Vibrations from the floor must not propagate to the telescope. Therefore, the concrete footing does not touch the floor, joists or battens. The pyramid-shaped footing is in contact only with the bedrock and the compacted gravel layer over the bedrock.
Together with the radial pattern of the floorboards, this complicates the layout of the floor joists. The joists can only be in contact with the cylindrical wall and the underlying compacted gravel.
The finished floor is located in the same plane as the top of the pier footing, forty centimeters above the gravel layer. The battens are 2x6” and joists are 2x8”. This is very oversized relative to the span between joists. The reason for using over-dimensioned lumber is that this combination puts the floorboards at precisely the correct height.
The telescope drive unit is now mounted on the pier.
The drive is the Astrosysteme Austria DDM 160, having an instrument carrying capacity of 300 kg. The counterweight mass is an additional 145 kg.
The model name DDM 160 is an abbreviation for “Direct Drive Mount, 160 mm”. Direct drive denotes that torque is transferred directly to the polar and declination axes without gear reductions. The polar axis is 160 mm in diameter.
The mass of the drive is 220 kg. The counterweight shaft is 24 kg, and will carry four counterweights totaling 119 kg.
Click on image to enlarge.
A milestone was reached in the project with the installation of the massive pier. Using the home-built travelling crane, I was able to carry out the operation alone. The pier's mass is 170 kg, the base plate is 38 kg.
Astrosysteme Austria (ASA) has designed this large pier with a goal of maximizing stability with a minimum footprint. Mated with the DDM 160, it allows 360 degrees rotation in right ascension without hitting the pier, even when pointed at the pole. Few GEM-type mounts are able to do this.
When the DDM 160 is mounted on the pier, the center of gravity will lie just eight centimeters north of the base plate's center. This is a very stable configuration.
Astrosysteme Austria (ASA) made the pier in 11 millimeter thick steel. It is designed as a "knicksäule", a bent column which is custom-made to the observatory's latitude and the telescope's back-focus and instrument package. The back-focus and instrument space determines the length of the pier's polar arm. This in turn determines the angle of the lower arm to allow continuous tracking past the meridian. The customer-specified height of the polar axis is also taken into account, which in this case is 1923 mm. The latitude of the observatory is 59 degrees.
A four meter dome provides little space for a 62 cm telescope with an f3.4 primary and accessible prime focus. Had the prime focus not been accessible, then the optical tube assembly (OTA) would have been considerably shorter.
Danger of collision
As it is, the prime focus camera and filter wheel will collide with the shutter drive motor if the mount is located at the center of curvature of the dome. The rotational center of the mount is the intersection of the right ascension (RA) and declination (DEC) axes. This point is normally placed at the center of curvature of the dome, making the telecope's sphere of rotation concentric to the dome.
The base of the pier footing covers one square meter, and extends all the way down to crystalline basement rock, about 50 cm deep. The footing is anchored directly to the basement rock. Steel reinforcing bars drilled and cemented deep into the rock ensure a strong bond.
Small margin of error
The upper part of the pier footing is cast on top of this cube of concrete. Because of the small clearance between the telescope and dome, the dome had to be in place before the upper part of the pier footing could be cast. Therefore, the pier footing had to be cast in two operations in order to determine the exact height of the pier.
The dome ring apron collar is made of fiberglass-reinforced polyester. It consists of five segments. There are pre-drilled holes for stainless steel countersunk screws and bolts that fasten the segments securely to the rotation ring. Later, the bolts holding the dome in place will extend through the apron collar and rotation ring.
The joints between the collar segments are sealed with a polymer sealant.
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The fine levelling adjustments of the sixteen rollers that support the rotation ring are now complete. Dome assembly work will commence as soon as the weather allows.
In order to lift the seven fiberglass and steel dome elements into position, we are building a gantry crane with a hoist trolley. The trolley carries an electric hoist having a lifting capacity of five hundred kilograms. The gantry will be securely fastened with guy ropes in order to withstand 20 meter-per-second winds common at this time of year.
Click the image to enlarge.