AIM: To prevent dew formation on optical unit of telescope.
This note will provide you with simple techniques to combat the problem in cold & humid environment. As everyone cannot afford the market price of dew heaters. I have come up with an idea for dew heaters which could be made with the easily available and cost effective materials.
Basic aim of making such a dew heater came up because of my 22 inch telescope which could cost my pocket a lot.
I managed to get ideas from scrap materials which were not used for long time.
I used Ni-chrome wire found in broken aquarium heater.
I found a 12V battery from old UPS & recovered battery by adding distilled water & charging.
For large size heat strap, ni chrome wire is the only solution,as the resistor attached in parallel produces enough heat but they can break& get disconnected on movements.
Ideal heat required is to be 2-3° above the environment temperature which is variable. For my 22 inches homemade telescope primary mirror, I required the strip of 190inches.
Step 1: Dew Heater Made From Aquarium Heater
List of things required for the project.
- 12 volt battery
- duct tape and duct pipe non conductable and non combustible
- heating wire, nichrome wire from old spare aquarium tank heater
- duct tape
- crocodile clamps for battery
- dc button on/off
- RCA plug pin male & female
- leather belt according to thickness of glass
solder iron,insulated telephone wires, insulating tape, rope used as a sling to support glass &retain heat.
Step 2: Calculating Length of Resistive Nichrome Wire :-
Spiral wire acquired from the aquarium heater was straightened mechanically by pulling. Now, that was attached to wooden board by iron nails. One end of wire attached to negative of battery (12V) & positive end was used as variable.
At least 2 metre nichrome wire should be used to prevent burns and injuries because smaller size would generate more amount of heat .Take an insulated copper wire from the positive terminal of the battery and attach it to the other end of nichrome wire. Wait for 30 seconds and check the temperature of the nichrome wire with the fingers.
Use a thin cotton glove over your left hand which checks the temperature of the wire and keep moving the positive terminal (copper wire) towards the negative terminal until your left hand feels comfortable amount of heat.
NOTE: THE AMOUNT OF HEAT GENERATED DEPENDS UPON THE GAUGE OF THE NICHROME WIRE AS WELL AS THE VOLTAGE OF THE BATTERY WHICH COULD BE VARIABLE IN OTHER CASES.
For my 22 inches mirror I required 190 inches of nichrome wire.
I calculated the length to be 190 inches for my strap. I divided it into 4 parts: 140 inches for primary mirror, 20 inches for secondary mirror, 20 inches for finder & 10 inches for eyepiece.
Now basic idea was to take all the four parts in series with the battery.
Step 4: Construction of Strap
First take leather or rexin cloth as a base according to thickness of the mirror. Stick the duct tape over the leather base. Attach the duct pipe which has the nichrome wire in it by securely sewing it on the leather base. The strap was attached to the mirror by placing 2 cotton ropes as seen in the photograph.
Precaution should be taken that the wire should not be in direct contact with the optics.
Similarly, the straps are made for finder, eye piece.
for secondary mirror strap is attached behind the mirror over the base plate to prevent obstruction.
Heat is retained between the leather strap and two ropes which help in keeping the glass temperature above environmental temperature
Glass acts as a good heat sink.
Step 5: Connecting of Heating Straps
if your mirror is of large size you may have to make two different connections for primary mirror and the other for secondary mirror, finder and eye piece.
put 2 d/c on/off buttons as per your convienience on the telescope.
use crocodile clips for battery attachment and properly insulate the positive and negative ends of battery with insulating tape.
take adequate length of telephonic wire so that the movements of telescope are not restricted
connect the heating straps to the battery by adequate length of insulated copper wire.
with this set up battery being fully charged we could use it for over 8 hours or overnight.