Refractor at the end of an observing session

[osbourne one-nil]

I've got a 4" refractor and I've got a heat band, which I place on the dew shield immediately behind the lens housing. So far dew hasn't formed on the lens. 

At the end of a session I bring it back into my office, which is in the garden and kept at about 20º. Should I leave the lens caps off while it comes back up to temperature (which is what I currently do, pointing the objective down slightly and turning the diagonal to the side so dust doesn't get in too easily) or should I replace the lens caps on first, then bring it back in? Or does it make no odds?!

Thanks

[John]

I recap my refractors before bringing them inside and leave them capped while they come back up to inside temperature. I don't find I need a dew band though.

[Jim Steele]

I leave the caps off; though I think the lens is generally well hardened to dew - I just dont want mould forming in the tube - you can also buy pretty nifty dessicant plugs that fit into your focuser from Flo; though i have not tried these...

Ben

[philj]

I always leave them uncapped till they have warmed up and dried out. It all depends on how wet they get. Leaving a dewed up lens capped is not recommended

[graemlourens]

Hi.

A rule i apply and that has served my well until now

- if dew has formed

leave it uncapped, facing down and let it evaporate until all the dew is gone (over night until the next morning)

- if no dew has formed

cap it up before you bring it in, and let it come to room temperature (until next morning) then uncap and see if everything is ok.

Furthermore: I have attached desiccant to the inside of the caps, so even if i cap it up, its still removing moisture while its getting back to room temperature.

Kind regards, Graem

[Stu]

If the scope is dewed up outside already then I would bring it in uncapped and let it come back up to temperature. I do normally try to bring it into a cooler/unheated part of the house but this is not particularly important.

If it's not dewed already, then I would cap it and bring it in and leave it to come up to temperature. Remove the caps after to give it some air and just to check it is dry before putting it away.

Dew conditions vary widely I think based upon your local circumstances.

I live near a river and between 3 large outer london reservoirs. Dew is almost always a problem sooner or later.

John and others using similar scopes do not experience issues so this can only be based on local humidity or breeze/wind conditions.

[osbourne one-nil]

Yeah - I live in Cumbria; moisture of some sort is always a problem too. 

Thanks everyone!

[mikeDnight]

I live in an old cotton mill town which is often damp and misty. Hellas on earth I call it!

I've used refractors for the last 35 years and have often ended up with dew on the lens. Sometimes it may even occur on the inner surfaces. I find it is more of a problem when I've been observing in freezing conditions as moisture can freeze on the elements.

Its never been a problem, even with the calcium fluorite elements of my FS 128 and FS152 Takahashi's. I have simply brought the telescopes back into the house and allowed them to warm up gradually with the lens caps off. I never heat the elements with a hair drier! If a drier is used it is best to have it set on a cool setting and hold it at a distance so the moisture evaporates slowly, which it will do anyway if its left alone uncapped.

Occasionally I have not noticed the lens to be slightly dewed and have capped the scope. The following day I've noticed dew stains on the front element. This does no harm to the lens or its coatings, so there's no need to panic and brillo the stains away.

If dew marks make someone feel uncomfortable, they can easily be removed by using isopropyl alcohol on lens tissue. I would probably clean my lens once a year or so. The lens always comes up as new and the coatings remain unharmed. I've never harmed an objective lens yet by caring for them in this way.

I suppose if in doubt uncap it and let it dry out thoroughly, and leave well enough alone.

Mike

[alfingido]

I have always stored my scopes in an unheated shed so if there is any dampness in the air and dew anywhere on the OTA, I leave all the caps off till next day, then cap everything if the surfaces are dry.

Seems to work and in 20 years haven't had any problems with fungus or internal condensation

Cool down times are pretty quick too....

Allan

[osbourne one-nil]

Well I think I should make use of the big shed I have - it's a 3-horse stable in fact (well it was when the people before us lived here) so there's plenty of air flowing about. Seems silly not to make use of it but some have said the mirror on my Dob, in particular, could suffer?

Anyway, I think I'll certainly do that with the refractor - thanks

[alfingido]

I kept a large dob in an outside shed and within a few days had a lot of dust on the mirror. It was windy outside and the shed was not very well draught proofed. Your frac will be fine in the stable, but I would try and ensure the dob mirror is stored as dust free as possible.

I cleaned the mirror easily enough then stored it in a different shed ( an unheated  summerhouse) with no draughts or dust and it was fine.

Allan