Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1165
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: 1165
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: 1165
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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Galaxyhunter
Pooh-Bah
Reged: 01/02/06
Posts: 1257
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Well Dan, I guess that we'll have to agree to disagree. 
But as Scott Horstman said in a post dated 1-3-09 @ 11:38ct
Quote:
Unless you have the roof open and the sun beating down on the slab, slabs tend to hold the ground temp and help keep the obs naturally cooler in the summer. Considering results from customers I see no particular benefit of one over the other.
He has built a few dozen Observatories across the country, so he is the closest to a "Resident Authority" that we have here.
"naturally cooler in the summer". That has been my experience.
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Carl
My lousy skies at Hawkeye Observatory
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csa/montana
Den Mother
Reged: 05/14/05
Posts: 40316
Loc: montana
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Quote:
He has built a few dozen Observatories across the country
I believe mine was the 99th, that Scott built.
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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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Bob Griffiths
Postmaster
Reged: 10/10/05
Posts: 6592
Loc: Frederick Maryland
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As for a roof...??
I never paid much attention when I started building my observatory but the only place I could put it was right next to a 12x16 foot garden shed..directly to the East-Southeast.
When I open my shutter door the bottom of the shutter is only about 2 foot higher then the roof and only about 2 foot away... So I am looking at a 16 foot long 12 foot wide roof only a few feet away from me.
The shingles are light colored and the dome itself shades the sheds roof in the afternoon ..(O lucked out) .. in the 3 years The Obs has been functional I have NEVER had a problem with heat currents coming off the sheds roof...
Heck after I noticed this potential problem I made sure my garden hose would reach close enough to the shed in case I needed to water down the roof but I have never had to...
On the concrete slab:
I restore cars as a hobby and before I went out and purchased a lift so I could work on them in comfort I had put them up on jack stands to lay on my back on the concrete floor (summer and winter)
To be honest I have never felt any heat "in" the floor in fact it is kind of cool in the summer.. BUT boy I sure felt the penetrating cold in the winter...
IF you keep the roll off roof in place until shortly before observing I do not see the concrete floor being any real problem...
Bob G.
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CPC1100
Nexstar 8i + GPS & Rays Brackets
Denk S1 power switch
Orion 100 mm Refractor
Meade LXD 55 ...AR-5 127 mm Refractor
Exploradome Observatory S.I.E. (Smiling Irish Eyes)
Gerbring Heated Motorcycle clothing in the winter
39*21'03" N
77*28'12" W
The sky over my head....
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andyschlei
sage
Reged: 03/05/06
Posts: 376
Loc: Los Angeles, CA
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Wouldn't the magnitude of diurnal temperature change make a difference? In a high desert area (like the Anza Valley in So Cal) we can get highs in the 70s and lows in the 30s to highs in 90s and lows in the 50s with a pretty steep nighttime cool down. In my shaded garage, the cement floor feels relatively warm by the time it has cooled off outside. Whether or not that is enough heat to make a difference for seeing, I don't know.
I was also thinking that the paved part of the driveway feels noticeably warm but it's in the sun all day. As has been said, one keeps the obs closed during the day.
I am building my new obs with a wood floor and concrete footings. I will have an exhaust fan to keep the temperature down in the day and a fan or two to keep air moving when the obs is open. Given the position I am in on the hill and the need to elevate the obs floor relative to the equipment room and warm room, a slab wasn't practical.
Those that know (Ron Wosaski presented this at AIC a couple of years ago) say that most seeing effects are close to the ground. If you go out and look a the web sites of the New Mexico observatories (Russ Croman's Dimension Point or JMSM) you can see they have significantly elevated their observing platforms. These would, of course, stand out a bit in the back yard. 
--Andy
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Observatorio de la Ballona
Mar Vista
Lake Riverside Estates
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andyschlei
sage
Reged: 03/05/06
Posts: 376
Loc: Los Angeles, CA
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I did notice that in the article Steve linked it has the text:
Quote:
The upper part of the column can be hollowed out to reduce the moment of inertia and the thermal mass.
So they seem worried about the thermal mass of a pier inside a domed observatory. Who knows? And wouldn't you want more inertial mass? I guess lower inertial mass rather than higher.
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Observatorio de la Ballona
Mar Vista
Lake Riverside Estates
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piaras
journeyman
Reged: 01/26/09
Posts: 6
Loc: Niagara
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That is correct that if you do not go below the frost line you will likely have movement. Whether wood or concrete makes no difference.
That was why the pier is below the frost line for our area. We did not care about seasonal movement of the slab as that will have no effect on the night time use of the observatory nor of the area outside. We were just looking for no vibrations when moving about while observing. In this we succeeded.
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ATM 13.1" Dobson design
Orion 80 ED
HEQ5 Pro
Modded Rebel XT
KWIQ Guider
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sfugardi
sage
Reged: 09/30/06
Posts: 332
Loc: CT
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As for a temperature gradient study, I plan to generate my own data. I have an omega 2-channel temp probe with RS232 ouput that came with its own software. It's an older model of this one, http://www.omega.com/pptst/HH506A_HH506RA.html
although, I wish I had purchased the 4 channel probe. My original issue was cooling my C11 mirror to within <0.5degC within ambient, however, after adding 3 11cfm fans to the back of my scope, mirror cooling is no longer a concern. 2 hours before I use my scope I am blowing air in/out of my dome with box fans. Then I shut the fans off and close the door (controlling the scope from outside) when I start to image. Strangely, the dome ambient temp increases approx 1degC and within 1 hr my seeing quality degrades. This week, I will disconnect my mirror tc and add a 2nd ambient tc. Then I'll place one near the scope of the other one just above the floor and collect raw data vs time. Precise scope resolution can be measured using metaguide, http://www.astrogeeks.com/Bliss/MetaGuide/
It is my theory that an ideal dome ventilation condition exists that minimizes thermal gradients which degrade local seeing and resolution. Every observatory is different. I will post some data as I collect it
Regards,
Steve
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1986 Compustar C11
Toucam, DMK21 + IFW Wheel
ST-4/C90, Nikon D70s
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HunterofPhotons
sage
Reged: 04/26/08
Posts: 266
Loc: Rhode Island, USA
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Quote:
Well Dan, I guess that we'll have to agree to disagree.
*** Sure, that's fine with me. I have no desire to tell people what to think. <g> I would hope that they would keep an open mind...
Quote:
But as Scott Horstman said in a post dated 1-3-09 @ 11:38ct
Quote:
Unless you have the roof open and the sun beating down on the slab, slabs tend to hold the ground temp and help keep the obs naturally cooler in the summer. Considering results from customers I see no particular benefit of one over the other.
*** Yes, I'm familiar with that thread. I certainly agree that the slab will keep the observatory cooler during the day. I'm just saying that If your nighttime air temperature becomes less than the slab temperature, then your slab will radiate heat. As air temperature drops, will the slab match that temperature drop? No, of course not. The thermal mass of the slab is much greater and the rate at which its temperature drops will lag behind that of the air. As much as we would like to, we can't contradict the laws of physics.
I'm assuming that what we're disagreeing about whether the radiated heat will affect the seeing appreciably. I freely admit that I haven't done the experiments to test this. I'm just relying on people who have done these experiments like the ones in Steve's article who conclude "To achieve good seeing, the observatory needs to be operated at the outside air ambient temperature.
This requires minimum heat generation, good ventilation, insulation, and low thermal mass construction " and "...the observatory floor should be of low thermal mass". If someone has contradictory research, I'd certainly like to see it. I have an open mind.
Quote:
He has built a few dozen Observatories across the country, so he is the closest to a "Resident Authority" that we have here.
*** I have no quarrel with the quality of Scott's construction. I've only seen pictures of his work but it certainly looks fine. As a former builder, I can appreciate the special complexities of building an unconventional structure. In fact, I believe more people here would be better served if they did use the services of someone like Scott. Rookies make rookie mistakes. A professional usually does a better job than someone who does something for the first time. That's why I didn't remove my own appendix.
Is Scott right 100 % of the time on 100 % of the issues (like I am)? I don't know.
As I said before, having a concrete slab doesn't make an observatory unuseable. You can still observe and image within such an observatory. In addition, there may be other overriding concerns that would make a slab preferable, like having a dob that you want to move around on the floor.
The original poster asked what would be the best base for an observatory, and, given a choice, I would opt for the raised wood deck which is much less inclined to interfere with seeing.
And I would recommend Scott to build it. <g>
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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I spent a lot of time living in NV and CA. Your bare feet tell you how well cement, asphalt, and masonry retain heat through the night. The best bet is a low total heat capacity material that will lose it's heat quickly. Wood is good. Metal is good if thin. Many of these materials will quickly cool to lower than ambient through radiation and as such can accumulate dew. Concrete etc. is less optimal.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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Scott K
Pooh-Bah
Reged: 09/13/07
Posts: 1413
Loc: Dallas, TX & Eufaula, OK
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Quote:
The original poster asked what would be the best base for an observatory, and, given a choice, I would opt for the raised wood deck which is much less inclined to interfere with seeing.
For what it's worth, I agree with you, although not quite for the same reasons you suggest.
To my mind, the advantages of a wooden deck are really simple:
1. If anything goes wrong with the build, a wooden deck is VASTLY easier to fix than a concrete slab. Dealing with a bad slab is a nightmare. In some ground conditions, building a slab is dead simple. In others, it just isn't.
2. If you ever need to remove your observatory for any reason, it's going to be much easier to remove that deck than to remove the slab sitting out in the yard.
3. Building on a deck, above grade, may improve your ability to water seal your observatory.
I haven't really noticed any seeing issues introduced by my slab foundation. When the seeing is good outside the dome, it seems to be good inside the dome. When the seeing is bad outside, it seems as bad inside. Sorry, not terribly scientific, but seriously, I think I've had very nearly every problem you could have with a slab - surely this one would've gotten me too? (Murphy's law tested here nightly...)
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Peter L.
member
Reged: 04/19/08
Posts: 13
Loc: PEI, Canada
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Quote:
Timber off ground platform is looking better ....
Good idea. I'd suggest putting down a layer of 1" foam between your two layers of plywood. Spray foam the perimeter and tuck tape the joints. That will help with the sound and provide some insulation as well. Screw the second layer of plywood through the foam, through the first layer, and into the florr joists beneath.
Peter
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