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Observatory build Questions


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As some of you aware I'm going to be starting an observatory build soon ( a roll on roll off roof type ), I've got most of the building materials & pier (Cheers Lenny) & hope to get cracking very soon. However I've got a few questions that I hope someone can help me with. I live in an area that has a lot of clay in the soil so in the summer the ground cracks & winter it tends to be saturated, what would be a good amount concrete to use for the pier? How big would the hole need to be? Would drainage be an issue & if it is then what could be done about it? If I think of anymore than no doubt I'll add some more questions but for me the pier is the main one at the moment as for obvious reasons I don't want it to shift.

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I suffer with clay - I went for a 3' cube of concrete isolated from the rest of the foundations.  The chap I got to do this for me clearly thought this was overkill.  I also put in some additional drainage around the observatory - the sceptical chap thought this a necessary step. 

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Hello Tich

To minimise the effect of clay heave you need depth rather than bulk. I would have thought an area to suit the pier base and then try to achieve at least a metre depth. Unless your subsoil is very wet you should not need specific additional drainage


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My soil is also very much clay and moist, again a 3' cube should suffice.

Reinforce with rebar as necessary to help pin in place and that should be ok.

That is what I'll be doing, as gnomus stated extra drainage can help as well - guttering and rain water saved in a bucket or sent to a proper soaraway will also mitigate the ground getting to wet as well.

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+1 for 3 foot cube. Mine was not reinforced, just a big lump.

If you do choose to go for this big an amount, I recommend getting a ready mix delivery. Saves hours and hours of graft and your back will stay in working condition.

Regards, Hugh

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My old (ex) Skyshed POD had a pier cube of 0.8m wide x 0.8m wide x 0.6m deep but it was inadequate and used to tilt if you stood on it while observing, even worse, when the night bus used to race up our road and hit an old sunken roadworks trench running across the road over fifty metres from my house a transverse shock wave used to shake the pier so badly that the guide star would be lost when imaging and the mount would need at least twenty seconds to stabilise before guiding would resume, the shock wave even moves wall hung pictures inside the house! The ground below the property is old river flood plain and very soft and the only place the observatory can fit is partially above an old filled-in and capped well which does not help.

For the new Pulsar Observatory pier I used a 1m cube isolated from the surrounding base by 50mm Celotex and the rest of the base is 200mm thick. Plastic drain pipe has been laid for cabling and is pipe-within-pipe with bubble wrap isolating the inner pipe from the outer pipe beneath the surrounding pad, only the inner pipe passes through the Cellotex shuttering and the pier cube so that the inner pipe is essentially "floating" and able to move independently from the outer pipe, this is to prevent vibration being transferred from the surrounding pad to the pier cube.

There are a few pictures below, I added 4 x 4m long pieces of galvanised angle iron bashed vertically into the ground in each corner of the pier cube and protruding into the pier cube by 600mm but this was done just before the concrete was poured and I forgot to get a picture, I hope this will add some resistance to the transverse shock wave that used to plague the old shallower pier cube. 

I had to use a pumped ready mix delivery which is the most costly option for concrete delivery because the road outside is only three metres wide and I would have had to pay a large fee to the county council to have a temporary road closure put in place for a regular ready mix delivery with wheel barrows etc, the pumped delivery took just fifteen minutes from the time they arrived and dropped the ready mix into the pump and piped it into the pier block hole and surrounding pad all in one go, the council were happy that a fifteen minute road obstruction would not require any special approvals and as it happened on the day only one vehicle had to wait for five minutes while the concrete pump was being washed out at the end of the delivery.

After the concrete set the pier block had contracted and lowered by around 5mm which was expected due to the differing volumes between pier block and surrounding base pad so I used a self levelling external latex screed mix to bring the pier block back up to the same level as the rest of the base.

North alignment was done by the plumb-line and local solar noon transit shadow method, I have no direct view of Polaris because of neighbouring trees and with the old well cap and angle iron in the ground a compass is useless, the noon transit shadow is the most accurate anyway and results in a polar alignment within a few seconds of arc.

Preparing the pad:


Pier block:


Just poured concrete:


Shuttering removed:


Setting the north alignment by local solar noon transit shadow:



Edited by Oddsocks
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  • 2 years later...

Evening all I thought I'd posted the start of my observatory build but it seems not or it got deleted for some reason, however the concrete slab got done a couple of years ago now & for one reason or another it never progressed much further. Drawing were made up of how the roof would work on the roof them my friend were saying do I want a warm room etc which I wasn't really concerned about.

Anyhow all those ideas never amounted to anything until I were browsing on eBay & noticed an observatory for sale in Milton Keynes I went & had a look & the deal was struck. I went to pick it up last month using a HIAB from a local haulage company only to be scuppered at the last hurdle due to a neighbours car parked in the way so another 300 quid later it finally arrived today.

So firstly I'd like to say sorry for the clouds folks.....





The arm was only just long enough & with some help from a neighbour it's positioned over the slab perfectly, just the pier to fit & some electricity & we're done, although I am away next week so won't be for a week or two.

The Dumpy Dalek is now at its new home.


The observatory is a hand built one & needs a few minor repairs sorting out although it's still usable, the dome itself moves around nice & smoothly. I can't wait to get up & running as it's been ages since I've done any astro from home. 

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2 hours ago, JimT said:

Better than my chicken shack  :D

Is it all repaired yet Jim? I were hoping to be down your way again this year but it didn't happen in the end unfortunately. ?



9 hours ago, L8-Nite said:

Hi Tich !

Looks good. :smile:

So, the Hiab driver didn't have a sling capable of temporarily hoisting the neighbours car out of the way ?  :angel9:

He did but if he had I think his job would have been on the line afterwards, I think this observatory malarky must have been stressing me out for a while as I've slept for most of the day today lol. In other news it'll be great catching up with the both of you next week at fforest fields.



18 hours ago, Freff said:

Glad you got it sorted Tich. 

It looks great in its new home, you won't know yourself during the cold nights.

Keep the photos coming.

Oh I'm sure I'll find something to do providing we have any cold clear nights lol, see you next week!!!

Back to the observatory after the rain we've had today (I can't think why?) I noticed some water had come in below the doorway so that would need extending downwards so will need addressing sometime, & the bottom of the ply board that goes around the base is going rotten so will need cutting out & replacing, other than that it's all good. The door is made from stainless steel so unsure what to use to extend it other than more stainless, I guess a PVC strip would do the job riveted & sealed onto it.  
I'll probably do some temporary repairs to it when I get back from Wales until next year then crack on with it next summer but it is usable as is which is the main thing.

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Tich, I am at the moment redoing the roof, realised a fault in it, other than that all is good.  Cleaned the place up but have not powered up, plenty of time for that.  Had a conservatory built, redone the garden and had a stroke, not in that order but all is well.  Taking my time with the observatory, little here and little there, hope to have it running before the years end.


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1 hour ago, JimT said:

Tich, I am at the moment redoing the roof, realised a fault in it, other than that all is good.  Cleaned the place up but have not powered up, plenty of time for that.  Had a conservatory built, redone the garden and had a stroke, not in that order but all is well.  Taking my time with the observatory, little here and little there, hope to have it running before the years end.


Crikey Jim take it easy then mate hope to catch up sometime soon.

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  • 4 months later...

Well the final piece in the puzzle has been fitted although I think the pier is a little bit low so may need an extension on top doh!!
As we were going to have a roll on roll off roof the floor would have been only off the ground by a few inches the observatory floor is about a foot off the floor so looks like I will be needing a pier extension, It still should be ok & good to go just targets lower down will be a bit tricky. It would have been fitted earlier but with events happening this wasn't the case until yesterday. It's even looking like it's going to be a clear night tonight so I may give it a go so I expect some swearing may be involved.

Thanks to Lenny for building the pier & thanks to Gerry for all your help in all this, still some tinkering to do to the observatory like the extra pod will have to come off as its leaky & getting damp so it will go back to just having a wall there instead.





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