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Hi all,

I'm a new refractor owner and had a question about dew. When I bring my refractor inside, I typically leave the cover off to let things dry out. This got me thinking, should I remove the field flattener and let it dry out separately? Or just leave the whole imaging train intact until the scope has had a chance to adjust to room temp?

Thanks

---Steve

I prefer to take everything apart to shorten the time to reach equilibrium. Perhaps leave the imaging train intact but out of the scope.

That would also depend on the status of your imaging run. After taking your lights, do you have your flats taken? If not, bad idea to break the imaging train down...

When bringing a scope in from the cold, install all covers outside in the cold. Leave the covers on indoors until the next morning. This will prevent the humid indoor air from contacting the cold glass and making the dew worse. Next morning, remove the covers for a few hours to led the original moisture evaporate.

KathyNS wrote: ↑Sat Sep 14, 2019 7:38 pm When bringing a scope in from the cold, install all covers outside in the cold. Leave the covers on indoors until the next morning. This will prevent the humid indoor air from contacting the cold glass and making the dew worse. Next morning, remove the covers for a few hours to led the original moisture evaporate.

I remember you recommending this Kathy, back on the 'other' forum I've been following this advice for four years now and my objectives are clean and spot-free.

Having done this more than a few times at temps from zero to -40C, I can vouch for and highly recommend Kathy's method

All the best,

I made a small wood caddy to just hold my whole imaging setup together. I tend to keep mine on a table in my garage. I can go months or even over a year on the same set of flats.

I would always recommend not bringing a cold scope regardless. We had a local do this with a large refractor. He was having issues focusing and finally took at look at it and found the lens cracked.

Dan

Thefatkitty wrote: ↑Sat Sep 14, 2019 8:35 pm

KathyNS wrote: ↑Sat Sep 14, 2019 7:38 pm When bringing a scope in from the cold, install all covers outside in the cold. Leave the covers on indoors until the next morning. This will prevent the humid indoor air from contacting the cold glass and making the dew worse. Next morning, remove the covers for a few hours to led the original moisture evaporate.

I remember you recommending this Kathy, back on the 'other' forum I've been following this advice for four years now and my objectives are clean and spot-free.

Having done this more than a few times at temps from zero to -40C, I can vouch for and highly recommend Kathy's method

All the best,

Wow... -40C.... As someone who observes from San Diego that's an entirely different level of cold!

starfield wrote: ↑Sat Sep 14, 2019 11:37 pm
Wow... -40C.... As someone who observes from San Diego that's an entirely different level of cold!

I'll happily trade with you any day... My DSLR start to get "slow" at around -35C... Press the remote shutter really hard cause it's almost frozen, then a second passes 'till the camera shutter reluctantly groans in a shot...

Well, still a few months to go until that

Enjoy your warmth and all the best,

If the temperature change is going to be quite drastic I will wrap the telescope in a heavy blanket or sleeping bag outside so that it warms up even slower when inside.
All caps on first of course

I find the suggestions from KathyNS and Gabby to put caps on first before bringing indoors interesting. I wonder if the level of indoor humidity is a factor? I have humidifiers running in the winter because I have very humidity sensitive wooden musical instruments. So I aim at keeping indoor humidity at 40% as a minimum.

My practice of keeping caps off is aimed at minimizing thermal relaxation time, getting the optics up to temperature and dry as quickly as possible. The corrector plate of my MK66 is over 16 years in service this way and looks quite good. So for my conditions (high indoor humidity) It's not clear that I'm mistreating my optics by leaving caps off. (I put them back on as soon as dew is gone indoors.)

Could it be that the "caps on first" approach works because the indoor humidity is low? No worries about entrained moisture?

I didn't pay much attention to the order of doing things until an acquaintance near Seattle (who put caps on first) found a spider web of mildew developing between the front elements of a TV Genesis. So that's what got me started on the dry first then cap routine. His house also had musical instruments and was quite well humidified. (Also, there's the Seattle issue where objective lenses have optional windshield wipers.)

No, the caps on first idea is based on the indoor humidity being high. Specifically, if the dewpoint of the indoor air is higher than the temperature of the glass. In that situation, additional dew will form on the glass when it comes indoors.

Keeping the caps on reduces the contact between the moist indoor air and the cold glass, limiting dew to what already formed outdoors. Once the glass has reached thermal equilibrium with the indoor air, it is safe to remove the covers and let the outdoor moisture evaporate. Don't skip the covers-off part after reaching equilibrium, or moisture will become a problem.

I'm going to have to give this further thought before I change what I am doing, which has worked well for me. I think that part of the problem is that we are optimizing different quantities under different conditions. I've pretty well talked myself into accepting that putting caps on where there has been no dew outside is fine and safe. I do not see that it is clearly a good thing to cap if the optics are already dewed when outside.

The correct metric to optimize is minimization of the total mass of dew that is accumulated during and after a session. What one wants to avoid is deposits of impurities on optical surfaces Whether the relative humidity inside is high or low might matter, the dew point might matter, temperature might matter. This is a more complicated optimization problem than I appreciated, now that I think of it carefully.

I think we can agree that "don't put it away wet" is a good idea. When I was a student 50 years with a large Clark refractor I didn't have to think. I just did as I was told and replaced the cap on the lens only when it was dry and that's what I've been doing. That was an observatory situation so temperature changes were always modest. Carrying kit between radically different temperature - humidity environments is complicated. I'm skeptical that one solution fits all occasions.

I agree with notFritz. I was taught that bringing a scope inside with caps on after dewing was verboten. The caps would keep moisture trapped on the objective. Probably is a case by case question.

Having become more curious, I did a literature search to see what science was out there. This looks relevant.

http://stc.fs.cvut.cz/pdf16/6535.pdf

I keep the caps on both over the eyepiece and the objective when I bring it in. When weather conditions permit and and the air is not too moist I sometimes leave the AR152 (caps on) outside, cover it with a tarp and retrieve it in the morning.

helicon wrote: ↑Mon Sep 16, 2019 5:30 pm I keep the caps on both over the eyepiece and the objective when I bring it in. When weather conditions permit and and the air is not too moist I sometimes leave the AR152 (caps on) outside, cover it with a tarp and retrieve it in the morning.

Fair enough. But whether you are caps on or caps off when you bring optics in, the OTA should be horizontal, optical surfaces vertical. Dust falls down. The 18.5 inch Clark was always put to bed pointing at the northern horizon. Leaving the scope pointing at the celestial pole invites any trapped dust to settle on the lens.

I'm going to stick with my bring indoors caps off approach. That TV Genesis I saw in Seattle really put the fear of moisture in me. So although I set up "caps on" I bring the caps indoors immediately so they stay warmer.

Interesting point nFA, I will have to give your method a try sometime.
My preference of capping and storing before bringing inside could just be years of habit.
The temperature inside is controlled as well, 40%-50% humidity and 20°C due to multiple wooden instruments as well.
One thing I have not seen anyone mention is desiccant, I have tea balls filled that go inside the focuser drawtube when the telescope is capped.
Another thing that may hinder fungus growth is the amount of solar viewing with the Herschel wedge, all of my telescopes get bathed in UV at some point so it may help a bit.
i have never (so far) had a issue with mold/ fungus on the lens or inside the tubes.

Desiccant is a good thing! The tea ball idea is neat. On my move I had doubled up on desiccant with tubs of it in the trunk of the car.

You make a good case for solar viewing.

The TV Genesis with the mildew was thankfully not mine. I had been hired to do calculus / physics tutoring and the student became aware that I did my own maintenance work on refractors. After one session I was shown the scope and was asked if I would fix the problem. I declined to clean and collimate anything more complex than a doublet, recommending it be sent back to Al Nagler. I won't collimate a Petzval. So I haven't a clue what the rear doublet was like.

When next at the druggist, ask them what they do with their desiccant packs that are in medicinal shipping boxes.
It is the really good grade, lightly used and typically they just throw it away since it cannot be reused.
Every couple of months I go to the local shop and get a box full of desiccant cylinders for me and my viewing friends.

Oops...
Apparently I have been doing it all wrong, the past half a century. I bring in the telescope and wait for the lenses being dry, before capping.Takes 40 minutes or so. Luckely none of my telescopes has been complaining