Using a telescope is sub zero temps

[PaulM]

Hi,

Plan to go to a dark sky location tonight but it looks like it will be sub zero temps - is this practical? will the temps just ice over\damage the mirror if I am out for a few hours and same for the eyepieces etc - I have a 14" dob

thanks

Paul

[andrew s]

Should not damage your scope. Years ago I had a brass RSA threaded eyepiece freeze to my eye! 

Too wimpy now observe remotely from a warm room.

Regards Andrew 

 

Edited December 1, 2019 by andrew s

[Physopto]

Nothing to do with telescopes but many years ago whilst riding a motorcycle (several times) in sub zero temps. When stopping at traffic lights or at the end of a fair journey,  my legs and arms were so cold I found I could not use them and just fell over with the bike. Motorbike clothes in those days were rubbish for the cold.

Same almost at Kielder a few years ago, as the nights imaging progressed into the early hours my legs just stiffened up to the point of not working. Found it dammed hard to get up out of the chair. By 0400-0430 both myself and Robbie Ince  had to call it a night as we both were physically not able to carry on. Ahhh!  the benefits of a warm caravan awning became apparent.

Derek

[RT65CB-SWL]

I would get everything setup beforehand and let it equalise to the ambient temperatures... and when you have put everything away, un-pack everything when indoors in an unheated room, (preferably away from heat sources; i.e. radiators, storage heaters, etc.), or in a garage and let it all 'dry out' naturally.  

[Space Hopper]

Just have a shorter session before you get too cold.

Simple. The scope will be fine. Use it, enjoy it.

I am at an age now where i feel the cold.

I was out at our darksite on Friday evening ; just a 3 hour session, but it was plenty long enough to see all that i needed to see. It was great.

Wrap up warm 😀

[Stu]

The scope will be fine, and to a degree it all depends on the conditions as to whether the mirror mists up or not. It certainly won't do it any harm, so long as it is dried out correctly as Philip says.

If you have dew heaters it is worth having these handy, particularly for the eyepiece to stop that fogging up when very cold. Keep the scope pointed away from vertical when not using it to avoid the mirror radiating too much heat and dropping below ambient.

[Paz]

I've found that the heat from my head helps stop eyepieces dewing up. 

If I'm not at the eyepiece for a while I'll either cap the eyepiece or take it off the scope and put it away to stop it cooling down so much.

[markse68]

2 hours ago, Stu said:

Keep the scope pointed away from vertical when not using it to avoid the mirror radiating too much heat and dropping below ambient.

Stu I read this in another thread and don’t understand the physics of it- would it really drop below ambient pointed up and is that because of convection currents cooling the air inside the tube more than ambient?

[andrew s]

19 minutes ago, markse68 said:

Stu I read this in another thread and don’t understand the physics of it- would it really drop below ambient pointed up and is that because of convection currents cooling the air inside the tube more than ambient?

When pointing to the sky the  mirror will radiate its heat to the sky which is close to 4k and vice versa.  When pointing to say a wall it will radiate to the wall at say 4c etc. It might reach equilibrium with the wall but not the sky!

Regards Andrew 

[markse68]

Ah- so the rate of radiance doesn’t change but the rate of absorption does?

[Stu]

38 minutes ago, markse68 said:

Stu I read this in another thread and don’t understand the physics of it- would it really drop below ambient pointed up and is that because of convection currents cooling the air inside the tube more than ambient?

Hi Mark,

Andrew understands this stuff far better than I do, but this relates to radiation rather than convection. The mirror will radiate heat away if exposed to a clear sky above until it is below ambient.

Dew shields on refractors and SCTs/Maks etc work by shielding the front element from exposure to the sky above for similar reasons. They are ineffective when you are pointing straight up, but work any time you are pointing away from vertical enough for them to provide some shielding from the cold sky above.

[andrew s]

2 minutes ago, markse68 said:

Ah- so the rate of radiance doesn’t change but the rate of absorption does?

Basically objects radiate proportional to T^4 so the sky radiates very little back the wall a lot more. So as @Stu said the mirror will continue radiating to the sky and fall below ambient.

Regards Andrew 

[Sunshine]

This is a very common question, living in Canada means the clearest skies are often the coldest, i have been out all night in -10 celsius. My toes and fingers suffered a lot more than the scope ever has.

Edited December 1, 2019 by Sunshine

[Physopto]

Hi, Sorry but this is not quite correct. quote " Dew shields on refractors and SCTs/Maks etc work by shielding the front element from exposure to the sky above for similar reasons. They are ineffective when you are pointing straight up, but work any time you are pointing away from vertical enough for them to provide some shielding from the cold sky above. "

If you view any part of the sky that is not shielded by clouds then you will radiate energy.

The dew shield will only provide some shielding against light and energy from outside the cone of its view. during the night that is why you need to provide some sort of heating to ensure that the objective is above dew point, as you are radiating energy outwards from the scope to the sky.

You can show the effect by filling a bucket with an inch of water and shielded from external influences. By that I mean insulation below and around the sides.  Leave it out on a clear night pointing in almost any direction at the sky. ( yes I know that water will flow to one side if tipped, so it works best if pointing straight up). The water will reduce in temperature because of energy radiation. Eventually if the night is long enough the water will freeze, but  the temperature of the frozen will continue to drop, until the sky is no longer acting as the receiver of the energy, i.e. morning light.

Hope that helps a bit , 😀

Derek

 

 

[Stu]

1 hour ago, Physopto said:

If you view any part of the sky that is not shielded by clouds then you will radiate energy.

Hmmm, I can now see the hopelessly flawed part of my logic Derek, thank you 🤣🤣

Is the effect of radiation most extreme when pointing up through the thinnest part of the atmosphere vs when you are pointing down lower? Otherwise I'm wondering why (if!) they work at all? Certainly adding dew heaters seems to be the most effective method of prevention.

[andrew s]

Dew shades both reduce the angle on the sky the optic can radiate into and limits the air flow near the optic which reduces the moisture that can condense on it from the air.

Dew heater heat the optic  counting the radiation loss.

Regards Andrew 

[markse68]

Perhaps they absorb more heat from the environment than they lose through radiation countering the loss through the open aperture to sky- a sort of passive dew heater? I still think condensing moisture in the air must fall and they would shield from that if not pointing straight up

[Stu]

22 minutes ago, markse68 said:

I still think condensing moisture in the air must fall and they would shield from that if not pointing straight up

Yep, that bit makes sense

[Physopto]

Yes, you will find that dew tends to form more on a surface pointing upwards. Due to gravity 🙂 So when at some angle it does reduce the condensation, at least that is what I have experienced. Stu is quite correct that upward pointing tends to be the worst case due to thinner air layer as opposed to the greater air layer the energy has to penetrate to escape when at some angle. 

What always amazes me is how two almost identical ( size of optics) scopes can act very differently in the same conditions with respect to dewing up. 🥶

Derek