Dew watermarks on primary mirror - should I be worried?!

[Swift]

Hiya folks,

Just had my first evening's viewing with my new 10" Dob, was really chuffed with the results - went for the showpiece planetary nebulae, as I've never had anything big enough to show them as anything more than fuzzy spots. My session was cut short by a sudden and massive buildup of dew on the finder and main optics after about 45 minutes, so I put the caps on and took the scope indoors. Now ... should I have left the caps off to allow the dew to evaporate?! I only ask because I just thought I might have another crack (since several hours have passed and Orion's now sitting pretty) but upon removing the main cover, I found the previously pristine primary mirror to be covered in nasty-looking watermarks/streaks that don't look like they want to steam off! I did wonder about leaving the caps off when I brought the scope inside, but my flat's pretty dusty and I didn't want to risk any scuzz getting inside the tube.

Any ideas on what I should do? I really don't want to have to remove the mirror and attempt to clean it myself after just one night's viewing! Have any of you guys with Newtonians experienced this? Maybe it'll be gone by tomorrow, but for the time being I'm a bit worried! (If you're wondering, it's a solid-tube Dob.) I'll leave the caps on for now and get some sleep, but any tips on fixing this or avoiding it in the future would be great. I've never had this problem before, but then I've never used a telescope in my current (apparently very dewy) location. Apologies if there's already a thread about this somewhere, I did a cursory search and found plenty about dew heaters and shields, but nothing describing what I'd found on the mirror or what to do about it.

Thanks in advance, good to be here, S

[umadog]

Dew... Nasty, huh? It's got to be pretty bad for it to have reached the primary mirror but don't worry, it's no big deal.

Do you have a fan behind the primary? If so, run it all night: that may help to keep off the dew. At the end of the night you can you can blow a hair-dryer over the optics once you're done. That will clean them up for you. Your primary will never look like new and it will look especially bad if you point a torch at it. This is normal! Don't get concerned: it will make no noticeable difference to the views.

Repeated dewings will make the primary look worse faster. If it gets dew on it often then perhaps wash it every 6 months or so. You will see a lot of instructions on the net on washing mirrors. Some of them are quite involved. I've found that all one needs to do is squirt half a litre of distilled water over the primary (and that's on an 18") and leave it to dry naturally. That cleans up most things. No need to touch it. No need to use cotton buds. No need to use detergent. If you have a truss, no need to take it out of the scope.

[Bizibilder]

Don't worry about dew marks. They will dissapear the next time dew forms on the mirror! No, really, don't do anything - its surprising just how "dirty" a mirror can be without affecting performance. You are right when you say you are worried about dust and scratches - these are much more lilely to cause real damage.

[Specman]

Buy a dew heater for that EP Swift, that's the main thing and be prepared to get your kit soaking wet some nights

Astrozapp stuff is good, I use it and very pleased with the results. Give FLO a call

First Light Optics - Dew Prevention

Oh, and stop taking it in and out of doors. Warming it up and cooling it down will make matters worse. Leave it out till you have finished.

Clear skies

Mark

[mark7331]

Dew is pretty common and as Biz says don't worry too much about it. At the end of the evenings viewing I place my tube with the mirror facing down in the house with the dew caps off. You've mentioned dust but you only have to leave it long enough for the scope to warm a bit and the dew will disappear. Do this every time and it won't be a problem.

If the dew bothers, as Umadog said, you a fan fitted to the back of the tube will help keep it at bay by drawing air over the mirror.

I really wouldn't wash the mirror unless there's major debris on it. I've only washed my primary once, shortly after manufacture, because of metal and other rubbish left in the tube.

Although Umadog suggests a hair dryer I would advise against it. It's just my opinion but I think that you can end up blowing muck onto the optics and compound any problems. Also be wary of blowing hot air onto cold optics - you don't want to risk a crack. Just my opinion.

Hope this helps.

Mark

[umadog]

It is, of course, your call whether to use a hair dryer. My 2 cents are that I regularly blow air over optics and have never seen dust accumulate whilst I'm doing it. Mirrors tend to fog over when brought indoors from a cold night. I have the impression that they get fewer dew stains when I've blown this off straight away than when I've left it to air-dry naturally. Like Mark, I was worried at first whether the hair dryer would be counter-productive, but it seems not to be. You certainly don't need to worry about cracking, however. Like the other say: don't over-do the washing and don't wash it now.

[acey]

Dewspots don't affect the eyepiece view - the only way you ever notice them is by looking at the mirror at the right angle in the right light. In theory, dew accelerates degradation of the mirror coating because of deposition of salts. In practice you really needn't worry about that.

I cap my scope before driving home from my site, and once it's back in the place where I store it, I remove the covers (invariably finding the primary to be dewed), then leave it exposed overnight, after which it's completely de-misted, though spots will remain and they accumulate over time.

Eventually I'll clean my mirror but it doesn't need it yet - and I've been using it for two and a half years. My last scope had a smaller mirror and I cleaned it more often (maybe once a year), but that was more than it needed. Meanwhile I just live with the dewspots and dust, none of which has so far made a noticeable difference.

In the course of observing I use a dewshield and, if needed, a hair-dryer - the latter can easily be brought close to the primary of my 12" Flextube, though it would be harder on a closed-tube scope. With my previous 8" I didn't get primary dewing while observing. Warming a primary from the back would be slow, and doing it while observing would negate the whole concept of letting the primary cool to ambient temperature where it should have the best figure. A fan set into the tube, blowing cold air across the surface of the primary, would prevent dewing - the laminar flow is also said to give better planetary views than the turbulent air naturally found inside tubes.

[umadog]

Indeed, it is the heat stored in a primary above ambient temperature that produces those turbulent currents. You can keep dew at bay to a large degree by blowing ambient air over a surface. I regularly de-dew my secondary with one of those squeezy camera dust-removers.

[Swift]

Thanks, dudes. In the end I settled for leaving the 50mm aperture stop off the dust cap, and the mirror was like new this morning. Phew! Probably a bit stupid of me to panic like that, I guess it must've just been a fine coating of streaky dew after all - but from the other end of the tube it looked horrific, exactly like the kind of tough watermark or mineral deposit commonly left on cutlery and glasses by washing them in "hard" water. I'm a minute's walk from the sea here, and the guys in the shop did tell me that I'd have to be careful with salt in the air, so I freaked out slightly when I saw it!

I'd been told that the Pyrex mirrors and solid tube design of this Dob would help with any dew issues, so it's a bit annoying to find that it's affecting the primary. I may have to look into dew prevention soon - for now, my bank account's taken a big enough hit from the scope! Even if I sort out a fan or dew heater or similar, I'll probably still need something that'll blow warm air over the finder, which also got really damp. I never used to bother with my old finders that much, but I'm finding this big fat scope much harder to point by eye without using the finder than the small thin ones I used to have, even at low power.

I reckon Specman's right, for the moment I'll need to plan whether I want to observe late or early in the evening and then get in as much time as I can before the dew sets in - I probably won't be able to take it out a second time during a single night. Shame, I never had this problem in Shetland ... maybe the lack of dew had something to do with the total absence of trees and bushes up there? I wonder whether it's worth finding some breathable material to fit over the aperture stop, just to keep out the dust as the dew steams off, maybe a chunk of some women's tights held on with a rubber band? Or maybe peering at the thin, gauzy nebulosity across M57's central "hole" last night is giving me daft ideas. Whatever, as long as M27 doesn't inspire me to start weight-lifting!

Thanks again to all, S

[umadog]

Dust tends to settle on the primary simply because it's pointing upwards. The secondary gets a lot less dust on it because it's pointing downwards. I'd not worry about the dust: it'll happen no matter what you do.

I don't see why Pyrex would be better for dew, but having a solid tube does help. Some nights are more dewy than others so perhaps you just had a bad one last night. The fan behind the primary is a must. It will cool the mirror faster and get you good views earlier. Plus it should help a bit with dew.

For the finder you can make a little heater out of resistors. Here's how: Heaters Run it with a battery. You probably only need one or two resistors hooked up to a 9V source. The alternative is to hook up a small computer fan and have it turning at slow speed, blowing ambient air over the finder. That will work. Don't blow warm air whilst observing! That will create horrible convection currents that will destroy the views.

I power stuff with a 12V deep-cycle marine battery. They're not cheap but you can re-charge them and they last ages and ages. In the long run it saves money.

[Specman]

I'm a minute's walk from the sea here, and the guys in the shop did tell me that I'd have to be careful with salt in the air, so I freaked out slightly when I saw it!

I may have to look into dew prevention soon - for now, my bank account's taken a big enough hit from the scope! Even if I sort out a fan or dew heater or similar, I'll probably still need something that'll blow warm air over the finder, which also got really damp.

I reckon Specman's right, for the moment I'll need to plan whether I want to observe late or early in the evening and then get in as much time as I can before the dew sets in - I probably won't be able to take it out a second time during a single night. Shame, I never had this problem in Shetland ... maybe the lack of dew had something to do with the total absence of trees and bushes up there?

Thanks again to all, S

I live within 100yrds of the sea. The sea salt air will ****** most things, but shouldn't be a problem during observing as I take it you select fairly calm nights. If its blowing a force 9 and whipping the sea spray into a haar all over your car and windows I guess your not taking your gear out!

Living in Shetland is like Orkney, constant fan of wind blowing over everything, sand blasting is the biggest issue!

I would go dew shield and heater tapes on your EP and finder, problem sorted. and as mentioned in previous posts a 12v drier for emergencies. If clouds come in and you pack up for a while use a cover, or tilt horizontal, that will keep dew off the optics but bringing it in to the house will make it sweat.

Cheers

Mark

[Speedy]

just noticed this thread, my refractor lens dewed up whilst observing, when finished i took the scope indoors and left the caps off, the dew slowly disappeared, but it seems it has left some kind of "foggy stain" on the inside of the objective, anyone else had this problem?. should i leave it or clean it?

[LeeB]

Just spring cleaned all the scopes, 2x10", 2x 8" and a 4.5", don't worry about a bit of dew, 2 of the mirrors had aviation fuel gel deposits and took a while to clean off. But even those have left no perm. mark, some people never clean the mirrors and i can understand that, but having just done it there is a real improvement post clean. It is true the image quality takes a lot to be affected but the gradual decline from build up of dust and etc. is hard to notice.

So my advice let a years build up occur then remove it ,,repeat!!