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As all owners of telescopes know, using them in the winter months means they will need to be kept free of dew on some of the coldest nights. I found out how this can affect viewing some time ago and so decided that a dew heater system would be better than using a hairdryer every so often. There are a number of options to controlling the build up of dew on the optical surfaces.
Using a hairdryer is one way of doing this but a better solution is
Introduce some heating via resistors or
Introduce some heating via nichrome wire
A quick search in the Internet revealed a number of sources of information including links to suppliers of the necessary nichrome wire I proposed to use. So this is how I did it.
I needed a minimum of two heaters. One for the secondary mirror, the other for the finderscope. Eyepieces were not considered a problem as most of the time I am using a camera for astrophotography. I purchased a 10m spool of nichrome wire with a resistance of 42 ohms per metre. I was planning on using a 12v supply so all my calculations regarding wattage are based on these figures.
The heater for the finderscope was the easiest to construct as it is simply a straight length of heater. To create this I measured the circumference of the finderscope. I allowed an inch extra for fixing with a self adhesive Velcro strip. For the heater element I used 800mm of nichrome wire which I laid on one half of a 200mm strip of duct tape. The wire was folded back and fore carefully avoiding short circuits. The second half of the duct tape was folded over the first to create a heater strip. I then added a piece of corrugated cardboard to provide insulation and covered the whole thing in more duct tape.
Nichrome can't be soldered by normal soldering techniques however with some ingenuity soldering the lead to the nichrome can be achieved by creating a loop in the nichrome and feeding the copper of the lead through the loop and then soldering the copper. The other end of the lead was a phono plug.
The heater for the secondary in one sense was the same as the finderscope with one exception. Because the secondary is at 45degrees the heater is shaped to fit the secondary holder. I created a template for this.
To create your own you will need to measure the diameter of the secondary holder. Draw a circle of that diameter using a graphics package. I use Autosketch. Divide the circle into sixteen. Then project the diameter of the circle and the lines dividing the circle, upwards. Cut off the lines at 45degrees. Then project the intersection of the vertical lines with the 45degree line horizontally. Draw a series of lines vertically equally spaced at one sixteenth circumference to the circle. At each intersection draw a freehand curve or if using a drawing package a sine curve.
This is my template, outlined in red. Suitable for a Sky-Watcher 200p. It should explain the paragraph above.
I added a tab on the side for fixing the heater to the secondary mount.
I created a former from a piece of plastic cut from a soft drinks bottle. I then used double sided carpet tape sticking it to the inside of the curve. A fiddly operation but by sticking the former down to the work surface was made easier to complete.
Ready to receive the Nichrome wire.
Using the curve of the former helps in fitting the heater to the secondary mount as you are not fighting to curve it to fit the mirror support so much.
800mm of Nichrome wire is then stuck to the carpet tape and covered with duct tape.
This gives approx 4w of heating, assuming 42 ohm per metre and a 12v supply; enough for the secondary mirror.
Nichrome wire is fiddly stuff to handle especially straight off the spool. I use this trick. Cut the length of nichrome you require plus enough to create two loops, one at each end. Hook the one end over something convenient like a cup hook underneath a shelf and attach a small weight to the other end. Take a cigarette lighter and heat the wire till it glows (it will be very quick depending on the gauge you are using) and go down the length of the wire heating it all the way. When you have finished Unhook the wire and it will stay straight until you bend it how you want.
Again the leads are soldered and the phono plugs which allow connection to the control box are added to the other end. Self adhesive velcro strips are added and the heater unit is tested.
So now we have two heaters which are secured to the telescope with Velcro. On the first really cold night the telescope had a film of frost but the secondary was clear all night.
The control box
As with the fan speed controller I used a PWM circuit based on a 555 timer chip. With the control box I fitted the controls into the bottom of the box. This means I have the top which can be removed without disturbing any wiring and allows easy access to the board and wiring. I Included a 2A fuse in the circuit as protection in the event of a short circuit.
The circuit diagram for this unit can be found at
http://www.backyard-astro.com/equipment/accessories/dewheater/dewheater.html
Ok so this final paragraph is a bit of patting myself on the back. I used the system one clear night with one strip on the finderscope and the secondary heater in use. I also had a guidescope on the telescope but with no heater strip. At the end of the evening the guidescope was misted over whilst the finderscope had remained clear.
I estimate the output of each heater to be around 4W and this has proved more than adequate.
I will be making a strip for the guidescope next. Fortunately I designed the circuit to take the extra load.
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