Thursday, November 20, 2014

Update on a Backyard Observatory!

Some of you with good memories might recall a post I made last February about my backyard observatory project.  Since our yard is rather small, I envisioned a design I'd not seen before, even making a small working scale model, shown at left.  It has the advantage of not requiring permanent space devoted to poles and rails to support the roof while removed.  I had also obtained a decent-sized telescope a few years back from the estate of Lloyd Horton - a beefy 12.5" telescope he made back in the 60's .  It needed an equally beefy pier, but was perfect for an in-town observatory for observing the moon and planets. 

Things have taken a big jump forward in the last couple weeks with my locating a building contractor willing to take a risk with my design.  And since he is also an amateur astronomer, he finds working on my little observatory more fun than his remodeling projects, so bumped me up in the schedule!

But first, I realized I'd not posted on the progress made way last Spring!  In April, just before it got ungodly hot in Tucson, work was started on the observatory over a LONG weekend by deciding on the exact location and installing a pier for the mount and telescope.  It has a pretty good-sized footprint, so needed a 24" diameter pier sunk into the ground, to be isolated from the observatory building.  From my reading, such a large amount of concrete needed some steel rebar, so I visited my local concrete supply place for cardboard sonotube, rebar and rounds for proper strengthening.  Starting with straight 1/2" rebar (#4) and a hefty piece of pipe, L-shaped pieces were made and wired together.  The round pieces were placed closer together at the bottom and top of the pier for added strength there.  The cross pieces of the structure shown at right helped keep it centered in the 24" sonotube form. 

The hole had been started before, but was deepened and enlarged to accept the form and rebar.  It was then leveled so that it was exactly vertical in the hole.  I also added some conduit so that electrical power could be run under the building slab up into the center of the pier to get power to the scope without cords to trip over...

In addition, before pouring the concrete, I borrowed a transit from work to take a sighting on Polaris, the north star, to establish a north-south line with which to align the pier and mount.  With Polaris at upper culmination, ie above the pole, but exactly north, I made a mark on the air conditioner and outer fence to stretch a N-S string when the time came...

The next day (Sunday, 13 April), Frank Koch came by to help with the concrete work.  He is the husband of a friend and work mate of Melinda's and he is quite the friend to have, volunteering to help with pouring over a ton of cement on a hot Spring day!  The one advantage of casting the pier was that the mixer could be parked in one place and just dumped into the pier location.  His mixer was a beefy gas-powered model, which hadn't been used in a while...  After spending the better part of an hour rebuilding the carburetor, it finally ran great and we got mixing.  Frank hoisted the 90 pound bags of concrete (26 of them!) into the spinning mixer and I added water to get the correct consistency. 

Of course, I had the easier job there, but I also used a vibrator (visible in the wide shot at left) that I'd rented for the occasion to make sure there were no air bubbles down around the rebar that would weaken the structure.  So between Frank's hoisting bags, and my adding water and vibrating the form, we were a well-oiled machine!  It seemed to take no time at all before we neared completion.

Of course, Melinda took most of these pictures, and got just the last couple batches of concrete mix to go into the form.  At left, Frank is dumping the mix, and at right, the last little bit goes in and we start work in smoothing the top surface and cleaning up.

After working the surface and allowing it to set up a little, it was time to insert the J-bolts that will fasten to the mount.  They had been installed on a plywood square pre-drilled with the correct bolt pattern.  In addition, lines were drawn on the square so that the bolts could be aligned N-S along the line laid out with the transit aligned to Polaris.  While a little nervous if I did it all right, in a day or two, the plywood was removed and the mounting adaptor slipped right into place - just like it was supposed to!

So that brings you up to date to April!  Like I said, lots has happened the last week, but that will have to wait for another post...

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