Astronomy Section
Articles
Dew Want A Piece Of Me, Punk?
For anyone who was there last night [18th February] when we were thwarted by dew here is a brief explanation of what is going on and how simply putting a piece of shiny plastic around the scope might have helped.
Dewing basically occurs when an object (i.e. the collector lens of the Nexstar) is at a temperature slightly lower than that of its surroundings. If the air is humid then moisture will condense out of it onto the cold object. In the winter this problem is compounded by the low temperature which causes the dew to then freeze forming a lovely semi-transparent layer on the lens through which we can see very little out of.
It may seem slightly weird that the lens falls to a lower temperature than the surrounding air (a bit of a disregard for the second law of thermodynamics) but the explanation is all to do with what the lens is doing - looking into space. The lens transfers heat in three ways - convection, conduction and radiation. Radiation is the main problem in this situation. Since radiation is an em-wave it can travel unhindered out into space. Now the problem is that space is cold (very cold - 3 Kelvin or so) and so does not radiate the same heat back to the lens (see the highly technical figure 1, right). This is where the equilibrium is lost, and why the lens can be colder than its surrounding air. It does try to restore the balance by means of convection and conduction, but being a large, glass object does not do this well enough to combat the losses through radiation. Convection itself can intensify the problem - air currents passing across the face of the lens can also take heat away from it.
Bit of a Catch 22 then - we want to look into space, but space is cooling down the lens meaning we can't look out into it. The solution is (ta-da!) the dew-wrap.
The dew-wrap should extend out from the lens by at least the diameter (in our case 5") of the lens. This reduces the effect of dewing in a couple of ways. Primarily it reduces the amount of sky the lens 'sees' which reduces the radiation and so reduces the cooling (see figure 2, right) . Also it sets up a column of air which the lens can maintain at a temperature much closer to its own. Finally it prevents cooling by conduction as air can no longer pass across the lens.
It also has other positive effects such as reducing stray light from Durham inveigling its way into our views so should enhance contrast making the sky look blacker and the stars shine that little bit more.
Now all we need to do is remember to use it!
First published: 20th February 2006 on the DUAS Website