Dew on the mirror?

[george7378]

Hi everyone,

I noticed that I was often getting a strange haze surrounding the objects in the EP tonight, which seemed to disappear when I rubbed the eyepiece, but strangely re-appeared whenever I blinked or tried to focus on the image. Also, when I got in, I saw that the primary was covered in a strange layer of mist, which I presume was dew. Does anyone have any tips on avoiding this? Is it normal?

Thanks.

[Altair]

Funny you should mention this but i was getting much the same experiences when looking through my eyepieces tonight, I wondered if it was atmosphere causing the disturbances - sometimes it was hazy and sometimes clear.

be interesting to know what others think I took my eyepiece indoors and gave it a clean I wondered if it was to do with the sky not being completely dark...my scope has just come back from being serviced and the corrector plate has been cleaned!

[barkis]

One of the hazards of our climate I'm afraid. Gets very annoying too. You could get a 12 volt hair dryer to clear it, or fit a dew strap around the tube in the area around the mirror.

Not easy to keep a Newt mirrorclear. I guess a heater strap around the mirror cell might work, but the temperature would need to be just above the dew point, as the last thing you want is the mirror becoming too warm, that would make matters worse.

Ron.

[brianb]

Dewing of the secondary is much more likely than dewing of the primary, unless your scope has an open lattice tube. (In which case try a fabric shroud round the tube framework.)

No observing for me last night due to strong wind, cloud & rain but the early hours of Weds morning were clear & transparent, nevertheless it was hard to focus my 11" SCT at even moderate powers because of the bad seeing smearing stars out into blobs. The effect was bad enough to reduce the limiting magnitude by about 3/4 of a magnitude. Dewing was not an issue, the scope was outside for 8 hours before the sky cleared and the outside temperature was fairly stable, so cooling was not an issue either.

[Beulah]

Last night was exceptionally bad for dewing, seeing and transparency, although the sky looked crystal clear before the clouds rolled in.

Excellent viewing conditions in the UK are quite rare, makes you wonder why we stick at it.

[brianb]

makes you wonder why we stick at it.

Sometimes I wonder that, too, but not often whilst actually observing. The good memories soon drive out the frustration.

[itchy]

I just tend to take the binos out unless good conditions are virtually guaranteed.

Last night for instance it went clear - cloudy - clear - intermittent cloud/haze. So i didn't even bother with the scope.

[george7378]

Thanks for the responses - I did notice wisps of this cloud going overhead, and the sky was not completely dark (it had a whitish glow to it) so I guess it was very bad viewing. I wondered if it was anything to do with collimation, because I completely de-collimated my scope last night while trying to improve the views. I also think I breathed down the tube more than is healthy while trying to collimate...

Despite this, I saw two dust lanes in Andromeda, M81 and M82, which were great despite the conditions and low altitude, a transit of Europa and a dramatic re-appearance of Io from Jupiter's shadow, and M42. It was amazing to go from one Jovian moon to four in under an hour!

[bish]

I use a dew shield for the mirrors and 12v hair dryer for the finderscope and eps. There have been times when everything has dewed up beyond use in less than an hour without them.

[JamesC]

Agree with the others. It is unlikely that your primary was dewing when it was outside. Of course as soon as you bring it inside, it is a very cold surface so will immediately form dew. Just as the primary mirror is the last thing to cool down, it is also the last thing to warm up, so it can take time to clear.

I used to have terrible trouble with my secondary dewing up. I now have a dew shield (tube extension) made out of black camping mat - it has solved my problem.

Another thing you need to be careful of is if your eyepieces are very cold, it's easy to accidentally breathe on them which of course mists them up. I sometimes even think the moisture from my eye and hands / face can dew them up if I don't let enough air in.

Another possibility is thin cloud. I usually just notice thin cloud as the stars looking faint. From Dartmoor the clouds can be very dark so you can just be looking at the stars thinking "I'm sure they should be brighter than that". Sometimes if you look at Jupiter you can see a bit of a halo caused by the light scattering off the cloud. There were definitely waves of thin cloud going through on Dartmoor last night.