The home made camping mat based Dew Shield for my LX200 worked quite nice, but under very moist conditions and especially when aimed at the zenith it lacked performance.
So I considered that the time had come to make an Electric Dew Heater: a heating strap around the OTA.
As basis for the Dew Heater I selected ... a bicycle tube (the inner one, which holds the air).
The local bike repair shop handed me a used [and probably punctured] tube for free ...
I searched the internet and discussed the matter with my friend Matthias Meijer and finally decided on a "Resistor Wire" based realisation.
Material used (Total costs: about Euro 15):
The electrical diagram, as designed by Matthias Meijer
Maximum Power consumption: 1.6 Ampere, which my AFX-2792 Power Supply can handle with ease, as the only other piece of equipment it has to service is my ATK16IC camera which only consumes 0.5 Ampere.
|Step 1: the bike tube laid out on a piece of timber.|
|Step 2: the bike tube, lengthwise split open and held open by nails.|
|Step 3: the resistor wire loops - one of 4 metres and one of 2.25 metres - have been installed, and nails help to keep them in place|
|Step 4: 2 loops of the resistor wire [13.7 Ohm at 12 Volt = 0.88 Ampere = 10.5 Watt and 7.5 Ohm at 12 Volt = 1.6 Ampere = 19.2 Watt].|
Step 5: the 2 loops of the resistor wire - 13.7 Ohm and 7.5 Ohm - are connected with the control box.
13.7 Ohm at 12 Volt = 0.88 Ampere = 10.5 Watt and 7.5 Ohm at 12 Volt = 1.6 Ampere = 19.2 Watt.
On top you see a layer of double sided tape.
Step 6: the resistor wires are covered with a layer of double sided tape.
After removing the protective cover of the double sided tape I folded the tube back into its original position, so that the resistor wires are neatly tucked away.
Step 7: I packed the whole assembly in duct tape and added a heat insulation layer on the outside [attached with double sided tape]
The heat insulation is just a strip of camping mat foam with an aluminium layer on the outside.
Please also note the control box and the 12 Volt connector.
Step 8: The Dew Heater Mark II installed: held in place by a lashing strap with buckle.
I decided to install the dew heater behind the piggy back brackets, so that I can still mount the telescope cover cap without having to remove the dew heater.
The dew heater control box: just a junction box from the DIY store, a switch and 2 12 Volt LEDs: one green and one red, so that I can see what is happening.
Just for me to remember: the longest leg of the LED is for the plus ...
The first two nights with the Dew Heater showed that it works very well: the outside of the corrector plate remains dry and there are no turbulence side effects.
But the INSIDE of the corrector plate has a 2cm condense circle around the central obstruction so I assume the air inside of the OTA is too humid; currently I am busy to tackle this issue. I started with using a hairdryer to blow hot air into the OTA via the 2-inch eyepiece holder: this worked OK and the condense disappeared, but time will tell if the problem has been permanently solved. After that I was outside for about an hour and the corrector plate remained crystal clear, both inside and outside.
The third night out was very humid: my scope was almost dripping. I had been imaging almost at the zenith via my piggy backed WS ZS 80 ED scope and had left the corrector plate of the LX200 uncovered so see how the dew heater would perform under these conditions.
After 2 hours I looked: there was the now familiar 2 cm ring of condense around the central obstruction, but only at the outside: so the hair dryer action had helped to dry the inside :o)
I do not worry too much about the 2 cm ring of condense around the central obstruction: there even were small 'pools' of water on the central obstruction itself and I think no dew heater can take care of that. So I installed the good old dew shield (which has been shortened quite a lot so the wind has far less grip now) and the combination of dew heater plus shortened dew shield copes very well, even under very humid conditions.