The pier hole
After careful consideration of the best place for my observatory, I chose the north facing
back garden, free from the village street and house lights on the other side of the house to the south. It
faces over the sea and hills beyond and gives excellent clear skies to the north-east and east especially.
The view of the southern horizon is unfortunately limited by the house being in the way, but the ecliptic
is visible. This part of the sky is good when every one has gone to bed and the village houses have cooled
down.
I built the concrete pad thinking I was going to have a dome and so its size and shape was
determined by the size of the expected dome which had a diameter of 2.76m. I made the pad 3.1m square. I
placed the pier 10cm (4") slightly to the south to give extra room to the north in the eventuality of
obtaining a wedge. This extra room is also quite useful when viewing along the ecliptic.
Staking out the shape of the pad and location of the pier
With any project, careful and extensive planning is required. I strongly recommend using
some long garden canes and plotting out different locations and configurations. When you start there is
little room for manoeuvre if you change you mind. I had the plan designed on paper but it took many goes
to get the location right, allowing for the surrounding trees, the house to the south (behind the camera
in the above picture). It's always going to be a compromise for most of us.
Starting the pier hole
Fortunately the house and garden are built on old sand dunes. Digging is easy once the turf
is removed. However, this was an old fishing village and our part of the village looks as though it was
used as a midden or rubbish dump. There was small random patches of gravel and old road tarmac in places
which was a nightmare to dig through. Sod's Law also dictated that I would find a huge lump of concrete in
the corner of the pier hole and so another adjacent hole had to be dug to remove it. You know how it is, a
two hour job turns into six!
A lump of concrete encroaching into the pier hole
That was annoying because it meant I had to rebuild that corner of the pier hole and it
didn't have any turf left on top to help keep it together. Before pouring the concrete I built it up level
with the turf with some wood.
See how the soil is only about 8" deep. Pure sand all the way down after that and easy
to dig and shape. The hole is just under a metre square (30") and just over a metre (33") deep.
I shaped the bottom of the hole so that it was wider than the hole above it undercutting the walls. When
this fills with concrete it will help prevent the concrete pillar rocking sideways in the soil.
The pier hole finished
Some people go crazy here digging a hole twice as deep, down to bed rock even, Well, I
don't have any bed rock (well not for some way yet!) nor see the need for such a huge hole to be honest.
All that matters is that the pier is solid, doesn't move in time and doesn't transmit any vibrations to
the telescope. The footing needs to be below the frost line but that doesn't matter in the UK where the
frost line is only inches deep at the most.
|
