Dome seeing, one of the most important problems in LAMOST due to its special optical path, mainly depends on
thermal distribution and temperature gradients in the enclosure. It is necessary to compute and then control the thermal
distribution inside the enclosure. The paper puts up many thermal analysis models with Icepak software, calculates their
thermal distribution under different thermal cases, and analyzes the maximal temperature differences in different cross
sections along the optical path. We apply many cooling methods, which include adding openings, forming ventilation,
forcing convection and local cooling. We also take the maximal temperature difference as an optimization object to
control the thermal distribution and optimize the cooling structure. Computation results demonstrate that improving the
thermal distribution can greatly reduce the temperature gradient. Through analysis we have obtained one cooling method
that involves a specific ventilating duct forming local cooling and intake cooling air. Simulation shows the maximal
temperature difference has been decreased from 3.8°C to 0.8640°C and in every cross section the maximal temperature
gradient has reached 0.125°C/m, which is better than the project demand of 0.4°C/m. All results confirm the thermal
control system of this project.
The optical path of LAMOST is 60-meter long with its main optical axis fixed and lying in close to horizontal orientation, that causes much more serious dome seeing problem than the one in conventional 4-meter class telescopes. The temperature gradients generated by many thermal sources in the dome induce the seeing problem. It is necessary to control of thermal distribution inside the enclosure to keep a good dome seeing. In this paper we introduce our computation and experiments. Through analysis we have obtained a method dealing with design of the cooling system to remove the effect of local heat source and reduce temperature gradients. The methods are outlined as follows: Cool the main heat sources with the cooling air with a temperature of 5°C lower than the ambient temperature. Because of the temperature difference between summer and winter, we have developed two sets of cooling systems to deal with respectively, particularly to keep the system workable in wintertime. We have designed a special structure for removing heat resources inside the enclosure. There is enough ventilation to keep the wind velocity at 0.5~1m/s in the optical path, and special fans generating low velocity wind provide good ventilation and avoid vibration noises.
Temperature sensors feeding signals back to the computer, which adjusts the control loop to maintain a good thermal distribution in optical path and to minimize the dome seeing problem.
LAMOST is a unique telescope, its optical axis is on the meridian plane with a 25 degree inclination to the horizontal, the optical path is 60 meters, much longer than that of traditional telescope, that causes much more serious dome seeing problems.
To improve the seeing around telescope, studies are carrying out on the enclosure thermal performance design, that include ventilation, air conditioning, cooling for heat sources, and calculation of thermal parameter of enclosure structure. Some experiments are taking for the remove the main heat from electronic.
This paper will concentrate on dome seeing improvement, related studies and experiments. The data obtained from calculation and experiments will be used for enclosure design to improve LAMOST dome seeing.