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tomato

Isolation of central pier plinth

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I’m starting to put plans together for a permenant observatory next year. I have a 2.2 m Pulsar Dome which will house a dual rig, one of which will be an Esprit 150. I have concluded there will be no room in the dome for humans, so there will be a separate shed warm room adjacent to it.

My question is that given there will be no human activity in the dome while imaging, is there any real need to isolate the central plinth from the rest of the concrete floor, even though this seems to be the accepted practice?

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I have mine isolated and it wasn't really a faff to do, just a little extra planning but then I do enter my observatory from time to time during an imaging run despite being able to operate it remotely!

In your case, it may not be required now but by having it isolated, you are potentially future-proofing your observatory if your needs change for some reason. I can't think of any disadvantage to having it isolated apart from a one-off bit of site preparation.

Good luck with your project.

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Thanks Steve, you are right to say it’s not much work to isolate during the build but would certainly be a big deal if it needed to be done retrospectively.

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I've got 2 piers bolted directly to concrete patio with no ill effects and I seem to remember all those at Ollys place are bolted direct to concrete / mountain.

But as said it's no trouble to do so may as well do it, I don't think the meter square of concrete under it is necessary unless you have really unstable sub soil.

Dave

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I isolated the pier block from the circular pad around it when building my observatory (well, nice Ben the handyman did anyway!) - pics here: http://www.gjmultimedia.co.uk/astronomy/rpo.html  I only did it because that's what people seemed to recommend, however, I'm glad that I did it. At least I know that everything possible has been done to reduce any vibration reaching the mount. It is amazing watching the PHD2 graph go mad if I move about in the dome while it is guiding... and that is with the separation! (Don't worry, it doesn't seem to actually spoil any subs!). I always think that it means that even the dome moving during exposures won't have any impact on things. Definitely better to do it in the initial build and not end up wishing you had done it!

Good luck with the project and I look forward to seeing progress updates.

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Thanks to all for the replies, for how much work is involved, the centre block will be isolated. I currently don’t seem to notice myself on the guide graph while tippy toeing around the mount, although it definitely does show up when I accidentally kick one of the pier legs in the dark.😖

Myself and Tomatobro will be pooling resources on this project, as I have the opportunity to move to a less light polluted location in Shropshire compared to where he lives.

There will be a build thread starting hopefully around Q3 2019, I can’t wait!

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My obs'y pier isn't isolated as I'm not in there when imaging but if imaging a planet and I jump up and down a good 6 feet outside the door the video image goes bananas :grin:

Dave

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If you can build a wooden floor above the concrete, that helps. I can jump up and down while imaging to no effect!

44600155930_077f462620_o.jpg

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My observatory has a large concrete block and pier with suspended wooden floor supported on six separate concrete blocks well away from the pier block.  I too can jump up and down on the floor and not affect imaging.  A foam ring round the pier isolates the floor from the pier and stops small objects falling through.

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I am becoming increasingly concerned by this worrying new craze for jumping up and down in observatories.  :huh2:
While I can well understand the need for privacy, there must surely be better places to practice one's calisthenics?

On a lighter note: I can't see the advantage of leaving a small gap between a massive concrete slab and a buried concrete block supporting a pier.
Both are effectively resting on the same bit of ground.
Attempts to isolate the pier by wrapping in a deformable material are more likely to result in a wobbly pier than true isolation.

I carried out some experiments, in my youth, on concrete slabs supported by soft foam, polystyrene and even springs.
The result was always that the compliance was completely short circuited by being flattened into a virtual solid. 

While floors "floated" on polystyrene board were apt to become wave-like if not identically treated [paint or varnish] both above and below.

An isolated wooden floor might well be considered preferable. Even if it does ring like.. er_um_er..  a suspended floor.
The distance between the floor supports and the pier base could be deliberately chosen for maximum isolation.

Avoiding the "trampoline effect" is highly desirable if it is not to encourage this grievous hopping and jumping about.
We are, after all, considered adults, even if it is only under the law.  :blush:

 

EDIT: This was supposed to be funny. In case there was any doubt. :angel9:

Edited by Rusted
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On 21/12/2018 at 16:15, Davey-T said:

I've got 2 piers bolted directly to concrete patio with no ill effects and I seem to remember all those at Ollys place are bolted direct to concrete / mountain.

But as said it's no trouble to do so may as well do it, I don't think the meter square of concrete under it is necessary unless you have really unstable sub soil.

Dave

You're right, all but one of mine are single slab. The one that isn't has a floor cast in two parts and this one does have some sensitivity to movement which can be picked up on the guide trace. I think that moving about on one of the two parts of the base makes the mount, which is attached to both, tip slightly. In all honesty the effect is never going to show up in a 10 minute sub but it would be better if it weren't there. One day I might do something about it but it's the one in which I do my high res imaging and I regard it as a very low priority even so. On the true single slab observatories (ie the Twin Tak, the 4 scope 'Per Frejvall' robotic shed and the 'Italian shed' there is no reason whatever to suspect that the continuous slabs cause a problem. I think it's a tough call. If you isolate a central part for the pier you reduce the total mass of the slab. Good? Bad? Has anyone done a control experiment?

The newest shed for Tom O'Donoghue's robotic rig has a separate pier slab but that's only because the slab was there already since it used to carry the 20 inch Dob. We just decided (or I did!) that pouring a perimeter-only foundation for the shed wall around the original slab would mean a sight less concreting. No-brainer!

Olly

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