What's Dew
April 11th 2011 03:14
Category: Telescopes
The Bain of most amateur astronomers is when the telescopes optics fog up.
Winter months can be a bad time for lenses to fog up. Fogging can occur in areas that have high humidity or just about any where there are changes in temperature.
Fogging can become inconvenience when you are just about to take a look through your telescope or binoculars and find the lense have a coating of dew on them.
Left unattended the accumulation of dew or condensation on glass optics can cause damage.
telescopes or eyepieces can accumulate condensation which can lead to the glass laminations separating and eventually form mold. It is good practice to dry thoroughly any piece of photographic equipment glass lenses etc before putting them into storage.
Binoculars are particularly susceptible to condensation even though they appear to be sealed they still have air circulating within the tube or barrel of the binoculars or telescope. If they were to be fully sealed condensation would become much worse. The only sure way sealed optics work is if the internal optics of purging condensation is by using nitrogen.
We can lessen the effect of condensation on our lenses in most case fully eliminate condensation altogether by use a dew heater. There are many heaters on the market that will take care of the condensation problem.
CURRENT MOON
You can make your own anti-dew device for a fraction of the price of a store bought unit. If you’re a bit handy with a soldering iron you can make one yourself. All you need is some resistors wire solder and tape.
Resistors, are readily available at any electronic supply shop. Resisters make ideal warming elements for your telescope , eyepiece holder or finder scope.
Assorted Resistors
You need to decide how much heat you will require. The starting point is 3 watts for an 8-inch telescope and 1½ watts for a finderscope objective or eyepiece. If your dew problems are severe you may need more wattage. In most cases it you may need to alter the amount of resistors used I order to get the required amount of heat. . All resistors have a wattage rating using resistors with a ½-watt-rating should be good enough, although using 1-watt resistors would provide a better margin of safety.
Resistors are available in multiple values and wattage ratings.
Electrical resistance is measured in ohms. In order to get a desired heat output in watts, the resistance you need is derived using ohms law expressed by the formula Amps = Watts / Volts
For example, if you use a 12-volt battery gel pack and want 4 watts, you need 48 ohms of resistance. You will require 8 resistors of 6 ohms each, wired in series. Resistors manufactured in a limited variety of values, so you may have to settle for a little more or less the figure you come up with need not be arbitrary.
A 4-watt heater running on a 12-volt battery draws roughly 1/2 amp. If you have a battery rated at 1 amp-hour it will run the heater for little over 3 hours before needing a recharge.
Using six resistors each having a value of 25-ohms, wired in series, will produce about 1 watt of heat when connected to a 12-volt power source.
It would be fool hardy and dangerous to connect a heater directly to the high voltage mains power.
Putting it all together is relatively straight forward you will need to get the ends from shorting out. You can place the resistors in some heat shrink wrap and attach to the requires optical device.
Final assembly in this case the home brew heater is attached to the finder scope.
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