Thermal drifts should not provoke changes in the optical alignment, which corrupt geometrical alignment. The mechanical structures should be sufficiently resistant to thermal deformations that the temperature stability requirements are realistic.

The Problem Beam alignment errors can result due to deformations or displacements of the optical tables and other optical support structures, and due to mechanical drift of optical mounts. To control deformation of optical tables, the thermal characteristics of the materials and of the environment must be considered.

Proposed Solution Spacing between elements mounted on an optical bench varies if the temperature changes cause the length of the bench to change. This is due to thermally induced expansion and contraction of the bench material. When a material is heated, it will expand according to the relationship,

D L = a Lo DT

where:
Lo = Original length of the material
D L = Change in length of the material
D T = Temperature change [°C]
a = Coefficient of linear expansion [°Cp> Dr. Miles suggested to the team to look into other applications were thermal alignment is of importance. He mentioned invar as a good choice and recommended the team to compare its CTE with other materials to see what the team found. The team discovered that invar is widely used in electronic equipment that needs precise alignments.

The team looked into different possibilities to control thermal drifts. To minimize the temperature gradients in the structure, construction techniques will be used which maximize, or at least do not hinder, thermal transfer across the structure. Materials with low thermal expansion coefficients (CTE) will also be used to decrease deformation due to changes of temperature of the structures. Correct determination of the right position for a component for the application depends on some knowledge of construction materials and information gained on how to machine optical structures.

The coefficient of linear expansion, a, varies with different materials. Typical values of a are given in Table 1 [1 803-806].

The environment in the case of the laser radar system cannot be controlled, but a temperature range is given for thermal analysis (10 - 40 C). The structural design is relatively inexpensive compared to the cost of the materials, but the time that is to be spent machining the structure is considerable and must be started as soon as the material is available. Thermally stable materials are rather costly, but for the increase in performance of the system, the cost should be worth the extra time that would be needed to realign all the parts if they happen to misalign during operation.

An examination of the displacements caused from thermal instability indicates that the use of invar support structures (preferably throughout the whole system) will work best for optical alignment. If the structure must contain other metals, then taking those components out of the system after alignment will be done if possible.