Reflector Secondary Dew Issues

[Jiggy 67]

Has anyone tried to solve the problem of dew on the secondary mirror of a reflector? 
I have a 200 PDS which suffers from this. I can control the primary with a dew strip but the secondary is a whole new ballgame. I was wondering if a dew strip around the OTA just below the focuser ( and secondary mirror) might help as it would heat the inside and heat would rise onto the secondary but I don’t want to invest if it wouldn’t work.....Anyone tried this??

[almcl]

I did suffer this problem for a while. 

Tried a dew heater round the secondary mounting block and while that worked as far as the dew went, the wires (which were pretty thin and carefully aligned with the spider vanes) played havoc with the diffraction spikes in the images.

In the end I settled for an extended dew shield, made out of thin(ish) neoprene foam rubber about 40 cm long.  It slides over the end of my 200P by about 5 cm and keeps the secondary dew free (at least until the primary starts to dew up).

[Uplooker]

You could create a dew shield using a foam camping mat or fit a dew heater - such as the Kendrick ones. I went for the latter option, works a treat

[joe aguiar]

yep its a common problem but not too many people think it is cause its a reflector

IF the 2nd mirror is close to the opening on hot dewy nights it will get dew.

I just use a dewcap and that should solve the issue for maybe 3 to 4 hrs max if you obseve all night it may have to be powered but for me 3 hrs is more then enough.

joejaguar

[Starflyer]

I didn't want to heat anything on my reflector so opted for a short camping mat dew shield. It only needs to be six to eight inches long and avoids the risk of introducing tube currents from heating your mirrors.

I've never had a primary dew up, but I have my cooling fans set to blow up the tube so that probably helps.

[Paz]

I use a dew heater on my secondary and it does the job fine but it also makes a little heat plume that affects the views so I've learned to use it as sparingly as possible.

There's something to be said for dew shields which don't have this issue.

Something else you could try is packing insulating material behind the secondary without obstructing the light path so that it cant so easily lose heat from that side.

[Jiggy 67]

Paz, how do you attach a dew heater to the secondary?....I looked at the Kendricks mentioned by Uplooker but they look complicated. I think I might try a dew shield for now, cheap and easy but I was hoping there may have been a more reliable solution. People think it's not an issue with newts but it definitely is.

Thanks for the advice guys

[niallk]

I've an Astrosystems dew guard: the battery is velcro'ed in a spot basically in the shadow of the secondary: so no wires routing along the vanes with any diffraction effects.  It also has an ambient and a back of mirror temp sensors for gentle automatic heating if required - no noticeable heat plumes.

I had to tweak a factory pot setting (covered in the instructions), and seems to do the trick now, dealing with dew.

[Paz]

3 hours ago, Jiggy 67 said:

Paz, how do you attach a dew heater to the secondary?....I looked at the Kendricks mentioned by Uplooker but they look complicated. I think I might try a dew shield for now, cheap and easy but I was hoping there may have been a more reliable solution. People think it's not an issue with newts but it definitely is.

Thanks for the advice guys

The heater on my scope was sorted by the previous owner but I think it is simply a heating element taped or glued to the back of the secondary.

In the picture below you can just make out the covering of the heater around the back of the diagonal glass. Also notice that there is a second dummy wire opposite the live wire - this was intended to ensure if there were any effects on the diffraction spikes that they would hopefully at least be symmetrical.

[Paz]

3 minutes ago, niallk said:

I've an Astrosystems dew guard: the battery is velcro'ed in a spot basically in the shadow of the secondary: so no wires routing along the vanes with any diffraction effects.  It also has an ambient and a back of mirror temp sensors for gentle automatic heating if required - no noticeable heat plumes.

I had to tweak a factory pot setting (covered in the instructions), and seems to do the trick now, dealing with dew.

That sounds like a better prospect than my set up - I will look that up!

[Captain Scarlet]

If I’m near a mains source I use a hairdryer ... a quick blast of it buys me tens of minutes or more. I find if I get dew on the secondary it’ll likely be on the eyepiece as well. Hairdryer does for both...

M

Edited January 13, 2020 by Captain Magenta

[niallk]

Yeah, I've a back up 12V hairdryer... which I can't use on a 'hot' setting due to the limited current sourcing from my Powertank! 😒  Luckily I've found that even moving air / blowing 'cold' is useful for clearing any light onset of dew on EPs etc - I've no dew prevention on them.

[Jiggy 67]

I have got a hairdryer that runs off 12V connected to my power tank but I’m always conscious of the neighbors at 1am and a noisy hairdryer 😗....they already think I’m a bit weird, hanging around in the garden in the middle of the night 🤣

[Uplooker]

I fitted a Kendrick secondary dew heater, added a fan to the primary, dew bands for the EP and both ends of my RACI 🙄. All run off a small leisure battery

[Jiggy 67]

2 hours ago, Uplooker said:

I fitted a Kendrick secondary dew heater, added a fan to the primary, dew bands for the EP and both ends of my RACI 🙄. All run off a small leisure battery

I bet you’re tripping over cables all night though 😀