Dewing assistance please

[cfranks (Charles)]

I have a DSI RC10C (10" RC telescope) mounted on a permanent pier out in the open, no shed nor observatory. I use a Dew strap at the opening and the secondary mirror heater is on. I image with the fans running all the time but even with a slight dew, the primary mirror dews up. Sometimes the dewing is heavy enough to make streaks as the water runs off the mirror! The mirror is 800mm down the tube so I doubt an additional dew shield will have any effect. I have ordered additional dew straps to place on the tube near the mirror but it remains to be seen if that is a good idea, obviously I don't want to affect my imaging.
If anyone has a thought for a possible solution, I would be greatly obliged.
Charles

[Chris85 (Chris)]

Could you try a shroud? I'd imagine DSI have some built for your scope.

Chris

[cfranks (Charles)]

Hi Chris,

I assume the suggested shroud is for a truss 'scope!. I forgot to mention that mine is a solid tube 'scope, about 900mm long and just over 300mm dia.
Thanks for the suggestion.
Charles

[Paul Haese]

Charles,

even down here in dew central (I am right near the Murray River and Lake Alexandrina) I only need a dew heater on the secondary of the RC truss. The fans are supposed to help prevent dew on the primary. And; the fans do that pretty well down here. You must be getting some very low night time temps and not much warmth during the day.

You don't really want to heat the tube too much otherwise you will introduce tube currents.

[cfranks (Charles)]

Hi Paul,

I'm ~40Km from the Murray so that shouldn't affect me but I certainly get my share of dewing! It does get ~5 degrees cooler here than say, Adelaide, but the days can be quite comfortable. The mount is under a TeleGizmo cover and gets warm enough to keep dry during the day. It's the mirror dewing up effectively at the bottom of a long dew shield that gets my goat. I'll try just having the fans on while the mirror temp stabilizes and turn them off for imaging and see if there is any change.
I note your comment about possible tube currents from another heater but, with the fans on all the time I had thought that might not be too much of a problem but if I turn the fans off it will definitely be as you say. I will wrap the exposed parts of the tube with some 5mm thick foam and see what effect that might have. The second dew heater can be tested if the wrap has no effect. Finally, I will try and get permission to put up a small RORO shed. I'm renting this house so it might be difficult although I did get permission to put up the pier.
Anyway, many thanks for the thoughts,
Charles

[Paul Haese]

Charles it is a bit of a problem for sure. Trial and error is going to provide an answer in the end I am sure.

[strongmanmike (Michael)]

Originally Posted by cfranks

I have ordered additional dew straps to place on the tube near the mirror but it remains to be seen if that is a good idea, obviously I don't want to affect my imaging.

Hi Charles

This is exactly what I have on my scope (but molded into the inside of the tube) along with a small Kendrick heater pad on the back of my secondary mirror and I have never seen any dew on either my secondary, primary or corrector face (exposed inside tube) in 3 years of using the scope. I too was concerned that having a tube dew heater just in front of the primary mirror might affect my imaging but as you an see from the following images I don't recon it does..?

Each of the three following image was taken with the secondary heater, tube heater and rear fans (that suck the air out of the tube) going throughout the entire exposure in a fully exposed position out in the open in Canberra.

Fornax A

NGC 253

NGC 1365

And even in the warm moist coastal air of Newcastle, while the seeing wasn't as good as in Canberra, I never saw any dew on the optics

Then once inside a dome and out of the elements with the same dew prevention running, the resolution only improved

Devils Mask

200 Million light year distant cluster

And again I have not seen any dew form anywhere

So I recon if you wrap a heater strap around the outside of the tube at the level of the mirror face and install a secondary heater pad then run your fans...all should be hunky dory I recon you should try making a dew shield as well though - everything helps

ED: I see you already have a secondary heater - good.

Mike

[DavidTrap (David)]

Hi Charles,

So I'm not the only one who has had dewing troubles with the RC10C! I have had similar experiences to yourself.

I've been corresponding with Kendrick about an RC dew heater pad that goes on the back of the mirrors, but they don't have one small enough for an RC10 and I don't believe you can cut the pad down to size…

Given a lack of other options, I've been doing similar to what Mike does - I've wrapped a dew strip (initially two to get the length required) outside of the tube, and run the fans +/- secondary heater. I've also installed a thermistor onto the back of the mirror when I took it out for cleaning. I used some thermal paste from Jaycar and stuck it down with aluminium tape. The wire feeds through one of the fan holes and I attach the meter outside the scope. The meter has two sockets so I can measure ambient and mirror temps. I'm working on the theory that keeping the mirror above the dew point should keep it dry. I've also sacrificed a camping mat to make a further dew shield around the front of the scope - can't hurt!

The above seems to help, but I'm haven't had the scope outside enough to really know if the problem is gone. Given the fans are running, I don't believe it causes big problems with tube currents.

Hope it helps. Let me know if you need further information.

DT

[DavidTrap (David)]

I do like the solution for Mike's scope, and I'm sure it would be more effective in heating the mirror. Might have to try sticking a dew strip inside the tube next time I clean the mirror - unfortunately, I've only just done that and recollimated the scope, so it will have to wait!