35 replies to this topic

[RobVG]

Starting to have nights below freezing here. 

Edited by RobVG, 05 December 2024 - 12:36 PM.

[Inkie]

At least and hour, but it could be closer to two if you go from home environment to fully outdoors.  I keep my 12" Dob in an out-building, unheated, garage, and I can use it within 30 minutes typically.

[Mike Q]

My scopes are in a well ventilated outbuilding.  As a result they are always within a degree or two of ambient air temp.  So by the time I roll it out and set it up its pretty much ready to go.  Going from the house to outdoors, you will have a solid hour at a minimum 

[sevenofnine]

Setting it out at dusk works for me. It's easier than trying to navigate in the dark too 

[sheath]

If you have a dew issue, waiting until after the dew point to uncover might not be a bad idea.

I've got a Gizmos cover, and one evening took it off a bit too soon. lol

It was in the low 20s by the time I got outside, and this was the result.

[Tony Flanders]

If you have a dew issue, waiting until after the dew point to uncover might not be a bad idea.

I've got a Gizmos cover, and one evening took it off a bit too soon. lol

It was in the low 20s by the time I got outside, and this was the result.

That looks the way a telescope should in the winter. There's a very slight accumulation of frost on the outside, which has zero effect on the telescope's performance. Frost gets to be annoying when the scope looks solid white instead of black flecked with white, and you can scrape large volumes of frost off with your fingernails. But even in such situations my Dobs' mirror usually remain unaffected.

 

In any case, there's no such thing as "waiting until after the dew point." The accumulation of dew -- or in this case frost -- increases steadily throughout the night in normal conditions.

 

I prefer to leave my Dobs out for at least an hour before using them, but that's typically not an option which I'm in the city, in which case there's nowhere safe to leave the scope outside, and in any case I will be driving to a remote location. If my scope isn't fully cooled, I concentrate on objects that can be viewed well at low magnifications.

 

A built-in fan can be a huge boon.

[sheath]

That looks the way a telescope should in the winter. There's a very slight accumulation of frost on the outside, which has zero effect on the telescope's performance. Frost gets to be annoying when the scope looks solid white instead of black flecked with white, and you can scrape large volumes of frost off with your fingernails. But even in such situations my Dobs' mirror usually remain unaffected.

 

In any case, there's no such thing as "waiting until after the dew point." The accumulation of dew -- or in this case frost -- increases steadily throughout the night in normal conditions.

 

I prefer to leave my Dobs out for at least an hour before using them, but that's typically not an option which I'm in the city, in which case there's nowhere safe to leave the scope outside, and in any case I will be driving to a remote location. If my scope isn't fully cooled, I concentrate on objects that can be viewed well at low magnifications.

 

A built-in fan can be a huge boon.

Try grabbing a frozen eye piece or focuser with bare fingers. lol 

If I wait until the temp crosses the dew point at sub freezing temps none of the components get frosted.

Might be different for your location?

Here is a picture from the night before.  Same basic conditions I just waited until later to pull the cover off.

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[Sebastian_Sajaroff]

My 6" F/8 Dobson took 30-45 minutes to cool down during Canadian winter nights.
In autumn and spring it was close to 15-20 minutes.
You’re ready when the view stops boiling like crazy.

[SeattleScott]

Larger aperture takes longer. For 8” or so, an hour is probably plenty. Personally I would start observing right away, and just focus on lower magnification targets first, then medium power targets, and after 45 minutes of that the mirrors should be acclimated for high power.

[BIG]

Another recommendation for setting up equipment at dusk with some helpful twilight, and coming back an hour or two later to observe. In my case, after my wife and I and our three pups have finished dinner, which usually results in all of them settling in for the evening and leaving me undisturbed.

[AlamoBob]

In the High Desert, I think the temps after sunset can fall faster than the glass can keep up with.  I guess I'll know when I actually BUILD my telescope!

[RobVG]

A new tarp is in order....

[rrpallechio]

I set my SCT up outside well before sunset.

[Sebastian_Sajaroff]

Another recommendation for setting up equipment at dusk with some helpful twilight, and coming back an hour or two later to observe. In my case, after my wife and I and our three pups have finished dinner, which usually results in all of them settling in for the evening and leaving me undisturbed.

That’s a great idea, it’s always easier and safer to setup your telescope with some light outside.

[Keith Rivich]

My "cool down" time is covered in the set up and collimation time. So once collimated I am ready to go. Keep in mind I have open truss scopes so there is very little chance of seeing tube currents that can plague closed tube scopes. 

 

If the temperatures are dropping so fast the mirror can't keep up the atmospheric seeing is going to be so bad no amount of mirror cooling will improve things. Here in SE Texas we normally have pretty good seeing right after sunset. Great time for planets! However, once the temps start to tank the seeing can be pretty bad until around midnight or so when the atmosphere finally starts to settle down. 

 

I run my fans mostly for dew control. I don't see any visual improvement once seeing settles down. 

[Jon Isaacs]

Starting to have nights below freezing here. 

 

If the temperatures are be slow freezing and your scope is at room temperature, it may never cool down adequately.

 

My conditions are very mild, I still allow at least an hour and it takes more. But I'm expecting a stable atmosphere and a scope that's providing it's best at high my magnifications.

 

Jon

[Napp]

As Tony pointed out, there is no such thing as crossing the dew point.  The dew point at any given time is the lowest that air temperature can drop to.  If the dew point continues to drop then temperature can drop.  If temperature equals the dew point condensation starts in the form of dew, fog or clouds.

[Tony Flanders]

As Tony pointed out, there is no such thing as crossing the dew point.  The dew point at any given time is the lowest that air temperature can drop to.  If the dew point continues to drop then temperature can drop.  If temperature equals the dew point condensation starts in the form of dew, for or clouds.

It's a bit more complicated than that. Dew generally starts to form when the air temperature is well above the dew point. That's because radiational cooling makes surfaces cooler than the air on a clear night. Moreover, the dew point down near the ground is often higher than the dew point at chest height, where it's officially measured.

 

But the underlying statement is true. As soon as the air temperature reaches the dew point, water condenses into some liquid or solid form -- either a film on a surface or droplets in the air, a.k.a. fog. The process generates a huge amount of heat, just as its reverse (evaporation) absorbs a huge amount of heat. So the air temperature never falls below the dew point.

 

It's possible to cool air slightly below the dew point in a laboratory setting, in which case the air becomes supersaturated with water. This can also happen in the upper atmosphere, where there's a shortage of condensation nuclei. But it never happens near the ground in everyday life.

[Mike Q]

If the temperatures are be slow freezing and your scope is at room temperature, it may never cool down adequately.

 

My conditions are very mild, I still allow at least an hour and it takes more. But I'm expecting a stable atmosphere and a scope that's providing it's best at high my magnifications.

 

Jon

Your normal condition are what most of us can only dream about

[Jon Isaacs]

Your normal condition are what most of us can only dream about

I agree.

 

Rob lives in White Rock, British Columbia and is wondering about cool down as the temperatures are dropping below freezing.  In San Diego where I observe with my 10 inch Dob, there have been 7 days in the past 160 years when the temperature was freezing or below. The average winter low is 50 degrees, the average winter high is 66 degrees. 

 

But I believe the value of my experience for someone like Rob is that even in such a mild climate, the time it takes for my 10 inch Dob with dual fans to cool down for optimal performance is more than an hour.  If it is freezing outside, going from indoors to outdoors, that is going to take quite a bit longer.  

 

Of course, optimal performance is the primary concern under optimal conditions. Other's, like Tony and yourself, have more practical concerns, will the telescope freeze over? 

 

Every climate has it's advantages and disadvantages.. If I were an ice fishing enthusiast, I would be one frustrated dude. 

 

Jon

[Mike Q]

I agree.

 

Rob lives in White Rock, British Columbia and is wondering about cool down as the temperatures are dropping below freezing.  In San Diego where I observe with my 10 inch Dob, there have been 7 days in the past 160 years when the temperature was freezing or below. The average winter low is 50 degrees, the average winter high is 66 degrees. 

 

But I believe the value of my experience for someone like Rob is that even in such a mild climate, the time it takes for my 10 inch Dob with dual fans to cool down for optimal performance is more than an hour.  If it is freezing outside, going from indoors to outdoors, that is going to take quite a bit longer.  

 

Of course, optimal performance is the primary concern under optimal conditions. Other's, like Tony and yourself, have more practical concerns, will the telescope freeze over? 

 

Every climate has it's advantages and disadvantages.. If I were an ice fishing enthusiast, I would be one frustrated dude. 

 

Jon

Yes ...the telescope will at some point be frozen, been there done that.  

[edsmx5]

I agree.

Rob lives in White Rock, British Columbia and is wondering about cool down as the temperatures are dropping below freezing. In San Diego where I observe with my 10 inch Dob, there have been 7 days in the past 160 years when the temperature was freezing or below. The average winter low is 50 degrees, the average winter high is 66 degrees.

But I believe the value of my experience for someone like Rob is that even in such a mild climate, the time it takes for my 10 inch Dob with dual fans to cool down for optimal performance is more than an hour. If it is freezing outside, going from indoors to outdoors, that is going to take quite a bit longer.

Of course, optimal performance is the primary concern under optimal conditions. Other's, like Tony and yourself, have more practical concerns, will the telescope freeze over?

Every climate has it's advantages and disadvantages.. If I were an ice fishing enthusiast, I would be one frustrated dude.

Jon

If you start ice fishing in San diego, or in the high desert, we're all in trouble

[Jon Isaacs]

If you start ice fishing in San diego, or in the high desert, we're all in trouble

 Probably only me.. 

 

"Jon has finally gone off the deep end. We knew it was coming, the only question was "when'". 

 

jon

[Mike Q]

If Jon goes ice fishing i will be there to watch, with a camera

[vtornado]

12 inch dob takes roughly 2 hours to cool for me with a 20 degree temp delta.  I store the dob in my garage, but my garage is insulated and attached to the house on two sides.  Winter temperatures can be 20 degrees warmer in the garage than outside.

Maybe someday I will erect a garden shed, that should stay much closer to ambient. 

 

Also some nights with a clear sky in the winter temps can fall 10 degrees per hour and the mirror never keeps up.  Counter to that seeing is probably bad anyways with that kind of temperature delta.

 

I suppose the same thing can happen if you drive to a dark site in the winter and want heat in your car.  .   Maybe you can swaddle the tube with blankets???

 

This is more important for high power than low.

Edited by vtornado, 06 December 2024 - 02:17 PM.