Cooling a telescope down before using?

[tombulgin]

I have a 250 truss dob and wondered what the ideal amount of time might be to cool the scope down before I use it?? I think I'm always impatient to crack on but want to try and add a bit of order to things!

And what about eyepieces, should they be cooled down or kept warm in my pocket?

The scope has a light shroud. Should I open the shroud up to hasten the cooling? Or try and keep some warmer air for longer in the primary end??

And should I remove the eyepiece 'plug' to increase cooling airflow up by the secondary mirror?

the sky is looking clear where I am right now and I really want good viewing without much moon around until later.

Any advise appreciated.

Cheers

tb

[blinky1983]

I always allow 1 or 2 hrs cooling as a min, Taking the end caps and EP dust cap off and also let my EP's cool aswel. I wouldn't worry about removing the shroud...

Once I bring everything back indoors I leave it as it is till the equipment has warmed up before replacing the caps etc..

[Stephen]

As above on the cooling, a couple of hours will set you right.

However I don't cool my EPs, they stay in their insulated/padded case or in my pockets.

[steelfixer]

Hi there

The amount of time to cool her down will depend on where you keep her.

If you keep her in your centerally heated home then it will take a lot longer than if you keep her in a garden shed or garage.

There is a fair old lump of glass in that 250 of yours so it is going to take a couple of hours to cool down if it is kept indoors.

I always remove my covers to cool mine down but mine is a closed tube rather than an open truss like yours.

Other truss owners will be along shortly to give thier opions and advice based on first hand knowalage.

As for the EP's I alwys keep them indoors before I use them. Mind my set up is in an obs so all my gear is around ambient temp all the time.

[blinky1983]

As above on the cooling, a couple of hours will set you right.

However I don't cool my EPs, they stay in their insulated/padded case or in my pockets.

I cool my ep's as most the time thats where I suffer most of the dew. I keep everything in a central heated house and its hot.

[Stephen]

I cool my ep's as most the time thats where I suffer most of the dew. I keep everything in a central heated house and its hot.

I'm not suggesting for a moment you are incorrect (what works for one may well not work for others,) but dew is the reason we have dew heaters, not dew coolers?

[Mr Q]

You might wonder just how long you have to wait before cooling down reaches a useable level. Every half hour, look at a bright star near the zenith with the focus racked out or in to produce a big "ball" of light in the EP. If you see any swirling of the light (like in a mirage), the scope needs more time.

Even if it's not cooled down completely, you can still get good images when looking at "extended objects", such as open clusters, large galaxies - objects where no sharp detail is needed. When it cools down, then you can observe detailed objects like close double stars, planets, etc.

Bottom line - you can start observing before the mirrors and optics cool down as long as you observe the right type of objects.

[David Smith]

I tend to find if I let the eyepiece get too cold the act of looking through it causes dew to form on it! Hence they always stay either in my case or in a pocket to keep them just above ambient temperature.

David

[blinky1983]

I tend to find if I let the eyepiece get too cold the act of looking through it causes dew to form on it! Hence they always stay either in my case or in a pocket to keep them just above ambient temperature.

David

this could be why then I suffer from ep dew. Thanks to you and

Kh3ldar...

[swamp thing]

You might wonder just how long you have to wait before cooling down reaches a useable level. Every half hour, look at a bright star near the zenith with the focus racked out or in to produce a big "ball" of light in the EP. If you see any swirling of the light (like in a mirage), the scope needs more time.

Even if it's not cooled down completely, you can still get good images when looking at "extended objects", such as open clusters, large galaxies - objects where no sharp detail is needed. When it cools down, then you can observe detailed objects like close double stars, planets, etc.

Bottom line - you can start observing before the mirrors and optics cool down as long as you observe the right type of objects.

Good advice here.

No need to wait around twiddling your thumbs just choose some low power, low def targets.

Have a brew first though eh? No sense in rushing.

Regards Steve

[Nexus 6]

like others, I let the scope cool down and the 925 does take a while to settle but the eyepieces stay inside until needed. Its not quite as drastic with the dob, I am out with that in 20 minutes if I want.

[twotter]

My 200P dob is a solid tube and I normally give it a good hour to cool before observing. End cap and focusser cap removed and the tube almost horizontal to avoid anything falling in onto the primary mirror. When I'm ready to go out I take my stool to sit on and my EPs in their case.

[tombulgin]

I left it 2 hours (went to the pub for dinner!) came back and after another hour the clouds cleared. Gin clear from then on as far as I could tell. I've only had the scope 2.5 weeks but best night yet, with no moon! Jupiter was so sharp and M42 stood out amazingly.

Thanks for all suggestions above. Not sure I need to leave it 3hrs next time!

[Moonshane]

I always use a rule of thumb of 5 minutes per inch of aperture, so for a 10" mirror about 50 mins - 1 hour should be ok. I agree with others though that sometimes you can start observing almost immediately if you choose low power/wide fields.

I keep my eyepieces in a foam lined case and this is retained in the house generally. when observing, and after choosing my eyepiece, I ensure I close the lid as this retains a little heat and seems to work for me.

[jimmyjamjoejoe]

As moonshane said, i always go with the 5mins per inch rule, which is doubled to 10mins per inch in sct/maks. Eyepieces are best kept warm apparently, some people even make DIY heated EP cases using either a heat matt, or just a couple of those gel type handwarmers thrown in the case.

As for the shroud/focuser etc, open it all up. The more circulation the better, keeping warm at the primary end is the opposite of what you want. Though tube currents are disruptive, the main culprit for destroying views is the "boundry layer" of warm air which sits just above the primary (in the same way you see heatwaves just above the asphalt in the summer).

[umadog]

Twotter, your scope will cool much faster if you leave it pointing upwards. This is because the mirror is able to radiate heat into the sky. Stuff is no more likely to fall down to the tube whilst it's cooling than it is whilst you're observing. I agree, though, that's it's disconcerting to leave a telescope pointing upwards.

A cooling fan behind the primary makes a vast amount of difference. The secondary cools very quickly: you don't need to worry about that. How long it takes to cool depends on the temperature difference and on the mirror's surface area to volume ratio. Mirror diameter is quite influential for sizes below about 12", but it becomes progressively less important for larger mirrors. Mirror thickness matters a lot, since cooling is proportional to its square. A difference in thickness of even 0.2 inches is significant.

Like the others, unless I specifically want to observe planets, though, I typically start observing right away and leave the mirror to cool in its own time. If you defocus a lot on a bright star then you can see the convection currents floating across the mirror and you can tell when it has cooled.