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I'm wondering what the procedure would be to clean the eyepiece of a telescope when it looks foggy or there's dirt either on the outside or inside it.

I can get rid of loose dirt with a camel hair brush or air. But how can I clear foggy eypieces? Or what should I do if somehow anything got into the eyepiece?

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    $\begingroup$ Can you post a photo of the eyepiece? How old is it? "Foggy" could be fungal growth, or maybe the coatings have disintegrated, or maybe even condensation (unlikely). When I clean my eyepieces I first use air, then a brush, and finally the Baader "Optical Wonder" lens cleaner on a cloth. $\endgroup$
    – Aaron F
    Commented May 25, 2022 at 15:15
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    $\begingroup$ @AaronF OK thanks. It's not really the case for my eyepiece, I was just wondering what I should do if it were to be. It's related to another question I posted (that my telescope looks blurry for some eyepieces) and people said it could be that the eyepieces dirty/foggy. But I don't think that's the case. But someone asked me anyways to ask that question seperately (how to clean an eyepiece). $\endgroup$
    – 299792458
    Commented May 25, 2022 at 16:56
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    $\begingroup$ ah I see, it's this one: astronomy.stackexchange.com/questions/49410/… As well as checking the collimation, another thing to check is the 'seeing': meteoblue.com/en/weather/outdoorsports/seeing If the value for seeing (in arc seconds) at your location is too high (over ~1.5) then things won't appear sharp. Try again when the seeing is <1.0. $\endgroup$
    – Aaron F
    Commented May 25, 2022 at 18:15
  • $\begingroup$ @299792458 If you've had some success (or not) please feel free to leave an informative answer. $\endgroup$
    – uhoh
    Commented May 27, 2022 at 0:43
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    $\begingroup$ The cleaning method is going to rely quite heavily on what's causing the "fog". Trying to detemine what it is and a method of removing it without affecting the coatings on the lens would likely cost more than the eyepiece. We had some success using an ultrasonic cleaner, but not 100%, and not really cost effective if you don't have one already. Unless it's a really pricey eyepiece, I think your best bet is to grab a cloth and hope for the best. $\endgroup$ Commented May 27, 2022 at 16:11

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Since you haven't mentioned what eyepiece you are using, I will go with the common scenario. Fog, fungus or dirt on outer part of the lens are easy to clean, air then a brush or a microfiber cloth is enough to do some good job. While getting inside of eyepiece, most eyepieces come with screw type lens mount, you can find some grooves in the eyepiece's lens mount that will help you figure out whether you can remove the lens to clean. You can distinguish between "fog" (haze) and fungus, fungus usually appear like small "streaks" or "scratches" or a patch in a particular area, where as in haze the eyepiece would become translucent. Haze is caused by several reasons. Best way to clean optics of eyepieces is to use some compressed air, then take a microfiber cloth give the lenses a little clean then clean the lenses with solution of distilled water and isopropyl alcohol (100%) with 1:2 concentration respectively, this solution do not erode any of the coatings on the eyepiece. (note: only a drop on the microfiber cloth, in case if your eyepiece lenses are plastic, use only distilled water to clean). I don't recommend using brush for inner part of the lens.

In case if you need help in removing the eyepiece. Here is a demo what to do

Take your lens. here I marked the barrel

enter image description here

Mostly you will be able to unscrew the barrel or the barrel will be held by two clips. after removing it. you will see two grooves to unscrew the mount to remove one of the two lenses in the eyepiece. I have marked the two grooves in the picture below.

enter image description here

enter image description here

Then keep the lens on your palm (will the opening tightly closed) a little bit of shake will get the lens(es) detached from the assembly

enter image description here

you will get the lens in hand for better cleaning. The ring next to the lens is used to separate the two lenses. Not all eyepieces have this. (Please keep the lenses on a soft towel even when cleaning, I don't don't keep it on other surfaces, I have kept it on my laptop's palm rest since I don't have soft towel right now)

In case if you the top lens doesn't detach, then you can use a brush and an earbud with drop of the solution

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I wonder if you're seeing condensation?

I ride a bike and wear glasses. In winter there's frequently fog issues when warm breathy air hits a cold lens surface, or a warm surface goes into a cold environment.

Is it possible your kit is not acclimated to the ambient temperature?

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