Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1166
Loc: Gateshead.NSW Nth Coast,Austra...
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I am leaning towards a concrete slab as the base for my planned DIYS backyard rolloff roof observatory.
I've a yard that has a slight slope where I plan on putting my slab (drop of about 25cm on a slab 4m long.
I am open to changing my plans and going for a off ground platform made from timber on 4 to 6 pedeestals made from concrete (in formatube (like Sonotube)) , just worried about extra costs ??? and keeping termite out of the platform and structure if I go this way acout two permanent (steel or concrete piers)).
What's the view of you guys ?
Which is better concrete or a timber off the ground platform surrounding the permanent pier(s) ?
I was going to do a poll but can't see a poll option .
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csa/montana
Den Mother
Reged: 05/14/05
Posts: 40316
Loc: montana
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Ian, I think you will get many answers for both. It probably will come down to cost. Mine is on a slight elevation, & I went with a wood platform, bacause of cost.
The platform rests on concrete footings.
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Carol
AstroTech 16" Dob (Thanks ASTRONOMICS!)
Vixen 80MF/AstroTech Voyager
Masuyama's 7.5, 15, 25W, 35mm,
Pentaxes; 5XW, 7XL, 10XW.
14mm Meade 4000 UWA
TV Panoptics; 22, 35
DreamCatcher Dobservatory, #2
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Joel
Carpal Tunnel
Reged: 07/08/04
Posts: 2586
Loc: Merrimack, NH
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I did concrete but my ground was pretty level. Only problem I've had is I got a water leak in one part from a poor caulk seal. I re-caulked it and no problem since.
I used concrete because I wanted a more "permanent building" feel vs a "shed" feel just for my preference and any future owner's use other than an observatory. Had I been on a slope I wouldn't have hesitated to build on a wooden deck. I put down foam tiles and it has a nice solid feel but easy on the feet. Doing it on a slab I was able to run conduits out to my pier for electric and data cables and they are protected from critters that way too.
Just be sure to isolate your pier well if you go with concrete!
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Joel
10" LX200GPS UHTC-SMT
Vixen 80EDsf
Canon unmodded 350D
QHY8
Turkey Hill Observatory
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1166
Loc: Gateshead.NSW Nth Coast,Austra...
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Quote:
Ian, I think you will get many answers for both. It probably will come down to cost. Mine is on a slight elevation, & I went with a wood platform, bacause of cost.
The platform rests on concrete footings.
You have a bit more slope than I have (in my back yard where I want it to go.
Very nice ROR shed though ....
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quantumac
sage
Reged: 12/17/07
Posts: 389
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I have my 10' x 12' peer and beam observatory floor supported by six concrete pads. The central pier is isolated from the structure and has a buried concrete base which is much more massive the other pads (to make the telescope orientation very stable).
The observatory is 13 years old and has weathered well.
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Scope: Meade 10" LX200R
Guide/Planetary Camera: Imaging Source DBK41AF02.AS
DSO Camera: QHY8
Guide Hardware: Celestron OAG, Shoestring Astronomy GPUSB
Software: Mac OS X, Starry Night Pro, Nebulosity, PHD Guiding, PixInsight, Astro IIDC. No Windows anything.
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csa/montana
Den Mother
Reged: 05/14/05
Posts: 40316
Loc: montana
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Quote:
I have my 10' x 12' peer and beam observatory floor supported by six concrete pads. The central pier is isolated from the structure and has a buried concrete base which is much more massive the other pads (to make the telescope orientation very stable).
The observatory is 13 years old and has weathered well.
I neglected to add; that the "base" for my 16" Dob is solid concrete, isolated from the floor.
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Carol
AstroTech 16" Dob (Thanks ASTRONOMICS!)
Vixen 80MF/AstroTech Voyager
Masuyama's 7.5, 15, 25W, 35mm,
Pentaxes; 5XW, 7XL, 10XW.
14mm Meade 4000 UWA
TV Panoptics; 22, 35
DreamCatcher Dobservatory, #2
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sfugardi
sage
Reged: 09/30/06
Posts: 332
Loc: CT
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Ian, I have a poured concrete pad and still struggle with degraded seeing as the floor radiates heat all night long upward at the scope. I've insulated the concrete and use a box fan in the door, even staying outside with my computers as I image. Still the seeing is not good enough. I would use as much wood as possible. Check out this link about observatories I found, http://www.dfmengineering.com/news_observatory_design.html#intro
Good luck,
Steve
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1986 Compustar C11
Toucam, DMK21 + IFW Wheel
ST-4/C90, Nikon D70s
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csa/montana
Den Mother
Reged: 05/14/05
Posts: 40316
Loc: montana
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Quote:
Very nice ROR shed though ....
Thank you; it's my pride & joy!
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Carol
AstroTech 16" Dob (Thanks ASTRONOMICS!)
Vixen 80MF/AstroTech Voyager
Masuyama's 7.5, 15, 25W, 35mm,
Pentaxes; 5XW, 7XL, 10XW.
14mm Meade 4000 UWA
TV Panoptics; 22, 35
DreamCatcher Dobservatory, #2
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roscoe
super member
Reged: 02/04/09
Posts: 192
Loc: Northwestern Mass
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Ian,
My choice was to build a wood floor on concrete piers, with a larger pier for the scope base. Here in the States we can get wood, both framing timbers and plywood, that is pressure treated with chemicals that keep the bugs and rot at bay. Not knowing the size of the floor you want to build, it's hard for me to guess if wood or concrete would be cheaper, but I agree that the heat issues of concrete could be a problem, and here in the frozen north, standing on a giant ice cube would also not be pleasant. The wood is somewhat noisy, though, it's a bit like a drum. I'm thinking of adding a second layer of untreated boards over the plywood to quiet things down a bit.
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Antares/Vixen 812 120mm f/8 on reworked CG-5,
Celestron/Vixen C-80 80mm f/11 on Polaris,
Sears/Towa 60mm f/15 on reworked Towa,
assorted Ultrascopics and Orthos
homemade tripods, chairs, EP cases, observatory
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1166
Loc: Gateshead.NSW Nth Coast,Austra...
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Timber off ground platform is looking better ....
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csa/montana
Den Mother
Reged: 05/14/05
Posts: 40316
Loc: montana
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Roscoe, Welcome to Cloudy Nights & especially this forum!
I laid some low nap indoor/outdoor carpet on my floor, & it's pretty silent. With the low nap, it doesn't collect a bunch of stuff on it; looks great, & warms the floor a little for winter observing.
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Carol
AstroTech 16" Dob (Thanks ASTRONOMICS!)
Vixen 80MF/AstroTech Voyager
Masuyama's 7.5, 15, 25W, 35mm,
Pentaxes; 5XW, 7XL, 10XW.
14mm Meade 4000 UWA
TV Panoptics; 22, 35
DreamCatcher Dobservatory, #2
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Joel
Carpal Tunnel
Reged: 07/08/04
Posts: 2586
Loc: Merrimack, NH
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Reading the posts on the heat radiating off the floor does not reflect my experience. My floor is covered with tiles and a roof so it doesn't get sun on it so there's no heat to radiate. I can touch my floor and it's cool to the touch even in summer. I've heard this argument before, usually from people who haven't built their observatory but not from people who have a concrete slab already.
I never open the roof until the sun has gone below the trees but I can see if you let sun on it before you went out then you might have a radiation problem. I just don't see radiation being a problem otherwise and it has never been a problem in my experience.
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Joel
10" LX200GPS UHTC-SMT
Vixen 80EDsf
Canon unmodded 350D
QHY8
Turkey Hill Observatory
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Chris Schroeder
Postmaster
Reged: 12/11/04
Posts: 6566
Loc: N.E. WI Sky Glow
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I used a wood platform for a couple of reasons. A nice thing a wood platform is you can use it to help ventilate the observatory. I laid aluminum bug screen on top of the joists before putting down the decking. I used about a 3/8” gap and you would be amazed on how much air flows through it on a breezy day. In the cooler/colder times of the year, I use a rubber cushioned floor mats on top of the decking and no breeze and warmer on the feet.
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Chris
Mallincam Color Hyper Plus
10" DSH with SC DSC, CPC 800 XLT
M110 Doublet ED, AT 102Achro, ZS80FD 10th Anniv, ZS66SD, PST
CG5-AGT, EZ-Touch, Voyager
POD XL3 http://POD.SchroederCity.com
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Galaxyhunter
Pooh-Bah
Reged: 01/02/06
Posts: 1257
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Quote:
Reading the posts on the heat radiating off the floor does not reflect my experience. My floor is covered with tiles and a roof so it doesn't get sun on it so there's no heat to radiate. I can touch my floor and it's cool to the touch even in summer. I've heard this argument before, usually from people who haven't built their observatory but not from people who have a concrete slab already.
Does not reflect my experience either. The only time that it could be warmer that the ambient air is in the coldest part of winter. I just went out & checked the floor temp and it is 20°. The current temperature is 39°. Think about it, The concrete is connected to a rather large heat sink.
If I was to build my OBS over again, I would use concrete again in a heartbeat. Nice & cool in the Summer time.
The biggest favor that you could do for yourself is to insulate the roof, to keep as much heat from getting in - in the first place. Also use steel for the roof instead of shingles (way less thermal mass).
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Carl
My lousy skies at Hawkeye Observatory
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HunterofPhotons
sage
Reged: 04/26/08
Posts: 266
Loc: Rhode Island, USA
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Hi Ian,
The article that Steve directed you to is a treasure trove of useful information.
Your choice of a floor depends a lot on where you live.
If you live in a place where the daytime and nighttime temperatures are the same and you don't have termites, carpenter ants, or such, then the question is moot. A concrete slab in this case has no disadvantages.
If, on the other hand, your daytime to nighttime temperatures are different, then the thermal mass of the concrete comes into play.
The ideal observatory setting is one in which everything is at ambient temperature. The thermal mass of an elevated, wooden floor compared to tons of concrete is pretty much insignificant. It's impossible for the concrete to adjust to changing temperatures as fast as the wooden floor.
If your nighttime air temperature is less than the shaded, daytime ground temperature, then the concrete will act as a heat radiator at night. There's just no way solid concrete can cool as fast as air.
You can now ask the practical question whether this heat radiation coming off of the concrete is strong enough to affect your viewing. We all know that heat plumes coming off of summer highway can be severely distorting, but how about in an observatory setting?
The needed experiment would be to build two almost identical observatories with identical telescopes (or measuring equipment) close together. One would be on an elevated wood deck and the other would be on a slab. Then you could measure the seeing effects. I don't know of any amateur who has done this, but, as you can see from Steve's article, professional astronomers, who have done these kinds of measurements, recommend avoiding concrete.
If you live an area that has some humidity, then the situation where the concrete is cooler than the surrounding air has to be considered. In this case, there are times when moisture will condense on the cooler concrete.
The other consideration is bugs. I don't know what it's like where you live, but in termite areas that I'm familiar with, building codes require concrete foundations to rise at least 10" out of the soil to lessen the chances of a termite infestation. To build a concrete pad that rises 10" out of the ground for an observatory just increases the mass and cost of the floor.
I'm not saying a slab floor for an observatory doesn't work, that wouldn't be able to see anything at all through your scope, but there will be some degraded seeing trade-offs to be factored in.
I hope this helps,
dan
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Galaxyhunter
Pooh-Bah
Reged: 01/02/06
Posts: 1257
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Quote:
You can now ask the practical question whether this heat radiation coming off of the concrete is strong enough to affect your viewing. We all know that heat plumes coming off of summer highway can be severely distorting, but how about in an observatory setting?
Dan, Do you know or has anybody did any analysis on how much temperature differential is needed to be able to say for sure that if you have "X" amount of temperature difference, That it will effect your seeing? Would somebody like to guess at this, would 10-15° cause a problem? If so, then how can some people with rooftop Observatory's claim that the shingles cause no problems? The shingles, roof decking & the atic all store energy, & that has to go somewhere.
Yes ,everybody has seen the heat coming off the highways in the Summertime. What is that temp differential? How many people leave there ROR in the open position during the day?
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Carl
My lousy skies at Hawkeye Observatory
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piaras
journeyman
Reged: 01/26/09
Posts: 6
Loc: Niagara
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We debated wood verse concrete and we decided on concrete due to low maintainence, no repairs for a long time. Also we had no idea how much the floor would move no matter how many piers we put underneath the wood deck.
The kicker was when we priced out the job for wood or concrete the wood deck with our labour would be $3200 - 3500 verses $2500 with concrete and a local contractor doing the job for us. No Brainer!
Stable platform for the observatory, the pier is on it own concrete block which was cast first. Due to minimum delievry charges the block for the pier is a 4ft cube. It is not going anywhere!
One thing as everybody has found. Put down mats to prevent shattered eyepieces and other stuff if they fall to the floor!
We wondered about convection currents but as the slab is oriented East to West and most viewing is to the south and on top of that we are on top of a hill where calm nights are not that common it was a non issue for us.
One advantage that we did find in the first summer of use that we get less dew on our equipment in the early evening as the slab cools back to ambient. The area of the guest scopes is where we found that to be true. Inside the Skypod dew has not been an issue yet.
During the winter it is a problem using the observatory as we have to move snow. This year has been a big snowfall year, over 4 ft in the backyard as I right this.
Pictures found at http://wrx-now.tripod.com
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ATM 13.1" Dobson design
Orion 80 ED
HEQ5 Pro
Modded Rebel XT
KWIQ Guider
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mikee
professor emeritus
Reged: 01/10/05
Posts: 701
Loc: Maine
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I would also think a disadvantage to putting a large concrete slab on your property is that if you were selling your house it would very likely be a drawback to the buyer (unless the buyer were an astronomer and you were selling the house with the observatory). Personally I wouldn't buy a house that had a 4 ton slab sitting in the yard but that's just me. I would however have considered a property with an observatory ready to go 
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HunterofPhotons
sage
Reged: 04/26/08
Posts: 266
Loc: Rhode Island, USA
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Quote:
Quote:
You can now ask the practical question whether this heat radiation coming off of the concrete is strong enough to affect your viewing. We all know that heat plumes coming off of summer highway can be severely distorting, but how about in an observatory setting?
Dan, Do you know or has anybody did any analysis on how much temperature differential is needed to be able to say for sure that if you have "X" amount of temperature difference, That it will effect your seeing?
*** I have no idea. I've never read the original journal articles, just the 'conclusion' articles like Steve indicated. I'm the cheating kid in class that copies off other peoples papers. I let the smart scientists do all of the heavy lifting and then I just copy their answers.
Seeing effects are extremely complicated. One would think that it would be hard to quantify and would be structure specific.
Quote:
how can some people with rooftop Observatory's claim that the shingles cause no problems? The shingles, roof decking & the atic all store energy, & that has to go somewhere.
*** What have they got to compare it to? Unless you move the setup away from the roof, mount it at the same height as the roof-mounted setup, and make simultaneous comparisons you can't truly say it's better since there's nothing to compare it to.
Quote:
How many people leave there ROR in the open position during the day?
*** If we're talking hotter days than nights, then no one that I know of. It's all about keeping the observatory as close to nighttime temperatures as possible. Most people find that opening their roll-off roof just after sunset works best for them.
There are thinks that you can do to kick start the process of cool-down like running an air conditioner. For myself, I did a passive cooling setup with a floor vent and functional cupola. Hot air rises out of the cupola while cooler air that has been drawn over an inground heat sink is sucked up through the floor vent.
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HunterofPhotons
sage
Reged: 04/26/08
Posts: 266
Loc: Rhode Island, USA
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Quote:
...... Also we had no idea how much the floor would move no matter how many piers we put underneath the wood deck...
There can be a bit of irony here.
A properly constructed wood deck, with supporting piers that go below the frost line, will only move a small amount, say about 1/8"-1/4", the seasonal cross grain movement of the wood beams and floor joists.
A 6" thick concrete pad poured directly on the ground can move much more than that. Despite its heavy weight, it's subject to the frost heaves of the frozen ground beneath it. You can guard against these heaves by preparing a base for your concrete pad. It usually involves removing the earth and replacing it with gravel that's been compacted in layers. I don't see many nonprofessionals doing this step, though.
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