11.03.2021 Supporting an "upstairs" Cross-axis.

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Thursday 11th. 39F. Very wet with storm force gusts later this afternoon. I have emptied the rain collection trays from the dome's base ring. Then pointed the shutters towards the west. From where the strongest winds are expected.

 The new observatory door position would have one of the [present] pyramidal pier's legs obstructing it on the left. I could move the entire pier a couple of feet to the WSW. Which would help. The pier needs to be adapted to the larger building footprint anyway. If only to maximise the clearances being offered by the larger dome. 

 Placing the existing pier more centrally in the new dome makes obvious sense. Just dig up four foundation blocks and move them over. Bury them again and the job is done. Except for removing the mounting, moving the massive pier, moving floor joists, flooring, access ladder, etc. Isolation is not exactly a simple exercise in an existing structure.

 Then there are the tentative English Cross-axis mounting plans. North and south A-frames, from 4x4, can be borrowed from the existing pier. The main problems arise from resisting the heavy thrust forces on the south bearing. The entire mounting must, of course, be isolated from the building. After living with the huge pier for a couple of years I would prefer a much less cluttered ground floor. A Cross-axis would have vertical A-frames. At least in the north. 

If only life were so easy. Triangulating the southern bearing, against the heavy thrust forces, requires solid structures be placed somewhere. Only placing beams within the ground floor, or outside, is possible. Any extension though the south wall would need to be massive to resist wind vibration. Triangulation beams inside the building would interfere with free movement. They cannot be tiny because of the considerable loads involved.

Getting enough clearance for the telescopes, within the dome, is highly dependent on the height of the north bearing. Otherwise the crossing of the RA/Dec axes cannot be high enough to be useful with long refractors.

 Unfortunately, the height of the northern bearing of a Cross-axis mounting affects several other factors. As the north PA bearing rises it pushes the distance to the PA south bearing across the observatory floor. 

The PA angle must obviously match the local latitude. Conversely, raising the south PA bearing also raises the north PA bearing. Ideally it should be easily possible to walk around the south bearing without tripping. This avoids ducking under the sloping PA and telescopes with likely collisions.

 There is an optimum PA north bearing height which allows for a northerly lean of the south bearing A-frame to match the PA angle. Having the south bearing anywhere near the south wall causes serious problems of thrust resistance. Bulky, thrust resistance beams are required inside the building. Or heavy beams outside. An external concrete [?] pier being a possible alternative. The obvious disadvantage being the much greater length of PA axis. Possible flexure?

 The images show most of the basic geometric problems. Unless the north bearing support is raised high enough there is no room for the telescopes to swing freely. Ideally, the RA/Dec axes should cross at the same height as the present GEM. Or, preferably, even slightly higher. Which the larger dome allows.

 Raising the north bearing greatly increases the telescopes' useful swept area in the dome. Though it pushes the south bearing further south. Denying a "straight shot" at resisting thrust on the south bearing. Raising the south bearing via an upper extension on its A-frame provides a much better resistance angle. Though at the cost of increased bulk and need for isolation from the observatory floor. 

The lean of the southern bearing support does not mean instant collapse of the southern A-frame towards the north. The long Polar Axis directly resists this toppling force. 

Meanwhile, the weight of the very long PA, plus all the telescopes, has to be taken into account where leaning A-frames are concerned. The sheer weight involved allows for a [supposedly] less optimum [i.e. more upright] lean angle. Without introducing an overturning [southerly] force on the top of the A-frame. The lean angle should ideally be calculated to lean at the resultant of all the forces at its top. Since it shares the loads equally with the north bearing then the vertical loads are halved. Though they are still not insignificant.

The importer has emailed to suggest tomorrow morning for delivery of the new "dome kit of parts!" 😎

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