Seeing caused by heat generation at the secondary mirror unit

In modern telescopes the secondary mirror is attached to electro-mechanical actuators which allow fine adjustment of the optical alignment and focusing. The latest telescopes projects also provide for fast motion of the secondary mirror, either to switch rapidly between the observed object and the sky background, or to correct guiding errors due to wind loading. All these systems produce some heat which, if it is not removed by a cooling circuit, will dissipate right into the telescope light-beam and may then cause a seeing degradation.
 

Figure 5: Seeing caused by a free convection plume from the M2 unit.
 

An empirical parameterization has been obtained by processing and scaling experimental measurements at the ESO 2.2-m telescope[1] [10]:

$$\theta = 0.018 \ Q^{4/5} D^{-9/5}$$ (18)

 where Q is the total heat flow from the secondary unit in Watts and D is the secondary diameter in meters.

1998-07-05