Major rethink ...

[Demonperformer]

I have been having a major rethink.

Until now, I have been fixated on the need for as much of the southern horizon as I can get.  But now I am thinking if I am willing to forgo some of that (from SE through S and W to WNW) then I can move the entire operation into the back garden.  This takes me away from all those pesky streetlights and the annoying "what yer doin'?"s from those equally pesky kids  I have done some measurements and played with CdC and realistically I will be saying goodbye to anything below the celestial equator (maybe not totally, but I want to consider the worst case scenario).  But the entire northern half of the sky would be well-positioned at some time or another during the year. And if there aren't enough galaxies in UMa, Dra, CVn, Com, Lyn, LMi and Leo to keep me going until I drop then I'm really not trying hard enough to find things to image.  Not to mention all the Open clusters in Cas.  And if I want some nebula challenges, there's always the north american, pelican and veils to keep me going.

So ...

The first part of the operation would be to set up the pillar (those plans are already well advanced) and put in a concrete "platform".  This would give me a base to set up my table/chair and keep my feet from getting too muddy.  It would also form a base for a building (aka shed/obsy) if I decide to put a building round the scope.  Otherwise I can still remove scope and mount/mounting plate each time, but because it will be on a constant pillar, the plate will end up in almost exactly the same position every time, which will shorten setup/alignment time to well within the cool-down time. I have attached a photo of the area where it would go (the garden is roughly 7' wide, and plenty long enough).

Now I am no engineer, so this could be where I drift off into complete fantasy, and if that is the case please feel free to drag me back.

I am thinking a 6'x9' platform with a 2'x2' hole for the pillar (I know enough not to have that connected to the platform). The boundaries are the black line on the attached diagram. I am also thinking of using a 'trench' foundation (the red hatched area in the diagram).  [The green line is the outline of the table/chair I use]

Now, numbers.  No idea really, but as a starting point I was thinking make the platform about 3" thick, and make the trenches 3" wide and 6" deep.  This would give 7.5 cu ft of concrete in the foundation trenches and 12.5 cu ft in the platform itself.

Does that sound anything like reasonable?  Any structural engineers out there?

Thanks.

[JamesF]

Is a building a genuine possibility?  I can't imagine you getting away with much smaller than 6' wide, but with a 7' wide garden won't that seriously compromise access to your gate?

James

[JamesF]

Ah, or do you mean the area to the right of the path is 7' wide?

James

[nmoushon]

I was about to ask the same thing James.

Let me try to undestand this. (Sorry brains a bit slow today) You're essentially going to dig your foundation for your pier and then pour a concrete slab around it? Not touch the pier, as you said. If thats the case then no need for interior support. All you would need is a footing around the outside edge and around the edge of the hole where your pier comes through and have the slab 3-4" thick. Make sure to have the proper ground cover underneith it. i.e. sand and gravel to help with settling.

If you are saying to pour the trench foundation then have a wood platform on top of that then I think that 3" is a bit to thin. Especially on the outside as thats were all the weight from the shed and roof is going. Plus snow loads and such. Concrete is actually pretty brittle so when it gets too thin it cracks very easily and then your shed will start to settle unevenly. I personally would never poor a foundation only 3". Someone might come on and say structurally 3" is fine but as an architect I just dont trust concrete enough. (When it comes to cracking not strength) If it was reinforced thats diferent but that gets expensive for us hobbiest and would rather spend that money on new equipement.

[pete_l]

I'd go with the concrete for the pillar and then put patio slabs around it for walking on.

That way you can get more / fewer and build up the right sized area, based on what you actually use. With a concrete slab, what you lay at the start is what you have to make do with - or realise it's bigger than you needed.

Once you have the pavers in place, then you'll have a summertime job to look forward to, to put down a proper sand / patio bedding and re-lay them level.

[JamesF]

I'd go with the concrete for the pillar and then put patio slabs around it for walking on.

That's my line of thinking too.

James

[Demonperformer]

Thanks for the replies, guys.

@James - yes it is 7' to the right of the path.  A building is definitely on the 'un' side of 'likely' but if I were to pour a concrete base, I would want to make it capable of supporting one from the start rather than having to amend/redo it later.

@nmoushon - your first paragraph is the option I had in mind,

but ...

@Pete - that is a definite thought.  Just done a quick internet search and could get 11* 2ft (600mm) square slabs + a couple of bags of builder's sand for the base for under £60 delivered.  Reasonable price and probably a lot less effort in putting it together.

Thanks.

[Tinker1947]

I would go with a Pier and slabs, if this works out ok, then build round this later......

[Demonperformer]

Major Rethink (Take 2)

I have been reconsidering the paving slabs.

Getting them delivered to the front door is one thing, but there is no direct access to the back garden from the front, except through the house. And 2' square paving slabs are not going to be particularly light. And I am not what one might call overly fit. I looked at 1' square slabs, but the price is much higher for these, and the thought of the job of laying them all in the correct place and level is frankly one that would probably put me off doing the job at all.

So my next thought is some sort of exterior wooden platform. (The idea of a building being extremely unlikely and getting more so by the minute!) The attached plan shows a framework of 1.8m and 1.2m timber (75cm-100cm square?).

For simplicity, the lengths would be 'butted' together (maybe glued?) and held in place with triangular angle pieces screwed into each piece at each corner (requiring 30 in all). The junctions marked 'X' would also have a straight metal plate down the outside to hold them together.

Alternatively, I could give this part of the job to a local carpenter - would depend on the price, but would probably be a much better job, and this is the bit that is most required to be rugged.

Would obviously need to get the ground as level as possible before placing this frame. Then use builder's sand as a 'bedding' material?

Once the frame is organised, two sheets of http://www.travisperkins.co.uk/p/marine-plywood-2440-x-1220-x-25mm-wbp-bs1088/612666/3893617 or similar, quartered, would give me 8* 4'x2' panels, of which I would need 7 to fill in the frame, to which they would be screwed.

All of this would be handle-able for me, I think. No idea of prices at the moment, but this is, afterall, a one-time job, so cost should take second place to durability.

So, do people think something like this be up to the job?

Thanks.

[Macavity]

After one slight "coronary episode" I am not overly keen of lifting heavy stuff. But I did risk "walking" (50lb?) 2x2 slabs over short distances and lifting (straight back!) below waist level. For the longer distance, I laid a track of old cardboard removers boxes and trundled them along on a:

http://www.maplin.co.uk/folding-trolley-97489 (Handy for other things along the way too)

You can even slide cardboard boxes + a load over grass! I wouldn't attempt anything with 3x2 flags though.

** BUT ** having built on a flat "patio", I concede there would have been advantages to have air flow (not water flow!) under the observatory? Perhaps, rather than a monolithic star-pad, some kind of PLINTH arrangement with joists (treated fence posts?) and an overlay of waterproof chipboard. Probably best to defoliate the entire area though! Hardcore + chippings. But (breeze blocks set in concrete?) Plinths need not be pretty, merely level at the top! If you do a google *image* search on "deck foundations" for ideas? 

Some other interesting ideas, maybe?

http://www.swiftfoundations.co.uk/

[jambouk]

How do you deter rats from setting up home underneath if there is a gap?

James

[Macavity]

Not much fauna around here at 200' up? Under temporary huts at workplace, the feral CATS were a pityable sight, but doubtless saw off any rats. But I always imagine some sort of perforated metal strip around the edges? 

Depending on cost, the: http://www.ecobase.biz/ looks rather intriguing now?

Modular... Infilled with gravel... Smaller joists (smaller gap) under the floor etc. 

[Tinker1947]

Putting down a wooden type floor there are a few ways to do it, the way a built mine although it did make the walls at the same time but could have shorter lengths so the length and width would then have a pre-built wooden shed placed on it at some stage, getting your scope mounted on a pier and ready to use just by opening a roof makes it a whole new hobby,,,this is how i would build the floor.......and the pier needs a deeper hole mine was 3' 6", used coach screws to hold the floor joists 3" by 3" with 4" by 2" on top to support the floor. I put 20mm gravel round the base to stop rodents getting underneath...

i used 4" posts put them in the ground around 2' and i used 10mm shingle (no cement mixing) 

[JamesF]

It is possible to get 450mm square slabs, which should be a fair bit lighter than 2' square ones (almost half the weight, in fact).

I'm not sure the marine ply is a good idea to be honest.  I think rainwater will tend to sit on it as slight dips form in the top and even if the surface is treated you may well get algae and suchlike growing on the surface making it slippery.  If you covered the ply with something waterproof and non-slip that might help, or there's the more expensive alternative of using decking.

James

[fondofchips]

I have opted for loose stone on my obsy path to avoiding slippery slabs when its freezing.  (slab base shown in my gallery pictures is for the shed only, path not constructed yet).

I also had to add more concrete when the pier mountings seemed too wobbly - see my gallery pics -  http://stargazerslounge.com/gallery/image/18174-pier-mount-2/

[jnb]

If you are going to make a wooden floor around the pier then I'd be tempted to make a wooden floor raised up on pillars. Same arrangement that it would surround but not touch the pier and I'd make the pillars from concrete blocks. Advantages are that it's potentially easier than a trench foundation, if you want to change your mind in future about location it's easier to remove (though the pier is another matter). Suspending the floor will also mean less heat loss through your feet than standing directly on ground or concrete. As for rats underneath it,you probably already have rats in your garden even you haven't seen them it's easier for them to set up home under a suspended floor  but by the same token it's easier to spot them and deal with the issue.

[wxsatuser]

This is what I did.

Four 4inch fence posts concreted in at the corners, used Postcrete for quick job.

Made footing between each post as wide as the post and about 6inches deep

Wickes readymix bags of concrete

Pic 1

Then built a deck on top with Wickes C16 95x45 treated timber and deckboards, solid even under my bulk.

Stud wall Wickes c16 70x45.....you could use smaller.

BTW obsy is 1.8m square.

pic2

[r3i]

Have you considered decking instead of slabs?

Edit: Typed too slowly, Mike beat me to it with the decking suggestion.

[Demonperformer]

I have spent all morning going round in circles thinking about this and am getting thoroughly frustrated with the reluctance of many on-line companies to actually tell you what their products cost.

What is also becoming apparent to me as a difficulty (please don't underestimate my ability to find things difficult!) is getting things level. Putting in a load of posts, attaching joists and placing decking on them strikes me as having far too many possibilities of me ending up with a wobbly floor. This could also be a problem with using paving slabs.

I like the idea of having a decking platform at the end, but I cannot see me getting a good result if I try to do it myself.

If I were to revert to Plan A - pouring a concrete platorm - that would be self-levelling as it sets. The disadvantages, of course, are (1) it has to be right first time, and (2) concrete tends to heat up and keep releasing it for quite a while after dark, which is not good for 'seeing'.

So I am now wondering if I should cut through the entire problem by applying the maxim: 'It isn't a problem if it can be beaten to death with a bag of money'. As I'm pretty sure what I want ideally (even if I'm not competent to make it myself) maybe my next step should be to get a few quotes. Even if that doesn't really qualify as DIY!

[Demonperformer]

 (2) concrete tends to heat up and keep releasing it for quite a while after dark, which is not good for 'seeing'.

Actually, thinking about it a bit more, the location rarely gets much in the way of sunlight - only really early on summer mornings, so maybe that would not be a big issue.

[JamesF]

I'm going to disagree that concrete self-levels as it sets.  It may to a limited extent, but partly it depends on how wet you make it, and the wetter it is the weaker the eventual end product.  If you really wanted it level you'd make up some shuttering, check that is level and  then pour the concrete inside it, using a board resting on the shuttering to tamp the concrete down to the right level.  And if you can create level shuttering, you can create level anything

There's probably no reason you couldn't do paving slabs in a similar way, to be honest, though you could just get away with a set of pegs with levelled tops around the outside of the paved area.  Drape a couple of pieces of string across the tops of the pegs at right angles to each other and you have a means to check the slabs are laid level.

James

[nmoushon]

I will agree with James and say concrete is not at all self leveling. You can form it to do crazy things and it will hold the form. Leveling is not as hard as you think. And since you only have to make one level level its easy. Its when you have to make sure every level is level is where it becomes hard. i.e. laying brick or CMU and having it all level at the very top.

[r3i]

What is also becoming apparent to me as a difficulty (please don't underestimate my ability to find things difficult!) is getting things level. Putting in a load of posts, attaching joists and placing decking on them strikes me as having far too many possibilities of me ending up with a wobbly floor. This could also be a problem with using paving slabs.

Achieving a level platform was probably the main reason why I went for decking, as there is a slight slope in the garden where my observatory is and I felt that levelling joists across vertical posts was going to be the easiest way for me to do it.  I got a decking kit from here: http://www.diydeals.com/.  They include a decent set of instructions on what to do. 

[jnb]

Achieving level or near level decking isn't as hard as you might imagine. Even if you were out by 1cm across the width of your platform that's still only 1/4 of a degree which is probably good enough for decking. That's one reason I like pillars of concrete blocks as a foundation as you build up a pier you can check level to one reference point and with a bit of judicious squidging of block to push the mortar down you can get reasonably easily get within a couple of mm. Also if you are working with wood then it will flex a little so you are unlikely to have something that wobbles even if it isn't quite level. (depends on how much weight you have above it)

And if all else fails you tell people it was deliberate so that it would shed water 

[yesyes]

Getting decking level that is supported on posts isn't hard. You put the posts in the corners but don't cut them to length yet. Then you fix the frame to it so that it is level, then you cut the posts. Much easier than trying to cut the posts to length before.

This is how I've done my shed base: