Cooling Down - Caps On or Off?

[DMH]

Pretty simple one, when leaving your scope outside to cool down for viewing, do you leave caps on or off?

My understanding: off to help gases escape.

My Scope: 1200mm 6" Dob

Cheers,

Daniel

Edited October 3, 2021 by DMH

[Carbon Brush]

The sensible answer depends on scope type & size. What scope are you thinking about?

[DMH]

13 minutes ago, Carbon Brush said:

The sensible answer depends on scope type & size. What scope are you thinking about?

6inch Dob. Edited first post in case anyone else wonders! I assumed the cooldown process was similar for most scopes!

[Nik271]

For a Dob or any open tube scope I would take the cap off to let the warm air escape, and in general this will take very little time, while the mirror is well protected inside the tube.

For a catadioptics I tend to leave my caps on because the front corrector plate will dew up pretty quickly just sitting outside. The colling of a closed tube scope is largely by radiation and not so much from convection, caps do not affect the speed so much.

Edited October 3, 2021 by Nik271

[DMH]

4 minutes ago, Nik271 said:

For a Dob or any open tube scope I would take the cap off to let the warm air escape, and in general this will take very little time, while the mirror is well protected inside the tube.

For a catadioptics I tend to leave my caps on because the front corrector plate will dew up pretty quickly just sitting outside. The colling of a closed tube scope is largely by radiation and not so much from convection, caps do not affect the speed so much.

Amazing, thank you!

[M40]

Logic suggests that as your DOB is cooling down, it is warmer than the outside air, therefore dew is not a problem so I am going to say off. I wouldn't point it straight up though. To be honest, I keep my telescopes in the shed so they are only a few degree's if that higher temperature than outside so it's not something I have really considered. I tend to leave a loose cover over the dew shield and by the time I have polar aligned it, it's good to go. Be interesting to see other answers.

[Peter_D]

 I have a 6 and an 8 inch SCT. For cool down,  I place them standing (with their caps on to protect the corrector plate) in a box. I place a piece of gauze over the rear opening just to stop anything falling or crawling in. Not sure this is the 'correct' way but it seems to work.

[callisto]

I leave the front cap on my Dob and take the focuser cap off, I have a DIY fan on the base so when its on you can feel the air coming out of the focuser

 

Mark

[John]

Caps off - focuser and tube cap for my 12 inch dob, diagonal cap off plus scope pointing downwards for my refractors.

 

Edited October 3, 2021 by John

[Mr Spock]

Caps off, tube at least level. My Dob is in a shed so cool down times are fairly short anyway.

[Dave scutt]

Home made fan and kept in the shed but I take the caps off.

Don't have it pointed upwards just incase a bird poop's  into it 

[dweller25]

Dob - horizontal, all caps off, fan on.

If you don’t have a fan you can make one……

[Dave scutt]

7 minutes ago, dweller25 said:

Dob - horizontal, all caps off, fan on.

If you don’t have a fan you can make one……

Do you get any vibration from the fan being screwed in I have mine hanging with hair bungees 

[John]

My 12 inch dob has a small fan but I don't use it. The mirror cell has an open design so it seems to cool quite quickly.

 

 

[scarp15]

As quite unanimously mentioned, tube caps off and with front end of tube slightly tilted downward, at least for a time. If set up prior to using for an extended period, I will return the caps, but leave the tube level / front end pointed downwards slightly. This is just to prevent any chance of condensation build up or any breezes that blow in dust participles (or chance of possible drifting smoke). Also have fans on mine, but never use.

[Tiny Clanger]

3 minutes ago, scarp15 said:

As quite unanimously mentioned, tube caps off and with front end of tube slightly tilted downward, at least for a time. If set up prior to using for an extended period, I will return the caps, but leave the tube level / front end pointed downwards slightly.

+1

I do the same to allow airflow to disperse any condensation when I bring the 'scope indoors after use .

[dweller25]

1 hour ago, Dave scutt said:

Do you get any vibration from the fan being screwed in I have mine hanging with hair bungees 

No vibration as the fan was mounted on rubber grommets. 
The important part is the aluminium baffle, which gives maximum cooling and pushes the warm air right out of the tube.

[Carbon Brush]

The important thing to remember on newt cooling is that the tube will reach ambient very quickly due to it having a large surface area to volume ratio, and (steel or ali) being a good heat conductor.

The main mirror is a different matter. Glass is an awful thermal conductor and the main mirror is a thick slab. It therefore cools slowly and the heat distribution is uneven during cooling.
This results in the possibility of thermals in the tube when first taken out, then the mirror not quite being the right shape.
Though having said this, pyrex glass has a very small expansion on heating so uneven temperature distribution is a small effect.
Also a 6" scope will have a fairly thin mirror.

In practice, for a 6" scope, the setup time for a tripod and mount, then fiddling with finder alignment, then aligning goto, and the rest, means your scope is well on the way to cooling by the time you are.
Don't forget if you are trying to align a goto, or check a finder alignment, thermals or lack of pinpoint stars don't bother you.

If you want to help cooling, then take the scope from your cosy 25C lounge and put it into your garage or shed for a while before going outside.

HTH, David.

 

[scarp15]

The other thing I do is always set the scope / dob up onto a mat, serves a few purposes but might just mitigate some condensation rising from the ground. 

[RT65CB-SWL]

When using my ETX105 and/or C6/SCT caps off and visual back pointing up, so any heat trapped can escape.

When finished and back in the warmth of a room, remount ‘scopes with the objective cell pointing down, so any external dew formation can run off without getting trapped inside the OTA.

I also leave my caps off my e/p’s and other accessories and place them in a draining rack while I take some sleep/rest, and pack away when I have recovered and refreshed.