Pier height and dome clearance.

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An important consideration is pier height. Depending on the length of the telescope this affects ground clearance and maximum height when pointing at the zenith. This, in turn, affects the choice of sill height of the dome slit. Meanwhile the top arc, scribed by the dewshield, must clear the inside of dome. So all these dimensions are inter-related. They directly affect the height of the fixed walls of the observatory and the height of the pier.

The image shows the arcs of the long refractor 180mm [7"] f/12. The Dec. axis, when horizontal, is at 2m from the ground. In a 3m diameter dome it has no need of a pier quite this high. Particularly when the Heavy GEM is employed. Note that the circles should really be concentric. I exaggerated the height to show the effect of adding the longer dewshield.

From direct measurement of the OTA, on the MkIV mounting, I can allow the Heavy GEM to sit on a 1m high pier. Which by happy coincidence, is the height I have chosen for a temporary stand. The Heavy GEM is 64cm high from underside of the base plate to the Dec axis center when horizontal. Again, by coincidence this pairing exactly matches my eye height [while standing comfortably] at 164cm.

I made some more eyepiece ground clearance measurements, while sitting on a handy [empty] beer crate. Which measures 27x30x40cm. On the lowest 'seat' height I can easily use a 70mm eyepiece height with its axis horizontal in a star diagonal. Being able to sit allows the observer to relax. Which means that I easily seat myself on a plywood observing box of [say] 20 x 30 x 40cm for those rare occasions when I actually want to stare at the zenith.

While the Moon can get quite high it is rarely that the planets reach such altitudes here at 55N. I can remember only one particular night when Saturn was high overhead. The seeing suddenly sharpened to single pixel viewing quality through my [normally indifferent] 6" f/8 refractor. It was just as if I was floating at a distance in a spacecraft with perfect windows. I stayed up just staring at Saturn until after 3am and was suitably admonished for my errant behaviour.  

Image showing the bare 7" f/12 refractor tube at 180cm long with batten marked at 3m high. The are two different lengths of dewshield. The fixed dewshield is 20cm long while the full length one is 40cm. [2.2xD and simply slides over the shorter one.] The 2" star diagonal and Vixen 2" focuser add another 20cm down at the bottom end. So the straight tubed refractor is about 240cm in length when complete.

Assuming I balance the refractor OTA in the middle, using the sliding brass weight, I can easily clear the 3m dome. This would be with a wall height of 150cm to the sill of the open slit. With the horizontal Declination axis at 165cm high I can view distant trees with the OTA horizontal should I feel the need without losing any clear aperture. The top of the long dewshield is at its furthest from the ground when pointing at Polaris with the tube over the Polar Axis. When looking overhead the rotation of the Polar axis lowers the OTA on the end of the Declination.

The OTA axis offset of a GEM, [German Equatorial Mounting] from the center of the pier, requires a little more space when pointing at lower altitudes. In a hemispherical dome this might matter. Since I have chosen to build a half cylinder it doesn't affect the clearance nearly so much. There should also be more room in the half-cylinder to move around the telescope in the confines of the observatory without bumping into it. This can be an important matter while imaging. Nudging the telescope can also require mounting re-alignment for Gotos should the drives lose track of the telescope's exact pointing position.

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