Building My Observatory

Part Five

Here is the top plate mounted on the post shoe on top of the 4" post.

I said earlier that the metal plate on top of the post shoe had a huge curve in it. This curve meant that the bolts were miles away from being parallel and this was giving me all sorts of problems with making the slots on the top plate. I tried everything to get rid of the curve but couldn't find a way to hold the post shoe firmly enough. In the middle of the night, I thought of a solution. I pushed one edge of the metal plate between two slabs on my patio which were up against something very substantial, and then I just heaved on the box part of the post shoe. It worked! I was able to take almost all of the curve out of all four sides of the metal plate so the bolts were almost parallel and the wooden top plate dropped easily onto the bolts and through the small centring bolt without needing any persuation.

Using a compass to line up the top plate to true north is practically impossible ( the USGS website says that the magnetic declination is 4deg. 15min. at my position on the Earth) as there is so much steel around but I was able to show that the slots were in the right place to allow me to adjust the horizontal position of the plate. Final positioning will be done by adjusting the plate to align the telescope with Polaris, making allowance for its offset from the celestial north pole. (Well, maybe!)

This is the wedge in position on the top plate. I made it about 12 years ago for my ETX90 and it is big enough to take the 125. Fortunately, the holes for the mounting bolts are on the same pitch. The wedge is hinged so I will be able to adjust it to the correct position for my latitude by using shims. The wedge has to be raised above the top plate to allow access to the nuts which need to be adjusted to align the scope with true North. This shows the rough position before the wooden blocks were cut to the correct length.

The wedge is offset from the centre of the pier as the telescope is cantilevered out from the wedge. The offset is so that the axis of the telescopes altitude bearings will be over the centre of the pier, marked by the small hole for the azimuth rotation. Hopefully, this will put the centre of gravity of the setup over the centre of the pier so reducing the turning moment on the pier itself to help it remain vertical.

This is the bolt that means that the wedge has to be raised.

And there it is, the wedge in its final position, blocks and wedge all screwed down ready  for the telescope to be mounted. One of the blocks raising the wedge needed to be offset to allow access to another bolt.

...and at last, Part Six, the last part!!