Dew strip position

[wookie1965]

I have read and seen a few posts of dew strips position behind the dew shield on refractors myself have always positioned mine on the dew shield just in front of the glass what do the more experienced astronomers think please. 

[Ronclarke]

I put mine over the glass for maximum heat transference..

Ron

[Dave scutt]

Sorry to jump in on your thread. 

What about a dew strip for a finder scope, the back and front dew up do I need to put one both ends or would one just be enough for the whole finder 

[wookie1965]

28 minutes ago, Dave scutt said:

Sorry to jump in on your thread. 

What about a dew strip for a finder scope, the back and front dew up do I need to put one both ends or would one just be enough for the whole finder 

Sorry cannot help I use a Telrad. 

[jock1958]

I’ve always wondered myself but I wrap mine around the dew shield adjacent to the lens 🤔Another thing why does my dew strap get warmer where it overlaps? 
Also do they cause warm air currents in front of the lens? 

Edited November 5, 2021 by jock1958

[Clarkey]

15 hours ago, jock1958 said:

Also do they cause warm air currents in front of the lens?

I guess that technically the answer is 'yes'. Always seems a bit odd to cool everything down before use, just to warm it up again! As long as the dew heater is set as low as possible to prevent dew but not so high to create significant air currents, the effect is minimal and better than dew.

WRT location, I put mine adjacent to the lens.

[Carbon Brush]

On a finder scope, excessive heating causing air currents in the tube is not a problem. It is low magnification.
If the tube is metal, then heat will conduct well.
I suppose the best answer is a heater anywhere on the tube, with lagging on the outside.

For a refractor, you are trying to stop the objective cooling below ambient (or dew point).
I think that heating either around the lens, or on the tube just behind it would give best results for minimum heat input.

Taking an extreme case. If you heat near the focusser, you rely on air currents convecting to warm the objective.
Not conducive to good views🥴

Just my ill informed and uneducated two pennorth.

David.

[Guest]

17 hours ago, jock1958 said:

I’ve always wondered myself but I wrap mine around the dew shield adjacent to the lens 🤔Another thing why does my dew strap get warmer where it overlaps? 
Also do they cause warm air currents in front of the lens? 

I think you have to make sure that you are using the minimum amount of heat. I put a dew heater on the secondary mirror in my newtonian and I can't remember the circumstances but there was a point early on where I could actually see heat rising from it through the optical system. Sometimes you can see something similar in the shadow on the wall behind a domestic radiator when the light is falling in a certain way. I use a dew heater controller which has LED indicators on each channel so now when I use it I turn the knob just to the point where the LED starts to come on. I don't think it needs much heat at all to stop dew forming.

[wookie1965]

I was thinking about heat currents and wondered if by applying the dew strip just behind the glass that would stop the glass dewing up but also stop any warm air currents in front of the OTA I just thought I would ask other astronomers what they thought.

[jock1958]

27 minutes ago, woodblock said:

I think you have to make sure that you are using the minimum amount of heat. I put a dew heater on the secondary mirror in my newtonian and I can't remember the circumstances but there was a point early on where I could actually see heat rising from it through the optical system. Sometimes you can see something similar in the shadow on the wall behind a domestic radiator when the light is falling in a certain way. I use a dew heater controller which has LED indicators on each channel so now when I use it I turn the knob just to the point where the LED starts to come on. I don't think it needs much heat at all to stop dew forming.

Very interesting I didn’t think a dew heater controller was essential I just plug my Dew Strap into an EC Power Bank battery which is clipped to my eyepiece holder on my AZ4 tripod.
To be honest it doesn’t get that warm just a bit warmer where the strap overlaps (11cm) and the cable runs down from. 
My original thinking was to save weight and adding a controller would add complexity, weight and something else to trip over in the dark.

Maybe I need to rethink my strategy and get a super light weight controller 🤔

[Dave scutt]

12 minutes ago, jock1958 said:

Very interesting I didn’t think a dew heater controller was essential I just plug my Dew Strap into an EC Power Bank battery which is clipped to my eyepiece holder on my AZ4 tripod.
To be honest it doesn’t get that warm just a bit warmer where the strap overlaps (11cm) and the cable runs down from. 
My original thinking was to save weight and adding a controller would add complexity, weight and something else to trip over in the dark.

Maybe I need to rethink my strategy and get a super light weight controller 🤔

One has just popped up on astro buy and sell 

[Guest]

My dew heater controller is stuck to the side of the mount (HEQ5) with velcro. It's pretty light anyway but this way it doesn't add to the weight of the OTA. I use one channel of the controller for the secondary mirror of the newtonian and I've just ordered another dew strap which will be for my guide scope which steamed up during my last session. Then I will be using two channels. The primary mirror of the newtonian has never steamed up (so far).

[AstroNebulee]

On my sw 72ed I place it just over the glass and as far back on the dew shield as possible and one also on my guidescope 

 

Edited November 6, 2021 by AstroNebulee

[MillHey Nebula]

Place adjacent to the front lens.  You want the surface of the glass above the dew point of the humid air.  If you just heat the air, its dewpoint will not be changed as the water content is fixed and so if it meets glass below this temperature, the condensation will form.