of the raw material, can be expressed as the stiffness to weight ratio divided by the cost per unit weight. Steel is 3.8 times more efficient than aluminum based upon this highly simplified analysis. A more detailed analysis must take into account the manufacturing process and the resulting labor costs associated with that process. Typically, the raw material cost for the telescope structure is only 20% of the total cost of the structure. The labor cost is the remaining 80%.

The cost of fabricating the telescope structure is type of material and telescope size dependent. Small telescopes can be made more inexpensively by using aluminum castings, particularly in quantities greater than 4 or 5. Moderate and large telescopes tend to be made in quantities of 1 or 2 so the cost of the patterns for the castings becomes prohibitive.

A casting eliminates much of the labor of transforming the raw material into the desired shape. Machining of mating surfaces and cosmetic finishing are still required and require about the same amount of labor as other fabrication techniques. The minimum wall thickness of castings provides an additional constraint. Thin sections will still have to be fabricated from flat sheet stock.

Large telescopes have been traditionally fabricated using flat steel plate which is cut out and welded into the desired shape. Round shapes, such as the polar axle, are either made up from available sized tubing, or the flat stock is rolled and seam welded. Presently, many of the steel suppliers have automatic flame cutting or laser cutting equipment for steel which can cut out the desired shapes very inexpensively. The practical minimum thickness plate for flame cutting is .25 inches (6 mm) while laser cutting can cut out shapes in much thinner stock. Cutting shapes out of aluminum plate is much more expensive.

Welding aluminum is much more expensive than welding steel, particularly when the aluminum plates are nearly 3 times thicker as is required to obtain the same stiffness. Welding a DFM 0.5 M telescope fork out of aluminum would require much heavier equipment and would cost about 3 to 5 times as much as the steel fork. The material cost would also be 4 times as much as steel.

Aluminum has many advantages for smaller parts and particularly for parts which require extensive machining. The best performance when considering cost is achieved when the proper balance between steel parts and aluminum parts is reached. This balance requires considerable knowledge and experience with the various fabrication techniques available to the telescope manufacturer. Also, quantity plays a significant part in determining the manufacturing process and the resulting cost. An "All Aluminum" moderate to large telescope of the same performance as a predominantly steel telescope will cost considerably more.

In the next DFM Engineering Article, we will discuss the effect of geometry on the telescope structure. The geometrical shape of the structural elements, and the details of how loads are applied and removed are far more important than the 5% difference between the stiffness to weight ratio of the commonly used materials.