12.06.2021 Another look at the English Cross-Axis mounting.

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Saturday 12th 64F and overcast. The following is largely a repeat of earlier posts on the subject. With additions from further thoughts on the subject. 

I have been making another mock-up of a potential English cross axis mounting. The scale is prodigious but is the only practical way to mount several of my larger instruments simultaneously. 

The white plastic pipe represents the greatly lengthened, but undernourished, Polar Axis. For scale: The larger telescope in the image is my 6" f/10 with H-alpha extensions at the eyepiece end. My 10" F/8 Newtonian and 7" f/12 refractor both wait their turn in the wings.

I have raised the Dec crossing point slightly above the GEM's axis crossing. This is to provide slightly more headroom for longer instruments. The exact point at which the Dec axis crosses the PA will also affect the working height of the mounted instruments. 

Which is an important factor for clearance but requiring a stepladder. Motor focusing is vital when dealing with a 2m long Newtonian so far out of reach. Fortunately my interests mostly lie with imaging rather than visual observing. 

The Newtonian could be mounted on a separate Declination shaft. On the same PA, but placed lower down for better access. Though unfortunately, this might have required individual Dec drives to each instrument. Belt drive and large timing pulleys between the separate axes? The pointing relationships of the Declination angle must be maintained between instruments for GOTO to function. The timing pulleys can easily be made out of birch ply. With the teeth gashed with a small slitting saw in the lathe.

My huge GEM flexes when overloaded. So I have been limited to remounting each instrument, in turn, as I remove another. Which is highly undesirable given the size and weight of the instruments. Each change also demands complex weight shifting.  Which is not without its risks working from a stepladder.

The cross-axis [almost uniquely amongst mountings] allows one instrument to balance another on opposite sides of the Polar Axis. Without the usual risk of OTA collisions with the pier so typical of a GEM. 

The Declination Axis pierces the greatly lengthened Polar Axis instead of [usually] sitting up on top of it. The much greater spacing of the PA bearings [of the cross-axis] is a huge improvement in carrying capacity.Without any risk of bearing shake. Provided, of course, the PA itself, does not flex. 

It is also vital that the PA bearings are rigidly supported. The height of both bearings are important factors in the design. My "second floor" observatory provides additional difficulties in maintaining the stiffness of the bearing supports. I'm still thinking in terms of splayed 4x4s of timber. Probably clad in plywood for extra stiffness. A pair of block-built "chimneys" would serve as solid bearing supports but would require extensive foundations. Not to mention the expense!

One of these "difficulties" is resisting the downward thrust towards the south PA bearing. A second is the towering north bearing support. By its very nature it is overhanging the observatory towards the south. This produces a downward force with considerable leverage. [Or moment] This southward and downward force must be resisted without interfering with free movement in the dome. Even more importantly the entire mounting must avoid contact with the building and dome.

The extended PA is best constructed as two, long, gently tapered, plywood boxes. Meeting the Declination bearing box roughly in the centre where the boxes are largest. Plywood damps vibration far better than metal. Solid wood would be prone to twisting and cracking over time. Plywood is stiff, very strong and damps vibration readily. It can also be made much lighter than solid wood. This is important when assembling such a huge structure while working alone.

Since the telescopes are balancing each other, this removes the need for conventional counterweights. So the overall loading on the mounting remains the same. Despite having added another large telescope. Though at the cost of increased moment with longer instruments over [normally very compact] counterweights. A cross-axis can use normal counterweights but that is not my intention.

The new dome and observatory are larger than the present one. [4.3m:3m] So there is a shift of mounting centres towards the camera of about 65cm. This will leave plenty of room all around the instruments as they swing to all points, anywhere in the sky. The Cross-Axis readily provides access to the North Pole. 

There is no need for a Meridian flip with a cross-axis mount. Though it can be advantageous to carry one out. Just to bring an instrument within easier reach. The Newtonian can be brought down to a more normal level beneath the PA. While the refractors can still be accessed from the observatory floor even when above the PA. Just as they are on a GEM. Unless you deliberately swing them under the pole. Which is presently a non-starter for me. My imaging monitor is hung from the north side of the massive pier. 

I have checked. The larger observatory's diameter should provide enough room for an imaging desk under the north bearing of a cross-axis. This is the ideal situation for a solar imaging monitor. No direct sunlight can fall on it and I can easily watch the movements of the telescopes. Which, by their very bulk shade me from direct sunlight. For those odd occasions when the light falls directly though the slit around lunch time. 

Even then I can slide my office chair to left or right to enjoy full shade. It would be dangerous and very unpleasant to sit in direct sunshine for hours on end. Since I never get a tan, from my countless hours of solar imaging, I must assume I am avoiding this potential health risk.

I tried imaging, this afternoon, when the cloud thinned. Most of my 3-second videos were spoilt by racing shadows. Those that remained clear showed how poor the seeing conditions were. I was even chilled from sitting in the cool NW breeze through the slit in the late afternoon. I had to fetch a fleece jacket.

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