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Mike, I made the 3 trapdoors on my scope very simply - each door is made by cuting out a square of aluminium from the OTA, and then reattaching it with cheap hinges from bunnings. The closing mechanism is just velcro, glued onto the door and OTA with 5minute araldite. Works fine.
Make sure the door opens so that you can direct warm air across the reflective surface of the mirror, you don't want to be warming the glass directly, just evaporating dew off its front. This has a negligible effect on the mirror temp.
You need doors so that you can keep them closed to stop dew from getting in. If it has to come all the way down the OTA then its effect is reduced. Just cutting open holes will likely make the dew problem worse.
When using the hairdryer it's nice to have the doors open all round so that the warm air can escape directly without having to travel up the OTA. Reduces the lost time waiting for the air in the tube to settle down afterwards. You point the hairdryer so that warm air comes in one open door, travels across the mirror and then blows out one of the other open doors.
I use these doors all the time, but my OTA is 1.8m long so I really have no other choice :-)
I wouldn't advocate heating straps, as they will introduce thermal effects into the air in the tube that you don't want.
cheers, Bird
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