Reaching Out to John Hayes

Dew Heater Equipment Physics
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GregsAstrobin avatar
GregsAstrobin avatar

I’d like to see if I can get John Hayes to join a thread that I am involved in on Cloudy Nights. The thread is in regard to EdgeHD telescopes and eventually led to some discussion about dew control. Another contributor to the thread and I both referenced John’s Astro Imaging Channel presentation on “The Physics of Dew Prevention”. One of the other participants insists that one should never put heat in front of the telescope. I listened to the presentation again so I would be fully up to speed for the discussion. The one thing that seems to be missing in the presentation why the low level of heat from a heated dew shield doesn’t cause image degradation. I believe the premise of the participant in question is that any heat in front of the scope will cause degradation. I have suggested to the participant that he view the presentation, but I don’t think he has done so.

I’d like to be able to explain why the low level of heat will not cause image degradation, but I can’t. Perhaps John could join the thread and explain why it is not a problem. Here is a link to the thread:

EdgeHD and Imaging - Experienced Deep Sky Imaging - Cloudy Nights

Greg

Salvatore Iovene avatar
Salvatore Iovene avatar

Hi Greg,

AstroBin has a private messaging function. If you find John's gallery, you'll see a Message button there.

Let me know if you need clearer instructions.

Salvatore

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Aaron Dalton avatar
Aaron Dalton avatar

John, we've been trying to reach you about your scopes extended dew shield!

Noah Tingey avatar
Noah Tingey avatar

If I’m remembering correctly, John does briefly address this in the video when discussing the ideal amount of power that should be sent to the dew heater. I believe he said it should be just enough to prevent dew, as too much heat will degrade image quality like the people on CN are saying.

That said, I don’t see any reason why having heat in front of your front element would be any worse than having heat behind your front element… especially if you need much much more heat to get rid of dew when the dew heater strap is behind your front element (as John showed would be necessary in his video).

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Arun H avatar
Arun H avatar
I believe the premise of the participant in question is that any heat in front of the scope will cause degradation. I have suggested to the participant that he view the presentation, but I don’t think he has done so.

Recognize that the only way to prevent dew formation on the front element is to heat its outer surface above the dew point.

This is accomplished most efficiently by heating the dew shield just enough so radiation heat exchange between the shield and the front surface of the lens takes it above the dew point.

Now consider what happens were you to place the dew heater behind the front element. You would have to supply enough heat to either:
  1. Use heat conduction to heat a larger section of the metal body of the scope and have that heat transmit to the dew shield so it can radiatively heat exchange with the front element and raise it above the dew point
  2. Directly conduct through an insulator (glass) and heat up the front element though conduction.


Both solutions require more heat than simply heating the dew shield directly.

So it isn't that heating the dew shield won't cause problems. It is that it will cause far fewer problems than any alternative method because those methods require more heat and are much less efficient.

I will add - if someone does not want to watch a well researched and useful video on the topic and still holds to an opinion - having John enter into the debate seems unlikely to change their mind. I personally prefer to understand and follow the physics rather than spend time convincing someone that does not want to be convinced!
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John Hayes avatar
John Hayes avatar

I’ve been out all day, which is why I’ve been so slow to respond. Someone else PM’d me about this so, I’ve seen that discussion over on CN. I no longer participate on CN so I won’t post anything directly to that thread.

The protagonist in that CN discussion claims to understand physics but he has also appeared to refuse to watch the presentation that I presented on TAIC. In that presentation, I gave very clear reasons why the method that he is promoting is not the best way to fight dew. Yes, what he is advocating will work (as I said in the presentation), but it requires a lot more power than what is needed to simply raise the temperature of the front surface to counter radiative losses to the sky. Furthermore, I showed some thermal plumes in an interferometer beam to emphasize why you want to minimize the amount of heat that goes to any heaters no matter where they are positioned. Remember that in a steady state, power in equals power out. So, if you are dumping 92W into the back of a C14 just to heat up the front surface by 6C, that’s how much power is being dissipated into the surrounding air—and if the scope is pointed straight up, that thermal plume is going to get into the light path. I showed that it takes about 60 times LESS power to simply heat a dew shield than to heat the tube for a C14. That calculation included a 3C margin so it might be a bit optimistic, but it’s not off by more than a factor of two! And..when you put less heat into the system, it is possible to generate laminar air flow, which will have a far smaller potential impact on image quality than turbulent flow. I’ve shown the calculations for a C14 so which is better? ~1W-2W on the dew shield (for an aspect ratio of 1.5) or 92W on the OTA tube?

For those of you involved in that CN discussion, please go back and review slide number 4. In my experience, the protagonist in that CN discussion is the most active food fighter on that site. He will NEVER agree with what I have to say and he won’t provide any data or coherent calculations to back up his position so, my advice is to let it go and re-watch the parts of my presentation that you might not remember or that you might not have understood completely. I’m happy to answer questions about it. What I’m not willing to do is to get into another meaningless food fight over on CN over simple principles that shouldn’t be at all controversial .

John

Freestar8n avatar
Freestar8n avatar

Gee - I think my ears are burning. I’m happy to have technical discussions but I prefer to avoid ad hominems. This thread is a strong mixture of both dysfunction and uncoolness, and Greg could have simply addressed me directly in a PM on CN, rather than take the CN thread off topic and then pursue it here in public.

The first thing I would point out regarding the CN discussion is that a person is asking for general advice from users, and the goal is to respond with advice people consider helpful. If a side issue comes up that gets belabored, it detracts from the thread and takes it off topic. So, once I gave my advice it’s perfectly fine for others to give theirs, and it is also perfectly fine if people disagree. But the point of the thread isn’t to change my mind; it’s to help the original poster. Unfortunately the damage was done and the thread was sidetracked, whereas it would not have been if people simply gave their own advice.

I don’t know Greg or his background, but it’s clear he isn’t aware I’m quite familiar with John’s take on dew prevention, and I have already provided feedback in the past. So when he said, “Please view the John Hayes presentation mentioned above. It will set you straight. Sorry, it's just physics.” That tells me a lot. Do I need to explain why it is quite presumptuous to think that just because one person finds a youtube compelling and truthful, it will automatically be received by others the same way? “You believe they landed on the moon? Watch this video and it will set you straight!” You see, it’s ridiculous to watch a technical youtube, declare it to be 100% true, and expect anyone else to agree, or they are wrong.

The topic of dew prevention and fans and insulation are somewhat unique in astronomy because they don’t translate well from professional work since observatories tend to be very well climate controlled, or they just shut down when dew is forecast. So you end up with amateurs having very different takes on what is the best way to do things. The opinions are very strong and varied, particularly recently with insulation on the tube and fans in the front of the telescope, with no heating at all. Lots of ideas out there, and youtubes.

Astrobin is primarily an image hosting site and the forum tends to be fairly low bandwidth and more social than technical. And the topics tend to be more about post-processing than imaging per se. So I don’t tend to post here. But given this very strange and dysfunctional public thread I will aim to start a separate one on dew prevention in the future. And people are most welcome to disagree, but please keep it technical, avoid ad hominems, and avoid appeal to authority.

Cheers,
Frank

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