Dew heater on a newt secondary?

[kbrown]

Just thinking aloud what might be the best way to have a dew heater on the secondary mirror of a Newtonian? In the past I have had nichrome wire wrapped around the stalk of the mirror holder but while it sort of worked, it wasn't very efficient way of doing it. Right now I've made an aluminum backing plate of the same size and shape as my secondary and the nichrome wire is sitting in a sort of a spiral groove I've routed into it. It works better but I still feel it's not the best solution since the mirror is glued to the plate with three blobs of silicone so the heat doesn't get conducted to it efficiently or evenly.

Has anyone tried to wrap nichrome wire directly around the edge of the secondary mirror so it would be in direct contact with the glass? Any other great ideas?

[ONIKKINEN]

Dew shield?

Never had dew or frost on the secondary in almost exclusively +90% humidity and between -25 and +15 with a dew shield. Dew everywhere else sure but on the mirror, never. Seems counterintuitive to heat the inside of the tube since tube currents with rising thermals are one of the issues newtonians want to avoid if sharpness is at all important.

[carastro]

I haven't owned a Newtonian for a while, but I used to wrap a couple of dew heaters around the tube (mainly because I didn't have a single one long enough, AND put on a dew shield.  I am not sure how well this worked, but I never got dew on the secondary.

Carole 

[Paul M]

My 10" Newt suffers from dew. I've even had frost on both mirrors on more than one occasion recently. So I use a heater band round both ends of the OTA and that cures it, completely. I find that I need heaters on for most of the year, except the height of summer when I don't set up due to perpetual twilight. The two straps shown here near the middle of the OTA are just Velcro straps to retain the cabling.

The Primary currently has condensation/water marks on it due to me powering down the scope after an all night session but not taking it indoors do warm up.!

The 10" Newt is being rested just now while I enjoy a new RC10. I'm taking no chances. Heaters on or it's indoors!!

[kbrown]

When I started with my 10" SW newt, I didn't have any dew heaters on it. But sure enough on a few clear but humid nights I got dew and even frost on the secondary mirror. Primary mirror has been fine so far...

Anyway. Dew shield on a newt... maybe... but sounds a bit clunky.

I already have heater wires and also wires to a digital temperature sensor running into the secondary mirror holder plus a DIY heater controller that can maintain a set temperature based on dew point calculated from measured humidity and ambient temperature. Just wondering about what would be the best way to heat up the mirror without too many things in the way.

Here's a picture of what I currently have and what I was suggesting in the first post. Thoughts?

 

[Guest]

I have a 10 inch dob and i have had a lot of success keeping dew/frost away with Orions Dew Zapper around the end of the OTA.  The only catch to it is getting just enough heat to keep the mirror clear and then adjusting it as the night goes on.  I find that 30 to 40 percent is usually plenty to keep it clear.  

There are times i will have to put a light shield on the scope and i will put it right over the heaters.  When i run like that i can back the power down as the shield insulates the heater from the air.  

[Zermelo]

I use a dew shield every time. Even if there's no dew, it helps to stop stray light entering the top.

You can get resistive heaters that are shaped to fit specific secondary ovals, but it's not difficult to glue some ordinary resistors on the back, of an appropriate resistance and rating. As for the heat transfer, you could use some of the stuff they use for mounting heat sinks onto CPUs. It's available in small tubes.

[PeterStudz]

11 hours ago, ONIKKINEN said:

Dew shield?

Never had dew or frost on the secondary in almost exclusively +90% humidity and between -25 and +15 with a dew shield. Dew everywhere else sure but on the mirror, never. Seems counterintuitive to heat the inside of the tube since tube currents with rising thermals are one of the issues newtonians want to avoid if sharpness is at all important.

This… I’d certainly try a dew shield first. I’m in the UK and since using a dew shield I’ve never had an issue with dew on the secondary despite the telescope being left out all night during some nights of very heavy dew. 

And as @Zermelo suggests it helps keep out stray light which for me, in an urban environment, is important too.