cool down or equilibrium?

[jeelan]

All,

Ive read many a posts around scopes requiring to achieve correct temperature to be optimal, however I'm confused as to whether they need to cool down or achieve equilibrium with their surrounding environment?

To me there's a difference between the two.

Is someone able to explain which one applies?

cheers
Jeelan

[casstony]

Equilibrium with the environment is what you want. Cooldown refers to cooling the scope down to the surrounding air temperature; until equilibrium is reached the temperature differential causes air currents and distorts optics which disrupt the view and limits your magnification. Atmospheric turbulence also limits magnification achievable on a given night.

[croweater]

Hi Jeelan . It refers to the need to allow your telescope to cool down to the ambient air temp. If the optics etc. are warmer than the air around them they will heat the air and create currents in your tube that will make your image soft or blurry/hazy and move about. Many owners with open tube telescopes use fans to help cool faster. Just stick your scope outside a bit earlier. Most times an hour to hour and half is enough. Cheers, Richard

[croweater]

Oops, Tony already answered :thumbsup:

[jeelan]

thanks for clarifying guys, I was interpreting cool down as scope being below ambient temp by a certain amount.

In Perth where its 25C+ most summer nights scopes will likely need to warm up when taken out of the house as everything's aircon!!

[JimsShed]

Does this mean that if we use dew heaters then the image can never be as sharp as it might otherwise be?

[janoskiss]

Under clear skies radiation will never reach equilibrium (not unless your scope's in outer space and cooled to about 3 kelvin) and that's why you get dew: because your scope radiates heat out into space and becomes cooler than the surroundings. (There is a thermodynamic tug of war between convection and radiation both preferring equilibrium, i.e., maximum entropy.)

Ideally, a dew heater would just make up for the heat lost as radiation and keep the optics at ambient temperature. In practice though it needs to bring what it's in contact with to just a smidgen above ambient so heat can flow to the rest of the system. With a good heating + cooling setup the image quality won't suffer: the job of heaters, coolers, fans is after all to get the best performance possible out of your scope. Best way to learn how is to hang out with experienced astro-imagers and watch them at work.

[MortonH]

It gets worse with Maks and SCTs. The closed design and large pieces of glass (mirror and corrector) make it difficult to cool down, or to maintain equilibrium if the temperature is falling quickly during the night (e.g. in winter). You can get tube currents because the air inside the OTA is at different temperatures at the top and bottom of the tube.

The vents in the Celestron Edge SCTs allow an add-on fan (e.g. TempEst) to circulate the air inside the OTA with so that it's all at the same temperature and this apparently results in a dramatic improvement to the image, even though the scope is not in equilibrium with its surroundings.