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Old 22-04-2018, 02:18 PM
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Joshua Bunn (Joshua)
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Raising the observatory

Hi All,

I'm currently in the process of designing a new remote observatory, and in my research I read a bit about "near seeing" (or ground seeing) and "Dome/instrument" seeing effects as opposed to "high altitude seeing". Apparently, 70 to 90% of your total seeing effects can be ground based seeing, and reading that 70% of the ground seeing is in the first 3 meters above ground level, has prompted me to build this observatory (which will be a roll off roof) as high off the ground as possible. Obviously, I will be taking other measures also to reduce the seeing immediately around the telescope and in the observatory.

In respect of this, I would be keen to hear of any others experience with building observatories high up and tall telescope piers.

thanks
Josh
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Old 22-04-2018, 02:40 PM
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Atmos (Colin)
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I have heard of the first 3m as being seeing mess due to the IR emission of the Earth but it isn’t something often discussed.
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Old 25-04-2018, 10:23 AM
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AstralTraveller (David)
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Josh,

I can't offer any advice but I am quite interested in your project as I also would like to get my future obs up off the ground, though for slightly different reasons. I'm in an urban back-yard and the raised obs would increase my access to the sky and greatly limit the effects of the neighbour's lights. I hope to get the floor 1.5 - 2.0m above the ground, most likely at the lower end of that range. The important thing is to get the top of my walls above the level of the neighbour's eves.

It seems the extra difficulties compared to a standard obs are getting the 'pier' right and working at heights during construction. I have a pier that came out of a ground-level obs and I plan to build base to go under it. I'm thinking of a solid top plate for the pier section to bolt to (15mm plate??) supported on 4 legs slightly splayed out to form a pyramid with cross braces between them. I'm debating whether to concrete the legs into the ground or have a base plate on each leg that bolts to the concrete. The former sounds more sturdy but the latter is easier to build (and remove if ever necessary).

Even if my knees didn't tremble while changing a light globe, working at heights makes everything more difficult. Wall sections will have to be lifted onto the floor and then stood up and I expect that much of the roof will be assembled in place. I haven't even contemplated the runners. I have some scaffolding but probably not enough.

Your post has at least made me thing about this project again, which is good. I'll be pleased to hear how your project progresses.

thanks,
David
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Old 26-04-2018, 01:06 PM
el_draco (Rom)
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I'm currently building a ROR with this issue in mind.

I'm working on the principle of reduced thermal mass and rapid temperature equilibrium for the building and equipment. The frame is all metal purlin with colour bond cladding. Floor is 40cm above ground and walls over 2m high. Roof fully retracts to facilitate cool down. A separate heavily insulated warm room is isolated from the observing floor; no thermal currents from other gear. The surrounding area is unpaved and the facility is 100m from any other building. Natural veg everywhere.

Touch wood, that'll do the job. I'd be concerned about an overly high pier due to resonance and flexure issues.
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Old 26-04-2018, 01:20 PM
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...
I'd be concerned about an overly high pier due to resonance and flexure issues.
G'day. I'm curious about the "resonance" - what's causing it, and how does it affect imaging? Is it from the mount motors during a slew? Focus motors? Ground vibrations? Wind?
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Old 26-04-2018, 01:53 PM
el_draco (Rom)
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G'day. I'm curious about the "resonance" - what's causing it, and how does it affect imaging? Is it from the mount motors during a slew? Focus motors? Ground vibrations? Wind?
Yes.... and probably 20 other things you have not heard about.

Dampening down any kind of vibration is important for imaging and viewing, especially at high magnification. Some people fill their pier with concrete or sand, but that adds thermal mass. Big "fins" on a pier are a good idea for a lot of reasons but I fill mine with expansion foam. Lots of threads about this on the topic. For gawd sake, avoid the "rats cage" thing...
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Old 26-04-2018, 01:54 PM
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Quote:
Originally Posted by el_draco View Post
... I'd be concerned about an overly high pier due to resonance and flexure issues.
Quote:
Originally Posted by troypiggo View Post
G'day. I'm curious about the "resonance" - what's causing it, and how does it affect imaging? Is it from the mount motors during a slew? Focus motors? Ground vibrations? Wind?
Draco^ is on the money. The longer the pier the lower the natural frequency of oscillation. And the lower frequencies can be much harder to damp or reduce. These lower natural frequencies are often excited by footfalls or any inadvertant knock/bump/low freq vibration in the area.

Broadly speaking, for a simple column (well vertical cantilever with uniform loading and no point load ) doubling its length without any changes to its cross section or stiffness, will decrease the natural frequency of oscillation of the structure by a factor of 4, making some vibrations very hard to damp or attenuate. As to how that all effects the image, well it certainly could if there is ground borne vibration or wind loading on the device or structure, not to mention footsteps. Surprisingly even a constant wind can cause / excite vibrations. Maybe it's just inbuilt dithering (not really )

Best
JA

Last edited by JA; 26-04-2018 at 02:04 PM.
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Old 26-04-2018, 02:15 PM
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I have heard of the first 3m as being seeing mess due to the IR emission of the Earth but it isn’t something often discussed.
Absolutely Colin - it's an incredible source of radiated heat. You can sometimes see that thermal boundary layer in a long lens over a warm field, mountain or most usually roadway. All those planning to go up should also consider the proximity of stone, brickwork, cement and terracotta and all things black to their observatories as they are all wonderful absorbers / retainers of heat.

I too like the idea of higher up and was thinking to one day turn our roof access / opening skylight in to something useful, but it's only 900x900mm so would need mods AND same problem as mentioned - tackling vibration in such a situation: 2nd storey timber construction/vibration damping/mass/pier mounting/isolating the floor yada yada yada......

Best
JA

Last edited by JA; 26-04-2018 at 02:25 PM.
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Old 26-04-2018, 02:57 PM
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I realise vibrations are not good for imaging, but I'm trying to get a feel for the source of them, and the order of magnitude. Natural frequency of vibration, oscillations etc are a consideration where the source of vibration is cyclic and the freq of the source matches the natural freq of vib'n of the pier/building/whatever. If the source is just a single bump, or footstep, then the vibration magnitude will drop to zero, and pretty quickly based on the sizes of piers commonly used. I would have thought this a nominal concern only.

I also realise that longer piers like Joshua is proposing are more susceptible. But really, how big are these vibrations? I know they exist, but do they cause an issue in the real world. Have you had an issue with them?

Many people say they put sand/concrete/foam whatever in the pier to dampen them. And that's fine. But did they do it because they experienced a real issue that needed to be dealt with, or did they do it because they read that it will dampen vibrations, so they thought it would be a good idea to do so. They may have been solving an issue that doesn't exist.

If you have some thumping or large impacts going on around your pier, significant enough to affect your imaging, you'd be best addressing the source of the thumping. I wouldn't have thought that slew/focus motor vibrations come anywhere near affecting images, and you're not imaging while slewing or adjusting focus anyway.
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Old 27-04-2018, 10:30 AM
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Hmmm... I wonder if you can could actively damp pier vibrations now that we have the computer power to do the measurements and respond in real time. Actually, I'm sure you can. The big question is whether it is economically viable. Perhaps a good AO unit would be a better option.

Cheers,
Rick.
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Old 27-04-2018, 01:31 PM
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Cant you just isolate the pier ? my floor dosent touch the pier in any way I can run around as much as I like
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Old 27-04-2018, 01:41 PM
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Originally Posted by RickS View Post
Hmmm... I wonder if you can could actively damp pier vibrations now that we have the computer power to do the measurements and respond in real time. Actually, I'm sure you can. The big question is whether it is economically viable. Perhaps a good AO unit would be a better option.

Cheers,
Rick.
Absolutely you can actively reduce vibration, with real time control systems. Active vibration control is used in labs for various scientific instruments and imaging gear , BUT I still think it's good to mitigate any vibration as much as possible with the various passive means: isolation, mass, damping ..... As to it's applicability to astro imaging that would be an interesting experiment.

I think the various Active Optics options would also be wonderful for all sorts of reasons. Now Adaptive Optics .... THAT would be great at some sort of consumer/non-professional observatory level for dealing with the atmospheric seeing.

Best
JA
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Old 27-04-2018, 03:05 PM
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A recent Silicon chip magazine had a really neat new "earthquake" detection system, using a cheap 3 axis accelerometer and arduino.
http://www.siliconchip.com.au/Issue/...h?res=nonflash
Might be fun to use something like that to "see" if you have ground vibrations to worry about.
It isnt fast enough to do vibration damping tests, but would certainly give an idea of low freq ground vibration.
Andrew
ed changed link to correct one

Last edited by AndrewJ; 27-04-2018 at 04:36 PM.
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Old 03-01-2020, 01:12 AM
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Just for anyone who might have been following this thread, I have started a new thread in the DIY observatories for this build.


Josh
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