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The extreme difficulties of properly supporting a [second floor] cross axis demands a total rethink!
Serious alternatives to a cross axis? A bent, polar [astrograph] pier. The upper section of the pier is bent over. It leans towards the north at the local polar angle of 55°.
This allows [some] freedom from the absolutely inevitable pier contact of
the GEM. Not quite as good as a cross axis but it has advantages. No need for a towering [isolated] pier to support the north bearing of the PA axis.
The main problem is the sheer scale required for the length of leaning pier. It must allow for at least 1m of overlap without any risk of contact between telescope parts and the mounting/pier until very close to the North Polar region. Preferably full sky clearance for all of the telescopes.
My
massive, timber and plywood cladding, pyramidal pier can easily cope
with such a total redesign. The top section could be made to lean towards the north
only above floor level. With the "downstairs" section redesigned to
balance the above floor structure's offset. Though the telescopes [and their
considerable mass] remain safely centered at the crossing of the mounting axes.
Centrally mounted in the dome. To maximize clearance of dewshields under the dome's inner projections. [Zenith board and the slit ribs.]
The
existing, pier foundation blocks are spread as widely as possible
within the original footprint. With room for expansion to the larger
footprint if required. [Very unlikely.] Or even additional foundation blocks
if required. Just dig and bury in self-stabilizing gravel. These simple drawings are only suggestions for a basic timber framework. It must be remembered that the base is huge at nearly 3m square. Lateral stiffness is as vitally important as vertical and torsional resistance to flexure.
The upper [leaning] pier section can be made slightly tapered. Or of
sufficient cross section along its entire length to clear the telescopes. Sturdy, internal,
plywood baffles can help to stiffen the PA "box." I have lengths of steel ventilation ducting which might serve for the leaning section of the pier.
Piers are
usually made of heavy, welded steel at the observatory level of
commercial mountings. The "bent" or "broken" pier can thus be made to
bend at any point. Some astrograph piers rise as a "Z" above the floor. While sitting on a
conventional, central pier. This requires very heavy sections of steel
for adequate stiffness. Otherwise the bend will act as a leaf spring. It is not usually possible to "cheat" and brace from the base up to the leaning section. This would simply block telescope freedom to pass under the PA axis.
Plywood cladding, over heavy timber
sections, just needs extra moment [depth] for adequate stiffness. Plywood
and timber are relatively affordable and wide open to design variations and
changes. Nor does the home made pier need to be as cosmetically
acceptable as a professional mounting. Perhaps costings many tens or
even hundreds of thousands. Rather than a few hundred at most. Plywood and timber are also self damping when excited by external shocks.
Medium sized, professional mountings often carry
many hundreds [perhaps thousands] of kilos. Mine only has to support
around a hundred kilos at most. Managing the length of my telescopes is much more
important than their total weight. The moment of all the instruments
involved has to be controlled and carefully balanced. Not quite as simple as sliding counterweights along a shaft.
The mounting loses its conventional PA, bearing housing. The PA shaft and bearings are hidden inside the sloping pier. While the 50mm, stainless steel shaft is made as long as possible. Length reduces the impact of bearing play. Increased spacing between the bearings improves rigidity. I have some spare 50mm shafts. One of the original Dec shafts could easily become a much longer, Polar Axis.
The next stage is to measure the required offset from the telescope mounting plates to the mounting head. A box section pier needs much more room to pass around the corners of the box. As the telescopes rotate around the PA while pointing at The Pole.
Not that I am ever likely to do so
deliberately. Though you never know when a Goto slew won't try to pass
close to the pole. My AWR drive system would often try to pass under the pole. Which would have been a total disaster had I not been constantly monitoring its every movement. I could never, ever leave the telescopes unattended during any slew! Even a routine Home & Park call might do a nosedive into the floor of the NE corner of the observatory!
Should I convert the new PA axis to tubular? I had a great deal on the purchase of the 50mm flange bearings. I'd only need one or perhaps two of much larger diameter. The price rises astronomically with bearing bore. All would depend on finding a suitable length of precision diameter and dead straight, thick wall, steel pipe. My lathe won't take lengths greater than about 18". So I can't turn longer pipes between centers.
The alternative is a disk based PA instead of a shaft. Though I prefer a combination of both. A disk [or rather cylinder] can provide a very deep cross section at the weak point between PA shaft and flange. I buried the PA, Tollok bush inside a very deep, 180mm Ø, aluminium cylinder. So that the cross section was maximized.
The Tollok bush expands outwards into the cylinder over its full length. The cylinder itself is bolted directly to the Dec housing over a large radius. Providing massive support at this vital junction. The same arrangement could not be achieved at the more lightly loaded Dec junction. This was due to the narrowness of the double channel saddle. [H-section] Here, the Tollok bush was buried inside a massive brass cylinder. See the image for an earlier set-up with the 6" f/8 H-alpha telescope. A Vixen 90mm f/11 is mounted for solar, white light observation and imaging.
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