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℃ 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.