View Full Version here: : Observatory on top of a concrete water tank?
17-07-2011, 09:42 PM
Please don't laugh!
After several years waiting, my wife and I are about to build a new home on a block of land we own. Unlike where we are now, this site will be more observatory friendly (even more so if some "northern die-back" occurred! :)).
My father-in-law (an architect) is currently preparing designs for the house. He is even willing to consider doing the observatory. But...
He is talking about placing the observatory (type yet to be determined) on top of a large concrete water tank. :shrug:
When I talked about problems of vibration, separating the pier from the surrounding concrete etc, he seems to think that this can be solved with enough concrete (my words, not his).
So the question is: can a pier that is attached to a large concrete water tank be used as an acceptable platform, or is vibration going to kill me? And yes, I'd like to do some imaging!
Opinions, informed or otherwise, are welcome!
17-07-2011, 10:13 PM
Not only that, but the thermal loading of exposed concrete will give you air turbulence as well.
Stay well clear -
all you do when you add more concrete is lower the frequency.
17-07-2011, 10:43 PM
Very bad idea. The vibrations will kill any attempt at a stable pier. Also no real solid anchor point.
18-07-2011, 05:38 PM
So I shouldn't be enjoying mine as much as I am :P
18-07-2011, 06:45 PM
Mike, so you do imaging, or just visual work?
Yep it will kill you,,do not do it.
18-07-2011, 08:42 PM
I can understand the problem if the mount/pier was just sitting on top of the tank, but if a concrete pier ran through the tank into the ground and had some isolation from the tank top would it still be unacceptable?
19-07-2011, 07:39 AM
David, Mainly visual but Imaging could be down the track. Obviously, when I am sitting looking through the eyepiece I am not also walking around on the tank.
I have only had the pier set up for a couple of weeks and have enjoyed it on the observing sessions so far. If radiated heat from the concrete proves a problem in summer I may just try things like hosing the surface to cool it or whatever.
Bottom line is that this was the best possible viewing site available to me so therefore it must be perfect.:)
You could try a temporary set up in your position to check for vibration issues. Even just on a Tripod. If you intend to cover the tank with an Obsrevatory, heating issues might not be a problem.
Kangaroos and Wallabies gather on the tank at night, but so far not when I am osbserving:rofl:
Pease keep us informed as to how you progress. I would be interested to see how you go with just a temporary set. Do you know someone close that has a similar tank that you could have a try on? My tank has a central concrete pier supprting the top from within and the top is 100mm thick with Reo bar in it.
19-07-2011, 12:56 PM
I agree with the other Andrew
If you can build an internal pier from the base of the tank
and have a hole in the top that is only sealed with say a rubber sheet gasket. Vibration wont be an issue.
If the top of the tank gets too hot, plant some grass.
19-07-2011, 02:48 PM
works well for me david..
have mine attached to top of tank very stable...threaded rod chemist to concrete tank then all the way through to bottom plate pier then bolted together.have timber base all around pod with 10mm gap against pier,same with floor of pod then silicon around pier... works bloody well for me..
see if i can dig out photo's and attach !!!!!!!!!!
or i'll bring my computer and show you at astrofest
19-07-2011, 03:02 PM
Beware of received wisdom of nay sayer's from this site , how many of these people have actually tried using a scope on top of a water tank ?
I use many different scopes for visual and imaging on top of a concrete water tank ( set into the ground ) just as they say you shouldn't , for visual and imaging .
My scopes are sitting on timber piers ( just as you some say you shouldn't ) which have loose gravel in the bottom half so they don't blow away work better than any tripod is cheaper ( and better ) than a metal pier and no vibration not even when the water pump runs.
My suggestion is to do a temperary pier try it for your self and then go ahead with the observatory.
All the best
19-07-2011, 07:51 PM
Somehow I knew this would be a contentious thread!! :)
Just to clarify a few things...
My initial point of view was that the suggestion was "just so wrong"! I mean we get told repeatedly about having a deep pier, separating the pier from the observatory base, etc. And after all, I'm in IT, not civil engineering, so I just take the advice as given.
However the arguments in favour are fascinating, including one given privately.
I should clarify though, the water tank doesn't exist yet, neither does the house (or as astro-friends refer to it: the observatory with attached bedrooms!). So a trial is out of the question yet.
I have heard a number of very reasonable arguments against the proposal. From a purely hypothetical point of view, one wonders how one might address them.
Mass: When we put in a pier normally, we put a pier foundation that weighs about 1 metric tonne (1 cubic metre of concrete). I am guessing that any reasonable size concrete tank filled with water (or even without) is likely to weigh at least the same. So is there any inherently problematic issue?
Separation: Normally we would not walk on the pier foundation - it would be separated from the observatory foundations/floor. If the tank were surrounded by a frame, and the floor were suspended from the frame, it could be separated from the tank.
Strength: Several people have suggested that the roof of the tank needs to be thick enough or supported underneath the pier to avoid vibrations further.
Thermal Mass: If the tank was contained inside the observatory (via the above frame), would this then reduce the issue?
Like I said, I am not committed to this, but I have found it an interesting thought experiment.
I was then even more intrigued after watching two videos about the AEC piers:
If I followed this correctly, it would seem reasonable that imaging should be undertaken while well away form the scope (eg inside the house!), as any local movement is still likely to affect the scope. Not sure of the validity of the comments, but it doesn't seem unreasonable.
Anyway guys, thanks for the comments, and I am keen to hear others thoughts (pro or anti), especially where actual experiences can be related.
19-07-2011, 10:00 PM
I have a cast iron pier that is bolted to the 100mm thick concrete floor of my observatory. I can jump up and down on the floor during an exposure and I can't detect any movement imaging at 1800mm FL. It would be nice to have a separate pier I suppose but it doesn't seem to make any difference for me.
20-07-2011, 06:29 AM
There You Go :thumbsup:
A solution for the Naysayers ;)
actually a concrete tank with water in it stays cool
21-07-2011, 09:02 AM
I have see a few observatories mounted on top of concrete roof slabs and the owners get great results both visual and photographic. If your water tank is solid and has a good reinforced concrete slab top I am sure you will find the vibration negligable as well as air turbulance.
I see many comments from people decrying vibration and air turbulance when in fact they have not taken other steps to reduce these problems. The biggest cause of these problems is usally the actual telescope mounting and the most common is overloading the mount.
These days of excellent guiding facilities and multiple exposure stacking can remove these problems entirely. When guiding from a separate guide scope the slow flexure type movement is a far bigger problem.
I say go ahead with your planned observatory. It will be a much envied structure by fellow astronomers. I would also suggest using a dome of the Sirius type. The long term practicality will be worth the extra cost.
21-07-2011, 02:54 PM
There is an awful lot posted about observatories, pier isolation and vibration problems. A lot of people use huge foundations for their piers and in most cases it isn't required. A good solid pier is much more important.
My observatory pier is a hole in the ground 4 ft deeoabout 8" to 10" square with the 6" pier sitting in the bottom and the remainder of the hole filled with concrete. Not much really.
In your case I would sugest a solid pier with a solid large base positively anchored to the top of the tank well and you shouldn't have any problems. I image with mine and have no problems with movement or vibration without a huge mass of concrete.
The height of the pier will be a much bigger problem with the vibration than the base it is bolted to.
Good luck with it all. Sounds great to me.
31-07-2011, 09:45 AM
Once again thanks for all the comments! This has caused a massive discussion both on and off forum. We are having a multi-speaker presentation at the next AAQ meeting (13/8/11) about building observatories, and I'll be presenting some of the feedback you guys have given as part of that.
Mossie, sorry we didn't get to talk longer at Astrofest - funny how a whole day can get completely consumed talking to people you see once a year! We'll chat offline mate.
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