Causes of dome seeing

  The occurrence of dome seeing has been linked by researchers to a variety and cumulation of several sources of heat transfer and air flow phenomena inside the telescope enclosure. Fig. below shows schematically the main causes that are generally considered to be possible source of dome seeing.

  
Figure: Causes commonly held to be source of dome seeing:

A

Convective air flow near caused by a surface-air temperature difference at the primary mirror: this causes the so-called mirror seeing

B

Convective air flow from the Cassegrain instrumentation located behind the primary mirror

C

Air flow through entrance doors to the telescope volume

D

Turbulence across the observation slit of the enclosure

E

Convective air flow from the members of the telescope structure

F

Convective air flow from the secondary mirror unit

G

Convective air flow from the heat generating equipment inside the enclosure

H

Convective air flow from the enclosure floor

I

Convective air flow from the dome and enclosure walls

Most observatories have taken actions in the course of time to reduce most of these effects by insulating wall and floor surfaces and by extracting the heat generated by electric equipment items by means of active cooling circuits. However the relative importance of the different causes, the quantification of their effect and consequently the definition of the technical actions best suited to provide a seeing-free environment has remained controversial.

In fact, the key for understanding the dome seeing phenomenon lies in a correct appreciation of the different free convection flow patterns which may be found in the air volume crossed by the optical beam. A main characteristic of free convection flow lies in the fact that temperature gradients and fluctuations are greatest close to the heat exchange surface while the far region experiences the largest velocities (see for instance [Townsend], pp. 381-392). Thus one shall expect that the seeing disturbance, being caused by temperature fluctuations, decreases rapidly with the distance from the heat exchange surface.