Thursday, December 28, 2006

Telrad heater done

Tonight I finished a custom Telrad heater.

I had brought down to Mom's the two large 2 watt 220 ohm resistors from Toronto. I used their heavy, thick lead wires to form a box frame that fit inside the Telrad uprights above the lens which support the glass plate. It was a perfect fit. There is a little bit of friction so the heater will stay in place but is easily removed, for use elsewhere, and for stowage.

I connected the two resistors in parallel so to yield 110 ohms. This is in turn connected by professional grade 4-wire microphone cable. I used 2 wires for each side of the circuit, for redundancy, and to reduce resistance from the cable itself. I borrowed my sister's heat gun to work the shrink tubing but I over did it on the high setting! Oops.

Finally, I connected an RCA male plug at the end. I had bought a pack of RCA plugs from The Source. Cheapo ones, not gold-plated, since—it occurred to me—this was not a high-end audiophile application. And also because I could not find nickel-plated ones (like Kendrick uses) or ones with openings large enough to fit the thick microphone cable. The plugs have coloured covers: white, yellow, blue, and red; two of each. Maybe I can use the colouring deliberately. I decided on white for the Telrad plug.

The assembly proper I completed a couple of days ago after some mock-up testing. It worked well.

Early today I bought some self-adhesive cable hooks from Canadian Tire (not shown). They work perfectly, fitting around the cable casing well. I only wish they were black. The hooks are designed well so to prevent the cable from slipping out. I used two of them to offer good strain relief as the cord loops from the side of the Telrad up into the lens/plate area.

Tonight, I did a field test. I put the Telrad outside at 8:00pm for about 45 minutes. Temperature was around 1 or 2°C, right around the dew point. The light breeze dropped the temp another degree or so. When I returned, it was dewed up. The glass and the lens were coated with a thin layer of water. I plugged my heater into the Kendrick controller which I turned up to its maximum setting. Within 30 seconds, I could see the dew clearing from the bottom edge of the glass. I dropped the controller setting to Medium and left it for a couple of minutes. On examination, the glass plate and lens were completely clear. Wahoo!

So, I completed my first custom dew-fighting heater. I'm quite pleased with the result.

Mom is happy too, oddly. Funny, she's never hauled out the telescope on her own.


Checked the fuse in my first generation Kendrick dew heater controller: 5 amp.

I understand I can bump this to 7 amp.


Replaced the casters on Mom's telescope dolley. The old casters were too small. There "new" ones are almost twice the size and work a lot better, a lot smoother.

No comments: