Before construction on the main structure of the observatory can start, I need to get the pier in place because this will be set into a concrete block in the ground under the floor. As I’ve said before, my aim is to be able to put my 127 Mak or C9.25 scope on the pier and have the eyepiece height suitable for viewing from a seated postiion (even if that has to be a slightly raised seat), but also to allow the use of my 200P Newtonian scope on the same mount, where the eyepiece is way above the top of the mount by some distance when viewing objects close to the zenith. The other consideration is that I don’t want the walls of the observatory to come too far above the horizon.
I’ve not had the chance to measure accurately, but I’ve assumed that I’d prefer the eyepiece height with the 200P to be no more than six feet (1800mm) above floor level and that generally the eyepiece will be no more than 425mm above the mount saddle. The OTA is just over 90cm long, but the eyepiece isn’t right at the end and I’m assuming the scope will be mounted somewhere close to the middle of the tube. That may turn out to be an optimistic assumption given the weight of the mirror, but if it turns out to be a few tens of millimetres on the low side I can handle that. For using my other scopes I assumed that I wanted the saddle height to be about the same as the wall height which I’ve decided should be a minimum of 4’6″ or 1380mm (for a pitched roof design) to allow me to walk around inside. The saddle-to-base height of the NEQ6 looks to be about 410mm, leaving me with an overall pier height of 965mm (1800 – 425 – 410) above the floor level. On an office chair at my desk, my eye height is about 1.2 to 1.3m, so with the SCT, Mak or a ‘frac on the mount I should be just below the level of the mount saddle (1375mm). I’ll have some height adjustment at the top of the pier anyhow, and I plan to make an adjustable chair in the fullness of time anyhow.
Dimensions fixed, after a great deal of deliberation I decided that some air conditioning type ducting would make a good for the pier. Hunting around online led me to ebay as a source for reasonably-priced 1.5m lengths of galvanised spiral ducting. I dithered over diameter (200mm and 300mm were both options) for a while and finally plumped for a 300mm diameter tube with a 300mm to 200mm reducer on top, to give a bit more clearance for the scope around the top of the pier. From the images the reducer appeared to give perhaps another 150mm of height as well, which would mean a reasonable length could be buried in the block in the ground.
The ducting has now arrived, and here it is (next to some stacks of beehives):
The reducer is a small amount shorter than I thought — only about 120mm, but that’s fine.
I’m now wondering if it wouldn’t be rather neat given that I have lots of room, to run some ducting up the inside before I fill it with concrete to put cables through rather than running them up the outside of the pier. I’ll ponder on that a little longer.