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Saturday 23rd Jan. 36F. The gales have dropped to light winds thank goodness. Largely grey skies but dry. I must try to check the ring template arcs for accuracy when they are butted together with the new angles.
I had a mad idea yesterday while contemplating the remounting of the 7" f/12 beside the 6" f/10 in the larger dome. Or, even mounting my 10" f/8 Newtonian opposite both refractors?
All of which are exhausting and enormous [sic] time wasters to remove and refit. When having to swap them around on the present mounting. Even though I have an overhead block and tackle to help me. Lifting the telescopes high enough without hitting the inside of the dome is a problem.
What about a cross axis style, equatorial mounting? Sometimes known as the Modified English Mounting. A very long Polar Axis is supported at top and bottom by individual piers in the North and South. The increased dome size offers me much greater freedom to play with this design.
I have a couple of 2m long, heavy, box section tubes in aluminium from the scrapyard. One is 5"x5"and the other 4"x8". With lots of shorter pieces in 4x8 and 2x8. So I can construct any PA length I may need.
Two piers would spread the load and be less prone to azimuth twist [if it exists.] Another advantage is that the counterweights can be hidden above or below the observatory space. I was repeatedly hitting my head on the counterweights with the longer PA shaft on the present mounting. That would need to be refitted with all the struggle that entails.
Perhaps more excitingly: All three OTAs could easily counterbalance each other. The OTA's individual spacing from the PA ensures balance. With no need for a meridian flip. Or even conventional counterweights. Provided it all clears the observatory and floor. The 10" Newtonian on one side of the PA. The two refractors on the other.
The need for the huge, pyramidal, timber pier in the centre of the observatory would simply vanish. I was certainly not relishing having to move the whole thing towards the SW!! Which would mean the total removal of the existing mounting [GEM] anyway. Then moving the massive 4"x4", four legged structure. Plus digging up and re-burying its four, foundation blocks. Not to mention the problem of fitting the huge pier between existing joists and flooring! Which would mean major surgery of the existing structure. The original joists were deliberately fitted around the pier.
I haven't done any proper scale drawing to see what is possible yet with the Cross-Axis design. I may well have overlooked a serious flaw. The huge PA would still need to be firmly supported at a considerable height from the ground. With the southern bearing resisting all the downward and PA+OTAs axial loads. No doubt a 4"x4" A-frame could provide lateral support.
The axial loads would need rigid resistance without hindering freedom of movement on the ground floor. Both A-frame piers could easily be tied together with timber well above head height on the ground floor. Which would simultaneously guarantee the N-S stiffness of both piers. As well as complete isolation from the building. The increased mass would be all to the good for rigidity of the telescope mounting.
The much higher, north bearing support could be a simple 4"x4" timber A-frame. Perhaps clad in ply for extra stiffness. It could be made of curved and laminated plywood within the dome itself for maximum telescope clearance under the Pole. With the supporting A-frame only reaching as high as the top of the observatory wall.
The northern [tall] pier would be placed very near the northern observatory wall. So no real hindrance to movement on that side of the dome. A very tall northern pier would allow a longer Polar Axis. Allowing a far greater choice of southern bearing placement. See the sketch above for the parallel lines. Showing alternative PA [Polar Axis] positions and lengths.
It would obviously be better to have a clear walkway around the southern end of the mounting. To save ducking [or even limbo dancing] under the PA at its lower points. Also to avoid having to move the telescopes just to escape to the stairs. Something to keep firmly in mind when trying different pier/PA geometries. The 55N polar angle is obviously fixed. Its very steepness being an advantage here, I believe. The PA would cover less overall floor area even when made deliberately very tall at the northern end.
I checked how extensions of the existing GEM mounting Polar Axis would fall: 2.2m to the floor below the axes crossing and 1.3 above to the present dome. A 3.5m long Polar Axis! There would be more room in the larger dome. An alternative, without offering the freedom form meridian flips, is a southerly extension of the Polar Axis down to the floor. Greater stability and resistance to torque effects on the present pyramidal pier.
Is it really worth the hassle? Probably not. The great difficulty of moving the present, massive pier is ample excuse to build the full Cross-Axis mounting. Far easier to dismantle it and use the four legs for the A-frames.
The three OTAs would be very bulky and sweep out quite a large volume in the middle and lower, north side of the dome. The height of the Declination axis will control headroom and OTA clearance from the dome above. No problems with clearance above or laterally in the larger dome. Particularly when the OTAs are parked horizontally facing East. Or when I am imaging the sun early in the morning. It should be quite easy to move about beneath the telescopes and PA under most, normal circumstances.
I enjoy the present, south-facing desk on the northern side of the dome. It gives me clear sight of the telescopes and their clearance from obstacles. Including the bare clearance from the desk top during a meridian flip! I am sheltered from direct sunlight by the instruments and the big GEM. The north facing monitor is fully shaded. While I deliberately wear black clothing to avoid any risk of reflections in the already low reflection, AOC monitor screen.
Can this successful workstation be duplicated to become free standing or even wheeled? Cables are already an issue. With severe limitations on length in some cases. Reaching the telescopes with USB3 cameras attached would need much longer cables. Unless it can be managed wirelessly. No problem having the RA drive at floor level and the Dec drive cable is usefully long.
I imagine the [horizontal] Dec axis would be arranged at much the same height as at present with the big GEM. That would mean the 10" Newtonian eyepiece will be rather high above the observatory floor at times. Though the plan is mostly to image the Moon and The Planets. Which just needs the camera to be fitted, with the telescope horizontal, before starting an imaging session. With remote, motor focusing. Thereby avoiding the need for tall stepladders.
I do not foresee the Cross Axis mounting costing me much at all. I already have all the [scrap] building materials to hand. Or to be sourced in the parts of existing GEM. Such foolish ambition in the middle of a pandemic at my age? Will this madness never end? 😊
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