Dew problems when Observing

[hawaiiruss]

I have recently purchased an 8" Newtonian Reflector and took it out last night for it's trial run - I have a dew shield, but after 30 minutes all I could see through the eyepiece was an opaque blur. Is this a dew problem with the scope or the eyepiece itself dewing up (Televue Ethos 13mm)?

Rather frustrating not seeing anything through the eyepiece, especially as the Moon was well placed for viewing.

[Kai]

I would thought the eyepiece would of been more prone to dewing up.

[FLO]

It'll be the eyepiece or secondary mirror. Do you live in an area prone to dew (low altitude, near water)?

[crepitis]

I would have thought it most likely to be the secondary mirror, theres a good post about making a dew shield for an 8" newt somewhere on the forum, search it out, or if you arent too "DIY" minded ( like me ) you could always buy one.

[ZOG]

If it happens again, hold a small mirror at the end of the OTA so it gives you a view of the secondary to see if it's dewed up. If it's the eyepiece, holding it up to the light will soon tell you if it's dewed up.

[Bigwings]

I get eyepiece misting. I have a 12v hairdryer which clears it at a blast. It also clears the Corrector plate of the SCT and the Telrad.

[Doc]

IMO it will be the secondry mirror. Mine always dews or freezes over. My eyepieces never do. My 18-55 lens on the camera always does as well. Nothing a hairdryer doesn't cure.

[James]

With a reflector it's usually the eyepiece or secondary that fog up fairly quickly. If you have a dewshield it's less likely to be the secondary and more the eyepiece.

If you have a Mak-Newt or other scope with a corrector plate then of course it could be the corrector that's dewing and a dewshield alone may not be enough in very dewy conditions and you may have to consider dew heater tapes.

Whichever it is it should be easy enough to check when it happens, pull out the eyepiece and have a look, if its fine then it's the secondary. If you have a corrector then the dew should be obvious on the surface.

HTH

James

[hawaiiruss]

Dew shield attached and with the eyepiece kept warm in my pocket and a clear night it was time to have another go. I left the scope outside for an hour to cool down (having been stored in the garage the temperature difference was minimal). Once I attached the eyepiece, I still could not view anything - I could either see the secondary mirror reflection or an opaque blur. I also tried the hair dryer trick but with no effect - I'm hoping dew heater straps will resolve the problem, as it definitely appears to be the corrector mirror.

[Taff]

When the dew is on my eyepieces or the mirrors i use a hair dryer at low speed it does the job.Mark

[The Warthog]

I find that dewing on my Newt is always the secondary or the ep. Most often it's the ep. I warm the eps in my hand sometimes, but I like the hair dryer idea.

OTOH, I find I don't get dewing when it's below -6º C, so if you confine your viewing to those temperatures...

[lunator]

HI Hawaiirus

It sound like your eyepieces are dewing up.

I have a Newt and the eyepieces always dew up before the secondary.

When you next set up look at the secondary using a torch. It will be obvious if there is dew on it.

Cheers

Ian

[hawaiiruss]

Cheers for all your help. In the end, a dew shield was all I needed. I keep the eyepiece inside whilst not viewing (waiting for the tube to cool down). Even in my light polluted area (annoying streetlamp - could do with some Harry Potter magic) I can still pick out the Orion Nebula and see the orange glow of Betelgeuse.

Russ