So, where is the best place for a dew heater? [Deep Sky] Acquisition techniques · Paul Lloyd · ... · 37 · 1470 · 1

Thank you, Steve and Jeff. Like you, I have, up to now, been heating the 'scope's metal tube at or near the objective of my several refractors, and this has largely served me well. Sometimes, it's the only option available to you, such as when using a standard camera lens which usually has a sloped, tulip-shaped lens hood which offers little room to mount a tape, and a slippery, sloped surface to hold it. I have been let down in heavy dew conditions with both a medium-sized and large objective, and wish ensure that this doesn't happen again.

What has transpired here, in this discussion, is the conclusion that mounting the dew heater tape on the dew shield/lens hood, offers advantages in efficiency (less power for the same degree of dew elimination), effectiveness (large surface area of dew shield providing radiative warming over whole lens surface vs less effective conductive warming from the edge of the objective), and with less potential image degradation (from heating the air immediately over the objective's surface). The former will pay off if you are reliant on battery power, especially if your objective is rather large, while the latter would show benefits where there is fine detail in, say, a nebula (generally frequently), and also in general image quality. I will be pursuing this approach in my future imaging activities, although it might take some time before I see a really heavy dew condition that I can compare results. Given our B.ofM.'s long-range forecasting, it is looking like a wet and humid summer ahead of us, this year, at mid latitudes in eastern Australia, so I might not have to wait too long.

If I have a definitive result from my trials, I'll post something to this discussion.

CS,
Paul.

Thanks, MaksPower, for your personal observations and practice description. I wonder if the difference between a Mak/SCT and a refractor needs a different approach to dew management?

Thinking about it, heating the OTA around its centre, with additional thermal insulation would provide radiant warming to the inner surface of the corrector, and also the primary mirror. If this is the mechanism working here, then the same could wotk for a refractor - apply warmth to, say 1/3 - 1/2 way along the OTA from the objective. It's an interesting thought. Wouldn't it be nice to have side-by-side comparisons? Unfortunately, almost all of us have to use the trial and error method.

I have tried running a fan gently wafting across the open end of the dew shield, but I haven't seen it improve things - I still got dewing. Perhaps my trials delayed the inevitable, but without a comparison test I can't know for sure. My heating tape was as close to the refractor's objective as I could make it, with no thermal insulation, on a few heavy dew nights.

I'll have to try out your suggestion and see if it works for me.

CS,
Paul.

Paul Lloyd:
Thanks, MaksPower, for your personal observations and practice description. I wonder if the difference between a Mak/SCT and a refractor needs a different approach to dew management?

Thinking about it, heating the OTA around its centre, with additional thermal insulation would provide radiant warming to the inner surface of the corrector, and also the primary mirror. If this is the mechanism working here, then the same could wotk for a refractor - apply warmth to, say 1/3 - 1/2 way along the OTA from the objective. It's an interesting thought. Wouldn't it be nice to have side-by-side comparisons? Unfortunately, almost all of us have to use the trial and error method.

I have tried running a fan gently wafting across the open end of the dew shield, but I haven't seen it improve things - I still got dewing. Perhaps my trials delayed the inevitable, but without a comparison test I can't know for sure. My heating tape was as close to the refractor's objective as I could make it, with no thermal insulation, on a few heavy dew nights.

I'll have to try out your suggestion and see if it works for me.

CS,
Paul.

Maks, SCTs, and refractors all share a common characteristic:  They all have a glass surface at the front of the telescope.  Dew forms when the temperature of that front surface is at or below the dew point of the ambient air.  Preventing dew requires keeping the temperature of that front surface at or above the dew point temperature.  As I said earlier, there are a lot of ways to do that but some are a lot better than others and that's just one reason why no one uses a blow torch aimed at the front element to prevent dew.  Sure, that would work but it's overkill.   Having some understanding of the three mechanisms of heat exchange, conduction, convection, and radiation, really helps to formulate an effective solution.

Heating the OTA works but it's closer to a "blow-torch" solution.  First, you have to apply enough heat to the surface of the OTA tube so that it conducts to the inside surface of the tube.  Then, that heat energy has to transfer to the air inside the tube through thermal convection..  Then, the rise in in temperature of the air in the tube has to transfer its heat energy by convection to the inside surface of the glass and that energy then has to be thermally conducted through the glass  to the outside surface of the optics.  Glass does not have a very high coefficient of thermal conduction so you'll need a fair amount of heat at the backside of the glass to see much of a temperature rise on the front side.  Things get a LOT worse if you are dealing with a multi-element, air spaced refracting objective.  In that case, the objective becomes like a multi-paned window with extremely low thermal conductivity.  In that case, you'll be looking at pumping maybe a couple hundred Watts of power into your heater strips on the OTA just to get even a degree or two of temperature rise at the front surface.  You won't get dew, but your whole scope will be producing a mess of thermal convection everywhere you point it.  It will become your own local "bad seeing" generator.

Finally, let's not forget that if you heat the tube, you are also transferring heat by radiation to the rear of the optics from the warm tube itself.  So why doesn't that heat also directly warm the front surface as well?   At the "normal" ambient temperatures that we are talking about (~300K), the peak wavelength is around 10-12 microns and optical glass is completely opaque at those wavelengths.  That means that the radiation from the warm tube  will be absorbed by the rear lens surface and the heat will then have to be thermally conducted to the front surface.  So, does heating the OTA work?  Yes it does, but you generally have to dump a LOT of heat into the system in order to hold that front optical surface just a couple of degrees above the dew point.

Sure, you can put heater strips around the edge of your objective as well.  In that case, the heat has to transfer by conduction through the tube, through the optical cell and the small air space between the cell and the glass before it heats the edge of the glass.  Remember that the amount of heat transferred is proportional to the area so if you are trying to heat a thin element such as a SCT corrector plate, the temperature difference has to be pretty high to conduct very much heat at the edge of the glass.  At that point, heat has to conduct all the way through the radius of the glass to keep the very center of the optic at or above the ambient dew point (usually by 1C - 3C).   Again, that approach works but it requires a lot of heat--and the larger the scope, the worse this approach is.  Heating the edge of the objective is generally a really terrible way to fight dew.

Simply heating the dew shield is the most energy efficient way to directly raise the temperature of the front element of the optics.  The thing that it doesn't do very well is to provide any significant heating to anything inside the tube.  That's not a problem with a refractor but if you are using a Mak or a SCT, you could still get dew (or frost) on the primary mirror--even inside the tube.  That's because the OTA tube is also exposed to the sky and it is subject to the same radiative heat loss as the front of the objective.  Once the tube cools enough, it acts like the sky and cools the primary mirror--as if it weren't inside the tube!   The two most effective ways to fix that are to wrap the OTA in a thermal blanket such as Reflectix and to add small air circulation fans to the OTA.  You don't have to circulate much air to be effective.  Tempest fans for a Celestron SCT are amazingly effective at eliminating tube currents and reducing the chances of internal dew.  If you want to add a single heater strip to the OTA to prevent this problem, it doesn't require much heat to prevent internal dew.  If you combine a heater with a thermal jacket, you probably won't need more than a couple of Watts to prevent internal dewing.   With a fan and a thermal jacket, you shouldn't need any heat.


John

Arun H:

John Hayes:
That's because the OTA tube is also exposed to the sky and it is subject to the same radiative heat loss as the front of the objective.  Once the tube cools enough, it acts like the sky and cools the primary mirror--as if it weren't inside the tube!

Agree with everything you said John, but I don't know that the tube would behave exactly like the night sky. In the absence of an atmosphere, that would clearly be true and demanded by equilibrium. But in the presence of an atmosphere, the temperature of the tube will still be higher than that of the night sky due to convective heat exchange with the atmosphere. But of course, your larger point that the tube, once it cools significantly, will become less effective in preventing dew for interior elements is correct, and it is for this same reason that a dew shield will eventually lose effectiveness.

Thanks Arun.  I'm assuming that the tube is sitting under an unobstructed hemisphere and I wasn't very precise.  My point is that the tube will cool significantly relative to the ambient air just like the front objective, which will cause radiative heat exchange with the internal optics.  You are totally correct that the amount of cooling will not be as significant as with the sky at say, -30C, but it can be enough to cause significant condensation on internal surfaces.  I used to see the inside of my C14 get completely frosted over from this effect before I started using Reflectix and fans.

John

Thanks, again, MaksPower. As my refractors range in the 60-120mm diameters, that explains why I haven't noticed any obvious image degradation that I haven't been able to associate with poor seeing (coastal situation = frequent sea hazes drifting 4km inland to us). I also have a 150mm refractor and am finding that I need better than average seeing for it to show its "stuff", and your explanation helps me understand why.

That's some Mak you've got there! Our club recently acquired a 11" RASA which it was intending to use near town, right on the coast. The number of nights with good seeing are, literally, down to 1-2 handsful/year in that situation. I'm sure common sense will prevail, and we will relocate it to our DSS which, while still on our eastern seaboard, has about the same number of good seeing nights/month. I'll mention your comments to the relevant people and await events.

CS,
Paul.

Thanks, once again, MaksPower. I still haven't got my head around f-ratio and its influence on a.p. It's good to discuss these things with knowledgeable people like yourself. It looks like the RASA was a good choice for our town obs. I suspect that was chosen more on serendipity than on knowledge, although our supplier might have had this in mind as they have a pretty decent reputation, here in Australia.

And, you're right regarding imaging a small galaxy with a RASA. I suspect that in the short to medium term, we will be imaging the brighter nebulae, globulars, and clusters. One use of the RASA is to show paying customers some of the delights to be seen in the heavens, and a WOW-factor needs to be built into this.

OK, back to the main topic: it appears that I have most of the dew-abation facts sorted out wrt refractors (filter & sensor dew problems to be decided). I, personally, won't be imaging with an SCT, Mak, or Newt., but our club has recently upgraded its main 'scope gear and have opted for SCT, RASA & refractor. The SCT & RASA are Celestron, and have been purchased with Celestron's ring-style dew heaters. How effective these heaters will be has to be determined. The SCT is a 14", so there is a considerable distance to the centre of the correction plate to warm. However, these 'scopes are housed in a dome of considerable size, so the atmosphere inside is going to change only slowly, and it will probably have air conditioning running prior to opening the shutter. It seems somewhat belt-and-braces to me, but there it is. (Note: Our club has the brief to run the town obs as a tourist attraction for our council, and recently the entire facility has been considerably upgraded. The upgraded 'scope gear is part of this facility, and not owned by our club.)

CS,
Paul.

Please let me know when your presentation/tutorial is ready for viewing. I'm sure quite a number of viewers of this discussion would be interested, too.