Dew control and insulation

[Magoo (Millie Magoo)]

I am planning ahead for the Autumn to Spring weather (since summer seems a dead loss for imaging these days). The plan is to travel to a cold and dark sky location over the new moon periods and spend a few nights imaging.

As part of the planning process, I am looking to see what I should be doing for dew/condensation control. The scope I am most likely to be using is a newtonian and that raises the issue of heaters for primary and secondary mirrors and a guide scope too.

My question is whether these are really necessary or beneficial on a newtonian. Could I achieve the same result (with no need for extra power supply) by wrapping insulating foil around the OTA. There are a couple of products around that are basically bubble-wrap with a foil skin. These would be relatively easy to cut and shape for the purpose.

I'd appreciate thoughts on this from folks who have "been there".

[Michael_Bibby (Michael)]

Asking the same question, have you had any success? Dew is a real problem at my location. Hate the idea of a powered dew heater, looking for 'passive' options. Currently searching bunnings catalogues and ebay listing for appropriate insulation material. Found this useful thread on Cloudy Nights: https://www.cloudynights.com/topic/7...-my-newtonian/

[Startrek (Martin)]

Ive learnt after many years trying to combat dew at my 2 locations ( City Suburban and Coastal rural ) with both observing and imaging with my 6” , 8” , 10” and 12” Newts that I don’t bother anymore.If the humidity is above 85% and wind is below 10km/hr then sure enough dew will form and get worse into the night.
Dew significantly limits your seeing with Imaging and Observing no matter what gear you use.
I originally fitted Kendrick dew controllers , heat straps , heater bands around the secondary mirror stalks etc… on all my rigs. Also tried fans , dew shields as well
I’ve since removed everything and now image and observe on nights without dew. Much more enjoyable viewing and much better images

Clear Skies

[Michael_Bibby (Michael)]

I've just been resorting to using a hairdryer periodically (basically before each object during a EAA session). I'm new to imaging, but more than once I have wasted time thinking I was gathering valuable data only to later realize that the secondary on my Newt was completely covered in dew...

[gaseous (Patrick)]

I made a DIY dew heater for the secondary mirror on my 20" dob for less than $15 (assuming you have a power source already). It's dead easy and should be a transferable skill to smaller secondary mirrors. You need a USB powered heating pad (dirt cheap off eBay). Peel apart the two sheets of fabric and carefully remove the heating wire stuck to one face. I then used a bit of silicone to spot fix the wire around the back of the secondary (it's a big piece of glass), then run the wire along a spider vane, fixed with black electrical tape. They come with a heat controller, and even the lowest setting is enough to keep the mirror above ambient temp. Best DIY I've ever done (the bar was pretty low though).

[Michael_Bibby (Michael)]

Originally Posted by gaseous

I made a DIY dew heater for the secondary mirror on my 20" dob for less than $15 (assuming you have a power source already). It's dead easy and should be a transferable skill to smaller secondary mirrors. You need a USB powered heating pad (dirt cheap off eBay). Peel apart the two sheets of fabric and carefully remove the heating wire stuck to one face. I then used a bit of silicone to spot fix the wire around the back of the secondary (it's a big piece of glass), then run the wire along a spider vane, fixed with black electrical tape. They come with a heat controller, and even the lowest setting is enough to keep the mirror above ambient temp. Best DIY I've ever done (the bar was pretty low though).

Great idea. Thanks for the tip. How does the wire effect the diffraction spikes on your stars? I suppose the spider vains on a 20" are pretty thick compared to a 8" Newt. I'm a big fan of pristine diffraction spikes...

[gaseous (Patrick)]

Originally Posted by Michael_Bibby

Great idea. Thanks for the tip. How does the wire effect the diffraction spikes on your stars? I suppose the spider vains on a 20" are pretty thick compared to a 8" Newt. I'm a big fan of pristine diffraction spikes...

Hi Michael, I haven't noticed any difference to the diffraction spikes. You're correct, the spiders on the 20" are a little thicker, but if you tape the wire flat with a long strip of electrical tape, I reckon you'd need a good eye to tell the difference. I also tend to use the 20" for faint fuzzies, so diffraction spikes aren't really an issue.

[Michael_Bibby (Michael)]

Just a quick update. I bought a cheap foam mat and made a dew shield for my 8" Newtonian, following the rule of thumb that is often mentioned of making the shield extend 1 and 1/2 times the diameter of the aperture. So far, this seems to have worked. I have only had the chance to use it twice under dewy conditions, during 5-6 hours of observation on both occasions, and at the end of each session there wasn't a hint of dew on the secondary (or primary) mirror, even though the telescope itself (along with everything else) was covered in it. So reasons to be optimistic. If in the end this doesn't prove adequate I might make a dew strap out of one of those usb heaters and instead of placing it on the back of the secondary, wrap it around the end of the OTA underneath the dew shield.

It is a bit cumbersome, and ads to the moment arm, so care should be taken to use light weight material, but material strong enough to hold its shape, and also, of course, to balance the scope with it on.