How do dew shields work, and do they eliminate the need for a dew heater?

[Trippelforge]

I have always been confused on how exactly a dew shield wards off dew from forming on the optics. My current scope has a retractable one, and when I was shooting last dew ending up forming on everything. The chairs, table, outside of the tube etc. But I didn't notice my images getting screwed up. So I got to ask, how does a sliding tube help? Secondly I don't own a dew heater, but the amount that accumulated was a bit alarming (it's usually really dry this time of year). So that made me wonder if I should be using a heater regardless.

And I might as well toss this in, since I have a dew shield where do you wrap the band? Around the dew tube itself, or on the scope where meets?

Thanks guys!

 

[CraigT82]

Dew will form on any surface that is colder than the dew point temp, usually this will be on  any surface exposed to the sky. The dew shield shades the objective from the sky and so dew is delayed from forming here by slowing the rate that the objective cools. 
 

Never used dew heaters so can’t answer you queries about those sorry! 

 

[Shimrod]

12 minutes ago, Trippelforge said:

I have always been confused on how exactly a dew shield wards off dew from forming on the optics. My current scope has a retractable one, and when I was shooting last dew ending up forming on everything. The chairs, table, outside of the tube etc. But I didn't notice my images getting screwed up. So I got to ask, how does a sliding tube help? Secondly I don't own a dew heater, but the amount that accumulated was a bit alarming (it's usually really dry this time of year). So that made me wonder if I should be using a heater regardless.

And I might as well toss this in, since I have a dew shield where do you wrap the band? Around the dew tube itself, or on the scope where meets?

Thanks guys!

 

It sounds like you have a refractor, so you would want to have the dew heater wrapped around the telescope where the objective lens is - so at the bottom of your dew shield. The idea is to keep the lens slightly above the temperature at which the dew will form. 

If you have an SCT, you would heat the corrector plate.

You can get dew straps to fit eyepieces, finderscopes and guidescopes as well - I only use one for the telescope.

[Trippelforge]

4 minutes ago, Shimrod said:

It sounds like you have a refractor, so you would want to have the dew heater wrapped around the telescope where the objective lens is - so at the bottom of your dew shield. The idea is to keep the lens slightly above the temperature at which the dew will form. 

If you have an SCT, you would heat the corrector plate.

You can get dew straps to fit eyepieces, finderscopes and guidescopes as well - I only use one for the telescope.

Ya I do have a refractor, thanks that clears it up. I see some "budget" minded ones on Amazon. SVBony has one for like 26 bucks, are those OK? I only ask because I also have seen them for over 100.

The only other powered thing connected is a DSLR, but I am going to guess they heat themselves up enough to protect from dew? I am going to assume motors, control units etc also keep themselves slightly warm. 

Thanks 😃

Edited January 12, 2023 by Trippelforge

[Shimrod]

2 minutes ago, Trippelforge said:

Ya I do have a refractor, thanks that clears it up. I see some "budget" minded ones on Amazon. SVBony has one for like 26 bucks, are those OK? I only ask because I also have seen them for over 100.

 I have only a DSLR, but I am going to guess they heat themselves up enough to protect from dew?

Thanks 😃

It depends a bit on how cool and how humid the climate is where you are. You may want one for your lens as well - I've had my camera mist up on photography trips - the objective of the camera lens is some distance from any heat generating components in the camera body.

I've no personal experience of the SVbony dew straps, but I have other equipment from them and it has been fine. A dew strap is certainly one of the simpler devices in astronomy, basically being a piece of wire which heats up when you run a current through it. The length of the strap will determine part of the price, as will the brand name. You may find the more expensive ones offer a bit more in the way of temperature control but not much else. None of my dew straps has cost much more than the equivalent of $26 in the UK.

[Trippelforge]

1 minute ago, Shimrod said:

It depends a bit on how cool and how humid the climate is where you are. You may want one for your lens as well - I've had my camera mist up on photography trips - the objective of the camera lens is some distance from any heat generating components in the camera body.

I've no personal experience of the SVbony dew straps, but I have other equipment from them and it has been fine. A dew strap is certainly one of the simpler devices in astronomy, basically being a piece of wire which heats up when you run a current through it. The length of the strap will determine part of the price, as will the brand name. You may find the more expensive ones offer a bit more in the way of temperature control but not much else. None of my dew straps has cost much more than the equivalent of $26 in the UK.

I see one with decent reviews (if you can trust them) for only 20, but it's powered via USB. Do you think a USB port would be able to output enough power to heat one up enough? I just noticed the Svbony doesn't come with a power supply, so it's prob more like 36 bucks. 

[vlaiv]

Two things happen to make dew shield work.

First is temperature. As has been pointed out above - something needs to cool down below dew point in order for dew to start forming on it.

Piece of glass facing up towards the sky can get colder than the ambient temperature under some conditions. This is because of ways heat transfers from body to body. One of these ways is heat radiation. Sky is very cold - it is about 2.7K if I'm not mistaken that is about -270C.

Two things that are next to each other are trying to reach equilibrium as far as radiation heat transfer is concerned - first object is giving off heat that hits the second and second is giving off heat that hits the first. Problem is - sky is much much colder than glass and that makes glass cool.

You can see this effect on cars in the winter. Ice will form mostly on the front windshield and on back - but not so much on side windows (depends where car is parked) - this is because front wind shield is facing towards the sky and side windows often face other objects like other cars and buildings.

Dew shield - well shields objective lens from cold of the sky - by restricting how much of it is exposed to the sky - it shields from broader exposure to the sky.

Second effect of dew shield is that it lowers humidity of the air. If things get cold enough for dew to start forming - that dew is water from the air and relative humidity drops in vicinity of formed dew. Dew shield traps pocket of air that has lower relative humidity than the surrounding air because it already deposited dew on outside of the dew shield and rest of your gear.

If there is little wind - that drier air will stay around the lens for longer. Stronger winds will blow it away and increase chance of dew forming.

[Shimrod]

3 minutes ago, Trippelforge said:

I see one with decent reviews (if you can trust them) for only 20, but it's powered via USB. Do you think a USB port would be able to output enough power to heat one up enough? I just noticed the Svbony doesn't come with a power supply, so it's prob more like 36 bucks. 

That shouldn't be a problem - there are plenty that operate that way and they don't need a huge power drain. I use mine via a dew controller, which is just a small box that allows me to have up to four dew straps attached and vary the power to each one. It's similar to this one dew controller You can buy some 'smart' controllers that will adjust based on current temperature and humidity, but they tend to cost £100's. If you go down the controller route, then you will have to be careful they you get the right type of connector on your dew strap - I've not see a controller that supports USB but maybe someone else can provide a link to one. Of course, if you are only using one dew  strap then you don't really need a controller at all.

[Trippelforge]

27 minutes ago, vlaiv said:

Two things happen to make dew shield work.

First is temperature. As has been pointed out above - something needs to cool down below dew point in order for dew to start forming on it.

Piece of glass facing up towards the sky can get colder than the ambient temperature under some conditions. This is because of ways heat transfers from body to body. One of these ways is heat radiation. Sky is very cold - it is about 2.7K if I'm not mistaken that is about -270C.

Two things that are next to each other are trying to reach equilibrium as far as radiation heat transfer is concerned - first object is giving off heat that hits the second and second is giving off heat that hits the first. Problem is - sky is much much colder than glass and that makes glass cool.

You can see this effect on cars in the winter. Ice will form mostly on the front windshield and on back - but not so much on side windows (depends where car is parked) - this is because front wind shield is facing towards the sky and side windows often face other objects like other cars and buildings.

Dew shield - well shields objective lens from cold of the sky - by restricting how much of it is exposed to the sky - it shields from broader exposure to the sky.

Second effect of dew shield is that it lowers humidity of the air. If things get cold enough for dew to start forming - that dew is water from the air and relative humidity drops in vicinity of formed dew. Dew shield traps pocket of air that has lower relative humidity than the surrounding air because it already deposited dew on outside of the dew shield and rest of your gear.

If there is little wind - that drier air will stay around the lens for longer. Stronger winds will blow it away and increase chance of dew forming.

 

Thanks for the great explanation! I feel like I now understand the entire process, the windshield example really cleared up the process. I am glad I asked for sure. So I assume this pocket of warmer air only can stick around so long and a dew heater is a must for longer sessions. My son and I have our scope out right now for about three hours or so. I setup prior to dusk, so the temperature swing sometimes is a lot. I am going to order one now regardless. 

 

26 minutes ago, Shimrod said:

That shouldn't be a problem - there are plenty that operate that way and they don't need a huge power drain. I use mine via a dew controller, which is just a small box that allows me to have up to four dew straps attached and vary the power to each one. It's similar to this one dew controller You can buy some 'smart' controllers that will adjust based on current temperature and humidity, but they tend to cost £100's. If you go down the controller route, then you will have to be careful they you get the right type of connector on your dew strap - I've not see a controller that supports USB but maybe someone else can provide a link to one. Of course, if you are only using one dew  strap then you don't really need a controller at all.

Thanks, ya the ones I see that are cheaper and use USB just have a simple controller for low-med-high. I guess for 20 bucks I can try one out and return if it sucks. How warm are they supposed to be? I was just curious so I know if it's actually working well enough.

 

[markse68]

One interesting thing I noticed (to me anyway) is that the dew forms faster and more heavily on my eyepiece case which is plastic than it does on my OTA which is painted aluminium and feels much colder to the touch. I guess the plastic being a good thermal insulator can have a colder temperature at its very surface than the metal tube that conducts heat better but has higher thermal mass so takes longer to cool ?

Mark

[vlaiv]

6 minutes ago, markse68 said:

One interesting thing I noticed (to me anyway) is that the dew forms faster and more heavily on my eyepiece case which is plastic than it does on my OTA which is painted aluminium and feels much colder to the touch. I guess the plastic being a good thermal insulator can have a colder temperature at its very surface than the metal tube that conducts heat better but has higher thermal mass so takes longer to cool ?

Mark

I think it has more to do with water vapor being heavier then the air. There is gradient of relative humidity and air near the ground is moist the most.

Dew first forms on grass, and that is organic material and will cool slowly, but it is closest to the ground and there is the least wind to stir things up.

If your case is placed on some low table or similar - it is going to dew up sooner than the scope that is higher on the mount.

[PeterW]

Apps like clear outside can tell you the risk of dew, depends on the humidity level. If the object cools below the dew point then dew will form, not all nights or locations are equal.  Maybe  only occasionally it strikes. 
A clear night sky is around -40C or so (measured with a thermal camera), still plenty colder than your surroundings, so things will radiatively cool if exposed to it. Some weather monitors measure the sky temperature as clouds will be warmer and so you close the dome as clouds risk rain. I bought some short dew strips for some binoculars and they plug into a use power and 2.4A per channel. You could get a d ew controller to allow you to vary the heating, but you’d need a 13v supply, which is less available than USB.

 

Peter

[markse68]

17 minutes ago, vlaiv said:

I think it has more to do with water vapor being heavier then the air. There is gradient of relative humidity and air near the ground is moist the most.

Dew first forms on grass, and that is organic material and will cool slowly, but it is closest to the ground and there is the least wind to stir things up.

If your case is placed on some low table or similar - it is going to dew up sooner than the scope that is higher on the mount.

Maybe but there's not a lot of height difference. Interestingly according to this table it seems that emissivity might be linked inversely to thermal conductivity- compare aluminium and copper to glass and plastics and black body

https://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html

Mark

[Trippelforge]

7 minutes ago, PeterW said:

Apps like clear outside can tell you the risk of dew, depends on the humidity level. If the object cools below the dew point then dew will form, not all nights or locations are equal.  Maybe  only occasionally it strikes. 
A clear night sky is around -40C or so (measured with a thermal camera), still plenty colder than your surroundings, so things will radiatively cool if exposed to it. Some weather monitors measure the sky temperature as clouds will be warmer and so you close the dome as clouds risk rain. I bought some short dew strips for some binoculars and they plug into a use power and 2.4A per channel. You could get a d ew controller to allow you to vary the heating, but you’d need a 13v supply, which is less available than USB.

 

Peter

I don't expect much from a budget USB powered dew heater. But it does have three heat settings on a little inline controller. I don't know HOW warm it's supposed to be to become effective though. I am going to imagine it's not going to have the same heating capability as a standard powered one. If anyone knows I can use my gun to measure it when it comes in. 

[ONIKKINEN]

23 minutes ago, Trippelforge said:

I don't expect much from a budget USB powered dew heater. But it does have three heat settings on a little inline controller. I don't know HOW warm it's supposed to be to become effective though. I am going to imagine it's not going to have the same heating capability as a standard powered one. If anyone knows I can use my gun to measure it when it comes in. 

I had a USB powered dewstrap when i used a guidescope and it was way too hot, not that it mattered since it still wasnt very power hungry but a larger one might be. That one did not have a switch of any kind so full power all the time, you might find that the minimum setting is enough with yours as it only needs to heat the glass by one degree or so.

[Trippelforge]

45 minutes ago, ONIKKINEN said:

I had a USB powered dewstrap when i used a guidescope and it was way too hot, not that it mattered since it still wasnt very power hungry but a larger one might be. That one did not have a switch of any kind so full power all the time, you might find that the minimum setting is enough with yours as it only needs to heat the glass by one degree or so.

Thanks for letting me know! 

[Cosmic Geoff]

Last night I was obliged to operate my CPC800 without a dewshield because the weather was so windy.  After some time I checked the corrector plate with a torch and noted that it appeared to have condensation on the inside as well as the outside, to the point where I wondered how it would affect the imaging.  I had not expected any dew to form in a high wind.

I left it for some time with the dewshield on, and the tube horizontal, and most of the dew (inside and out) seemed to go away.

I recall another member complaining that his observatory-mounted SCT got condensation on the inside of the corrector plate.

I think what was happening here is that (as I read elsewhere) the sky is very cold and the faceplate radiates heat to it, cooling down to the point where condensation occurs from moist air inside or outside the tube.

The moral of this story: always use a dewshield even if you don't think you need one.

[PeterW]

… and consider adding a desiccant plug to sealed tube scopes to avoid the inside having issues. Heavy dew can yield some pretty “spectacular” artistic imaging effects…

 

Peter

[doublevodka]

20 hours ago, Trippelforge said:

I don't expect much from a budget USB powered dew heater. But it does have three heat settings on a little inline controller. I don't know HOW warm it's supposed to be to become effective though. I am going to imagine it's not going to have the same heating capability as a standard powered one. If anyone knows I can use my gun to measure it when it comes in. 

It does not need to be hot, the theory is around .5 to 1 degree warmer than the ambient air temp is the ideal. Making it too warm can cause it's own issues as you could get thermal currents around the objective.

Personally, I only use a shield, but my sessions are quite short visual ones so that's fine for me

[Elp]

I find even though some dew heater bands can be hot if holding them, once they're on a scope of metal and glass, all that heat quickly gets sucked up out of the band. Never had a thermal issue with them.

[Stu]

I recently bought a USB dew strip and have been quite happy with it. It came with a tiny little controller which works well. It was this one off Amazon, ideal for my 4” scope. Was quite amusing really, I confidently advised a chap on a FB group that they didn’t exist, then he posted links to these 🤣. Felt a bit daft but swallowed my pride and bought some, pleased I did.

Lens Dew Heater Fog Remover Strip with Temperature Regulator 5V USB Warmer for Universal Camera Lens Telescopes Eyepieces Heating https://amzn.eu/d/e7yD8Q7
 

I also got another with two short straps which work well for eyepieces, or can be joined together for a longer strap. This one is without a controller but doesn’t get too hot.

USB Fast-warm 2in1 XL Spliceable Astronomical Telescopes DSLR Camera Lens Active Anti Fog Dew Heater Shield Strap, Warmer Bands Strip for Canon Nikon Sony Gskyer Celestron(Max 6 inch/180mm Diameter) https://amzn.eu/d/3EPX2wN

 

[Pixies]

On 12/01/2023 at 17:53, Trippelforge said:

I see one with decent reviews (if you can trust them) for only 20, but it's powered via USB. Do you think a USB port would be able to output enough power to heat one up enough? I just noticed the Svbony doesn't come with a power supply, so it's prob more like 36 bucks. 

The main thing is that you use a USB socket from a proper power supply device or controller. A USB port on a laptop, say, won't be able to supply enough power.

 

 

On 12/01/2023 at 17:22, Trippelforge said:

I have always been confused on how exactly a dew shield wards off dew from forming on the optics. My current scope has a retractable one, and when I was shooting last dew ending up forming on everything. The chairs, table, outside of the tube etc. But I didn't notice my images getting screwed up. So I got to ask, how does a sliding tube help?

 

As for a dew shield keeping dew away - it does this because is keeps the lens warmer than it would if the night sky was directly visible from the lens itself. If you point the scope directly up, it will dew up as quickly a without the dew shield. It's radiative thermal transfer - space is cold, so there's a great thermal difference between sky (space) and anything under the sky, so the radiative transfer is greater. This is heat radiation (electro-magnetic), NOT convention through a medium, which tends to balance out temperatures locally.

That's why the ground can be colder than the air temperature above it overnight. That's why dew forms on cars in the street and not under a car port.

[Pixies]

1 hour ago, Pixies said:

As for a dew shield keeping dew away - it does this because is keeps the lens warmer than it would if the night sky was directly visible from the lens itself. If you point the scope directly up, it will dew up as quickly a without the dew shield. It's radiative thermal transfer - space is cold, so there's a great thermal difference between sky (space) and anything under the sky, so the radiative transfer is greater. This is heat radiation (electro-magnetic), NOT convention through a medium, which tends to balance out temperatures locally.

That's why the ground can be colder than the air temperature above it overnight. That's why dew forms on cars in the street and not under a car port.

Ah - it has a name: Nocturnal Surface Cooling

https://en.wikipedia.org/wiki/Radiative_cooling#Nocturnal_surface_cooling:~:text=energy to space.-,Nocturnal surface cooling,-[edit]

 

[vlaiv]

13 minutes ago, Pixies said:

Ah - it has a name: Nocturnal Surface Cooling

https://en.wikipedia.org/wiki/Radiative_cooling#Nocturnal_surface_cooling:~:text=energy to space.-,Nocturnal surface cooling,-[edit]

 

Well, wiki does say:

Quote

The effect is well-known among amateur astronomers.

[knobby]

I've been trying the USA method of thermally insulating the tube and a very low power (0.3 amp) DIY nicrome strip just behind the corrector plate on my C9.25 and a short (10") dew sheild ... Looking good so far 🤞🏻