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Impact of dust and dew on filters and optics?

Beginner Equipment Eyepieces Filters Maksutov Optics Refractor
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#1 Escape Pod

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Posted 19 October 2021 - 03:29 PM

TLDR: Dust and Dew. 

  • How worried should I be about dust on a nebula filter, or dew stains on my objective lenses?
  • Are the black spots in my unsealed eyepieces really mold? Should I avoid buying such unsealed eyepieces in the future, or how can I most effectively prevent mold from forming in the first place?
  • Does everyone really wrap their telescopes and gear in big plastic bags when moving from indoors to outdoors in areas with humidity? I often drive to and from observation locations. This would be a real blow to my grab-and-go process.

Three unrelated events have me wondering (and worrying) about the proper care of telescope optics. 

 

The first is my purchase of a used 2" Lumicon Oiii filter from CN classifieds for $155. The seller disclosed to me that it had some dust spots on it, but upon inspection there were more than I expected. I took the rubber rocket bulb air blaster that I use for my photo gear to it, but most of the dust persists. 

Overall the filter and its coatings appear to be in great shape, but I am wondering how dust spots that late in the optics train will impact final images. I know that when I have spots on the diagonal those often jump out when put against bright targets like the moon. Are nebula filters more forgiving?

 

Second, while observing the full moon last night I noticed several dark spots in two of my cheaper, non-sealed and non purged eyepieces, the Orion 7-21mm zoom and a Paradigm 8mm. I tried blasting them with air also, in case it was just dust. But to no avail.

 

I am concerned that it may have been caused when I got ambushed by a wave of evening dew a week or so ago. I took great care to dry out my OTA, binoculars, and my eyepieces before putting them away. Is it possible that these less-expensive, non sealed and purged eyepieces developed internal moisture and mold from the fog event? By comparison, my Explore Scientific 82 eyepieces look perfect. 

 

Third, as a result of the dew assault, the objective lenses of my telescopes and binos did develop some dew stains. I know many folks on here advocate against ever touching objectives, but I was surprised to see just how many folks in this thread (virtually all) advocated attacking such spots with microfiber cloths, distilled water, and perhaps a specialty solution that is ammonia-free. https://www.cloudyni...f-how-to-clean/

 

 

 

 



#2 TOMDEY

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Posted 19 October 2021 - 06:33 PM

Well... those things are concerns... but not really worth losing sleep over. Optics --- especially optics that actually get used ~in the field~ will eventually get somewhat blemished. The best investment is decent cases with fresh desiccant... and storing that stuff in a controlled environment, like in the house, not the garage or basement. I keep stuff in the domes, but my little secret is Tupperware containers (not the cheap substitutes) that sit on dog bed heating pads (extremely low wattage just enough to raise the temp slightly above ambient). Those draw just a few watts and stay on... keeping everything above the dew point, even though the observatories are otherwise ambient. Even my telescopes have that little blush of warmth --- always.

 

I learned that approach when stationed in the jungles of Panama back around 1970. I was collecting insect specimens for the boss (insect vision research) and those few watt heaters could keep the lockers pristine. To clean your stuff just use the standard simple procedures.

 

Far more optics have been ruined by aggressive overcleaning than anything else.

 

PS: If you choose to use chemical desiccant --- refresh it as needed. Otherwise it becomes worse than none... can become a saturated water sink not unlike a wet sponge! At that point your container can turn into a steam bath!    Tom


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#3 Escape Pod

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Posted 19 October 2021 - 06:41 PM

Thank you so much for the thoughtful reply, Tom. Several genius hacks in here. Fresh desiccant, blushes of heat, and “honey, I need to build an observatory” :D


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