2022 January 13
Newtonians are cool
How to avoid image distortions caused by heat in a Newtonian telescope
Reflecting telescopes are often kept indoors and brought out for a night’s observing only to result in the view through the eyepiece shimmering. This can be caused by many things such as unstable air currents rising from the ground or nearby buildings, a common cause is air currents inside the tube. This article explores ways of cooling the main mirror with a fan and managing the air currents inside the tube to get thermal equilibrium with the ambient temperature.
During the time the telescope is indoors the mirror stores heat which it then loses at a constant rate when placed outside. During the process of the mirror cooling down a boundary layer of warm air forms over the surface of the main mirror. Tube currents are caused by this warm air slowly rising up the inside of the tube, further degrading the image. As the night progresses and the ambient air temperature falls, a thick mirror cannot lose heat quickly enough to match the outside air temperature or at least get within 3°C. Even open truss tubes suffer from a boundary layer of warm air forming over the main mirror.
To complicate things further in humid conditions dew forms on cool surfaces. (An excellent tutorial by David Arditti explains how to deal with dew). Fitting a fan can stabilise the air currents and improve the heat transfer from the main mirror in a relatively short time. This can also help prevent the formation of dew on the secondary mirror.
Occasionally it’s been suggested fitting internal baffles helps, but unless the position and size are carefully worked out this can make the problem worse. If baffles are already fitted to the tube they may need to be removed for the cooling fan to work efficiently.
Where to put the fan
The decision to either blow cooling air onto the main mirror or extract the warm air, either through the side of the tube or from the back of the mirror, depends largely the environmental conditions.
Fitting a fan on the side of the tube and extracting the air out of the opposite side can occasionally be found in the DIY telescope community. This can cause problems with ingesting dust and pollen to be deposited on the mirror surface, but can also generate or amplify the existing tube currents. It’s also not so efficient at cooling the mirror as a rearward mounted fan.
Fitting a fan to suck air from the back of the mirror is also fraught with problems especially in humid conditions. Drawing moist air into the top of the tube and onto the reflective surfaces can result in dew forming prematurely. If there any baffles in the tube it can cause extra tube currents and trap the warm air you are trying to get rid of. In addition, it’s not so efficient as the method I propose below.
It’s quite common for telescope manufacturers to fit threaded holes on the back of the tube above the main mirror as in figure 1.
Care must be taken in choosing a fan, you need the smoothest running fan possible to prevent vibrations making things worse, mounting the fan on rubber washers can help.
Over tightening the bolts holding the fan to the telescope can alter the collimation so it’s always advisable to check this and adjust if necessary afterwards.
Mounting the fan on the back of the telescope, blowing cooling air onto the back of the mirror and around the edges, helps break up the boundary layer forming on the front of the mirror. This enables the tube currents to move smoothly up the tube. As well as cooling the mirror it can slow down dew forming on the secondary mirror.
Fitting a bigger fan does not necessarily result in the main mirror cooling down quicker, it also can result in unwanted vibrations. For mirrors around 300mm diameter more than 1 fan is desirable, 3 is usually the number used, 120 degrees apart.
Fitting a fan on a baffle making sure that access isn’t blocked to the collimating screws helps distribute the colder cooling air evenly around the back of the mirror further aiding thermal equilibrium. A smooth even flow of air around the mirror also helps to achieve even cooling and prevent hot spots in the mirror. Figure 2 shows the version I have fitted manufactured by the Ukrainian firm Asterion, they also do a version for Schmidt-Cassegrain telescopes as well as different sizes depending on the size of the mirror.
Choosing the power source for the fan is an important consideration, low voltage fans powered via an AA or AAA battery pack is one solution where direct power isn’t available. However, the batteries might not last as long as the observing session and the fan might have a lower airflow. The set up I use is powered from a power tank which provides 12v, 1A this power tank also provides power to the mount and accessories. Switching the fan on at least 30 minutes before observing and letting it run while I’m using the telescope I have found does improve the view through the eyepiece.
Dobsonian telescopes can have their mirror very low to the ground by the nature of the design which can cause extra problems. By putting a clean dry sheet on the ground prevents pollen or dust being ingested and depositing particles on the mirror surface. Also it prevents any moisture rising from the ground being deposited onto the mirror which helps preventing it dewing up.
Teaser image: A group of Newtonian telescopes at Perkins Observatory, Delaware, Ohio, by Analogue Kid at Wikimedia Commons