How to thermally optimise your telescope

We show you how to combat the irritating effects of tube currents.
 
Written by Martin Lewis.
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Image Credit: 
Martin Lewis
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 really want the best performance – whether you are imaging or observing visually – it is worth getting to grips with these issues so you can minimise their impact.

Tools and materials

Fans and batteries - For Newtonian mirror cooling choose a low-vibration ball bearing or Hydro Wave bearing DC computer fan. Power the fan from an external power supply or an onboard battery pack.
 
Radiator foil - The aluminised insulation sold to fit behind radiators or line lofts can also be used on the outside of a telescope tube.
 
Space blanket - Aluminised Mylar space blanket is an alternative to radiator foil and can be bought from camping and outdoor shops. Ideally, use both products.  
 
Tools - Scissors and insulating tape will be needed to cut and hold the radiator foil and space blanket in place.

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 telescope and, because the warmer air has a different refractive power than the cooler air, they introduce differing delays to light passing through. This upsets the ability of the telescope’s optics to bring the light to a sharp focus. 

The longer the optical path inside the telescope and the more unequal the air temperatures, the greater the problems caused by tube currents are. For a scope with a 1m focal length, even a 0.1°C temperature difference over the length of the scope 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.
 
In principle, to eliminate the problematic convection currents you just need to allow all parts of your telescope to cool to ambient temperature. Thermal effects really subside when the optics and other parts inside your telescope have a less than 1ºC difference to the ambient temperature. When within 0.5°C, not only do the tube currents die right down, but the layer of warmer unstable air that is otherwise tenaciously 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 is often not so easy. If it has been stored in the warmth indoors, then taking it out into the cold night air and expecting it to perform at its best straight away is a mistake. It is much better to store it outside the house before use and allow it to cool down properly beforehand. 
 
A small or medium telescope might cool down fully in 30 minutes, but a large one can take several hours to properly acclimatise. Because air temperatures usually continue to fall through the night, the scope may stubbornly 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 really help here.

After cooling actions

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 radiatively 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 continually 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 such effects by wrapping parts of your scope in a poorly radiating material such as shiny aluminium or add a layer of insulation.
 
The best way to check for any residual thermal issues before starting to observe 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 to the right, 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.

Step 1 - You can speed up the cooling of a Newtonian mirror by fitting a DC 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.

Step 2 - A low-tech alternative to fitting an internal fan is to place a free-standing fan nearby, blowing on the mirror end. This will help to get it closer to ambient temperature, at least until you start your observing session.
 
Step 3 - Allow plenty of time for the scope to acclimatise to the outside temperature before use. To help speed things up, store your telescope in an unheated place, like a shed or garage, when not in use. If it is stored in a warm place, it will need longer to cool.
 
Step 4 - Fit the scope with a layer of aluminised radiator foil to reduce inverse tube currents caused by the chilling effect of the cold night sky, especially during still and transparent nights. This will ensure the exposed parts of the tube stay closer to ambient temperature.
 
Step 5 - Another way of reducing inverse tube currents is to wrap Mylar space blanket around the body of your scope. Like the more permanent insulated foil, this reduces the radiative chilling of the telescope by the night sky.
 
Step 6 - 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.

This How to originally appeared in the February 2015 issues of BBC Sky at Night Magazine.

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