Seeing caused by heat  DOME SEEING  The "steady local air"

Dome seeing caused by a warmer dome floor

An estimate of the dome seeing $\theta_d$ is obtained by the following relationship, derived by Zago[10]:

$$\theta_d \approx 20.9 \ D_d^{-1/5} q_s^{4/3}$$ (17)

 where D_d is the dome diameter and q_s the surface heat flux.

q_s is the normalized surface heat flux (i.e. divided by air density and specific heat, typically a factor 1000) with dimentions [K.m/s].

Equation (17) is plotted in fig. 4 for two typical values of enclosure size: free convection flow in the enclosure volume will begin to cause significant seeing effects (about 0.4 arcsec) in the light beam with a heat flux of the order of 20 W/m². Considering that a typical free convection heat transfer rate would be 3 W/m²K at the floor, one should then expect a seeing contribution of about 0.06 to 0.08 arcsec per deg K of floor-air temperature difference.

Relationship of the same quantitative order between heat flux, distance from the exchange surface and seeing are likely applicable also to other potential sources of free convection located inside the enclosure (e.g. items G and I in fig. 3). However, in view of the smaller exchange surface areas of walls and other heat dissipating objects with respect to the inner air volume, the seeing rate per deg K of surface-air temperature difference will be quite lower than for the floor.

 
Figure: Order-of-magnitude value of seeing FWHM caused by a warmer dome floor.

August 3rd, 2010