A permanent observatory needs a permanent foundation. There's no point spending time and money on a structure if it's going to shift, settle, or crack over the first winter — and in Lancashire, the ground moves more than you'd think.
Planning the Pad
The observatory footprint is 4m × 3m, but the pad needed to be slightly larger to give the walls somewhere to sit with a proper overhang. I settled on 4.4m × 3.4m, with a depth of 150mm throughout. For a structure this size that's comfortably over-engineered, which is exactly where I wanted to be.
I also had to decide early on whether to include a separate pier pad for the telescope mount. A lot of observatory builds integrate the pier into the main slab, but this transfers vibration from footsteps directly into the mount — not ideal for long astrophotography exposures. I opted for a completely isolated pier pad, physically separated from the main slab by a 10mm expansion gap.
Excavation
I hired a mini-digger for the day — one of the better decisions of the project. Hand-digging a 4.4m × 3.4m × 300mm excavation (you need to go deeper than the finished pad to allow for a hardcore sub-base) would have taken the better part of a weekend. The digger had it done in three hours.
The soil here is clay-heavy, which means it needs a decent sub-base to prevent heave. I put down 150mm of compacted MOT Type 1 hardcore before the concrete went in.
The Pour
I used a ready-mix truck rather than mixing on site. For this volume of concrete — approximately 2.2 cubic metres for the main pad plus another 0.15m³ for the pier — mixing by hand or even with a small electric mixer would have taken all day and produced an inconsistent result.
Mix specification: C25/P4 — a standard structural mix, suitable for external ground slabs.
The pour itself went smoothly. Key points:
- Laid polythene DPC directly on the hardcore before pouring
- Used a screed board to level the surface, working in sections
- Steel mesh reinforcement throughout the main slab
- Left the pier pad fractionally proud to allow for fine levelling later
Curing
Concrete needs time. I covered the pad with polythene sheeting immediately after finishing to slow the cure and prevent surface cracking, and left it for a full seven days before walking on it. The pier pad got a full 28-day cure before the mount was anywhere near it.
The result is a dead-flat, solid base. Next up: the walls.