Quote:
Originally Posted by Camelopardalis
I’m sure you have already thought of this Stefan, but what’s the plan for keeping the dew at bay? Just because the corrector plate doesn’t exist, doesn’t mean the dew remains suspended in the atmosphere.
Not being funny btw, I’m genuinely interested, but lack the means to fund an interesting scope such as this. My first Newtonian dewed then frosted up first time out bush  |
A very good question.
Dew in fact does remain suspended until it contacts a surface that is cold enough for it to settle on. If a surface is a bit above ambient temperature it tends to repels the dew.
Anodized surfaces are very good black body radiators and, when pointed towards a very cold sky, quickly cool to below ambient temperature, attracting a lot of condensation.
Shiny metallic surfaces, like the mirrors in a telescope, are not good radiators, at least on their metalized sides.
Therefore a DK secondary will lose most of its heat though its back surface.
On my 16" DK, to prevent dew on the secondary, I added a 3mm thick foam disc topped by a layer of plain aluminium foil that also wrapped around the edge of the mirror. So, by slowing down the cooling of the secondary, I never had dewing problems although the mirror was still able to closely track the ambient temperature through convection cooling.
It is important to take into consideration the different use case scenarios too between planetary and deep sky observing sessions.
Planetary sessions, in my experience, never exceed a couple of hours while deep sky ones can go all night and that also helps with dew management.