A slightly unusual observatory build

[LuckieEddie]

After nearly two years of planning, preparation and talking about it, I've finally made a start on my observatory build. 🥳

I thought I'd share the progress here as there are a number of "off the beaten track" features involved.

The first issue is location. Our urban back garden is not great for astronomy - we're on a corner plot which is triangular, there's a two-storey block of flats to the South with permanently on security lighting, and there's also a street light outside our front garden. The house is towards the NW side of the plot but the NE - SE is larger obstruction free. We're not likely to be moving anytime soon (if ever) and I can't afford a remote site. All this combines to make only one viable location for my observatory - the extreme corner of our rear garden, where my existing shed is located. Given that I built it to last and it's as sizeable as will fit there, I think it makes sense to convert the existing structure.

And the problem? Remember I wrote above that the plot is triangular? Well so is the shed.

[Bivanus]

Something like this ?  https://skyandtelescope.org/astronomy-equipment/a-compact-back-garden-observatory/

[LuckieEddie]

No, not as compact as that one...

The shed/future observatory is 4.5m X 4.5m X 6.35m. It's also tucked up against the boundary so I'm limited to 2.5m in height to meet the requirements for not needing planing permission. I want to keep maximum practical height so that:

1. I get the best sight lines/horizon possible.
2. I don't have to duck down to enter or move about inside. Luckily I'm not that tall.

Here's a snap from Google maps showing the plot, the observatory is the red triangle.

[Bivanus]

That looks better, and the dimensions seems ok - the height probably will not be such a bother once you open it I would think.

There were also some pictures with a propper observatory  shed tucked similarly to yours , it was a bit more wide from the looks , alas I can't seem to find them anymore on Google.

[LuckieEddie]

The first job was (nearly) emptying the shed. This has been my main workshop and wood store for many years, as well as housing all my astro kit, so this was a major undertaking. Luckily my requirements for major DIY projects at home (this build excepting) are largely done with now. The attention has now shifted to helping my children out with their place, so the least they can do is store all my wood. With that out of the way. a serious purge of accumulated toot, repurposing another shed to be the 'dirty' workshop and moving my kit into the conservatory temporarily I was able to strip out most of the benchwork & racking, and get to the floor.

Preparation for my pier came next but this isn't a green field/new build so I had to start by cutting out a section of the floor boards between the joists to expose the existing concrete base.

I then lined the sides with some treated CLS timber and set to with my SDS drill and breaker bits to knock a hole through the concrete. Once I'd removed that and the supporting hardcore I was about a foot/300mm below floor level by the time I reached undisturbed soil.

I'd kept the removed sections of floorboards so I could screw them together with some more CLS to make a trapdoor to cover the hole until I could get to the next stage.

[Alan White]

Ed, nice to see the build started 👍

That trap and hole made me think of the Great Escape, I hope you didn’t put the dirt removed down your trousers and shuffle about outside to distribute it 😉 

[LuckieEddie]

As I want this to be a 'done once, done right' job and I like astrophotography I felt that I needed to err on the side of caution and allow for future equipment upgrades. This means maybe over-engineering my pier, rather than accepting what others have tried and found acceptable. I bought myself a copy of the Telescope Piers book by Jim McCathren in order to educate myself as to how to design a pier.

It's got some good information including typical design choices, compromises, and achievable performance (deflection under load). On the down side it doesn't necessarily travel well being written for an American audience and full of left-pondian terms, measures and standards. Anyway I got some good pointers and adapted the information to my situation.

As I'm going to have a fairly tall pier, I need an equivalently deep hole for the footing. It doesn't need to be wide as the loading on a pier is all about angular deflection, not weight load bearing. I hired myself the largest one-man Augar I could get hold of locally.

It took a full day, due to having to keep extracting the loose soil and the fitting on the hired Augar being partly broken, but eventually I bored a hole, 14"(355mm) diameter by about 6ft(1800mm) deep. Unfortunately within an hour of finishing I had a small well as due to all the rain the ground water level is high, but this can be pumped out just before I pour the concrete. The hole is deliberately offset to be in the correct location - it's the preexisting floor joists that don't line up.

[Ouroboros]

Interesting. I think (maybe) that’s the first time I’ve read someone boring a hole for their pier on SGL rather than digging a hole. 
 

How much do borers cost to hire?

[Alan White]

Ed, from our conversation at Astro club, this all now makes sense.
That is quite a footing and I like the method of achieving it, is the top going to have a slab with the  ore under?

Oh and I love the term, left pondian, made me chuckle.
But of course anything we say is right pondian!

[LuckieEddie]

Ouroboros - The Augar was £120 for a weekend hire, delivered & collected. Not cheap but less than a mini-digger, a neater hole, and I could get it up the steps in the garden.

Alan - No slab. 14" footing, 12" pier above ground. Will be poured in one go with a rebar cage running right the way through.

[LuckieEddie]

A slow and fiddly job next. I already had armored mains cable run out to the shed but now I need network. WiFi is just too subject to variation for me to trust it for this application.

I've bought an outside rated Cat5e cable, not armored but double walled and UV proof. This had to be buried and run right round the garden so it can come in near to the router. Then I stripped out nearly all of the current electrics in the shed as I won't need the sockets or lights where they are currently. Then I removed some more floorboard, fitted some trunking from the pier location to the side wall under the floor and replaced the floorboard. Finally, for today, I've added some new socket positions, trunking, and a metal network outlet.

I won't need many mains sockets as all the astro gear is going to run off of distribution from  UPS.