Dew on the eyepiece - is it OK to wipe off with tissues?

[Martin-Devon]

Great viewing conditions last night, but increasingly cold, and dew was a perpetual problem. As usual the binos start with the problem, I don't know why, but by 11.00pm, I had to keep continuously wiping the lenses. Then the eyepiece on the scope started misting up. I wiped it gently a few times with soft tissue paper to dry, but was worried that this might actually be doing some damage - I keep reading that the coatings are very delicate. I know that sooner or later I have to get some heated dew strips, but this must become a nuisance if you often change the EPs, and I still have to get my head around how these heated strips would all rig up around the EPs, end of the scope and the power source needed.

So my question - is it actually harmful to wipe the surface of the eyepieces with a soft tissue?

[Llamanaut]

yes and no.... as long as you use something like a lens cloth and not tissues (abrasive after a while) you should be ok.... its better to avoid dew in the first place by either keeping them above the dew point temp (in a covered case) or use some form of anti dew heating, personally i put a selection of eyepieces i mainly use in my fleece pocket and keep them warm till use, as long as theyre covered they dont get too manky... i know the purists will all gang up on me but in the last 7 months of regular observing ive only had one dew issue and that was because i DIDNT use my pocket method.... over to the forum >>> shields himself from bottles and stones>>>

[Rui]

Just take one of those ubiquitous shoes boxes and put the EP's your not using inside. The cardboard material they're made of prevents the dew. I always do that and never have issues with dew.

[John]

It's best to try and prevent the dew forming in the first place - frequent wiping of lenses / coatings, even with lens tissues, is best avoided.

[Moonshane]

you can also buy a small 12v hairdryer which many use to remove dew from lenses without touching them.

hey John, you big show off with your new double helping of Ethos avatar

[Ags]

Is it mad to toss a hand warmer in the box with the EPs?

[tobot]

I tried the pocket method tonight and it worked a treat

[barkis]

A good Idea might be to make a heat pad to line a box where the eyepiece can be kept warm, not hot.

A suitable heat could be about 5 watts perhaps, Easily calculated using Nichrome wire of a resistance per metre, with a supply of 12 volts. a network stitched between two felt pads would be OK.

A Phono socket on the box to plug in the power.

Less than .5 Amp current draw.

Ron.

[great_bear]

When people talk of dew, they are often discussing two different things.

"Condensation" forms when moist air hits a cold surface and can happen at any time when the conditions are right. "Dewfall" on the other hand - simply falls like silent rain, suddenly and at a specific time of night, and is quite a dramatic effect. The time can vary wildly - all of a sudden everything just gets covered in a condensation-like moisture.

This was demonstrated to me on my camping trip a fortnight ago by my wife. "I can feel the dew is about to fall" she said, as she jumped up and moved most of our stuff under the large Gazebo. "What are you doing?" I asked - pointing at the billowy, open-sided Gazebo, "That's not going to protect anything!". I believed that all dew was merely condensation.

A few minutes later, I discovered that (annoyingly) she was right. Everything around our general camp area was damp and covered in a condensation-like dew - but everything under the open-sided gazebo on the other hand was bone dry.

In practice, I find that merely flipping the eyepiece case lid closed after each EP change is enough to stop dew.

(I still suffer big problems from eyeball-induced-fogging though...)

[John]

...I still suffer big problems from eyeball-induced-fogging though....

Does using longer eye relief eyepieces help with this issue ?.

[John]

..hey John, you big show off with your new double helping of Ethos avatar

OK Shane - I've gone for something non-astro now

(still bad for your wallet though - and your head )

[great_bear]

Does using longer eye relief eyepieces help with this issue ?.

Not as much as one might expect - but a little bit.

With binoculars too, I find if I am to use them on an evening, I have to keep them around my neck whilst not in use - in order to keep them warm. If I leave them on the table when unused, they fog up later after about 1/2 second's use.