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Avoiding internal condensation in refractors (doublets, triplets, quadruplets)

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#1 db2005

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Posted 17 August 2019 - 01:32 AM

Hello all,

 

I'd like to hear your experiences with best practices for avoiding internal condensation in refractors.

 

Allow me out explain my question by outlining this scenario:

  1. A grab-and-go APO is brought outside from room temperature to colder air for an observing session.
  2. The air inside the refractor slowly cools down below the dew point...
  3. ... and moisture in the air condenses on internal surfaces, including the inside of the main objective lens.
  4. At the end of an observing session, the dew cap is put back on the dew shield to prevent fogging up the objective when the telescope is brought inside again.
  5. When the telescope is brought inside again after an observing session, the main objective lens is protected against fogging by the dew cap, and the cool, moist air inside the tube warms up again to bring the temperature above the dew point.
  6. Any dew inside the tube evaporates, but a small amount of dew residue (air contaminants?) possibly remains on the rear surface of the main objective lens.
  7. This is repeated for most observing session for a while, and the objective after a while seems slightly hazy on the inside.

Over the years I have cleaned the rear lens surface on several of my refractors due to finding the objective to be somewhat hazy on the rear element. Cleaning it removes the haze, but I'd prefer avoing having to do, so I'd appreciate if you would share your own experiences with this.

 

Have you experienced the same problem? How do you avoid it? I have heard that some observers prefer to uncap the telescope focuser and pointing the OTA down for a few minutes prior to each observing session to replace the warm air with cooler outside air, but I have until now been under the impression that this practice was mainly driven by the desire to cool down the telescope quicker, and wasn't motivated by concerns about dew. But I can imagine that replacing the warm, moist air with cooler air might be helpful for controlling internal condensation, too.

 

Any thoughts on this? For instance, how would this work with an APO triplet/quadruplet with widely spaced lenses keeping a large volume of air to be trapped inside the tube?

 

I recently acquired CQ 1.7 extender module for my Tak FC-76 DCU, turning it to an extremely nice, flat field quadruplet, but I expect the addition of the extra lenses also increases the potential for trapping moist air between optical surfaces.

 

I'd appreciate your thoughts!

 

CS

Daniel

 

 

 

 


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#2 PirateMike

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Posted 17 August 2019 - 02:39 AM

Get a dew heater. I have a Dew Buster, but there are less expensive options.

 

http://www.dewbuster.com/

 

 

Miguel   8-)

 

.


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#3 db2005

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Posted 17 August 2019 - 03:00 AM

Get a dew heater. I have a Dew Buster, but there are less expensive options.

 

http://www.dewbuster.com/

 

 

Miguel   8-)

 

.

Check waytogo.gif I already use a dew heater strip, but I'm not yet entirely convinced that the dew heater will keep the air inside the tube above dew point temperature.

 

But I guess if the dew heater will keep the lens (including the rear element) just above ambient temperature, maybe that would prevent condensation on the lens surface when the air inside the tube cools down?


Edited by db2005, 17 August 2019 - 03:01 AM.

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#4 Redbetter

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Posted 17 August 2019 - 03:47 AM

Ideally what you need is some intermediate location that has air that is as dry as the outside's cooler conditions, protected from the elements, but not necessarily as cool at the start.  This would be the place to store the scope before use.  Colder northern climates in winter and some other months can be quite dry outside, but are considerably more humid indoors where our breath, cooking/washing, showering, and sometimes humidifiers are used to keep humidity at a more comfortable level.   [Climate where I am now is the opposite, wet in winter, dry all summer, but I grew up with more traditional cold and dry winters.]

 

The idea of pointing the OTA down and uncapping the focuser is one I was going to suggest...without knowing that it was already a common practice.  It is quite logical as exchanging the humid air mass within the tube with drier ambient external air is what you are after.  With the focuser facing up (not necessarily fully vertical as it might not be the best angle for the natural convection pattern to establish itself since the two layers have to slip past one another in opposing directions) the warmer air will rise on the upper side and cooler air will slip down into the tube on the lower half of the focuser, slowly exchanging air masses.  I would try this.  I don't know how long it would take, but you could try some period of 5, 10, or 15 minutes and see what was sufficient and what wasn't.  If it doesn't work, or initially works but the weather/indoor humidity changes and stops working, then add another 5 minutes and see if that does the trick.


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#5 astronz59

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Posted 17 August 2019 - 04:05 AM

I made a 'dehumidifier' out of a empty plastic pill bottle. This is a neat fit in the eyepiece holder. Drilled holes in the bottom, filled with silica gel beads and replaced the cap. Stays in the eyepiece holder while I'm not using the telescope.......waytogo.gif


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#6 Stephanh

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Posted 17 August 2019 - 08:02 AM

I made a 'dehumidifier' out of a empty plastic pill bottle. This is a neat fit in the eyepiece holder. Drilled holes in the bottom, filled with silica gel beads and replaced the cap. Stays in the eyepiece holder while I'm not using the telescope.......waytogo.gif


+1 works well and it acts as a reminder to take your night meds. :-))
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#7 LDW47

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Posted 17 August 2019 - 09:17 AM

In my northern climate I don’t really have that big a problem but I do get a bit mostly on the outside of the front lens ! What I dew after a session is put my scope in a protected, covered area, take off the front cap and take out the diagonal and by the next morning any bits of moisture has dissipated by the air circulation ! Works well ! Sure am glad I don’t live in a more tropical climate but I do have the cold frosty conditions and I do the same thing !



#8 MalVeauX

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Posted 17 August 2019 - 09:20 AM

Dew heater.

 

If the optics are warmer than ambient, it won't dew over. The rest of the air in the tube isn't a problem. The glass itself just needs to stay warmer through the session and to the point of being packed up and taken inside (if this is a G&G). If it's going into a case or something, then warm the case and throw in some dehumidifier packs.

 

Here in Florida, our humidity is 99% every night by 10pm and we all have air conditioning running way cooler than ambient. So all our glass is instant dew over the moment we step it outside unless we heat it up prior and keep it heated while using it.

 

Very best,


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#9 drd715

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Posted 17 August 2019 - 10:00 AM

I made a 'dehumidifier' out of a empty plastic pill bottle. This is a neat fit in the eyepiece holder. Drilled holes in the bottom, filled with silica gel beads and replaced the cap. Stays in the eyepiece holder while I'm not using the telescope.......waytogo.gif

Best idea I've heard. You might consider a longer 2 inch container for a larger amount of desicant. We use a product called 13X or molecular sieve in the air compressor filters for scuba diving air tank filling. You can get it from Lawrence Factor Company (Miami area) on a air compressor after filter we usually discard the used product because it is contaminated with oils and water. But as a desiccant you can put it on a cookie sheet and bake to 300 degrees then reuse it. Store the product in dry air tight containers when not being used.

The only other thing I can think of is put a fitting on the ota to purge it with dry nitrogen or a bottled high pressure dry air (scuba tank) with regulator restrictor and purge the moist air out of the tube then store it in an airtight case with desiccant (an old cotton sock filled with 13x) the real secret is keeping the moist air from getting to the scope, not so easy here in Florida.

Sent from my SM-N950U using Tapatalk
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#10 junomike

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Posted 18 August 2019 - 07:02 AM

IME the best way to avoid any internal Dew is to MAKE SURE you don't leave the back of the OTA open for even a minute or so.

When changing eyepieces (or even Diagonals) always cap/cover the Focuser or Diagonal to prevent outside air from entering.

This had always worked for me.


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#11 Nippon

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Posted 18 August 2019 - 11:21 AM

Another Floridian here and can testify to  MalVeauX's comments. Our issue a lot of the year is reverse in that we risk dewing as soon as we uncap a scope outside. What I do is leave it capped start the dew heater and have a cup of coffee while I wait for it to warm up in the Summer. Winter is actually rarely a problem because there is little difference between outside and inside temperature. But when we have a cool snap and we do a dew heater takes care of dewing, actually in Florida a dew heater is needed year around, and I have never seen an objective dew on the inside during a session. But I have when the cold scope is brought back in the warm house. I make sure to keep my lens cover in a dry place in the same temperature as the scope. When it's time to come in I cap the scope with the dew shield still extended so I don't disrupt the climate inside the dew shield and bring the scope in, lay it on the sofa and just leave it to warm up slowly


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#12 John Fitzgerald

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Posted 18 August 2019 - 12:06 PM

In summer, put the G&G (still in its case) outside an hour or two before starting the session to allow it to warm up slowly from being in A/C inside the house. 



#13 Redbetter

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Posted 18 August 2019 - 02:19 PM

The problem as described is the opposite of Florida.  The air in the tube has a higher water vapor pressure than the outdoor air in the described case.  Unlike on the outside, it is trapped within, so to keep it from dewing while in use one will have to keep the optics hotter while in use rather than just keeping them at ambient temp (dew heaters are to prevent subcooling of the glass relative to the ambient.)  Hotter isn't conducive to a good image.  When I bring out my 110ED and the outside temp is 10+ F cooler it takes 15 minutes or so for the image to stabilize at 150 to 200x, the damage for planetary is pretty obvious.  I sure wouldn't want to have to keep the glass hot to prevent condensation inside the tube.

 

That is why it makes more sense to me for this case to exchange the air in the tube with what sounds like drier outside air by tilting the scope and opening the focuser for a few minutes.  [The relative humidity of that indoor air was likely lower than the outside relative humidity, when at the indoor temperature; but when taken outside that trapped air cools and its relative humidity reaches the dew point and exceeds the relative humidity of colder outdoor air.]  The dew heater should work fine after that since the air inside will be of similar humidity to the outside.  One will only need to keep the glass from subcooling substantially relative to the ambient rather than keeping it hotter because one air space has a higher dew point.


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