So I learned an important lesson in telescope position tonight...

[MKHACHFE]

Hi all, first off, Merry Xmas to you all, I hope you are having a lovely day.

So, nice and clear (ish) in SE UK tonight and I left my telescope to cool outside about 1.5m from the house. After Xmas dinner and presents, I went out to enjoy a  few hours with my XT8 and every single piece of metal and glass was literally dripping with water. And obviously I also left my box of eyepieces out there with it. Fortunately, i had left the cap on the top of the scope, so the main mirror was still OK....but there was nothing doing, I wiped and wiped and wiped, but everything was still flaring. I assume the secondary mirror was wet, but didn't want to touch it.

I'm also going to assume this happened because of the 0 degrees outside and the heat from the house (esp the kitchen which gives onto the garden and which was going full steam all evening. 

So, evening ruined...i moved the scope to the end of the garden now, but I doubt it will recover tonight.  Now I understand the importance of dew heaters for some people. 

Darn it! 😣

 

Cheers

[DaveL59]

central heating boiler venting out on the garden side too, perhaps?

I guess I'm lucky that mine blows out the front so no moisture laden air blowing out on the garden side, plus the conservatory is cold so the scopes don't have much temp change if I do move them out to the garden. I managed a little peek at Orion earlier having got back after spending the day at my dad's  tho clouds coming over now so I guess the view will soon be obscured.

Still at least you planned to have a go, hopefully no harm done 🙂

[MKHACHFE]

28 minutes ago, DaveL59 said:

central heating boiler venting out on the garden side too, perhaps?

I guess I'm lucky that mine blows out the front so no moisture laden air blowing out on the garden side, plus the conservatory is cold so the scopes don't have much temp change if I do move them out to the garden. I managed a little peek at Orion earlier having got back after spending the day at my dad's  tho clouds coming over now so I guess the view will soon be obscured.

Still at least you planned to have a go, hopefully no harm done 🙂

No harm done ... although I very nearly fell flat on my face while lugging the scope back in, I thought I would save 18 seconds and not tie my shoelaces. Big mistake. 

But on the plus side, I learned TWO lessons today, even though I should have learned about the second one about 40 years ago. Doh.

 

I'll look into where the house  boiler gets vented as well. Thanks for the tip. 👍

[DaveL59]

I'll admit I take great care carrying the scope down the couple steps back into the conservatory. Only got that wrong once and thankfully not carrying a scope... well not so far...

[banjaxed]

Same problem for me, I was looking forward to an evening of a brilliant night sky at my daughters in Southern Ireland with my 127 Skymak outside cooling ready for use. We went outside expecting some fabulous views but could only make out small blurry stars until I noticed the lens was covered in moisture along with the eyepieces 😖 However all was not lost because I had my binoculars which gave us a lot of great viewing. I was amazed at how clear the Orion Nebula was with the naked eye as the view where I live is barely noticeable.

[IanL]

Placing the scope close to the house will delay the onset of dewing not make it happen sooner.

The air contains a variable amount of gaseous water vapour, which will condense in to liquid water when the atmospheric pressure and air temperature meet certain conditions. We can't control the amount of water vapour or pressure, so primarily we care about temperature. When the temperature falls far enough, water condenses in to clouds or fog. In temperate latitudes with less water vapour, the air has to get quite cold for this to happen, but in the humid tropics, condensation can occur when it is much warmer.

In the case of the telescope, if the lens or mirror cools enough, the adjacent layer of air also cools and water condenses on to the surface of the scope since it needs a nucleation site to start condensing.

The mirror or lens mainly cools by radiation. Heat is lost to the environment in the form of infra-red radiation, but it is also gained from the  environment by the the same means.

The scope is radiating at + or - a few degrees or tens of degrees C, as is the ground and nearby objects like buildings.  When the sky is clear at night, space is radiating at about -270 C. So there is a rapid net loss of heat from the scope to space.

Placing the scope next to the house blocks some of the sky and will greatly slows the net rate of  heat loss since the house will actually heat the scope by radiation. If you park your car next to the house, the windows facing it will dew/frost far less than those facing away since they experience a net heat gain whereas those facing the sky experience a net loss unless it is cloudy (clouds are warm vs. space).

Placing the scope by the boiler vent would replicate the tropics, hot humid air, but otherwise the house is not the issue, quite the opposite.

The keys are:

- An insulated dew shield (e.g. foam) since this blocks the lens from seeing most of the sky (except the bit you are observing) and thus reduces the rate of net heat loss. Metal dew shields cool quickly so are less effective.

- Cap the lens or tube until observing for the same reason.

- Keep eyepieces in a closed, insulated box until using them.

- Use a dew heater to offset the net heat loss. Heater is a bit misleading though, it should only be warm enough to keep the surface above the dew point. If it feels warm then it is too warm and will create local 'seeing' issues.

-  A bit of breeze is ideal, since air is a good insulator and if it is moving it won't stay in contact with the cold scope long enough to reach the dew point. You car won't dew up and (radiation) fog doesn't form on windy nights for the same reason.

Edited December 27, 2019 by IanL

[MKHACHFE]

14 hours ago, banjaxed said:

Same problem for me, I was looking forward to an evening of a brilliant night sky at my daughters in Southern Ireland with my 127 Skymak outside cooling ready for use. We went outside expecting some fabulous views but could only make out small blurry stars until I noticed the lens was covered in moisture along with the eyepieces 😖 However all was not lost because I had my binoculars which gave us a lot of great viewing. I was amazed at how clear the Orion Nebula was with the naked eye as the view where I live is barely noticeable.

Damn. It's disappointing isnt it? The worst thing is that my wife bought me 2 lovely BST Starguider eyepieces and an Astronomik UHC filter....and I haven't been able to try them out yet due to dew formation. Sadly, I live in Biggin Hill and we are very prone to fog here. 

 

You must get some lovely dark skies in Ireland. 

 

Cheers

[MKHACHFE]

3 hours ago, IanL said:

Placing the scope close to the house will delay the onset of dewing not make it happen sooner.

The air contains a variable amount of gaseous water vapour, which will condense in to liquid water when the atmospheric pressure and air temperature meet certain conditions. We can't control the amount of water vapour or pressure, so primarily we care about temperature. When the temperature falls far enough, water condenses in to clouds or fog. In temperate latitudes with less water vapour, the air has to get quite cold for this to happen, but in the humid tropics, condensation can occur when it is much warmer.

In the case of the telescope, if the lens or mirror cools enough, the adjacent layer of air also cools and water condenses on to the surface of the scope since it needs a nucleation site to start condensing.

The mirror or lens mainly cools by radiation. Heat is lost to the environment in the form of infra-red radiation, but it is also gained from the  environment by the the same means.

The scope is radiating at + or - a few degrees or tens of degrees C, as is the ground and nearby objects like buildings.  When the sky is clear at night, space is radiating at about -270 C. So there is a rapid net loss of heat from the scope to space.

Placing the scope next to the house blocks some of the sky and will greatly slows the net rate of  heat loss since the house will actually heat the scope by radiation. If you park your car next to the house, the windows facing it will dew/frost far less than those facing away since they experience a net heat gain whereas those facing the sky experience a net loss unless it is cloudy (clouds are warm vs. space).

Placing the scope by the boiler vent would replicate the tropics, hot humid air, but otherwise the house is not the issue, quite the opposite.

The keys are:

- An insulated dew shield (e.g. foam) since this blocks the lens from seeing most of the sky (except the bit you are observing) and thus reduces the rate of net heat loss. Metal dew shields cool quickly so are less effective.

- Cap the lens or tube until observing for the same reason.

- Keep eyepieces in a closed, insulated box until using them.

- Use a dew heater to offset the net heat loss. Heater is a bit misleading though, it should only be warm enough to keep the surface above the dew point. If it feels warm then it is too warm and will create local 'seeing' issues.

-  A bit of breeze is ideal, since air is a good insulator and if it is moving it won't stay in contact with the cold scope long enough to reach the dew point. You car won't dew up and (radiation) fog doesn't form on windy nights for the same reason.

I just bought a dew heater for my eyepiece tube. Do you think that will suffice or would I need one to wrap around the body of my XT8 as well? What's frustrating is that my RA finder lenses are also dewing up so badly I can't use them. 

It's funny, when you buy a telescope, you never think one day soon you will need to buy 2 or 3 heating straps and the power supplies to hook them to. I guess I could just wait for a more favourable evening instead of spending more money. LOL

 

Edit: I forgot to thank you for the great advice at the start of your reply. It's very much appreciated.

 

Cheers

 

Edited December 28, 2019 by MKHACHFE

[IanL]

8 hours ago, MKHACHFE said:

I just bought a dew heater for my eyepiece tube. Do you think that will suffice or would I need one to wrap around the body of my XT8 as well? What's frustrating is that my RA finder lenses are also dewing up so badly I can't use them. 

It's funny, when you buy a telescope, you never think one day soon you will need to buy 2 or 3 heating straps and the power supplies to hook them to. I guess I could just wait for a more favourable evening instead of spending more money. LOL

 

Edit: I forgot to thank you for the great advice at the start of your reply. It's very much appreciated.

 

Cheers

 

With regard to the RA finder, the dew shield is usually totally inadequate, mine dews up very quickly. You could try making an extension out of some craft foam or similar, just cut a length, roll it up and use an elastic band to hold it on the end, or self-adhesive velcro for a better job. The eyepiece end is a total disaster, facing straight up at the sky usually with no protection. Keep it capped unless using it and ultimately yes you may need a dew heater for it.

For the rest of the eyepieces, you can certainly use a dew heater, but keeping them in warm-ish pocket and swapping them regularly usually works for me. Some people make heated eyepiece boxes which do the same thing more effectively.

For the XT8, it's probably only the secondary you need to worry about in most circumstances. That tends to get cold more quickly but to be honest I don't have many issues with my 10" Dob. The primary mirror in a closed tube Newt generally won't dew up unless conditions are extreme or you have been out all night since the tube acts as a giant dew-shield.  You don't really want to be heating the primary since it can create a boundary layer of turbulence on the front of the mirror ruining the view. Some Dobs have built in fans to keep this layer moving and also cool the mirror. The moving air also prevents dew forming in the the bargain. Mine has one but I have never needed to use it (yet).  Search for "Newtonian Fan" and you'll get the idea.

Definitely do not heat the tube body, that will set up air currents that will ruin the views.

Edited December 28, 2019 by IanL