Black Oak Observatory Construction
Welcome to a gallery narrating construction of my astronomical observatory. Entries were added in chronological sequence. Click on photos for a larger view. Schematic plans and detail drawings are available here.
foundation framing finish
Confluences, inclinations, circumstances, chance ... that's how decisions are made in the human realm. At least, my human realm.
In the winter of 2011 I decided to build an observatory at my home in west Sonoma County. Why? Wasn't my telescope dolly enough to make observing with my Meade 12" convenient and rewarding? Yes, it was.
But the driveway is a rather large expanse of asphalt in two directions, and in the summer this would heat to cooking temperatures. Worse, the entire south and west compass directions lay under the span of my residence, which exhaled heat in the early evening from shingles and attic. And the rooflines block the entire southern sky below declination –20˚ or so. (The horizon is at around –50˚.) I could do better!
In addition, I ordered a 254mm, /20 Dall-Kirkham from Robert Royce, due for delivery in early summer. This would put two telescopes in the garage, which was already cluttered. And require building a second dolly ... An impediment curve was rising before me.
Then there was my dear wife, who had decided to get a hot tub installed in our back yard to soothe her sometimes mordant arthritis. It seemed like a great opportunity to fold two projects into one, and construct the observatory at the same time.
There was a bit of peer pressure, too. A generous and very knowledgeable acquaintance, Phil Sullivan, mentioned that I should have an observatory. Later, when he came over to my house to stargaze and share eyepieces one night, he groused about a neighbor's porch light shining through the trees above our site. "You should have an observatory."
And finally ... I did some browsing of amateur observatories online at nine planets the "Amateur Astronomer's Dead Link Site", where over half the links return a 404 and I was surprised at how crude and makeshift many of them were. Really? Rot lumber on cinderblocks? An outhouse observatory? A doghouse observatory? And I don't have an observatory?
So I spent a couple of months researching different observatory designs, visiting local observatories and talking with their owners about their experiences with the buildings, and gradually developing my own concept for a building in our back meadow.
Contractor and friend Bob Corcoran came over today to begin work on the site. Our agreement is that I will contribute labor, he will provide expertise, and we'll get the job done.
We first carefully examined the location I had chosen against a map of the leach field below the garden, and discovered that the leach lines extend under the previously selected site between two lovely, medium sized coastal black oak trees. So we relocated to west of the western tree (in the background of the photo, left) instead.
We fastened two 8 foot poles to the corner stakes, to show the finished height of the roof, and got approval from my wife to proceed.
A few days earlier, a tree care contractor was out to trim back the oak tree and other trees around the property. I'll have to continue trimming the oak to keep it at an acceptable height. Although its canopy blocks part of the eastern horizon up to about 30°, that is a good thing it partly shields the site from the light dome cast by Petaluma, a rural city to the southeast. In tribute to its influence, I'm calling the building Black Oak Observatory. (Thanks to Phil Sullivan for the correct identification.)
We staked out the approximate location of the caisson and pier holes, and talked about the project logistics going forward.
Later, I took a photograph of the site from our south facing deck and used Photoshop to rough out a concept panorama of the finished building ... the stake locations gave the scale and perspective.
I've designed the structure to conform to the slope and be compact, functional, comfortable, thermally stable and robustly weatherproof against our fierce winter winds. Only 10' by 12', it requires no building permit ... but will be able to house two separately mounted Schmidt Cassegrain, Dall Kirkham or Ritchey Chrétien telescopes up to 16" aperture, or classic Newtonians up to 12" at /8, in any combination.
There will be a bench swing from the roof track supports, trellised vines and bushy flowers, a small patio of packed sand, and maybe a staging area downhill for a big Dobsonian. We'll see.
the observatory concept image
Bob arrived this morning at 9am. He used a transit to bracket the site with batter boards, then strung the exterior dimensions and checked the accuracy with diagonal and side measurements. He took care to get everything within a quarter inch or so of the plan dimensions.
Bob marked the pier/caisson hole locations with construction paint. We spent some time discussing the design of the metal telescope piers and the electrical requirements for a telescope drive, dew heater, hair dryer, CCD camera and so on.
Most observatories seem to work with spaghetti wiring improvised around the pier, but I'm trying to figure out something cleaner and easier to customize.
Caisson marks in place, all is ready for the auger contractor tomorrow.
Bob arrived at 9am and shortly after came Carl Van Dyke and his crew. They offloaded the tractor and took it down to the site through our neighbors' property, with their kind permission.
Seven men then congregated around the usual lumber and tools, had some discussion about the site, the soil, a nearby effluent line, landscaping and a french drain to carry the winter runoff around the foundations and minimize the tendency of the wet slope to creep downhill ... and work got started.
The 18" holes for the telescope caissons went first, 9 feet down into what passes for bedrock in this area, the "Sonoma Gold" rotting yellow sandstone. That's Carl at the far right, watching as Bob drives stakes to hold a plywood cover in place over a finished hole.
Bob and Carl left for other duties as the crew finished the 12" foundation holes, 10 in all, and after lunch under the eponymous black oak dug the trenches for grade beams between the caissons. A productive day.
You know the caisson is deep enough when you strike water! But it's only a puddle, so my decision to pass on an extensive french drain system seems appropriate.
Bob arrived around 10am with rebar, lumber, dobies, tools and sonotube. We unloaded everything down to the site and began installing rebar in the foundation cuts. That's Bob manning the rebar cutter (below), and the cage for one of the telescope caissons on the ground.
We tied dobies to the cages and set them in the foundation holes, 10 in all, then tied everything together with grade beam rebar. Alignment was checked against the batter board string with a tape measure and plumb bob. We used the transit to identify the highest rebar cage among the uphill foundation caissons, then cut and set its sonotube collar.
Then we got the telescope pier caisson cages into their holes. Bob checked the height of the slope and estimated height of the subfloor, using the uphill foundation caisson sonotube as a reference.
One of the cages was a bit short, so I shoveled up some crushed rock from the utility path around the residence, and dumped that in the hole to bring the cage up a few inches. We double checked the slope measurements and installed the two pier sonotubes. The top of the tubes identifies the floor level of the finished structure. Then we called it a day.
Rain is forecast for Sunday and the middle of next week, with a break on Monday. I'm working toward a pour on Monday, which gives the concrete a two day rain forecast to cure.
Now it's looking like we might get the subfloor framed before I leave for a week to attend the RTMC.
Bob arrived around 10am with more rebar and lumber, and we spent the day leveling, centering and staking down sonotube around the 12 piers. We finished around 4pm and cleaned up the site so that the weed whackers can clear the meadow on Saturday.
Not much particular to report ... the work was steady and arduous. Because I had skipped breakfast I quit for a half hour lunch, but Bob pushed through to the end.
My main tasks were fetching tools, holding the survey measure, checking centers and measurements, digging out the flat area for the concrete pad to support the observatory steps, tying off rebar, and so forth.
The evening before I spent time revising the telescope pier specifications, and I've decided our best approach is just to tie the pier anchor bolts to four sides of the rebar cages, let the concrete set around them, then drill the pier base plates to match the bolt positions. Since I'm having the piers custom made, I don't have to worry about getting the bolts exactly aligned to a standardized hole spacing.
Also, the eight V casters for the rolling roof were delivered this week from Hamilton Caster. The red paint may not last, but the rest of the construction seems quite solid and durable. Rated for 550 pounds each, 4 should be plenty to support each half of the rolling roof, which I calculate will weigh about 500 pounds.
Rain is forecast for Saturday night, tapering off by Monday. The concrete pour is scheduled for Monday at 10am, and we should be finished with troweling, setting anchor hardware and checking the telescope pier alignments by 1pm.
Saturday I spent a few hours shopping anchor hardware for the telescope caissons. I decided on simple threaded steel rod rather than the usual construction hardware because the finish on the steel was much cleaner.
I bought three 5/8" rods, 36" long, plenty of washers and hex nuts, and lumber to make a form to hold the rods during the pour. With a rod Bob had brought earlier in the week, I had enough to make eight 18" anchor rods.
I reconsidered my plan to wire the rods to the rebar cage, as this would make the process of fabricating the metal piers more complicated. So I made two forms of 2 by 3 fir stud lumber instead.
I cut the fir into four 23" sections and routered out the central 21/2" so that two pieces would fit crosswise with flush top and bottom. I glued and nailed them together, and added 1 by 2 blocks at each end, spaced exactly 18" apart, the outside diameter of the sonotube. Finally, I drilled four 5/8" holes 63/8" from center, using a compass to set the crosslines perpendicular, and secured the rods through each hole. A large washer sandwiched between two hex nuts served as the anchor mass at the buried end of each rod, and the rods were fixed to the form with two more hex nuts. The forms fit snugly around the top of the sonotubes, the threaded rods just cleared the coiled rebar on all sides, and the form arms left ample room for concrete to be pumped in.
Luckily the rains relented Sunday evening, the clouds parted, and Polaris was visible around 10pm. I took my HoTech flashlight down to the site and used its green laser to align the north/south side of each form, a discard Snapple bottle serving as a mallet to rotate the form in place. The moon was up and nearly full, and its diameter helped me judge the visual location of celestial north 0.7° from Polaris, on a line between Polaris and Kochab.
Pour day at last. The sky was bits of blue through huge broken clouds, with rain showers forecast for the afternoon.
I was down at the site around 8am pulled away and folded some tarps, cleaned up scattered scrap lumber and tools, took down the measurement string, gathered gravel from a utility path to spread in the bottom of the concrete pad that will support the entry stairs.
Bob arrived a bit later and set to drilling forms to hold the caisson anchor bolts during the pour, checking alignment of all the sonotubes, reassembling braces on the north telescope caisson that I had to disassemble to align the anchor rod forms, cutting pigwire for the entry concrete pad, realigning the measurement string to match the building outer dimensions, and spreading tarps on the front drive.
The pumper arrived after 10:30 and began unloading hoses for the 200 foot run from the front drive through the garden to the site. The mixer truck arrived a few minutes later.
There was a lot of agida around getting the concrete down to the site. Brian the pump guy had to clear out several dry clogs of gravel, pounding on and shaking out the hose, getting frustrated and red faced with the effort. The mixer driver diluted the concrete with water, and pumping began in earnest around 11:30. Bob and Brian squirted mix down at the site while I stood on the residence deck with the pump remote control and chatted with Jan and Phil Sullivan, who stopped by to watch the operation.
We ended up with just enough concrete six yards of six sack mixed with fly ash and began the cleanup. I broke a wheelbarrow hauling a heavy load of the scrapings, and we made the transfer to Bob's construction barrow on the garden steps.
I paid Brian and the mixer driver, Phil took off after assisting with some of the concrete finishing, and Bob and I spent an hour smoothing out the grade beams, leveling the tops of the caissons, setting anchor hardware, checking alignments, and cleaning off the concrete tools. Bob folded up the driveway tarps, cleaned everything with a power sprayer, and took off around 2pm.
Meanwhile the skies clouded up again, and after a few light showers a winter drizzle began around 4pm. By then the concrete had hardened enough to resist it, so all is well.
A light cool rain fell during the night and the morning of the next day.
While touring the site, Phil identified the nearby tree as a black oak, not a live oak as I originally thought. I found out that oaks can hybridize, and I had some trouble finding references online that showed exactly matching leaves and bark (they're shown below). Later Phil recanted, thinking it is a Garry Oak instead ... but too late. It's now a black oak in name, whatever its nature and it looks like a black oak in the dark.
Tuesday was overcast the whole day, and rained in the morning and evening.
Wednesday the sun broke through. Bob had the day off, and I spent several hours cleaning up the site breaking away the mould supports, stripping off the sonotube, chiseling off excess concrete at the base of the tubes, picking up scrap lumber and paper and piling it up for disposal, carting broken concrete down the hillside, shoveling up concrete splatters, and restringing the building outline.
I didn't bury the grade beams, to give them a day or two in the sun to cure.
The forecast is for a week of sun ... Bob and I will have time to frame the floor before I leave this Sunday for the Riverside Telescope Makers Conference at Big Bear.
next : framing