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16" lightbridge primary dewing up


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Has my first dewey night with the scope last night and the the dew heaters fir the secondary,finderscope and eyepiece worked admirably but my primary dewed up badly a few times.I have a battery powered fan on the back of the primary but the batteries ran out really quickly so would sorting the fan out cure the dew or am I going to need a dew strip on the primary?bit gutted as my 300p primary never dewed up.

Also,was collimating my primary and when I the scope vertical the collimation moved quite alot.do I just need to secure the primary clamps a bit or is it likely the cell springs?

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Great scope, should give wonderful views.

Do you have a shroud to fit over the truss tubes ? My club's big trusstube Dob has a shroud, plus a fan that blows across the face of the primary, works very well to suppress dew.

It's not usual to fit a dew band on a Newtonian primary, but a dew heater on the secondary is often fitted as it's much more prone to dew up.

If the collimation is shifting, could be the primary springs not strong enough, or you could try putting more tension on all three with the collimation screws. Could be the truss tubes not clamped firmly, I'd try this first before you investigate further. I wouldn't clamp the mirror itself, could cause issues.

Hope you sort it, a 16" is wonderful especially under a good sky, Ed.

Edited by NGC 1502
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I have an astrozap light shroud and a secondary heater.the dew was quite bad last night as I was down a track with fields on all sides but I still didn't expect dew on the primary.it's still soaking now so going to have to clean and dry it off later.as I said,the fan batteries went really quickly so I'm hoping that's all it was.

It's turning out to be a nightmare this scope,it takes so much modifying to get right.

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Sorry you have these issues. If you have a shroud, and that's not enough to keep the dew off, and we are not anywhere near the cold season when dew is usually worse, then the only thing I can think of is a fan (or 2 ?) above the primary and blowing across the optical surface. As I said, that seems to do the trick on my club's big trusstube Dob. You could keep the fan below the primary as well.

I'd also be thinking of a rechargeable battery pack to work the fans and dew heaters.

Do hope you find fixes that work, Ed.

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Im thinking of a second battery now one for the fan and one for the dew heaters and proofing the light shroud with a tent treatment.don't really want to get into doing too much wiring as I'm trying to avoid an electric.if I have a bad winter with it then I will get rid in the spring and get something else.cheers for the ideas fella :)

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I'm not sure if my primary cooling fans help against dew but I keep them running all night anyway. I also have a 12v hairdryer but I've only ever used it once when there was a really heavy dew, I can easily lift the bottom of the shroud up and give it a quick blast but you may have to wait a few minutes until you can use your scope again because your primary is enclosed.

Hope you get it sorted.

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oddly, with my 10", there were nights when I was running the primary fan and the primary fogged up bad. Dont know if it was going to happen anyway, or the fan assisted the dewing process. Its not what I would have expected. I've only had the primary dew up once or twice on the 16" but that was when it was REALLY dewy out and almost misty where I could see the moisture in the air with my headtorch. I see what you wrote about breaking the observing. When I blew away the dew from the primary with a hairdryer, I only lost 5-10mins. Not too bad, really.

Barry

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I'm not sure if my primary cooling fans help against dew but I keep them running all night anyway. I also have a 12v hairdryer but I've only ever used it once when there was a really heavy dew, I can easily lift the bottom of the shroud up and give it a quick blast but you may have to wait a few minutes until you can use your scope again because your primary is enclosed.

Hope you get it sorted.

.....welcome back mate...

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could it be that the primary was cooler than the ambient? it was incredibly warm here last night and maybe the fan actually cooled the mirror a little more than expected before it packed up? were you observing at or near the zenith for a long time? this can also affect how quickly things dew up.

a mirror blowing across the primary might help and would actually disturb the boundary layer of air, creating less of a lens effect off the primary and increasing sharpness potentially. I have not bothered putting them in any of my scopes since the 12" I sold that had these fans and have not really missed them.

even my 16" solid tube dewed up last year at PSP but that's because we were observing Jupiter, high in the sky while basically in the middle of a cloud it was so misty.

dew is a funny one. some nights things seem to dew up for no apparent reason and others when you expect it, nothing happens.

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ps

there's really no need to clean a mirror just because it has dewed up. mine does so every time I bring it in during the winter months and I rarely clean mine. just let it dry naturally.

Yep - best NOT to clean it unless absolutely filthy.

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It literally had standing water on it and bits of dust were settling on it.I was observing at zenith all the time(about 2 1/2 hrs) looking around Cassiopeia,cepheus,ursa minor area.not sure the primary would have been cooler but can't rule it out.I think another contributory factor would be not putting the scope on the carpet I usually take.

Cleaned the mirror but didn't dry it properly so will redo next week.

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Have you made a dew shield for it yet? This may help absorb some moisture, which may tend to trickle down the tube, when the scope is almost vertical, observing as you remarked near to zenith. In between use, i.e if checking charts, changing eyepieces between targets, having a coffee break etc, I will point the scope more horizontally. I always place my scope on a mat to, which may also help a bit with any rising damp.

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It literally had standing water on it and bits of dust were settling on it.I was observing at zenith all the time(about 2 1/2 hrs) looking around Cassiopeia,cepheus,ursa minor area.not sure the primary would have been cooler but can't rule it out.I think another contributory factor would be not putting the scope on the carpet I usually take.

Cleaned the mirror but didn't dry it properly so will redo next week.

I left mine out last night and it was a tad wet this morning! I've done this before and just left it to dry, as said before, these mirrors have to be pretty filthy before needing a clean. Can I ask how you cleaned and dried it ?

Phil.

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Cleaned in soapy water as always,rinsed off and blow dried.I never use distilled and this is the first time it's streaked in 10+ cleans so thinking my water supply is a bit more sedinenty than normal.have ordered 5 litres of distilled now anyway

Have you cleaned the 16" mirror 10+ times then ?

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Just an additional thought, but have you flocked around the primary? As I expect you know, the material acts a bit like a towel, in that it will absorb some moisture which may otherwise run down the tube. Dont expect that there is a lot else you can do in these circumstances, except of course, aim to go observing on slightly more exposed ground to pick up any light breeze and obviously as far as poss, well above water.

I suppose that as you go for scopes larger aperture, consequently the exposed open nature of the newtonian tube, will become inevitably more suseptible to internal moisture build up, and as you mentioned 16" + begins to be too high to attached a dew shield.

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