Cooling the Scope

[Cornflake]

The weather (according to the Met Office) is goin to be clear here on Sunday. How long should I put the scope out into the garage for to make sure it has cooled to the ambient temp?

I've got a Meade LT8 Cat so presumably this will take longer than say a Dob?

Chris

[IanL]

There isn't a simple answer, as it depends on the temperature difference between inside and outside, and also how rapidly and for how long the outside temperature falls.

- I assume the garage is unheated? If so you could probably put it in there early in the day; provided it is cooler than your normal storage location and secure no reason not to do so.

- Get it outside as soon as you reasonably can, as it will lose heat to a clear sky much quicker than in the garage. You don't necessarily have to wait until it is dark to move the scope outside. In winter usually put my SCT outside once direct sunlight is no longer shining on my observing location, which can be a couple of hours before sunset. The air temperature will usually be dropping long before sunset in the winter and if the sky is clear it will start radiating away even during the day (find a spot out of direct sunlight but with as much view of the clear sky as possible, and away from the house/walls/anything else that stores heat and re-emits it.

- A closed tube scope like an SCT or Mak will take longer too cool than an open tube design like a Newt. The main thing that needs to cool is the primary mirror but in an SCT/Mak the central baffle tube also creates a lovely spike of rising warm air from its end which tends to create more noticeable distortions than in a Newt.

- Bear in mind you are looking for the scope to reach thermal equilibrium with the environment, and the temperature will continue to drop after sunset, sometimes for the whole night and you may not reach equilibrium at all on some nights, or only after many hours.

As a rule of thumb, an 8" SCT will probably be useable after about an hour if you do the things above, and within two hours it should be pretty good. If it's still giving you poor views after three hours (and you have it properly collimated), it may be one of those nights where it is just not cooling down (or you may just have really bad seeing). Start with low-power eyepieces early in the night, and crank up the magnification later on.

[Mr Q]

I agree with Ianl and offer the additional tip - when you think the scope is cooled down, pop in a medium power EP and aim at a bright star, then rack the focus out until the "airy disc"of light fills a lot of your FOV. If the scope needs more cooling down time, "waves" of light across the disc of light will be evident as they shift with the warm air currents inside the tube.

But you can still use the scope for observing "extended" objects (nebulae, galaxies, star clusters, etc) while waiting for the scope to cool down. These objects are more forgiving in details when viewed with a scope that is not cooled down. A simple out of focused star as noted above every half hour or so will let you know when the scope is ready for detail work.

When doing an out of focus star test, be aware that when the light "waves" are moving around fairly fast, that's a sign of upper atmosphere turbulances ("bad seeing") but when the waves move slowly, that is from slow moving air currents inside the tube indicating the scope has not yet cooled down all the way.

And like IanL says, some nights the air temp can drop faster than the scope can cool down - making those detailed views of planets and other objects impossible.

[yesteryeargames]

Hi guys, i have been leaving the scope outside with the main lid on, should i take it off so that the aire from inside the tube comes out?

would things like bugs , grit ect get into the tube and onto the main mirror left unattended for too long,

anything i could cover it with to let the air out and to stop things from falling in ?

thanks

[IanL]

If it is an open-tube type (looks to be so from your signature) then yes you should open it up so that air can circulate and cool it. If you point it straight up at the sky you may find the secondary cools too quickly due to radiative heat loss (primary shouldn't be a problem with a long closed tube like yours unless you have extreme conditions), plus I guess there is a risk of something falling in from passing birds or maybe bats! Angling it down a bit will help on both fronts.

Dust on mirrors, correctors and lenses is a fact of life and whilst you shouldn't store your scope in excessively dusty conditions (or cover it well if you have no choice), dust from normal usage is not usually a problem and won't affect views. Don't be in too much of a hurry to think about cleaning even a fairly dusty set of optics, as it is easy to do more harm than good if you don't know what you are doing. I have cleaned my SCT corrector plate once in about ten years after having to store it for a very long time in less than ideal conditions, and never touched the primary or secondary mirror other than using a dust blower (and removing one dead spider who had set up home).

Again your mileage may vary if you live in a really dusty/gritty environment or if it has been really dry for months and the wind is whipping up a dust storm.

[swamp thing]

As has been said, you can start observing straight away. Extended objects can be observed from the word go.

Not a planet man myself so, I put my Dob down, have a quick brew then start observing.