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correctors and dewpoint

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#1 Mike G.

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Posted 26 June 2019 - 11:21 AM

Ok, so everyone knows that with correctors and summer, we have to deal with dew.  my question to the enlightened people of CN, is what is the typical amount of temp difference that a corrector will cool when exposed to the sky?  if I can get some idea of how much lower temp my corrector will be than the air temp, then I can decide whether I want to use dew heater/shield because I can know the dewpoint from published weather info.

 

I understand that how high he scope is pointed and the size of the corrector will both affect the amount of heat that radiates form the corrector at night when exposed to the sky.  but is there some rule of thumb or some simple way I can say, 'well the dewpoint will be 8 degrees lower than the airtemp - I should/shouldn't need my dewheater'.

 

local temps of course vary from predicted temps, and local geography has a huge impact on this as well.  but just would like to know what typical radiative losses are in degrees if it's a viable question.

 

thanks



#2 jhayes_tucson

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Posted 26 June 2019 - 12:33 PM

I’ve done a fairly careful calculation to answer this question but I still haven’t made any measurements to confirm the results.  This is a tough calculation because it has to rely on a sky model to determine the temperature of sky—and there are a LOT of different sky models (around 100.)  The folks working on passive cooling systems have made a lot of measurements to determine which models work the best and the data doesn’t always agree.  Still, there are some fairly widely accepted models and I used one of those.  Assuming that your system is completely passive (meaning no heat is being applied,) the answer depends on a LOT of factors including clarity of the sky, ambient air temperature, relative humidity, dew shield length and azimuth angle.  In general, my calculation shows that the temperature of front surface of a corrector pointed straight up without a dew shield can fall by as much as nearly 7C relative to the ambient air temperature.  Again, I won’t claim high accuracy to that number but it seems reasonable.  

 

BTW, I looked through a lot of papers to try to confirm this number and I couldn’t find one that answered this relatively simple question.  That doesn’t mean that it’s not out there but it’s actually a hard thing to compute with any accuracy and no one seems to have addressed this specific question.  I have in mind a simple experimental configuration to make some measurements to confirm my results but I’ve never gotten around to gathering all the equipment and getting it done—but maybe I will one of these days.

 

I’ve handled the question that you’ve asked about when to turn on the heaters by building an Arduino controlled “dew predictor” to automatically activate my heaters when conditions exist for dew formation.  The Arduino measures the temperature of the front of the corrector plate with a black temperature sensor positioned right next to the corrector surface along with the ambient air temperature and humidity using a “precision” RH/temperature sensor.  When the temperature of the corrector sensor is within 6 deg C of the predicted dew point temperature, the system automatically turns on a Dew Buster temperature controlled heater system.  The system is “latched” so that the temp-dewpoint spread has to fall to 7 degrees before the Dew Buster is switched off (once it’s been turned on.)  I use two heater straps on the dew shield and one behind the corrector plate on the OAT—mounted under Reflectix.  I also circulate air in the OTA with Tempest fans.  This whole arrangement has worked extremely well to prevent dew under some really tough conditions at my remote location.  Furthermore, it minimizes air local turbulence to allow imaging while the system is running.

 

John


Edited by jhayes_tucson, 26 June 2019 - 01:29 PM.

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#3 Mike G.

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Posted 26 June 2019 - 12:42 PM

thanks John, I was actually hoping you would respond as I have read many of your posts on similar topics and I consider yours an expert opinion.  8C seems quite a lot for a piece of glass that should seemingly have low emission characteristics.  but apparently that is an incorrect assumption.  still, I would guess if I use that number (about 15F) I'll probably be pretty safe if I leave the dew heater back int he house and the dewpoint is larger than that.

 

again, thanks for the reply and all of your extremely helpful and informative posts!

 

Mike



#4 jhayes_tucson

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Posted 26 June 2019 - 01:28 PM

thanks John, I was actually hoping you would respond as I have read many of your posts on similar topics and I consider yours an expert opinion.  8C seems quite a lot for a piece of glass that should seemingly have low emission characteristics.  but apparently that is an incorrect assumption.  still, I would guess if I use that number (about 15F) I'll probably be pretty safe if I leave the dew heater back int he house and the dewpoint is larger than that.

 

again, thanks for the reply and all of your extremely helpful and informative posts!

 

Mike

 It's been a while since I worked on this problem so I went back to confirm my number and as usual, my memory was wrong.  The drop actually approaches 7C not 8C.  (I'll correct my post above.)  Here are the plots showing the absolute temperature drop and the drop relative to the dew point temperature.  As you can see, anytime the RH is above about 55%, dew can become a problem and above 90% it's always a problem.  Remember that this is for an unheated corrector without a dew shield pointed straight up.  I've been meaning to get back to the article that I have about half done on this subject so I'll see if I can find some time to get it done.  It shows results for both heated and passive dew shields of different dimensions as well as discussing the physics behind the whole thing.

 

John

Attached Thumbnails

  • Thermodynamic Temp Drop of Front Surface.jpg
  • Thermodynamic Temp Drop Relative to DP.jpg

Edited by jhayes_tucson, 26 June 2019 - 01:31 PM.

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#5 Mike G.

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Posted 26 June 2019 - 02:15 PM

Although I’m a strictly visual guy, I do always use dew shields, more to prevent incident light from the neighborhood getting into the OTA than to prevent dew. We have lots of streetlights and porch/patio lights that are high enough they noticeably affect the image. A dew shield kills 99% of that. Unless I’m really low in the sky which I try to avoid. 

 

Thanks again!

Mike


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#6 jhayes_tucson

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Posted 26 June 2019 - 02:24 PM

Mike,

That's good.  Aside from the stray light issue, a passive dew shield is very effective at preventing dew--all the way up into the mid to high 90% RH range.  The longer the better, although a long dew shield will act like a sail in the wind on a large scope.  The next step up from there is to heat the shield.  It does't take much heat to be quite effective.  Heating the OTA tube itself can also be effective but that can introduce other problems and that's where the controversy often begins so we'll leave it at that.

John



#7 whizbang

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Posted 26 June 2019 - 03:09 PM

Here's my "no math" system:

 

Always install a dew shield and separate heater strip.

 

When you notice the shield getting damp, turn on the heat strip.



#8 HxPI

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Posted 29 June 2019 - 02:29 PM

Here's my "no math" system:

 

Always install a dew shield and separate heater strip.

 

When you notice the shield getting damp, turn on the heat strip.

Can I just turn it on and leave it on or will that be too much heat?? I’ve always believed it is easier to prevent dew than remove dew!

 

Ciao,

Mel



#9 whizbang

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Posted 29 June 2019 - 05:16 PM

In the wintertime, I actually run the heat strip full time, full power.  NO controller, just on or off.  Yeah, it is easier to prevent than eliminate.

 

In Seattle, once stuff gets wet, it stays wet.  The scope is wet, the tripod is wet, your observing chair is wet, your eyepiece case is wet, your PC or tablet are wet, YOU'RE wet.  With the dew shield and strip doing their job, the corrector is dry.  So what?  It's time to pack it up.


Edited by whizbang, 29 June 2019 - 05:17 PM.


#10 wrvond

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Posted 07 July 2019 - 10:37 AM

I have an app on my phone called Astrospheric that shows all kinds of useful information, including temperature forecast and dewpoint.

My take is that when the lines for temperature and dewpoint meet, the dew heaters should already be on.

 

IMG 1796


#11 Mike G.

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Posted 07 July 2019 - 09:21 PM

I use Astrospheric as well. What I was looking for was how close they need to be before the heater needs to be on. Looks like 10-15 degrees F is as close as you want to get without a dewheater 



#12 Napp

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Posted 07 July 2019 - 09:28 PM

I'm in Florida so dew is a constant.  Dew shields are a must as are dew heaters.  When I use my refractor or SCT I use a dew heater.  Generally I start the night with the controller set at about half power but almost always must go to full power by midnight.




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