Cooling down?

[johngccfc]

I read about some scopes having better or worse cooling down times, why do they need to cool down, what makes them heat up?

thx

[faulksy]

they heat up just with the general ambient temperature during the day. they need to cool to reach equilibrium same temperature as outside , if they don't cool you get heat currents, so when your viewing at high power you will look through all this heat which will make your image shimmer, just the same as you look in the distance down a road and see the image shimmer of the heat on the road.

[Matt Scunthorpe]

They heat up from the sun if you put them out before sunset, or if they are kept in your house they will heat up.

Ideally, the scope needs to be at ambiant temperature when your outside viewing or imaging, so your telescope will need to cool down from the temperature from where it is kept, to the temperature it is outside.

I hear that the general rule is 5minutes per inch of aperture. So my 8 inch telescope, should take 40minutes to cool down.

Now the reality.. I just start viewing as soon as i am set up and dont allow for cool down times, mainly because i dont want to waste time, as it cools down, the views can only get better right?

[michael.h.f.wilkinson]

I store my SCT in an unheated garage for this reason: it stays closer to ambient temperature. With their short, closed, and wide tubes SCTs and Maksutovs are most prone to thermal currents. Narrow tubes (especially with baffles against reflection) do not allow strong thermal currents to develop, and lose excess heat faster through their larger surface area relative to their volume. Open tubes also allow excess heat to escape rapidly.

[johngccfc]

ambient temperature makes sense

i thought it was dew prevention, or the motors or something (a bit thick, see)

thanks all

[Jim Steele]

Dew settles on mirrors and lenses much like it does on a car wind screen on a cold night. This can be prevented by using a dew shield or warming the mirror or lens slightly above the dew point much like a car fan heater does

[Moonshane]

in conditions where your scope is inside, you might find that the outside temperature is higher and therefore you might find your scope needs to heat up not cool down. therefore 'reaching equilibrium with ambient' is a better description.

[johngccfc]

So how do you warm the mirror and how does a dew shield work.

Thx

Sent from my GT-P7500 using Tapatalk 4 Beta

[Moonshane]

the same as cooling. put it outside and wait (use a small fan to reduce the wait).

a dew shield provides additional shade on the end of the telescope to reduce the chances of the optics dewing up. I make mine from cheap 2mm foam sheets.

[umadog]

Cool-down is more important in reflectors and SCTs because of the design of the scope. In a reflector, say, you get a warm "boundary layer" of air that hovers over the front face of the mirror. This refracts light both as it travels towards the mirror and as it goes back up. That refraction introduces optical errors that you can see at the eyepiece. In addition to that, a cooling mirror is a different shape from a mirror in equilibrium because different bits of it are different temperatures. Consequently, the scope will perform better when it's cooled.

Cooling depends on mirror mass. The critical variable is thickness, as cooling depends on the square of the thickness. A rear fan will massively decrease cooling time. It needn't be turning fast. Even at low speeds, a fan can halve the time it takes to cool a mirror.