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Eyepiece cool or not?


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As some observers use heaters on their eyepieces I was wondering about the temperature of the optics vs the same temperature of the rest of the glass in the optical chain.

Is it best for views if the eyepiece is the same ambient as the diagonal and telescope optics or not?

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It doesn't really matter because the glass in them is generally small enough that temperature differentials don't cause stresses in the glass that would distort the image.  The reason people put dew heater strips on them is to prevent them from dewing up when they cool below the dew point.

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And, we aren't actually talking about making them hot, or even warm.  If done right, the eyepiece is merely heated up to a temperature slightly above dew point.

which is usually still less than room temperature.

This will, of course, alter the wavefront passing through the eyepiece because it is not at the ambient temperature.

But, as is the case with heating a secondary mirror or star diagonal, if the alternative is no observing at all, it is a small sacrifice to make.

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I keep mine a bit above the outside temperature during a session. They are either inside or in their foam lined boxes. The foam seems to retain the heat a bit - keep the lib down between eyepiece changes and that seems to do the trick.

If an eyepiece gets cold it tends to mist up in the colder weather as your eye approaches the eye lens. Very frustrating as I've found when I've left an eyepiece in the scope for sometime during a cold session.

 

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I've recently taken to covering the glass surfaces then putting the eyepieces in my jacket pocket whilst swapping between them during a session. There's just enough body heat gets through to warm them up sufficiently to avoid fogging problems. Ten minutes in my pocket is looking enough to achieve this.

Make sure there's nothing else in your pockets to damage the eyepiece though!

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On 10/09/2020 at 14:46, Louis D said:

It doesn't really matter because the glass in them is generally small enough that temperature differentials don't cause stresses in the glass that would distort the image.  The reason people put dew heater strips on them is to prevent them from dewing up when they cool below the dew point.

Yes  to stop dew, or in my hazy recollections of 80's Winter observing, i actually had a eyepiece or two  frost over 😟

Back then the EP case was pretty minimal so  effectively end of session!

.... & I actually stuck to the EP one night. (rubber eyecups were then not the norm)

I guess Stateside it still can happen?

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Frost is a common problem at the high altitude site I frequent.

Often in the winter, I have to wait for the sun to melt the ice off all my equipment (chair, table, battery) before I can pack it in the car and drive home.

Eyepieces, though, never frost up if they are left in a closed case until used, then returned to the case after use.

they can fog up from your breath, but a small fan will solve that.  I've taken to waving a small Japanese folding fan at the eyepiece when it fogs, and it always unfogs the eyepiece.

You have to be careful not to breathe on the eyepiece when using them in cold weather.

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