On Fri, 11 Jul 2003 20:56:05 GMT, "Michael Barlow"
wrote:

Everything I read tells me everything I want to know about buying a pier
and bolting it to the floor of my observatory. But I wont have a floor,
just dirt, and I don't want to buy a pier. So, what should I do? My plans
were to dig 4 foot down (frost line) and plant a 10 inch Sono-tube + 4 foot
above ground. Fill it with cement topped off with 4 J bolts to bolt the
mount to. 10 inches seems awfully big!

I'm going to be setting a Meade 127mm F/5 refractor on this with a heavy
mount (30ish lb.) and an OM-1 for astrophotography. What's the best size
for a pier with that in mind, 6", 8", 10"? and what perameters should I be
considering?

There's no reason at all to buy a pier- none of the commercial models offer
anything you can't easily provide yourself, and some have things like level
adjustments that you absolutely want to avoid. Your decision not to put in a
floor is also a good one, and will increase your performance both mechanically
and thermally.

My pier is 6" well casing (6.5" OD) filled with oiled sand. It is rock solid
with a 12" LX200 and a lot of additional equipment on it. Cement by itself can
support some interesting vibrations, so I think your instinct to use 10"
sonotube is good. Be sure to put some rebar in there, and I'd suggest a fiber
reinforced concrete. You could save some concrete by putting a 6" sonotube in
the center of the 10", and pouring only in the outer area. Then fill the inside
with sand, possible oiled, to help further damp vibrations.

The foundation is even more important. I'm down only about 3 feet, above the
frost line but sitting on solid rock. So I only have about an 18" round
foundation at the bottom, maybe 6" high. If you can't get down to rock, consider
a 3 to 4 foot square foundation at the bottom of your hole. With a permanent
polar alignment, you don't want the angle of your pier drifting by even a few
arcseconds.

Are you building a platform floor around the pier? If not, consider it. This
will be much more comfortable to work on than the ground. It only requires a few
hours to put one in, but you need to plan on your pier being 12-18 inches higher
if you do that.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com