How to avoid dew when bringing telescope inside?

[CasualObserver]

Like the topic states, I am looking for a good way to avoid dew when bringing my telescope inside.

I have just gotten my Skymax 127 AZ-GTi, and while I have found various tips and tricks, there seem to be very little information about how to avoid dew building up when bringing it inside after observing (the clouds delivered with the telescope has hindered me from actually going outside yet).

The telescope will be stored in an IP65-case with cutted foam, and I hope to also be able to fit the AZ-GTi-part in here. Would it work to just leave the case outside to cool down while observing, and then pack everything up while outside, and then just bring it inside? The idea being of course that the equipment would then be cold, and gradually warm up inside the case over a long time period. Or would this have the opposite effect where the humidity will be stuck inside the case, making it dew up anyway?

As I understand it, the reason for dewing up when going from outside to inside, is that the cold equipment will instantly attrack all the water in the warmer air, and then making it wet. So very gradually warming it up should maybe be a good way to avoid this?

And what about other pieces like diagonal and eye-pieces? I suspect I can not fit them all in the case, so is there some good way to avoid those dewing up?

[Nyctimene]

Your Skymax 127 is a compact scope, so the simple routine for my 80/400 FH Vixen frac should work quite well:

Still outside, leave the scope assembled. Cover the eyepiece and the front aperture (Maksutov corrector) with the appropriate plastic covers. After that, put a large piece of moisture-absorbing cloth (a bath towel or similar; for my little frac, the R-Sky observing hood fits the bill quite nicely), and bring the whole stuff inside. Let it warm up slowly for the rest of the night. In the morning, take off the towel and the plastic covers of eyepiece and corrector plate, separate diagonal and eyepiece, and let it dry again for an hour, to remove any rests of moisture. Then you can store everything in the case; additionally, you can  put a small pack of silicalgel inside to prevent moisture.

Hth.

Stephan

[Jiggy 67]

Your scope and all your other equipment will dew up whilst being used outside, sometimes to the point of being soaked. You can minimize this with dew strips etc but at the end of the day, there’s no way of stopping it. To place it in a case will just keep it wet for longer, it would be the last thing I would do. Just take the scope in and leave it with covers off outside the case to warm up gradually and it will be fine

[M40]

1 hour ago, CasualObserver said:

As I understand it, the reason for dewing up when going from outside to inside, is that the cold equipment will instantly attrack all the water in the warmer air, and then making it wet. So very gradually warming it up should maybe be a good way to avoid this?

And what about other pieces like diagonal and eye-pieces? I suspect I can not fit them all in the case, so is there some good way to avoid those dewing up?

Having been outside, your telescope and all the other stuff, when you bring it inside, will be colder than the inside air temperature, and as long as where you keep it remains a few degrees or more higher temperature than outside, the telescope and other items will dry naturally .... here comes the caveat .... as long as air can get to them. If you put it inside a case then there will be no air circulation and the telescope will either remain wet or take longer to dry out. So as Jiggy 67 says, don't put it in the case or if you do, leave the lid open. When I bring my stuff inside, I keep it in a shed, leaving it uncovered and just cover it up the following day.

Edited September 20, 2021 by M40

[Tiny Clanger]

3 hours ago, CasualObserver said:

Like the topic states, I am looking for a good way to avoid dew when bringing my telescope inside.

I have just gotten my Skymax 127 AZ-GTi, and while I have found various tips and tricks, there seem to be very little information about how to avoid dew building up when bringing it inside after observing (the clouds delivered with the telescope has hindered me from actually going outside yet).

The telescope will be stored in an IP65-case with cutted foam, and I hope to also be able to fit the AZ-GTi-part in here. Would it work to just leave the case outside to cool down while observing, and then pack everything up while outside, and then just bring it inside? The idea being of course that the equipment would then be cold, and gradually warm up inside the case over a long time period. Or would this have the opposite effect where the humidity will be stuck inside the case, making it dew up anyway?

As I understand it, the reason for dewing up when going from outside to inside, is that the cold equipment will instantly attrack all the water in the warmer air, and then making it wet. So very gradually warming it up should maybe be a good way to avoid this?

And what about other pieces like diagonal and eye-pieces? I suspect I can not fit them all in the case, so is there some good way to avoid those dewing up?

The foam in your case will insulate the mak . Just like a sweater insulates you, it will slow the flow of heat energy (which always goes from the hot thing towards the colder thing) Putting your kit out inside the case will slow the cooling, just like a sweater slows your cooling. Putting the mak out sealed in a foam lined case will work, but take far longer than putting it out without the case.

The foam is probably open cell, like a sponge, not closed cell, if so, the holes in it can and will hold moisture, so to protect your kit, you need to ensure that foam stays dry. Sealing any damp in would be a really bad idea . 

Unless you live in the desert (and there's none of those nearly nightly sudden thunderstorms I experienced in Arizona ) our air holds moisture, and when you have any good conductor of heat (things which feel cold to the touch in moderate conditions, eg metal, glass , but not plastic or cloth) which is colder than the air around it, water will condense on that good conductor.

Taking a telescope (especially a mak with the front glass corrector plate) out at night means it will need to cool for at least half an hour outside any box, much longer if inside (as I explained above, the box will insulate it) It will cool further while being used , and it will attract dew. You can slow the formation in various ways, but you can't avoid it.

Bringing the kit inside , to a home at a comfortable temperature , will mean it is in warmer air which will cause the condensed water to evaporate. You want this to  happen, you want to encourage a flow of air around the telescope and eyepieces and diagonal etc etc.

You don't want to have the kit warm up and the condensation evaporate inside a sealed case. Where would the damp go ? Into the foam, into small crevices in the equipment, forming water drop marks on the glass, rusting any vulnerable metal and encouraging fungus to grow in those dark damp conditions it loves.

So, cool the kit inside the sealed case if you want : it probably won't  hurt, but it will take ages.

After use, do not pack the kit in the sealed case, it will be wet with dew. Instead, bring it indoors, leave the mak with the caps off (you do not want to trap any moisture where it cannot easily escape ) until the morning. I just carry mine in as one on the tripod , and leave it in the corner of a well ventilated room overnight. Same for the diagonal and eyepieces , leave them uncapped , on their sides,  leave the case wide open too so the foam dries .

Only put the caps back on and pack the kit away in the sealed case when you are confident it is all completely dry.

[Zermelo]

A slight extension: I always follow the advice above with my eyepieces, diagonals, finders, and OTAs.
What about filters? I don't usually bother with them, although they go out in my case and return with everything else, whether they're used or not.
I suppose I may have been thinking that it's just a single piece with two exposed surfaces, nothing internal to worry about ... but?

Do people follow the same routine with their filters?

[Tiny Clanger]

4 hours ago, Zermelo said:

A slight extension: I always follow the advice above with my eyepieces, diagonals, finders, and OTAs.
What about filters? I don't usually bother with them, although they go out in my case and return with everything else, whether they're used or not.
I suppose I may have been thinking that it's just a single piece with two exposed surfaces, nothing internal to worry about ... but?

Do people follow the same routine with their filters?

I leave the supplied flat plastic case ajar on my one expensive filter (UHC) , it lives in the eyepiece case, so it gets treated in a similar way to the eyepieces My small collection of cheap coloured filters and NDs are standing in slots in a tiny case all of their own, again I leave the lid ajar. Maybe overkill, but it fits with my routine of packing everything away next day, and it can't hurt.

[Louis D]

It all depends on the relative humidity inside and outside, and if you're using a watertight case (which an IP65 apparently is).  For instance, in the winter, our outside air in Texas is bone dry most of the time, while the air inside my house is rather moist.  Thus, I cap and pack my eyepieces into my Doskocil watertight cases before going inside to prevent dewing and simply store them away.  I typically cap my OTAs before bringing them inside to slow the otherwise instantaneous dewing, allowing them to warm up with the trapped, dry air inside them.  Once warmed, I remove the covers just to be safe and wait to put them away.

However, in the summer when it's a hot swamp outside and relatively dry and cool inside my house thanks to AC, I bring everything inside uncapped and let it all cool down and dry out before packing it away.

[CasualObserver]

From what I can find both in this thread and other places, the concensus seem to be that the conclusion largely depends on the humidity and if any dew has already formed on the scope.

As long as the air outside is relatively dry (and less relative humidity than inside), AND the optics/scope does not have any dew before packing up, then it would be wise to put the caps on the scope/eyepieces, and put it in an airtight container. This should mean that as the dried, cold air gets warmed up, no dew should form as the air is "dry", and cold air holds less humidity than warm air. Still, it would probably be wise to open up the case the following day to make sure nothing is wet regardless. This also makes sense as locally, the air outside in the winter is usually considerable dryer than inside (typically a difference of 10-15%). I have only had the scope outside for about an hour so far, and it was still dry when packing up. But please let me know if I am at fault in my reasoning here.

 

However, if there is any dew on the equipment, the best solution would obviously be to make sure it dries up before storing it. I would assume that depending on the length of the session, dew is impossible to avoid regardless of how dry the air is? I will probably opt for leaving caps off when going inside in those cases, and putting a towel around it for gathering the moist (and try to limit dust gathering on the optics).

Just to be clear, I was only talking about going from outside to inside. The other way I will just take out all the equipment at once, and let it cool down as quickly as possible.

[Louis D]

6 hours ago, CasualObserver said:

I would assume that depending on the length of the session, dew is impossible to avoid regardless of how dry the air is?

If the dew point is well below the ambient temperature, dew formation is highly unlikely.  If they are close, dew formation is likely.  Just check the relative humidity.  In my experience, anything above 70% can be suspect.  Above 90%, dew is almost guaranteed.

[Carbon Brush]

Something to include.

Cold air holds little moisture. Warm air can hold a lot of moisture.
When the 'relative humidity' reaches 100%, the air cannot hold more moisture and you get dewing.

Parts of a scope pointing at a clear sky can be below ambient temperature.
This is why a scope can dew on a night when other things seem to be dry.

Suppose your house is open to the elements. If the outside is cold and there is no dew, the inside will be the same.
Now you close the doors and turn on the heating.
The house is warm and very dry.

Unfortunately your house does not stay dry for long.
You and your family breathe out water vapour. You boil a kettle, or a saucepan.
This means that indoor air on a cold day holds more moisture than outdoor air.
But it is warm, so the air can hold a lot of moisture without dewing.

Now you bring a cold dry scope indoors. Note the scope contains cold dry air.
There is therefore no point in opening the tube to allow in moist house air.
However, as soon as the cold scope body chills air nearby, dew forms.
The important thing here is that the dew is only on the outside of the scope and will evaporate as the scope returns to house temperature.

If your scope, while outside, is dewed up, the comments above probably apply.
In some cases there may be dew on inside surfaces.
As soon as you warm up the air in the scope, the dew evaporates.

You can just leave the scope to warm by itself. But you can be waiting a long time for a big scope to dry so it can be put away.

A waft over with a hair dryer will quickly bring your scope up to house temperature and allow you to put it into a box.

A good long term precaution with any scope in storage is silica gel.
Keeping the air dry - even without dew - will discourage fungus growth.
I have a few of the eyepiece plugs that contain small silica gel bags.

Sorry for the long post. But dew formation, removal and prevention are very big subjects.

HTH, David.
 

[Stephenstargazer]

Thanks David. I avoid bringing cold scopes indoors and find this beats the problem too. I built a secure wooden ventilated store in the garden and use silica gel in the cases. Has worked well for some time now.

[nephilim]

Whenever I bring the scope/ camera etc indoors after an evening/ night imaging during the colder months I never put it all straight into a warm room.

Im lucky to live in a 200+yr old farmhouse (although it does have drawbacks) with very thick walls of around 2ft. I have a storage room downstairs that's only a few degrees warmer than the temperature outside, so first it goes in there until the morning & then over a period of time goes into warmer & warmer rooms.

This may seem a bit convoluted 😂 but it works very well. I appreciate many will live in a house that has an equal temperature all the way through, that'd cost a fortune in my house (one of the drawbacks I mentioned 😬) but if you have a garage that'd work or a room noticeably colder than the rest of the house. The key is, is to let it warm as slowly as you can. It's similar to if your using a dedicated cooled camera, I never just switch it off when it's at -10⁰c, I let it warm slowly by a few degrees at a time by resetting the cooler temperature. I'm honestly not sure if it makes a difference but I've read about it in a couple of AP related articles so better to be safe than sorry.

Steve

[Zermelo]

2 hours ago, nephilim said:

first it goes in there until the morning & then over a period of time goes into warmer & warmer rooms.

This may seem a bit convoluted

That reminds me of my school history lessons, when we learned how the Romans used to organize their bath houses.

[nephilim]

11 minutes ago, Zermelo said:

That reminds me of my school history lessons, when we learned how the Romans used to organize their bath houses.

They probably had warmer houses than mine 😂

[gilesco]

On 20/09/2021 at 10:07, Jiggy 67 said:

Your scope and all your other equipment will dew up whilst being used outside, sometimes to the point of being soaked. You can minimize this with dew strips etc but at the end of the day, there’s no way of stopping it. To place it in a case will just keep it wet for longer, it would be the last thing I would do. Just take the scope in and leave it with covers off outside the case to warm up gradually and it will be fine

Exactly, I stagger this and bring it into the conservatory first, then when I've had a rest, and the heating has come on,  I cover the scope and put it back into storage.