Report about a DIY heating system for the sencondary mirror of a Dobsonian telescope
Dr. Achim Tegeler, December 09, 2023
Just like the always somewhat sad-looking, sympathetic magician Pan Tau, who was portrayed by the Czech actor Otto Šimánek in the 70s and was a somewhat enigmatic companion of my television childhood, there are still constant companions in my life as an amateur astronomer today.
If you don't look at the name "Pan Tau" from the point of view of the Czech language, in which "Pan" means nothing other than "Mister" – "Pan Tau" means "Mr. Tau", other points of view are also available: The prefix "Pan", borrowed from the Greek, also means "whole, total, all-encompassing..." – and in exactly this definition, Tau, the German word for dew, is a constant companion of the astronomer: On mirrors, lenses, objectives and eyepieces, the dew settles everywhere and clouds the view of the universe – it’s Pan Tau (dew)... and in contrast to the magician "Pan Tau" it is very disturbing.
At the last "Sternstunden" with my 12" Dobsonian in November, despite the dew shield attached, dew formation on the secondary mirror. So I decided to take care of that problem by installing a self made heating system. Of course there are ready to use systems available on the market but can it be so hard to build them in a DIY project? It is not...
However, before taking action, it is necessary to clarify how the power supply should run, what power is needed and what type of heat generation is to be used.
Since my Goto-Dobsonian, just like my mounts, are supplied with 12 V direct current, the mirror heater should also be operated with a voltage of 12 V. Since the prevention of dew fogging is already effective if the mirror surface is kept even just above the dew point, there is no need for a lot of heating – it is enough to exceed the dew point. Greater heating would also lead to thermally induced air movements in the tube and interfere with observation – this should be avoided at all costs.
Basically, there are several ways to generate heat at the secondary mirror – flexible constantan wire can be used as a resistor or ready-made wire resistors, which are available in a wide variety of designs. I decided to use wire resistors in ceramic here, as they are available in a wide variety of resistance values for very little money. Depending on the size and total mass of the mirror, only a small amount of power is usually required to keep the temperature of the secondary mirror above the dew point. However, if you forget to switch on the heating early enough, then it is good if you have some power reserve, which you can then use to "defrost" and then turn it down.
Usually, secondary mirror heaters have between 0.5 and 1 watts of power – I would like to have a small power reserve and aim for 1.5 watts. The power P of resistors is calculated according to the well-known formula [2]:
P = U2 / R
with the voltage U and the (total) resistance R. In order to generate approx. 1.5 watts with 12 V, 96 ohms of resistance (122/1,5) - divided into three resistors connected in series (the power should be distributed over a larger area), it should be resistors with approx. 32 ohms. Since 33 ohms of resistors were at hand, they should be used accordingly – so the power would be about 1.45 watts. In addition, there should be a way to regulate the heating as needed. The control should be carried out via a potentiometer/pulse width modulation (PWM) [3], which can be implemented quickly with small, cost-effective modules. A small test setup showed that the resistors got warm and that the control worked very well.
So everything should be installed together in a small housing and the resistors should be glued to the back of the mirror and connected via a thin enamelled copper wire.
For better heat conduction, some thermal paste was used between the resistor body and the glass surface. The resistors were then glued in place by applying cyanoacrylate adhesive.
T prevent the heat to evaporate into the air the whole resistor-system was encapsulated with isolating foam (Armacell) that was cut into shape to ensure that no foam was in the lightway.
In order to make the cable routing as visually ineffective as possible, I decided to use 0.5 mm enamelled copper wire, which was routed along the top of the secondary mirror spider facing away from the primary mirror and attached with adhesive tape. To pass the wire onto the outside of the tube, one of the screws holding the tube edge was pierced lengthwise and used as a channel.
The small box with the PWM module was then provided with a socket for the 12 V power supply and attached to the outside of the tube with Velcro. Initially, the cable to the heater was only routed to the box under adhesive film – whether this can remain that way remains to be seen.
As a small side project, the M4 hexagon socket screws, which are used for mirror alignment and which are always a bit "fiddly" to adjust due to the hexagon socket, were replaced by self-made M4 knurled screws made of brass. They simplify the adjustment and thus the collimation of the telescope considerably – if you can't make the screws yourself, Bob's Knobs [4] are recommended.
The next cool night with dew formation will show how well the secondary mirror heater proves itself – I am very satisfied with the result!
[1] The dew point, or better, the dew point temperature, is the temperature at which, depending on the absolute humidity and the ambient pressure, the water vapour present in the air condenses and thus dew is formed. The exact determination of the corresponding. parameter is relatively complex and is most accurately measured with an (aspiration) psychrometer and subsequent calculation based on corresponding. tables (Sprung's formula). For astronomers, the exact determination of the dew point is irrelevant – the main thing is that the mirror temperature is correspondingly above this mark!
[2] The calculation of electrical power is shown very well here: https://de.wikipedia.org/wiki/Elektrische_Leistung
[3] The principle of pulse width modulation is explained here: https://de.wikipedia.org/wiki/Pulsdauermodulation