45 replies to this topic

[EsaT]

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.

Also observer breathing adds some extra water into air.

And have certainly noticed tube surface near focuser getting frosty first.
 
 
 

 

 Probably only me.. 

 

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

 

jon

We have this same joke in Finland...
https://www.reddit.c...ery_first_time/

[AlamoBob]

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

Not to worry, the Ice would be in my glass, and drowned in Pendletons.  AKA "Turbo Focuser Fluid"

Edited by AlamoBob, 06 December 2024 - 05:11 PM.

[Inkie]

A new tarp is in order....

Even a doubled-up unused bedsheet works wonders. Laundered, natch.  Use the tarp money for a nice red flashlight or something.

[Echolight]

If it's below freezing, I'm probably not setting up a dob. Or an SCT.

The yard cannon is stored outside and does pretty well with keeping up with the slow temperature drops around here.

Just a little frost on top.

 

[daveb2022]

At this time of year I like to set up by 4:30pm and am observing in an hour. I observe with a C925/NP-101. The refractor is much more forgiving and I don't see much of a difference in cool down. I can generally pull it out of a 70 degree storage temp and into 40-50 degree temps w/o issues.

[RobVG]

I assume the ideal would be to let all components cool to the ambient temperature. Less than that should take care of tube currents but do the optics also need to cool to settle into their final shape?

[Echolight]

I assume the ideal would be to let all components cool to the ambient temperature. Less than that should take care of tube currents but do the optics also need to cool to settle into their final shape?

If by "reflector" you mean your C11, I think it is common practice, SOP, to combat the cold with insulation (reflectix). Effectively slowing temperature swings moreso than trying to reach ambient.

[Tony Flanders]

I assume the ideal would be to let all components cool to the ambient temperature. Less than that should take care of tube currents but do the optics also need to cool to settle into their final shape?

It turns out that the main problem isn't tube currents, as once thought, but convection right near the surface(s) of the objective. Which can be a problem even with a temperature differential as small as 1 degree C (2 degrees F).

 

The idea behind insulating an SCT is to keep the temperature inside the insulation uniform, so that while there may be a temperature differential between the inside and the outside, there's no differential between the mirror and the air touching it.

 

With an open-tube reflector, by contrast, you want maximal air flow along the surface of the mirror. As far as changing the shape of the optics is concerned, that's usually only a problem when the temperature is changing very rapidly.

[spaceoddity]

I usually set up about an hour before dusk and I have the fan on until I am ready to observe. I then run the fan for a few minutes periodically throughout the night. Of course that depends on how quickly the temperature is dropping.  

[UnityLover]

0 seconds.

I'm of the belief that cooling is actually optional in 5 inch and below. I've gone out in the cold, then immediately started observing (my time is usually limited to an hour). Stars are pinpoint in the center. I can even go straight out and immediately start planetary, with no obvious decrease in quality.

[daveb2022]

With minimal cool down time, my 7" Mak seems most affected followed by my SCTs. Last night I got out my XT-8 dob and it didn't seem to need much cool down time at all. But it was only going from 70 degrees to temps in the mid 50s. 

[doyers017]

I guess it depends on what scope you have. SCT and maks probable need the most time.

[Sebastian_Sajaroff]

6" F/8 Dobson : 45 minutes cool down during Canadian winters (from 22C inside to -25C outside).

[scotsman328i]

My 8” Orion XT8…around 20-30 mins depending.

my ES Discovery 12”…an hour or so.

[dnrmilspec]

As I live in Arizona my answer is far from the average one. 

 

If I have time I set it outside for an hour or so. 

 

If I am in a hurry I look at stuff anyway, knowing that the view will improve with time.  A tour of open clusters will usually be nice even in a scope that has not reached equilibrium.  

 

Even when I lived in Montana and Utah I still followed this course.  If you are enjoying what you are seeing, even if it could be a bit better, you are not wasting your time. 

 

We sometimes obsess about stuff, and let perfect become the enemy of good enough.

[JayinUT]

I’m in Utah and I am much like dnrmilspec. I usually have my scope set up for an hour and have the fans on and then I cut the fans off. I usually don’t need them. Dew is not something I usually contend with, perhaps two or three times in either fall or spring. I have a dew system for that if needed. 

[izar187]

...you drive to a dark site in the winter and want heat in your car.  grin.gif.   Maybe you can swaddle the tube with blankets???

___________________________________

 

If you are driving to a dark site in winter,

then you have cold weather clothing.

Put some of it on, for the drive there.

Then open the newt ota, and the space

around it, to some flow thru ventilation

on that drive out to the darker sky.

 

IME the newt will be ready for near to

max magnification, by time you're there.

Obviously some bit of experimentation

is involved in timing this, and widows to

open or keep closed during the drive out,

as well as the cab heat level for comfort.

 

But this does work for this circumstance,

and eliminates waiting for scope cooling

on site, after driving out to winter dark sky.

Really helpful on worknights, where time

constraints deter deploying larger scopes.

But also works for all smaller scopes.

Edited by izar187, 17 February 2025 - 03:36 PM.

[izar187]

I guess it depends on what scope you have. SCT and maks probable need the most time.

I do not know, as I have no SCT's nor mak's.

But if I understand it, reflectix insulation is

the workaround for this long time classic issue.

I believe it eliminates much or most of the

cooling issues for these closed tube reflectors.

 

If you have not already done so, check with

the folks in the SCT forums on this.

[Keith Rivich]

0 time. My 25" scope is stored in a trailer so its always at ambient. My 18" is in my garage and hauled to our site in the back of my truck. I do have a sectioned cover over the bed but I leave one panel open so air flows through. 

[star acres]

Cool down. A 114/500 ? Impatience is a virtue. A few minutes at most.

[areyoukiddingme]

For quartz 8" F7, and deltas of temperature of perhaps 20F, I find I'm doing about as good as I am going to usually under 15 minutes.

 

For quartz in 12.5" F5 Portaball, same temperature delta takes a lot of time with fan running, usually over an hour before things are looking settled.