Dew Heaters and Fighting Humidity

17 replies699 views
Chase Davidson avatar
Hello everyone,

Apologies to the beginner level question in advance. I've been imaging in south Texas for the last year and lately I've experienced issues with humidity and my lends gathering condensation around 3am, so 2 hours of shots are wasted. I have recently upgraded from a 51mm refractor to a 130mm refractor capturing with a ASI2600mm camera cooled to 0 degrees C (i'd go lower but -10 degrees makes my ASIAIR run at 100%). The cheap amazon dew heaters worked well for my small rig but now they don't seem to make any difference.

Has anyone else experienced this issue or have any ideas to resolve it? If it's as simple as buying better dew heaters or a heater controller, are there any recomendations for a 130mm scope?

Thanks,
Chase
Respectful Engaging
Vasile Unguru avatar
Hi Chase,
you will need a dew heater and a controller for it. something like this https://www.astroshop.eu/heater-bands/astoptics-dew-heater-60cm-6-telescopes-/p,47199 and controller for it; there are a lot of options in the market but something like like this https://www.astroshop.eu/controls/pegasusastro-dual-channel-dew-heater-controller-dewzap/p,51918 will be more than enough. There are also the much more expensive power boxes which are capable of driving the dew heaters automatically based on the external temperature and humidity. The choice will be yours. Make sure you have good power source in very humid nights.
Cheers 

CS,
Vasile
Helpful
Chase Davidson avatar
Vasile Unguru:
Hi Chase,
you will need a dew heater and a controller for it. something like this https://www.astroshop.eu/heater-bands/astoptics-dew-heater-60cm-6-telescopes-/p,47199 and controller for it; there are a lot of options in the market but something like like this https://www.astroshop.eu/controls/pegasusastro-dual-channel-dew-heater-controller-dewzap/p,51918 will be more than enough. There are also the much more expensive power boxes which are capable of driving the dew heaters automatically based on the external temperature and humidity. The choice will be yours. Make sure you have good power source in very humid nights.
Cheers 

CS,
Vasile

Thanks Vasile, I’ll get some new dew heaters and a controller and let you know how it goes.

thanks,
chase
Ashraf AbuSara avatar
I image from south Texas too and in fact I have the exact same scope as yours. I have been using the SV192 dew heater strip from Svbony and it seems to be working well for me. Typically I just set it to about 40-50% power on the ASIair.
Concise
Anthony (Tony) Johnson avatar
Used this guys stuff for my 12”SCT since he went into business back in the 90’s, great stuff and even better now. At the time was the premier dew heater supplier. I have the controller, bands for my 12” my ETX125, and eyepiece heater. Used them for over 25 years with no problem at all.
https://www.kendrickastro.com/index.html
apennine104 avatar
For a cheaper than a standalone controller, but better than Amazon option in my opinion, I use theZWO Y-Cord which allows for two RCA style heaters (I use Dew-Not) to be plugged into one of the ASIAIR's 12V ports. If you have an ASIAIR Plus, it then allows for PWM (%) control of the heaters similar to a standalone controller. If your Amazon heaters were USB, the downside there was it was 5V vs. 12V, which makes a large heat difference per given heater resistance.
Helpful
JohnAdastra avatar
Chase,

In the NE USA where I am, dew is a major problem and dew heaters are a must every night. In the winter there is frost too but that's probably not a concern for you.  I use the DewNot bands and controllers but have also have used Kendrick accessories in the past. The band should be wrapped around your OTA as close to the objective lens as possible for best heat transfer.

But besides the advice already given, an extended dew shield will also help keep moisture away. A longer path from the outside air to the lens is the idea. You can get some cheaper flexible ones that bend into a tube secured by Velcro which will work just fine.

But as far as heater control, you get a feel for the settings by looking at the dew point versus outside temperature, as given on weather websites or Astrospheric. When the two values start getting close, as in the morning hours, that's when condensation will occur and your settings should be increased. There are controllers which also measure dew point too, but they are more expensive. Shedding the dew is key to extended imaging sessions and not having the droplets collect dust and spot the lens, which can wreck your flats if done post hence.

Good luck!

John
Helpful Engaging Supportive
Matteo Beretta avatar
In the NE USA where I am, dew is a major problem and dew heaters are a must every night. In the winter there is frost too

Some nights there's frost even here in Italy 🥶

I use a DIY dew heater based on this project (you can see a spreadsheet to calculate the sizes) and always a dew shield that helps to reduce light leakage too.
I’m using a PWM motor speed controller to manage the power, but in future it will be managed by Arduino linked to a meteo station and a temperature sensor.
Helpful Engaging
Dan Brown avatar
Some people wrap their dew heater around the dew shield. I've always felt that it is more effective to wrap it around the telescope tube just behind the lip of the dew shield. This config will allow better conduction of the heat to the objective.
Dan
Well Written Insightful Respectful Concise
George  Yendrey avatar
I'm in South Texas as well and I can tell you humidity/dew is a problem except for in Winter (mostly).  I purchased my velcro strap type dew heaters from High Point Scientific and use a Pegasus Astro Ultimate Power Box v2 (the current model is v3 but doesn't effect the dew heater control).  The UPv2 has its own environmental sensor that velcros to the OTA (the UPBv2 is mounted on the OTA framework).  That enables the 'smart' dew heater control that Pegasus Astro has so that it matches outbput to the dew heaters based on the environment.
Helpful
Bill Powers avatar
I have been using Kendrick dew heaters and controllers for over twenty five years with great results.
Well Written
Quinn Groessl avatar
I also recommend the Dew-Not brand heaters. I can for sure say they were working the other night when the glass on my camera dewed up in the early morning hours, but my scopes were dew free.
John Hayes avatar
Dan:
Some people wrap their dew heater around the dew shield. I've always felt that it is more effective to wrap it around the telescope tube just behind the lip of the dew shield. This config will allow better conduction of the heat to the objective.
Dan

Unless there is fog, dew forms on the front optical surface because of radiative heat loss to the sky, which lowers the temperature of the front surface below the dew point temperature.  The most effective way to protect the front surface is to heat the shield itself.  A properly heated dew shield replaces the heat lost to the sky to hold the temperature of the surface at or above the ambient air temperature.  Because radiative heat loss can also occur on the sides of the telescope, dew can form internally so it’s a good idea to either wrap the tube in something like Reflectix, to add a heater or two (set to very low current), or to do both.  Your approach of heating the tube behind the objective will certainly help prevent internal dew.  However, applying heat behind the object to conductively heat the front surface is a poor way to prevent dew—particularly with an air spaced objective lens.  An air-spaced objective acts like a double pane window to prevent much heat from getting through the gap to the front surface.  In general, relying on thermal conduction to heat an element requires more power than simply replacing radiative heat loss—and the total amount of heat applied ultimately goes into producing turbulence in the imaging path, which is bad for image quality.

I’ve had a paper with the calculations showing all this stuff works sitting 80% finished on my PC for years.  I’m going to have to finish it and get it posted so that you guys can see more clearly how all this stuff works.

John
Well Written Helpful Insightful Engaging Supportive
Greg McCall avatar
John Hayes:
Dan:
Some people wrap their dew heater around the dew shield. I've always felt that it is more effective to wrap it around the telescope tube just behind the lip of the dew shield. This config will allow better conduction of the heat to the objective.
Dan

Unless there is fog, dew forms on the front optical surface because of radiative heat loss to the sky, which lowers the temperature of the front surface below the dew point temperature.  The most effective way to protect the front surface is to heat the shield itself.  A properly heated dew shield replaces the heat lost to the sky to hold the temperature of the surface at or above the ambient air temperature.  Because radiative heat loss can also occur on the sides of the telescope, dew can form internally so it’s a good idea to either wrap the tube in something like Reflectix, to add a heater or two (set to very low current), or to do both.  Your approach of heating the tube behind the objective will certainly help prevent internal dew.  However, applying heat behind the object to conductively heat the front surface is a poor way to prevent dew—particularly with an air spaced objective lens.  An air-spaced objective acts like a double pane window to prevent much heat from getting through the gap to the front surface.  In general, relying on thermal conduction to heat an element requires more power than simply replacing radiative heat loss—and the total amount of heat applied ultimately goes into producing turbulence in the imaging path, which is bad for image quality.

I’ve had a paper with the calculations showing all this stuff works sitting 80% finished on my PC for years.  I’m going to have to finish it and get it posted so that you guys can see more clearly how all this stuff works.

John

John,
id be really interested in that article.
Even better if I was able to include it in our local magazine or pass it onto Astro society members. 
cheers
Greg
Michael Gruenwald avatar
John Hayes:
...dew forms on the front optical surface because of radiative heat loss to the sky...

 OTBS warning. (Outside The Box Solution) 

Totally agree with John Hayes, so I am not wrapping a heater strip at the objective cell, or around the dew shield, and here is why:
John correctly states heat loss at the front lens is radiative, so it seems counterproductive to heat the shield to heat the objective lens surface.

Radiate right back at it! Therefore I 3D printed a contraption that sits on the inside of the dewshield, outside of the light path, obviously.

Said contraption holds some NiCr heating wire that directly radiates "down to the lens" and replaces that heat loss using some 4W, around 0.3A at 12V. easy and cheap. that whole thing cost me maybe an hour of work, and certainly less than 10 bucks.... and what I think is important, its clearly an OTBS.

There is more dew around telescopes, though. Especially here in the middle of Arkansas. Especially when one is using the Risingcam TR3 (imx571 sensor). A great camera but that thing tends to form dew on the sensor window quite quickly when cooled. The idea is to use a day with low humidity, open the system to replace the humid air, and to trap dryer air in the volume between field flattener and sensor, and that includes the filter wheel. there are plenty of air gaps in this system to allow within just days enough humid air to intrude to create that dreaded dew signature, and ruin the night.

OTBS:
first:        seal the gaps. tape. caulk, teflon tape for the adapter threads.
second:  drill a hole in the filter wheel housing, try not to hit the stepper
third:      insert an iv set with 3way valve into that hole
fourth:   instill welding gas Argon (I am a hobby welder)  into that now closed chamber.  Problem solved. now I need a refill about every 100 days...
               (Welding argon is extremely dry, less than 0.004% water content. unfortunately Nitrogen, Butane Propane and similar industrial gasses are "wet")

sounds like BS but its not. its OTBS

Mike
John Hayes avatar
Mike,
That's a good approach but you are probably pumping in way more power than you need and all that excess power gets converted into turbulence, which usually isn't good.  To understand this, I'm going to leave out the math and reference just a couple of examples excerpted from my paper:

Example: Radiative heat loss to the sky from a C14 corrector plate
We can get a feel for how much heat flows to the sky if we look at how much heat flows from a C14 corrector plate if it is pointed straight up and exposed to a full hemispherical sky.   We assume that the corrector plate has an emissivity of 93% and its front surface is at an ambient temperature of 5C and that the sky has a temperature of -20C.  At that temperature difference, the corrector plate will experience a radiative heat loss of 9.83 W to the sky.  This heat loss is why the front surface of a corrector plate (or any other surface) directly exposed to the sky will have a lower temperature than the surrounding air and it is why dew will form under a clear sky before fog forms.

Example: Reducing radiative heat loss to the sky using a dew shield kept at the ambient air temperature
The purpose of a dew shield is to limit the angle of the sky visible from the front element of the telescope simply to reduce heat lost to the sky.  A common recommendation is to make a dew shield 1.5 times as long as it is around.  With those dimensions, the half angle of the opening is 18.4 degrees and the net power lost from the front surface to the sky will be just 10% of that lost without a dew shield.  Making the dew shield only as long as it’s diameter will reduce the net power lost to the sky to 20%, which is two times less effective as making the shield just half again longer!  Making the shield twice as long as its diameter reduces the loss to the sky to 6% but at that length, it begins to make the telescope more susceptible to wind—particularly for larger aperture scopes.  So, there is some wisdom behind the common recommendation of making a dew shield 1.5 times as long as its diameter.  Still, if dew is a major problem and there’s no wind, remember that a long dew shield is better than a short one.  Just remember that this result assumes that the temperature of the dew shield is held at the ambient air temperature and that the shield itself is not cooled by radiative heat transfer.  That can be accomplished by making the outside of the shield highly reflective and by insulating the shield to support a higher temperature differential between the inside and outside surfaces.

For the C14 example, a heated dew shield with an aspect ratio of 1.5 only needs to uniformly radiate about 1W to exactly replace the radiation lost to the sky.  If you have a 125 mm refractor with the same aspect dew shield, the heat loss goes down to only about 1/8 W (the heat loss scales with the area).   If you compute the temperature rise needed at the dew shield to exactly compensate for the heat loss, the size of the objective drops out and the result looks like the following:



The take away is that you only need to raise the temperature of the dew shield with an aspect ratio of 1.5 by about 3C to replace the radiative heat loss to the sky.   You may want to pad it by maybe +1C and it's very important to make sure that radiative heat loss from the tube doesn't create additional cooling from the inside of the scope, which is why I generally recommend insulating and adding just a little heat to the tube.  In your case, 4W will certainly keep your system free of dew, but I suspect that if you turned the power down to say 0.2W, it will work just as well.

John


PS. I have to point out that it doesn't matter whether the radiated heat comes from a nichrome wire or the dew shield itself.  You didn't say how your wire is arranged but in the case of a single circular turn of  wire, the temperature of the wire is going to have to be considerably higher than a heated dew shield to dissipate the same amount of power.  Concentrating the heat into a single wire will almost certainly produce both more turbulence due to the Reynolds number of the wire.  Whereas heating the shield is more likely to produce a laminar flow--up to a certain point.  Turbulence right in front of the objective produces the "shower curtain effect", meaning that it has a bigger effect on image quality than if the turbulence were from a more distant source.
Well Written Helpful Insightful Engaging
Michael Gruenwald avatar
Great post, and I am geeky enough, would love to see the math…

I had a more hands on approach, as most dew shields for my 5” refractor have somewhere between 15 and 20W I thought I should be able to get away with some 25% of that value…

design detail: my 3D contraption is a ring that contacts the dew shield at three points only. The air gap behind that avoids 1) heat loss to the dew shield, and also gave me some space for a Mylar strip that again reduces energy that hits the dew shield, but also reduces loss towards the sky…

i will build a 0.5W version and report back….

turbulence. Of course a major concern, but I am sure a 20W heater creates more than a 4W. In that context I never understood the RAZA concept, that puts the cooled  camera as little heat source right in the middle of the light path. 25W in the case of a ASI1600…

seems like the effect on image quality is negligible…

thanks
John Hayes avatar
turbulence. Of course a major concern, but I am sure a 20W heater creates more than a 4W. In that context I never understood the RAZA concept, that puts the cooled  camera as little heat source right in the middle of the light path. 25W in the case of a ASI1600…

seems like the effect on image quality is negligible…

thanks

You are right about RASA; however, it’s a wide field system that is almost always under-sampled.  RASA and Hyperstar imaging is all about field and although you want a crisp image, achieving seeing (or optics) limited performance isn’t a priority.  Every optical system has pros and cons and having a cooled camera that exhausts heat in the middle of the entrance pupil is clearly a con for these types of imaging systems.

I’m pretty busy right now but this thread (along with your comments) is an inspiration for me to resurrect my book chapter.  I’ll see if I can pull together an extract that is appropriate to post here.  When I get it done, I’ll put it in it’s own post so stay tuned.  It has a lot of good stuff and plenty of math for all of the geeks among us!   )))

John
Engaging