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Martin Lewis shows you how to combat the irritating effects of tube currents
Thermal dynamics can have a big influence on the views you get through your telescope, blurring planetary detail, distorting star images and degrading contrast in detailed objects such as globular clusters. If you want the best performance - whether you're imaging or observing visually - it's worth getting to grips with these issues so you can minimise their impact.
Inside a cooling telescope, the warmer (less dense) air rises from hotter parts of the instrument as they lose heat by convection. These 'tube currents' are trapped inside the body of the telescope and, because the warmer air has a different refractive power than the cooler air, they introduce differing delays to light passing through it. This upsets the ability of your telescope's optics to bring the light into a sharp focus.
The longer the optical path inside your telescope and the more unequal the air temperatures, the greater the problems the tube currents cause. For a scope with a 1m focal length, even a 0.1°C temperature difference over its length is enough to degrade images.
Larger telescopes are more affected than smaller ones due to their longer light path, but also because they have a smaller ratio of area to mass, meaning they take longer to cool down. Reflectors also tend to suffer more than refractors because light has to make one passage of the tube before being collected by the mirror and bent inwards away from the tube walls, where the worst convection currents often lurk.
To eliminate the problematic convection currents you just need to allow every part of your telescope to cool to the ambient temperature. Thermal effects subside when the optics and other parts inside your telescope have a less than 1°C difference to the ambient temperature. When that temperature differential is less than 0.5°C, the tube currents diminish and the layer of warmer unstable air that's otherwise stuck to the front of the mirror or primary lens almost completely melts away — allowing maximum optical performance.
Unfortunately, getting the scope to cool down sufficiently isn't always easy. If it's been stored indoors, then taking it out into the cold night air and expecting it to perform at its best straight away is misguided. It's much better to store it outside before you begin observing and allow it to cool down properly.
A small or medium telescope might cool down fully in 30 minutes, but a large one can take hours to acclimatise. Because air temperatures continue to fall through the night, the scope may remain a few degrees above ambient. Big telescopes also have big mirrors, and the large mass and the poor conductivity of glass mean they don't give up their heat readily. Fans blowing gently on the back of the mirror can help here.
Insulate against the cold
Even if the scope has lost all of its heat to the air, you can still get convection issues of a different kind. Parts of the telescope that face the cold night sky, particularly the top face of the telescope's tube, can drop several degrees below ambient temperature, inducing convection currents of cold air that cascade down inside the tube. Unlike normal tube currents, which tend to die down with time, such 'inverse' tube current processes can plague you all night. The good news is you can combat these effects by wrapping parts of your scope in a poorly radiating material, such as shiny aluminium, or by adding a layer of insulation.
The best way to check for any residual thermal issues before starting your observations is to perform a star test where you rack an eyepiece well inside focus. This allows you to clearly see any thermal currents in the tube silhouetted against the bright expanded disc of the defocused star.
By following the steps below in the Step-by-Step Guide, you will see how badly thermal issues affect your telescope and will hopefully be able to reduce their severity to give you sharper views of the night sky.
Tools and Materials
Fans and batteries
For Newtonian mirror cooling choose a low-vibration ball-bearing or Hydro Wave-bearing electric computer fan. Power the fan from an external power supply or an on-board battery pack.
The aluminum-coated insulation material sold to fit behind radiators or line lofts can also be used on the outside of a telescope tube.
A space blanket is an alternative to radiator foil and can be bought from camping and outdoor shops. Ideally, use both products.
Scissors and insulating tape will be needed to cut and hold the radiator foil or space blanket in place
Ways to keep your scope at the right temperature
You can speed up the cooling of a Newtonian mirror by fitting an electric fan to the rear cell so that it blows onto the rear face of the mirror. If possible, mount the fan on soft rubber washers or string it between elastic bands to isolate its vibrations.
A low-tech alternative to installing an internal fan on your telescope is to place a desk fan nearby so that it blows on the mirror end. This will help to get the scope closer to the ambient temperature so you can start your observing session.
Allow plenty of time for the scope to acclimatise to the outside temperature before you use it. To help speed things up, store your telescope in an unheated place, such as a shed or garage, when not in use. If it's stored in a warm place, it'll need longer to cool.
Fit the scope with a layer of aluminium-coated radiator insulation to reduce inverse tube currents caused by the cold night air, especially during still and transparent nights. This will ensure the exposed parts of the tube stay closer to ambient temperature.
Another method of reducing inverse tube currents is to wrap a space blanket around the body of your telescope. Like the more permanent insulated foil, this reduces the radiative chilling of the telescope by the cold night air.
Rack the eyepiece far inside focus to expand the image of a bright star to one-third of the field diameter. Tube currents will be seen as swirling patterns of bright and dark, trapped within the circular disc. Experiment using your hand at the front end to see the currents.
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