TAIC: The Physics of Dew Prevention coming up Sunday, 9/7/25 - Forums

On this coming Sunday, September 7th, I’ll be back on the Astro-Imaging Channel to talk about “The Physics of Dew Prevention”. This is a new and improved version of the talk that I presented at NEAIC last spring. Here’s a brief description:

It seems that everyone has their own ideas about how to prevent dew but do you really understand how to maximize dew prevention and still get good image quality? In this presentation, we go over the difference between dew and frost and examine the 3 mechanisms of heat exchange to understand why condensation happens—and various ways to prevent it. Then we’ll look at specific strategies for minimizing dew while maintaining image quality. We’ll see why some of the most popular methods are the worst for image quality and we’ll show what works well…and why! We’ll also show when dew is unlikely and how it may still shut down your imaging system—even when things seem completely safe. This will be a fun session with some surprising answers and solid recommendations. You will leave this presentation with a deeper understanding of how to best keep your gear dry when everything else is getting soaked or covered in frost.

You can join in here: https://youtube.com/live/5Wp2bkz85-o?feature=share. Along each coast, the Channel goes live on Sunday evening at 9:30 pm EDT and at 6:30 pm PDT. That’s 8:30 pm Central and 7:30 Mountain. I’ve got some really good stuff to share and I’m looking forward to it so I hope to see you there!

John

Noted, I'll be watching. Already very intrigued.

I’ll be tuning in!

Looking forward to it…

Looks to be interesting. Not too much of a problem here in Central Oregon except in the winter when it is cloudy anyway, but my setup in California pays for the good seeing with dew and even fog. I will be sure to watch.😶‍🌫️

Hi John,

I was surprised to see you recommend to heat the dew shield, and you gave an F to placing the dew heater on the body.

In my experience with my refractor, and I have seen others on CN say this, putting the heat on the dew shield creates unwanted heat plumes right in front of the lens.

Karl

ks_observer · Sep 10, 2025, 01:46 AM
Hi John,
I was surprised to see you recommend to heat the dew shield, and you gave an F to placing the dew heater on the body.
In my experience with my refractor, and I have seen others on CN say this, putting the heat on the dew shield creates unwanted heat plumes right in front of the lens.
Karl

Hi Karl,

No matter where you apply the heat, turbulence can get into the light path and the more power you apply, the worse the potential problem. The point of applying heat to the dew shield is that you don’t have to apply very much power. As I showed with the C14 example, it requires at least 60x more power when heat is added to the tube compared to adding it to the dew-shield and with an air-spaced refractor, it will require a lot more. I very gently heat the dew shield on my GTX130 in Chile when the conditions warrant it, and I’ve seen virtually no effect on image quality. Remember that you only need a delta of 3C to eliminate any chance of dew. If the conditions aren’t right on the edge of the dew-point, just forgo heat altogether and simply rely on the dew-shield. If there’s no wind, extend it to an aspect ratio of 2, and you’ll make it twice as effective. As for heating the dew shield, I suggest that you give it a try. I think that you might like it.

John

I was surprised to see you recommend to heat the dew shield, and you gave an F to placing the dew heater on the body.

I am sure John goes into this in his presentation which I plan to watch.

At the end of the day, the outer surface of the lens has to be heated above the dew point to avoid condensation. Putting the dew heater on the body (behind the lens) is incredibly inefficient, because you are relying on conduction through the body of the scope and then through the lens (or radiation exchange between the back of the lens element and the inner body of the scope and then conduction through glass and air gaps) to heat the outer surface of the lens. And simply because this will require a lot more heat to be effective, it is likely to cause problems.

It is much simpler to consider how the lens cools below the dew point in the first place - through radiative exchange with the sky and/or surface of the dew shield, whose temperatures are much lower than the air temperature. So, heating the dew shield directly is very effective because it prevents this heat loss. Incidentally, this is also why you don't get dew if it is cloudy (the black body temperature of the clods is higher than the sky) or if there is a breeze (convection is very effective in raising the temperature of a surface to the air temperature and hence above the dew point).

After watching the (excellent) video, it turns out I actually had it right although more by luck than from knowledge or skill. 😊 The dew shield was about the right length and had heat.

That was a great talk John, thank you, and it answered some questions that I’d been asking without joy for a a few weeks, namely, how much above dew point should I set my dew point monitoring dew heating controller. That lone was worth the time and gave me a 20% increase in battery life and decreased my guiding error by an appreciable margin.

So many things for a newb to learn, and that’s what makes it such an absorbing hobby.

I also learned a lot from your galaxy processing lecture. Thanks again.

Great talk, John.

I was really surprised by the vast differences in efficiency - amazing.

Two questions:

why would a Celestron heat ring on top of the corrector plate (where it is well isolated from the glass as you pointed out) not be equivalent in replacing the off radiated heat like the heat straps around the dew cap? (Surely I am NOT asking why it does not work by heating the glass :-) )
Why did you apply several heat straps across the dew cap ie spreading over the entire length?

Arny

Arny · Sep 12, 2025, 07:07 PM
Two questions:
why would a Celestron heat ring on top of the corrector plate (where it is well isolated from the glass as you pointed out) not be equivalent in replacing the off radiated heat like the heat straps around the dew cap? (Surely I am NOT asking why it does not work by heating the glass :-) )
Why did you apply several heat straps across the dew cap ie spreading over the entire length?

Arny,

Thanks for your comments and questions.

1) The dew shield heats the entire front glass surface more uniformly than a dew ring. Remember that with an aspect ratio of 1.5, it is covering 90% of the hemisphere viewed by the front surface. That’s essentially why it only requires a small temperature increase to replace radiation lost to the sky. On the other hand, the dew ring is only heating the glass directly below it and that heat has to conduct from the edge all the way along a radius toward the center of the corrector plate. That’s a huge thickness—and it requires a LOT of heat to raise the temperature of the center region by 3.5C.

2) The calculation assumes that the dew shield has a uniform temperature. You could use a single heater strap, but it would have to operate at a higher temperature to replace the power lost to the sky. Using 2-3 straps and insulating them with a piece of Reflectix, helps to reduce temperature variation on the dew shield, which in turn, allows using a lower operating temperature.

Hopefully that make sense.

John

John Hayes · Sep 10, 2025, 04:05 AM
ks_observer · Sep 10, 2025, 01:46 AM
Hi John,
I was surprised to see you recommend to heat the dew shield, and you gave an F to placing the dew heater on the body.
In my experience with my refractor, and I have seen others on CN say this, putting the heat on the dew shield creates unwanted heat plumes right in front of the lens.
Karl
Hi Karl,
No matter where you apply the heat, turbulence can get into the light path and the more power you apply, the worse the potential problem. The point of applying heat to the dew shield is that you don’t have to apply very much power. As I showed with the C14 example, it requires at least 60x more power when heat is added to the tube compared to adding it to the dew-shield and with an air-spaced refractor, it will require a lot more. I very gently heat the dew shield on my GTX130 in Chile when the conditions warrant it, and I’ve seen virtually no effect on image quality. Remember that you only need a delta of 3C to eliminate any chance of dew. If the conditions aren’t right on the edge of the dew-point, just forgo heat altogether and simply rely on the dew-shield. If there’s no wind, extend it to an aspect ratio of 2, and you’ll make it twice as effective. As for heating the dew shield, I suggest that you give it a try. I think that you might like it.
John

I will give it a try.
I likley had the heat way too high.

Thank you as always!

Thanks John. Just watched it on YouTube. It was very educational and an eye opener.

Cheers,

Kartik

Kartik Atre · Sep 17, 2025, 10:04 AM
Thanks John. Just watched it on YouTube. It was very educational and an eye opener.
Cheers,
Kartik

Thanks Kartik! I’m glad to hear that.

John

A very informative and educational talk John. One of best parts, the congratulations at the end… 😄

As an engineer, I always tend to think in physics principles. Thermodynamics explains it all, with total sense. I always enjoy this kind of talks. Thanks for doing this kind of divulgation.

Regards,

Alfredo

Alfredo Beltrán · Sep 18, 2025, 02:47 AM
A very informative and educational talk John. One of best parts, the congratulations at the end… 😄
As an engineer, I always tend to think in physics principles. Thermodynamics explains it all, with total sense. I always enjoy this kind of talks. Thanks for doing this kind of divulgation.
Regards,
Alfredo

Thank you so much Alfredo! It is always good to hear from you and I am delighted that you enjoyed the talk. It is especially nice to hear that you tend to think in “physics principles”! It is always good to know that we have so may engineers, scientists, and other technical folks involved with imaging around here.

- John

Took me a bit of time to get to it but watched this yesterday. Thanks for the interesting and informative presentation - I’m so glad you are willing to put the time into doing this and sharing. Now it is time me to reposition straps on my refractor and redesign my C11Edge setup!

Dave

David Jones · Sep 18, 2025 at 11:03 AM
Took me a bit of time to get to it but watched this yesterday. Thanks for the interesting and informative presentation - I’m so glad you are willing to put the time into doing this and sharing. Now it is time me to reposition straps on my refractor and redesign my C11Edge setup!
Dave

Thanks Dave! I wish you the best of luck with it…

John

Watching! Very useful! Thank you very much!

Thank you Igor. I’m glad to hear that!

John

Dear John,

thank you for another amazing presentation! I only wish I’ve seen it 6 months ago when I got my Edge 92.25 🙂

The fan blown dew shield will be a good DIY project…

Richard Mak · Sep 25, 2025 at 01:32 PM
Dear John,
thank you for another amazing presentation! I only wish I’ve seen it 6 months ago when I got my Edge 92.25 🙂
The fan blown dew shield will be a good DIY project…

It’s my pleasure and I’m happy to hear that you found it valuable! The “fan shield” is indeed a good project. I know of one person who built one and he tells me that it works well—up until fog formed. Then things got wet. Good luck with it and let me know how it works if you build your own.

John

John Hayes · Sep 5, 2025, 03:15 AM
On this coming Sunday, September 7th, I’ll be back on the Astro-Imaging Channel to talk about “The Physics of Dew Prevention”. This is a new and improved version of the talk that I presented at NEAIC last spring. Here’s a brief description:
John

Thanks to this wonderful presentation I have moved the dew strap on my refractor to the dew shield in front of the lens and have metal dew shields for my C9.25 and C14 on order. I also just picked up a couple of “smart” controllers. I do have what is probably a dumb question (I hate it that the controller may be smarter than me) about temperature probe placement. In his presentation John mentions that the dew shield should be about three degrees Celsius above the dew point. But where is the best place to attach the temperature probe? I saw a suggestion to stick it under the dew heater band and have the band hold it in place, but unless the dew controller has changed the laws of physics to produce 100% efficiency in heat transfer from the band to the shield I can’t imaging this will be accurate! At the base of the dew shield between the lens or corrector plate and the band? Further out on the dew shield between the band and far end of the shield? Between the bands if using more than one? How far out beyond the band if only using one?

The battle continues, but this time I’m better armed!

Arnie · Sep 26, 2025, 05:09 PM
But where is the best place to attach the temperature probe? I saw a suggestion to stick it under the dew heater band and have the band hold it in place, but unless the dew controller has changed the laws of physics to produce 100% efficiency in heat transfer from the band to the shield I can’t imaging this will be accurate! At the base of the dew shield between the lens or corrector plate and the band? Further out on the dew shield between the band and far end of the shield? Between the bands if using more than one? How far out beyond the band if only using one?
The battle continues, but this time I’m better armed!

It’s the corrector plate temperature you’re concerned about, so I place the probe on the glass at the periphery of the CP. I also put a piece of foam rubber on top of the probe to both hold it down, and to protect it from radiative cooling.

For a dew shield with fans, the issue I ran into with the smart controller I got (Pegasus DewMaster2) is that the PWM power outputs it has work with dew heaters, but aren’t configured for fan motors. They’ll still run the fans, but not until the power output exceeds 86% or so. So, the fans (I have three, two blowing in, one blowing out) don’t come on until later than I’d like. The Pegasus controller also doesn’t let you set the delta T between probe and dew point. The Celestron smart controller might offer more control, but I haven’t looked at it very closely.

As John mentioned, I’ve only had dew form when humidity actually hits 100%, and that at a point (dawn) where imaging hasn’t been impacted. The Cadillac model, though, would also have a dew heater on the shield as backup for those extreme humidity situations.

Cheers,
Scott