Platform structure questions

[dhill]

I'm getting my son a refractor telescope for Christmas, and I told him I'd build a platform on our land so he could star gaze from an advantageous position :D Actually, I'm building the platform more for fun than purpose. I want to build it about 15 feet off the ground. If I use 20 foot 3" steel pipe and sink it about 4' in the ground, will that be enough to support a platform of, say, about 4x6 or maybe 6x8 size. Another thought is to use 16' pt wood posts. The platform would only be about 12 feet high, but that's okay too. I'm building in a clearing of a bunch of granddaddy oak trees. Any suggestions on how to do this? I planned to build it like I do decks, but under a deck I usually put lots of extra posts for bracing. I don't know that I'll be using that many posts on this platform. If anyone has any plans, I'd appreciate the shared information. Or, if you think I'm crazy for even thinking of doing this, tell me that too.

Thanks,
Dusty

[Rich]

A little crazy but that's ok :)
Probably the biggest that I see is with wind. Get that thing swaying with 3" pipe and it could walk itself right out of the ground. You might want to think of putting some good sized chunks of concrete and have the 3" pipes with a base plate bolted to the "footing". Similar situation with using wood posts. Then at the top to give a little more rigidity I would look at putting some diagonal bracing (top 4-5')

[dhill]

Thanks, Rich. Yeah, for sure cross bracing. I'm sure there's lots of other stuff I haven't thought of. I hadn't even considered wind swaying that sucker. It might be a little tricky pinpointing venus if the platform is trying to sway out from under us.