Building My Observatory

Part Four

I cut two pieces of 19mm Contiboard to the right size for the plate which will mount on the top of the upturned post shoe. The wedge for the tripod will sit on this plate. I used "no-nails" glue to bond the two pieces together, clamped them and left them to cure for three days.

Contiboard may not be ideal but I had plenty of it, hopefully it will never get wet and it would have cost me 20 for a 1m square of 19mm marine plywood.

I scribed on the positions of the holes on the post shoe, drilled holes and elongated them into slots so that I can alter the azimuth of the wooden plate. Its long axis needs to be aligned to true north but the patio slabs and hence the drive-in post anchor are about 27 degrees east of north, that's why the slots are offset. The slots allow me to rotate the top plate by about +/- 5 degrees.

The top plate is fixed to the post shoe by 4 M10 bolts, each 100mm long. The small bolt sticking up in the centre is in the very centre of the post shoe so each bolt hole is equidistant from this position. The small bolt will act as a pivot about which the top plate will rotate when I adjust the azimuth position. The nut on the small bolt is just to stop it falling through.

The top plate mounted on the post shoe. The small hole has the small bolt sticking up through it to act as a pivot and you can see how much leeway I have in azimuth from the slots.

The post shoe is far from square and the metal plate on the top is crudely welded on. As you can see, the metal plate has a huge curve to it. The bolts are fixed to the metal plate using a pair of nuts locked together so they are neither vertical nor parallel. The slots in the wooden plate are wide enough to make allowances for this. The nuts immediately under the wooden plate are used to adjust the plate so that it lies horizontally. The nuts just underneath are lock nuts. You can also see the small pivot bolt in the centre.

The top pair of nuts are used to push the top plate down against the nuts just underneath. The top nuts can be left just loose enough to make the final adjustment in azimuth before being tightened. 

By adjusting the nuts just below the top plate, I can make the plate perfectly horizontal. I used a very small bubble level here but I will use a 1m length spirit level when everything is in the final position.

Almost there, click here for Part Five