Before the thirty three 16 millimeter bolts are cemented in place, a lot of work is required to make templates. The lower rotation ring comes perforated with more mounting holes than necessary. It is therefore easy to find holes which are located over cavities in the blocks, which will later be filled with concrete.
Two tasks remain to complete the observatory foundation.
1) The crown, a 12 cm thick ring of reinforced concrete, will cap the top of the masonry. The crown serves two functions: it binds the structure together and it secures thirty-two bolts which hold the dome rotation ring's steel track in place.
2) Both exterior and interior walls must be plastered. This forms a barrier against moisture, and it looks nice to.
Work on the foundation has progressed this far today, 28th May 2013. No, that is not an oil derrick in the middle, but a temporary structure that secures the center axis pole vertically. This pole marks the exact center of the cylindrical foundation. Each cinder block is mortared 150 centimeters from this axis, giving an inner diameter of three meters.
If this mirror blank was on display at a museum of modern art, connoisseurs would think it was an abstract installation. More beautiful, stiffer and lighter mirror blanks do not exist. This is the primary mirror in Norway's largest optical telescope. It is made of Schott borosilicate glass, 620 mm in diameter, 73 mm thick and weighs only 17 kg. Only Dream Cellular, LLC, can produce so lightweight and stiff mirror blanks.
The concrete formwork for the foundation of the observatory's cylindrical base is now complete. With both internal and external bracing (only internal bracing is shown in the photo to the right), this is a stiff formwork that is very easy to construct.
Wooden spacing rods are fastened with screws between the two plywood rings. These spacers are 25 cm long, forming a concrete ring that is the same width as the cinder blocks. The spacers will be removed when the form is anchored with concrete blocks around its perimeter.
Two long poles, each three meters long, intersect at the center of curvature. They serve two functions: 1) They center the formwork on the rotational axis of the dome, and 2) they ensure that the structure is circular instead of ellipsoidal.
The baseplate of the telescope mount is offset 9 cm north of the dome's rotational axis. At the intersection of the two centering poles is a long wood screw which extends down to the concrete base, resting in a cavity drilled at the axis of the dome. This keeps the formwork centered on the telescope pier with a very small margin of error. Accurate centering is important because the four meter enclosure leaves very little clearance between the prime focus camera and dome.
With an outer diameter of only 3.5 meters, this is a very small observatory for a 62 cm telescope. The cinder blocks have a wide cross section, so the floor space is only around 9 square meters.
Unfortunately, I ran out of gravel yesterday. Three more tonnes will be delivered on April 4th. Then I can complete filling, levelling off, and compressing with a vibrating plate. I hope to complete the ground work during the weekend.
An unusually stable region of high atmospheric pressure has remained over southern Norway for four weeks. Although night temperatures are down to minus 6 C, daytime temperatures are suitable for concrete work, provided a nitrate solution is added to the concrete mixture to accelerate curing. Covering the concrete with a tarpaulin is further protection against frost.
Some of the reinforcing steel is in place. This will anchor the concrete base to the basement rock. Thick ice has filled the crater all winter. Temperatures are on the increase, allowing the concrete work to commence soon. Half a cubic meter of concrete will form the base of the massive steel pier of the telescope mount. The base plate of the pier is 50 cm in diameter and 4 cm thick. Ten expansion bolts secure the base plate to the concrete base. The observatory building and floor will not be in direct contact with the concrete base, thus minimizing the transfer of vibration to the telescope.
"Nr 26 NORWEGIA" is still visible on the plastic packing. Maybe this is the 26th 4 meter ScopeDome? There were many installed in school observatories in Poland. Admirably, the astronomy curriculum for Polish schools is given high priority.
Half of a snowed-down observatory dome snuggles on its pallet beside stacks of cinder blocks. The cinder blocks will form the 3.5 meter diameter cylindrical foundation of the observatory. There are two obstacles along the road to a completed observatory: the weather and the day job.
Rain, snow and ice have delayed the foundation work. The construction area is a frozen, 20 centimeter deep pond at the moment. The first milestone on the road to completion is to drain surface water away from the basement rock where the concrete pier of the telescope mount will stand. Deep ground frost is delaying this work.
There is too much to do, too many hours go to commuting, and not enough hours in a day.