Truss (8 Members)

Wind deflection compensated, zero -coma telescope truss geometriesAden B. Meinel* and Marjorie P. Meinel*Optical Sciences CenterUniversity of ArizonaTucson, Arizona 85721*Currently at Jet Propulsion LaboratoryCalifornia Institute of Technology4800 Oak Grove Drive, Pasadena, California 91109AbstractSix telescope truss geometries are evaluated for both wind and gravity deflectionstiffness. The existence of simultaneous solutions for gravity and wind deflections isdemonstrated. A baseline design for the Texas 7.6 -meter telescope with an F /1.4 Angellightweight borosilicate primary mirror is shown.IntroductionThe deflection behavior of a number of telescope truss configurations was explored aspart of the engineering design studies of a 300 -in. (7.6 -m) telescope for the McDonaldObservatory, University of Texas at Austin. It is standard procedure to design a truss sothat the gravity deflections of the upper and lower trusses are balanced and thus to main-tain optical alignment. Further, since a well- constructed structure is perfectly elasticin behavior, Hooke's Law is maintained; and this compensation applies to all zenith dis-tances because the force component normal to the truss axis varies as the cosine of thezenith distance for both the upper and lower trusses.Wind deflection, on the other hand, represents an asymmetric forcing function on thetruss, which mainly affects the upper truss because it has more direct exposure to windforces than has the lower portion of the telescope. Early in our study it was recognizedthat it is difficult to make the upper truss stiff enough to keep the wind deflection ofthe image below +0.2 arcsec ( +0.1 arcsec of the structure) for an effective force of 180 kg(400 lb) applied at the upper cage and ring of the telescope. We will here review these