National Astronomical Observatory of Japan

Thermal design concept of JNLTIzumi Mikami, Noboru Itoh and Satoru KotohMitsubishi Electric CorporationAmagasaki, Hyogo, Japan 661Hiroyasu AndoNational Astronomical Observatory of JapanMitaka, Tokyo, Japan 113ABSTRACTThe Japanese National Large Telescope (JNLT) will use a hemispherical domemounted on a cylindrical substructure, which might be rather of classical type.Based on this dome concept, various studies have been carried out experimentallyand/or analytically to see if the seeing degradation induced by the dome can bereduced to 0.1 arcsec FWHM range budgeted. The studies have indicated thebudgeted value would be attainable on condition that the empirical conversionmethod, which converts an averaged temperature difference between dome internalair and the ambient air into the seeing quality, is still applicable to such asmall seeing size study. If a distribution of air density and air temperatureis given dynamically in a light path of the telescope, one may be able toconvert it into the seeing quality. The JNLT Working Group therefore decided tointroduce a three—dimensional heat flow analysis which is thought capable forthe above requirement. Although the analysis will need another 4 months tooutput the first calculation result, the result is expected exquisitely usefulIn better understanding not only the seeing degradation mechanism but also thewind buffet effect on the primary mirror.INTRODUCTIONThe seeing degradation is mainly contributed by the free atmosphere, themicrothermal turbulences in the boundary layer and the artificial sources suchas a dome, a primary mirror, etc. The seeing quality of the free atmosphere isinherent to the telescope site and the JNLT's candidate site, Mauna—Kea, is oneof the best in the world. The seeing degradation in the boundary layer,budgeted as O.06 arcsec FWHM, is found attainable through the microthermalmeasurement carried out at a candidate spot' when the telescope elevation axisis mounted at 27m high from the ground level. The seeing degradation induced bythe artificial sources has been budgeted as O.12 arcsec FWHM. The budgetedvalue has been studied and found attainable through a dynamic thermal analysisof the dome and the telescope2, wind velocity measurement using the CFHT dome3,wind tunnel test using a scaled dome model and so on.These studies, however, seem to have some uncertainty, such as a windvelocity assumption used in the analysis, empirical conversion of a temperaturedistribution obtained by the analysis to the seeing quality and translation ofwind velocities measured in the dome of a different size and concept. To