Personal Reflections on the Design of the Webb Space Telescope

L. Van Warren MS CS, AE, PhD. Candidate CS

4.4% Bounce Cost

The Webb Space Telescope has an optical path consisting of a concave primary mirror, a convex secondary mirror, a concave tertiary mirror, a flat steering mirror and a final focal surface. A ray of light impinging on the primary mirror thus has 4 reflections before final instrument entry on bounce 5. The reflectance of each mirrored surface has been measured to be within a neighborhood of 98.5%.  Multiplying the loss at each bounce gives us a final effective signal strength of 94.1%.

Contrast the current design with one that stations the instrument entry in pThis would enable an improvement of 98.5-94.1 =4.4% in signal strength corresponding to an equivalent effective surface area increase, or a corresponding weight reduction if the primary mirrors were scaled down. It would also result in a significant reduction in mechanical complexity and cost if the three intervening mirrors were eliminated, at some cost in versatility.lace of the secondary mirror.

The convexity of the secondary mirror implies that the primary would also have to be reground if it were to immediately feed an instrument focal plane, which itself could be a difficult endeavor. So that is item one.

Solar Panels vs. RTG

The Voyager spacecraft have exhibited longevity that exceeds 44 years, due in part to the use of Radioisotope Thermal Generators whose performance is not dependent on solar distance. The planned orbit of Webb around the L2 Lagrangian point is a million miles from the sun, ~1 percent further than that of the Earth itself, so the solar irradiance is like Earth’s. I wonder if solar panels will have the longevity of RTG’s. In any case fuel to remain on station about L2, and to unload the reaction wheels would seem to be the factor limiting telescope lifetime, rather than power source. I also wonder if an ion thruster could offset the need for expendable fuel, resulting in increased spacecraft lifetime.