Jovets Observatory build

[Ivor]

My observatory build was meant to start in the spring of 2014 however a few unexpected events delayed the start and I found myself starting the project in October instead. Clearly autumn isn't the best time of year to start so there's going to some busy weekends trying to kept ahead of the weather and dwindling light.
I'm lucky in many ways with my garden I can see Polaris and I have a 270 degree southerly view, light pollution isn't too bad for an urban location so I'm very grateful. The big challenge with my garden is the depth the picture below shows the challenge and you soon start to understand my wife's hesitance for an observatory so close to the house.




The view of garden above is due west and as I've mentioned I can see Polaris but it's only just over the ridge tiles of the house.





A roll off roof solution was quickly ruled out due to space and there is only one spot in the garden where I can locate the Observatory and see Polaris so many of my decisions have been made for me. I have a space of 3m² to work with and to add to my challenge the garden is sloped towards the house. I also want the design to be portable, there is small chance I will be able to buy some extra land behind the house in a couple of years and I'd want to relocate away from the house if the opportunity arose.

I've looked at various designs and ideas people have created and decided to make an octagonal cylinder with a rotating top section. l considered building a dome but I now feel it adds an extra complexity which is going to exceed my DIY skills and time available. I also set myself the goal of designing a solution which could be built using standard power tools (Jigsaw, Circular saw  & Drill), with no angle more complicated than 45 degrees.

Rather than build a large concrete base I chose to build footings for the posts this way it reduced the amount of concrete and also made it easier managing the levels.  Using 3mm plywood I built 4 frames for the footing to support the uprights in pairs.



For those who have tried to dig a little deeper than the initial top soil will understand it is quite amazing what you discover the further you go down. I live in a former farm site on top of a hill among the Victorian milk bottles, metal pipes, 20cm metal disc, a few dog bones I also have a wall buried about a foot below the surface.


 


With all the obstacles and the fact I've started the build whilst training for a marathon progress has been slower than planned. Last weekend I managed to rope in some help and concreted the footings which was where I realised I'd made a few oversights. The first was 3mm plywood wasn't sufficiently rigid  enough to contain the concrete so a couple of the footings bow at the top, this was contained using luggage straps which passed the building inspectors checks.



I'd originally planned to just have the supporting posts resting on the footings figuring that the frame and roof combined should be heavy enough to prevent any movement. Whilst in the timber merchant I saw some metal fence posts brackets designed for being put on top of a flat surface and decided it wouldn't any harm to add them. My footings are oversized 10cm long and 15cm depth which wasn't enough extra space to cater for the brackets as well so the brackets overhang the footing. In addition I'd made the foundations on 1 to 7 cement to ballast mix which would have been fine for taking the weight but it couldn't cope with holes being drilled close to the edge of the footing and they were breaking in up. After much cursing a cunning plan was derived, I drilled a hole under where the post would go and countersunk the bolt top into the bottom of the post which appears to be sufficiently solid to hold.  This took up most of Sunday so I only managed to get four post and cross beams in before the end of the day. This is where I am as of the 19th October.

[MarkS]

Nice report!  I've only one question: "building inspector"?

Mark

[Ivor]

The dog! Clearly I need to rewrite it, my humour bone needed more coffee when I wrote that.

[MarkH]

I got it Ivor, excellent man a fellow jack Russel fan. Lean mix in concrete is always a mistake.. if you want to borrow a chop saw with graduated mitres I have one that you are more than welcome to borrow.

[Mike]

Why is the pier leaning over like that?

[Ivor]

The conduit is merely in the pier hole, I've not cut it to size yet as I've waiting to get hold of Graham from AstroTec to get a cost for the pier top. Unfortunately he appears to be on holiday so I'm thinking about approaching it differently now.

[Canadian Roger]

I'm curious, since I built my own here in Canada about 10 years ago, but how far down did the footings go, and how deep is your pier?

Here, we have to put everything more than three feet deep (and preferably four).  This is so the bottom is deep enough that the ground isn't frozen solid in the winter.  Otherwise, it'll shift when the ground thaws in April.

[Rick]

Otherwise, it'll shift when the ground thaws in April.

Not a problem we have here, but ground drying out and shifting is, particularly with the clay-rich soils we tend to get round here.

[Ivor]

I have mix of soil types about 15cm of top soil, 10cm of gravel/ debris (I assume comes with the former site's use), 10-20cm clay and then sandstone or sand. For the footing I went about 40cm deep and for the pier I've created a 3D trapezium shape 120cm sizes at the bottom and 100cm at the top.
I'm a more concerned about subsidence rather than frost but I'm not sure what else I can do.

[Rick]

I'd guess the clay would probably contribute most to subsidence. When they underpinned my house they dug right through the clay to put the concrete onto the sand below it...

[Ivor]

27th October - update

I'm sure I wasn't the most severely hit by Hurricane Gonzalo however the gusts in Kent were sufficient to knock over the start of my frame pulling the fixings out of the footings leaving my hard work flat on the ground.



With the whole frame up it will be self supporting but post Gonzalo I want to fix the frame down so I've decided to cement a lintel on top of the footings and fix the posts to that. I didn't have time to buy the lintels on Saturday as I ran Beachy Head marathon so I focused on the roof design and the top of the pier on Sunday. I've not got much to show as progress as I was strangely a little tired and these two tasks require a lot of accuracy and planning.
The roof will rotate on ball castors sunk into the wall tops, to prevent the castors creating grooves in the bottom of the roof frame I had a metal disk made by a local Engineering company, this will be screwed into plywood and form the base of the roof.



As the building is made of wood with time the structure will change and warp so I'm still undecided on how rigid to make the base of the roof a little bit of flexure could allow the weight of the metal to bow down and stay connected to the casters. Equally too thin and when the roof is lifted onto the roof it will bend or break; more thought is required. In photo below shows the ring suspended with a 11mm plywood top, I'm thinking 18mm will be required.





I've been re-thinking the top of the pier, my original plan was to use two metal plates supported by 4 M12 bolts however having read various articles and spoken to a number of people I'm concerned this design is flawed as it means any vibrations will occur between the plates while the concrete pier remains rigid. I've no idea of the magnitude of the problem but I can't see any real negatives from taking a different approach. My approach will be to have one plate on top of the pier and create a cavity in the top of the pier to be allow fixing the EQ6Pro to the plate. The prototype looks like this.

[mickw]

any vibrations will occur between the plates while the concrete pier remains rigid

That means that it is the mount that will be creating the vibrations, which means you will be stuffed no matter what modification you make.

[Carole]

Just wondering why you think there will be vibrations Ivor.  If your pier is isolated there won't be vibrations from you walking in the obsy.  I have a pier with 2 plates and no vibrations there. 

Old photo but same pier:

[MarkS]

Looking good.

That's an interesting owls' nest idea for the pier top.

any vibrations will occur between the plates while the concrete pier remains rigid

That means that it is the mount that will be creating the vibrations, which means you will be stuffed no matter what modification you make.

Not necessarily the mount but external factors (such as wind) might set up oscillations.

My approach was still to have a plate but virtually no distance between the plate and the pier top. 


Mark

[MarkS]

Just wondering why you think there will be vibrations Ivor. 

It's explained in this video. 
http://www.youtube.com/watch?v=AOeLnecOMeg

To be honest though, it probably only becomes a problem for high magnification planetary imaging.