Cooling a reflector telescope - caps on or off

[Gohan75]

Worried about the secondary mirror dewing up is it best to cool a dob down with the caps on or off? i have a dew shield.

[Spec-Chum]

Just leave it horizontal with cap off, that should stop any dew forming.

Personally, I use mine from the minute its outside, but I stick to lower powers for half hour or so. It's not unusable whilst its cooling it's just better when its cooled.

[Delboy_Hog]

Very interesting question, as i've been dabbling with leaving the lens cap on my scope. I was hoping it would delay dewing, but I'm now wondering whether it just prolongs cool-down time, and then potentially increases the chance of dew because the lens could still be warmer than the surroundings, and therefore cool faster once you take off the cap? I think Spec-Chum might be right - horizontal and cap off, then dew shield on before you start viewing. I've even seen people mentioning pre-cooling their dew shields to prevent air currents from that! Every little helps I guess.

[swamp thing]

I do the same as Spec-Chum and start using my dobs straight away, just taking a break for a quick brew normally.

Mirrors work fine on low res deep sky objects when cooling, as said only high power observing need be avoided.

Have fun

[Gohan75]

thanks for the advise all.

[LukeSkywatcher]

Ditto to Spec-chum and Swamp thing.

[Mr Q]

The idea is simple - when you first bring the scope outside and it's not cooled down, there's no need to wait to observe. Before it's cooled down, observe objects that don't need good resolution (galaxies, nebulae, etc.) After a while you can switch to objects that do require good resolution (planets, star clusters, double stars, etc.)

When to know when your scope is cooled down? Observe a bright star overhead and using a fairly high power EP, rack the focus way out or in so that a big circle of light appears. Providing the sky is steady (no turbulants aloft), when the cool down is complete, there will be no "watery" waves of light in the image.

[LukeSkywatcher]

I take my scope out and generally start observing from the get go. I start with solar system objects or big (DSO) gradually move on to FAINTER/SMALLER DSO as the scope cools.

[Catweazel]

I leave mine out for 30 mins or so with the caps on, but it's just a strange habit that I have. I think that the above advice is sound; start with the Moon, Jupiter etc, and move out as it cools down.

[stonemonkeylives]

+1 as LukeSkywatcher, tend to stat with planets and the moon straight of the bat and then once the scope has cooled enough, go hunting for fainter objects.

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[John]

I take the caps off and point the tube upwards at around 45 degrees. My aim is to get the warmer air currents out of the tube as quickly as possible. On really cold nights this can take some time - some stubborn currents hug the tube wall and seem to take longer to dissipate. My 12" has a fan behind the primary mirror which speeds the process up a bit.

If you look at an out of focus star image it's easy to see the currents remaining in the tube. On cold nights you can see the heat from your body entering the tube through the tube walls too sometimes as you stand close the the scope.

[swamp thing]

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+1 as LukeSkywatcher, tend to stat with planets and the moon straight of the bat and then once the scope has cooled enough, go hunting for fainter objects.

Sent from my Nexus 7 using Tapatalk 2

You'd be far better waiting for the scope to cool before trying to view planets. Planets are high res objects and require the optics to be thermally steady. Galaxies are low res objects that aren't effected by cooling optics. Faint fuzzies don't require cool optics, planets do, you'll get better planetary views if you wait for your optics to cool.

[jimmyjamjoejoe]

I stick mine out with the cap off but resting on the top, and the fan running for at least 40minutes.

[Moonshane]

I don't really think about it. I put the scope out at about 45 degrees with no cap on and start the fan. I then collimate and then get the rest of my stuff, sort out a brew, get my book and hat etc. by then you can start looking at lower powers and build the power as conditions allow. sometimes I leave the fan on and sometimes I forget it's on.

it's only in extreme cases that you'll have dewing of the mirrors in a newt (perhaps more in truss dobs with no shroud) or if you leave it pointed straight up at the sky for a long time and it's quite damp - the mirror might lose so much heat to space that it falls below dew point temp. this is not something that happens to me though so don't worry about it. it happened once recently when I was observing Jupiter in fog at 6am. it was pretty much at the zenith and after an hour or so the primary dewed up - pretty extreme circumstances though, unlikely to be repeated unless desperate for views of anything in any conditions at a star party which had been fogged out.

[paul schofield]

I leave the cap off but put a hair net over the tube because I know if I don't, a big dusty moth will fly into my OTA and leave an outline of itself on my mirror.

[Episkey]

I keep my dob out for 30 minutes caps off before I start observing. During the time, I go over possible targets, observing lists, etc. You can also get your eyes dark adapted while getting familiar with the sky. If you use your scope directly, I wouldn't start with planets because of reasons above. You need to get rid of the thermal gradient inside the 'scope.

[Eastridge]

Caps off, just down from horizontal for 30 mins. Didn't realise I could look at fuzzies whilst still cooling so thanks to those that suggested that.

[Gohan75]

thx all

[Mr Q]

With my 250mm newt, I tried all different OTA positions and found that keeping the tube (caps off) in a horizontal position resulted in the quickest cool down time. As the heat in the tube rises out the top edge of the tube, cool air enters the bottom edge. Never used a fan but suppose it would decrease cool down time some.

[knobby]

With my 250mm newt, I tried all different OTA positions and found that keeping the tube (caps off) in a horizontal position resulted in the quickest cool down time. As the heat in the tube rises out the top edge of the tube, cool air enters the bottom edge. Never used a fan but suppose it would decrease cool down time some.

Wouldn't that be vertical...? As in pointing up.

Horizontal would be laying down

[Moonshane]

I can see that horizontal would actually allow the tube to lose heat more quickly as a wider aspect is facing the cold sky. not sure it would affect the cooldown of the mirror though which would probably take as long no matter what position.

[Mr Q]

No, horizontal to the ground. Pointing up, there is no easy way for cool air to replace the warm air rising out of the tube and point down, the warmer air would rise up to the primary end and take longer to cool unless it can escape from vent holes. By being horizontal, warm air can easily escape out the top edge of the tube's end while cooler air can be sucked in to replace the exiting warm air at the bottom of the tube end edge

[acey]

Caps off and start observing straight away. A less-than-perfect view is better than no view at all. While you're sitting indoors for half an hour with your scope unused outside you could be outside for half an hour getting properly dark adapted.

[knobby]

No, horizontal to the ground. Pointing up, there is no easy way for cool air to replace the warm air rising out of the tube and point down, the warmer air would rise up to the primary end and take longer to cool unless it can escape from vent holes. By being horizontal, warm air can easily escape out the top edge of the tube's end while cooler air can be sucked in to replace the exiting warm air at the bottom of the tube end edge

Horizontal is lying down... As in the horizon, I'm sure you mean vertical... And yes, I agree vertical allows the air to go up and out of the its, just watch out for bird poop

[Moonshane]

I'm positive he meant horizontal!