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Do EPs need to cool down?

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#1 Tropobob

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Posted 25 May 2019 - 05:59 AM

The conventional wisdom is that telescopes need to cool down; does this also apply to EPs?

 

Should I be leaving some of my EPs outside before beginning an observering session?


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#2 db2005

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Posted 25 May 2019 - 07:09 AM

Not in my own experience; actually I prefer to keep eyepieces in my coat pockets to prevent them from cooling down too much during an observing session. It they cool down too much they tend to attract dew during use, and I don't want to use dew heaters for my eyepieces. I have never experienced any adverse effect on optical performance by keeping the eyepieces somewhat above ambient temperature. However, I rarely use extremely large eyepieces (Pentax XWs are the largest eyepiece that I use on all observing sessions), and I don't know if thermal issues could cause problems with larger eyepieces. Some people report that inserting an eyepiece warmer than the telescope can cause tube currents.

 

I find it more important to leave the star diagonal outside in the scope for acclimatizing.


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#3 izar187

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Posted 25 May 2019 - 07:28 AM

The conventional wisdom is that telescopes need to cool down; does this also apply to EPs?

 

Should I be leaving some of my EPs outside before beginning an observering session?

I second the vote for ep's in pockets when at a scope.

I have done this for decades.

Heat in the ep case works too, via heating pad in or under the case, or a hand warmer in the case.

It doesn't take a lot of heat. 


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#4 TOMDEY

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Posted 25 May 2019 - 07:29 AM

Good question! And the answer is pretty much "No"; the temp etc. of the eyepiece doesn't degrade the view. The reason we are immune is that each field point uses only a tiny footprint on each element within the eyepiece... so the image will look sharp, regardless. On the other hand, the image of the System Aperture Stopping Element (most often the Primary Mirror)... what we call the ~Exit Pupil~ --- that gets degraded by an athermalized eyepiece... but that does not hurt imagery.

 

[There are a lot of funky things going on in optical systems, as used --- we amateurs (and most professionals) are blissfully unaware of most of those gremlins. And, thankfully, most of those don't attack us.]    Tom


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#5 bbqediguana

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Posted 25 May 2019 - 07:30 AM

I don't suspect so - the issue with cool down in telescopes is thermal currents that appear and disturb the view as the scope acclimatizes to the temperature difference. I don't believe you will experience this within the eyepiece itself.

 

Having said that, I usually put my scope and my eyepieces (in their small case) outside prior to viewing. Like db2005 and izar187 above, I also tend to stash my eyepieces in my pockets while observing to keep them from dewing up. 

 

I have had winter sessions where I got the scope and all the equipment nicely cooled down to like -20°C and then when I look in the eyepiece it instantly fogs up due to my body heat! LOL! For those situations, I use my dew straps to actually introduce heat to the eyepiece.


Edited by bbqediguana, 25 May 2019 - 07:32 AM.

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#6 Ernest_SPB

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Posted 25 May 2019 - 07:58 AM

Optics of eyepiece less sensitive to its temperature difference then objective lens as much as magnification factor reduce diameter of light beams passing the optics.

 

On other hand warm optics resists to dew falling.  


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#7 X3782

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Posted 25 May 2019 - 03:45 PM

The reason why telescopes need to cool down is because the thermal currents of the air and thermal deformations of the main objective lens or mirror cause some fraction of the light rays to deviate from their intended paths as they are focused. This fraction of deviated light rays strike your retina (particularly the small central area called the fovea which is responsible for sharp high-resolution eyesight) and causes a "blurred" or "deformed" image.

 

Although similar unwanted deviations do occur in the glass of a warm eyepiece lens as well, they happen so much closer to the retina, that the blurring effect on the image is much smaller. Generally speaking, in a telescope the large-diameter optics located furthest away from the eye have to be most carefully figured and cooled down. The small-diameter optics located closer to your eye do not (though they are sensitive to other effects, like needing better AR coatings and polishing).

 

Fogging of the eyepiece lenses would be the more practical problem.


Edited by X3782, 25 May 2019 - 06:52 PM.

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#8 Miranda2525

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Posted 25 May 2019 - 06:07 PM

The conventional wisdom is that telescopes need to cool down; does this also apply to EPs?

 

Should I be leaving some of my EPs outside before beginning an observering session?

Eyepieces need to stay relatively warm or they will fog up when your warm eye gets close to the lens.

 

Spring time - can be a problem, depending on temperature and humidity

Summer time - no problems

Fall - Same as Spring time.

Winter time - lots of fogging problems, unless you use an eyepiece warmer.


Edited by Miranda2525, 25 May 2019 - 06:09 PM.

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#9 cuzimthedad

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Posted 25 May 2019 - 06:26 PM

I've fogged my eps by inadvertently breathing on them. I always used a dew strip when it was expected, which was almost always in Sonoma, and I'd throw one or those throw away hand warmers into my ep case and close the lid to keep my eps warm. Most the time in the summer I didn't have to worry much, but was always prepared for it.


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