Secondary dew (i think)

[bendiddley]

Hi all. I was out last night and noticed by the end of my session (about 1.5 hours) that there was a large round foggyness in the centre of my fov and i could see the spider arms showing in a similar grey appearance. Is this dew on the secondary and is there anything I can do to prevent it from happening? I've got an 8" dobsonian. Cheers.

[Tinker1947]

I use a hairdryer on low heat, blow the air through the focus tube for a minute or 2, if its main supply hairdryer don't leave it outside, mine goes straight back in the warmroom, next step is a dew heater....

[Luke]

I also use a hairdryer, a low power one on low heat setting.

[NGC 1502]

You can check to see if the secondary mirror on a Newtonian is dewed up by removing the eyepiece, looking into the focuser, and shining your red torch into the open end of the tube.

The posh solution is a secondary heater like this http://www.altairastro.com/product.php?productid=16275&cat=243&page=1

Many of us (including me) fit a dew shield to extend the end of the tube (camping mat foam is a favourite to use) and if that is not enough use a dew gun. I use a 12v "hair dryer" and a rechargeable battery pack. The 12v so called hair dryer is next to useless for its intended purpose, but sorts a dewed secondary if pointed down the focuser, but only if it is misted up, if droplets of water have formed, then it's time to pack up.

Be VERY careful if using a mains voltage hair dryer, damp conditions and 240 volts are a lethal combination.

Regards, Ed.

[LukeSkywatcher]

Could it have been the EP that was fogged up?. Sounds like it could have been. I'm not saying the secondary wasnt fogged up, but a foggyness in the centre of FOV sounds like dew on the EP to me (not the end you look through.........the other end of the EP.

[bendiddley]

Thanks Paul. Yes I checked the EP and it was fine. Ed I'll have a look through the focuser tube next time to see if it is or not. Quite like the 12v hairdryer idea, would be useful for sorting out misted up ep's and finderscope/rigel too so may look into that as an option. Just spent the evening fashioning a dew shield for my rigel quikfinder, looks a bit of a bodge lol, but I hope it does the trick. Thanks all for your comments.