Your home observatory (part 7)

Posted by Michael Bakich
on Monday, June 04, 2007

In this last installment of my mini-series on building your own observatory, I'll talk about permanent piers. Most amateurs install a pier upon which to place their main telescope. If this is the route you're taking, you'll need a footing, a block of reinforced concrete set into the ground. Any concrete column (or, in certain cases, steel tube) above the ground is called the pier. The telescope mount attaches to bolts pre-set into the concrete pier. Alternately, you can attach a steel plate to a steel-tube pier. In either case, don't install the bolts too close to the pier's edge.

As a rule, a footing should be twice the pier's width and as deep as the pier's height. Make the footing square. The stiffness of a square section is 1.7 times more rigid than a circular section of the same length. Check your local building code for the proper footer depth.

Many amateur astronomers use an oversized round concrete pier. Most use a form into which they pour the concrete. In the United States, this form is a thick-walled tube sold under the name Sonotube. The tube comes in widths of 8-56 inches (20-140cm) and lengths up to 18 feet (5.5m). After the concrete dries you can leave the tubing in place or remove it. If you install a Sonotube pier above a footing, "pin" the pier to the footing with rebar or similar material that protrudes a foot (30cm) or so into the footing.

Whether the observatory has a floor made of cement or wood, the floor must be isolated from the pier. Prospective imagers must adhere to this rule or suffer vibration problems. Leave a 0.5-inch (1cm) gap all around the pier. If you do, you'll be able to dance in your observatory without affecting your viewing.

A lot of builders hand-mix their piers for economic reasons. Unfortunately, you can never hand-mix bagged concrete to the strength of quality-controlled, ready-mixed concrete from a batch plant. So, buy your concrete, and have it delivered.

Some observers off-center their piers intentionally to maximize the view in a certain direction. In the Northern Hemisphere, that direction is almost always south.

In my observatory, I made a wooden accessory shelf, which I slipped over the pier prior to installing the mount plate. The shelf was square (sand sharp edges and corners) with a 2-inch (5cm) lip all around. It protruded about 4 inches (10cm) from the pier and was my most-used feature.

Finally, after considering all the points in this 7-part blog, the ultimate question is whether you do-it-yourself or have the observatory built for you? This may not be a matter of finance. You could be wealthy beyond imagination and still choose to build your observatory. Whichever path you choose, become familiar with all the points I mention. Don't rush, use quality materials, and have a overall plan that includes a diagram.

When you finish, e-mail some pictures to me at mailto:mbakich@astronomy.com.

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