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An order-of-magnitude estimate of dome seeing

Equation (gif) may also be used for a simple order-of-magnitude estimate of the effect on seeing caused by free convection from a warmer dome floor.

Consider the geometry of fig. gif. If the dome diameter is , the mean height of the air volume crossed by the optical beam with respect to the heat exchange surface will be about . Assuming for the constant b the value of 2.68 determined for the atmospheric surface layer (cf. equation (gif)) and taking C, we find:


Considering that the light beam travels three times between the primary and the secondary mirrorgif, the total light path inside the protected volume of the dome will be and from equation (gif), a rough estimate of the dome seeing will be given by

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

This is plotted in fig. gif for two typical values of enclosure size: free convection flow in the enclosure volume will begin to cause significant seeing effects ( 0.4 arcsec) in the light beam for heat fluxes of the order of 20 W/m. Considering that a typical free convection heat transfer rate would be 3 W/mK 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. gif). 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.

The rate of dome seeing per floor-air of 0.06 to 0.08 arcsec/K which is estimated here indeed explains the high seeing values experienced during the first years of operation by many telescopes of 4-m class built in the 70s, like the ESO 3.6-m and the CFHT. These telescopes are enclosed in large concrete/steel buildings with no natural ventilation and, as the inner dome thermal environment was hardly or poorly controlled, the different heat capacities of the concrete base, the primary mirror, the telescope structure and the dome inevitably caused large differences of temperatures. Measurements at the ESO 3.6-m telescope done by the author [Zago 84] and [Schmider] before the installation of an effective thermal control system in the dome, show that positive differences of 5 K between the dome floor, and dome ambient air are typical and raise at times up to 10 K.

As we have already mentioned in section gif above, the thermal conditioning of these domes has been improved in the meantime, so that surface-air now hardly exceed 1 or 2 K in the worst cases, while the newer telescopes have been built inside lighter enclosures which also allow natural ventilation. As a consequence, dome seeing, in the proper meaning of seeing created by large convection flow patterns in the dome enclosure, is no more a critical factor of telescope performance nowadays. Therefore there is not much opportunity nor perhaps interest to investigate in detail a matter which appears technically solved.

next up previous contents
Next: Mirror seeing Up: Dome seeing Previous: Scaling variables and

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995