The "steady local air"  Engineering formulae for local  The free convection case

DOME SEEING

Dome seeing, in its general meaning of seeing generated by turbulent fluctuations of the index of refraction inside a telescope enclosure, can be distinguished into different phenomena:

• Seeing caused by free convection in the entire enclosure volume, generated typically by a floor warmer than ambient air.
• Mirror seeing, caused by a temperature difference between the surface of the primary mirror and air.
• Seeing from the secondary mirror unit.

     
 

Figure 3: 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

The key for understanding the dome seeing phenomenon lies in a correct appreciation of the different free or weakly mixed 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[5] 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.
 

1998-07-05