A question about secondary heaters.

[Doc]

If I run the wire across the top of the spider vane will the view through the eyepiece be degraded in any way?.

I have seen a few heaters that use a copper strip to carry the electric current. How practical is this?

[Milamber]

Not significantly. I had the heater wires for the 22" cable-tied to the vanes and never noticed them.

Very practical, until it's been on for a while and starts to come loose... which is why the cables for the 22" were cable-tied!

Arthur

Arthur

[Doc]

Thanks Arthur just what I wanted to hear.

Another question folks....

By using either a capacitor or a proper heater pad attached to the back of the mirror without removing from the stalk how can I direct the heat into the mirror and not into the air around it.

Is there a special heat insulating foam you can buy or do I glue foil onto the back to keep the heat in.

[blinky]

just glue a resistor to the back of the secondry.

[barkis]

So, If you tack a 47 Ohm resistor to the rear of the secondary,

and your supply is 12 Volts the resistor will dissipate 3 Watts.

The current will be .25Amps.

Ron.

[Milamber]

... and cover it with a bit of that thick silver foily-type stuff they put on exhausts

Arthur

[Doc]

So, If you tack a 47 Ohm resistor to the rear of the secondary,

and your supply is 12 Volts the resistor will dissipate 3 Watts.

The current will be .25Amps.

Ron.

It's a big secondary so is 3 watts enough or could I add two resistors one either end of the secondary.

And what would the maths be Ron

[barkis]

With two in parallel, you would have .5 Amps flowing, and the wattage would double to 6 Watts 3 per resistor.

[Psychobilly]

Watts = Amps x Volts

Watts = Amps x Amps X Resistance

Resistors in series just add them together

2 identical resistors in parallel halves the resistance

3 gives a third etc....

[themos]

I thought less than a Watt would be enough. Remember, you are not trying to fry eggs, you just want a tiny bit of heat so that the secondary will be a couple of degrees Celsius warmer than it would otherwise be. It's the equivalent of touching a few fingers to it.

[barkis]

Absolutely, you do not want warm air currents swirling about inside the tube. I would expect a controller to be in charge of the temperature. .

Ron.

[albedo0.39]

All good stuff - Mick, I have already worked out how I'm going to heat up my 2ndary and it's going to be a surface mount thick film resistor powered by whatever I decide as the voltage.

To ensure you minimise potential optical disturbance of the power wires to the dew heater, I'd get hold of wire smaller than the 1mm vane thickness and run one wire up one vane and the other away from the heater using another. The current is small, the wire is outside (plenty of cooling !) so not much to worry about.

If you are into precision, then you could secure the wire(s) with clear silicon bead on the top edge of the vane and a very small sliver of back tape in strategic points of stress.

[Doc]

All good stuff - Mick, I have already worked out how I'm going to heat up my 2ndary and it's going to be a surface mount thick film resistor powered by whatever I decide as the voltage.

To ensure you minimise potential optical disturbance of the power wires to the dew heater, I'd get hold of wire smaller than the 1mm vane thickness and run one wire up one vane and the other away from the heater using another. The current is small, the wire is outside (plenty of cooling !) so not much to worry about.

If you are into precision, then you could secure the wire(s) with clear silicon bead on the top edge of the vane and a very small sliver of back tape in strategic points of stress.

Steve we will have to discuss this at Salisbury as I'm pretty good at mechanical stuff but electrics baffle me a bit.

But I do get the jist of your idea.

[albedo0.39]

No worries.. after all, by the time our scopes get together, we might have found a way to use them as a bino-bridge ! (if only !)

And also, by then, I might have caught up on a couple of your other mods. (I'm now thinking of adding red LED "running lights" a-la- Calibos!..)