Before starting to construct a pier, several issues need to be sorted out:
– What material should be used for encasing the concrete ?
– What should be the total height of the pier and what length should be put into the ground ?
– Which inner/outer diameter should be taken for the tube ?
– What should be the exact location of the pier inside the observatory ?
As Sonotube is most commonly used, it was of course the first choice of material to be used for encasing the pier when pouring concrete. There was however only one small problem, Sonotube is not available in the Netherlands !
I had to find another solution to the “Sonotube-problem” !
Luckily, what we do have are large PVC tubing. I found a store called Wildkamp, which actually sells PVC tubing from 32mm sizes up to a ridiculous 630mm diameter tubing.
As the base of the G-11 is rather large and in order to get a more or less correct spacing for the rods and bolts in a steel plate on which to place the G-11 (using attach the Meade Field tripod adapter) I decided to go for a 2m long, 250mm diameter tube for the pier. The 250mm diameter tube should provide enough concrete around the rods to have them enclosed in a proper way.
In order to get a sufficiently large base and still have enough pier height available inside the observatory, 70cm of the pier was placed into the ground, leaving 130cm of pier height for the observatory. As shown in the image to the right, part of the concrete base was put slightly (approx. 10cm) above the soil to further increase stability. Please note that the observatory floor should NOT rest on this base !
Now that we have got the material, height and dimensions figured out, the next issue which needs to be addressed is where exactly to put the pier ? – should it be put exactly in the center of the observatory or apply some kind of offset ?
As I do not have a clear view of the northern sky (I can see Polaris however), imaging in this part of the sky was not really an option. I decided to not put the pier at the center of the observatory but offset it towards the north.
An offset towards the northern part of the observatory would free up more space for other equipment in the observatory behind the mount and scope. Besides this, it also provides a larger imaging area in the southern sky, as the pier is further away from the southern observatory wall allowing me to image objects in the southern sky at a lower angle.
Once we have determined all of the above, it is time to fill the PVC tube with rebar and concrete.
As shown in the image above and to the left, the rebar was cut and bent into the correct dimensions and was welded together in order to get a more solid construction before pouring the concrete.
When using a rebar frame, remember to never allow the rebar to make contact with the soil, as this will cause corrosion of the rebar and will cause severe stability issues over time with the concrete base of the pier !
After pouring the concrete allow it to dry out for several weeks. Remember to fix the rods in the concrete in order to prevent any movement when the concrete is drying (I used a piece of multiplex wood cut to the correct dimensions to keep the rods fixed inside the concrete when drying – visible in the image to the right).