The D-word (dew)

[Pixies]

Hi,

I started my telescope experience back in April of this year. An 8" Bresser dob. So I've been experiencing warm spring and summer nights, although a bit less warm recently!

Dew has never been an issue, so far. To be fair, I'm only a few 100 metres from the sea, so less prone to it anyway. I'm also on the East coast of Scotland, so the air is a little dryer too. But I'm sure it's just a matter of time until I suffer from it. I see lots about dew-bands and power supplies, dew shields, Telrad heaters, hair-dryers, etc. I know that SCT/Maks suffer more, as do some refractors.

With a large-ish dob, would I be OK with just a big dew shield and perhaps something for the finders? I'm going to get hit sooner rather than later and I don't really mind playing catch-up and trying to sort things afterwards. But really, I'd like to be prepared to some extent.

Cheers

[johninderby]

Never had any dew problems with any dob I’ve owned so never had to do anything about it. 

Have heard of some people using a dew heater band on the secondary though. Or an eyepiece heater orTelrad heater.  A light shield on the end of the tube may help a bit with dew but mainly will help with stray light.

[jonathan]

Dew mainly affects exposed cold surfaces, but especially ones that point directly towards the sky.  An 8" dob / newtonian might be fine as the primary is down at the end of a long tube, on a really heavy dew night you might get some on the secondary so a dew shield should sort that, if that happens your eyepieces are likely to be affected too if they are left with the glass exposed to the sky.

I have an 8" SCT so know all about dew and where it lands.  The main thing is to keep stuff covered when possible (i.e. keep your eyepiece case closed), keep them just above the dew point (so keep eyepieces in a warm pocket with the caps on), in my experience dew normally falls directly downwards but can also cling to surfaces not directly facing the sky.  You'll know if there's a dew as it will normally form on the cold outer casing of the scope tube first, or on a car roof.  The heat from your skin can cause dew / misting of an eyepiece on a very cold night, I usually find that when that happens it's time to pack up!

I run a 4 channel dew heater controller with dew heater tapes on the objective, 9x50 finder (both ends), and eyepiece, and sometimes it's still not enough!  I've never had to run a dew heater on my 6" newtonian or 4" refractor though.

 

Edited August 18, 2020 by jonathan