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Concrete floor or deck over grass?

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#1 Footbag

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Posted 11 December 2012 - 01:13 PM

What are the advantages of using a raised wood floor rather then a concrete floor? I know you could separate the wood from the pier, but all things equal would a concrete slab be better?

#2 Norm Meyer

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Posted 11 December 2012 - 02:21 PM

Adam,
I don't know if one is preferable over the other. I used
cement for my 8' x 8'. It took about a cubic yard of cement.
I spent about a day mixing with a mixer. I dug out all the topsoil and put down a layer of gravel about 8 or 9 inches.Then mixed and poured the cement. The perimeter
of the slab is deeper than the rest of it but minimum thickness is 4". It has been there for 15 years or so
and is still good shape. There is one hairline crack but
it isn't getting any worse. There is re-bar in the slab.
I haven't noticed any lifting due to frost. Cement has a feeling of permanency. The disadvantages are:
More physical work initially.There is more mass so it will
retain summer heat and it is quite hard if you drop an eyepiece.You can alleviate these problems if you put
wood or carpet on top of slab. a wooden platform is physically easier to build.It won't be a problem with
heat built up because of lower mass. You wouldn't need to
cover it to protect dropped items. If you use wood it should
be pressure treated and you have to be careful with that
because the newer PT does not keep insects out of it especially in direct contact with soil. If I were to do it again I would definitely consider a wooden platform just
because I couldn't do the cement work anymore. There are
benefits to either way. Keep us posted on what you decide to do.

Norm

#3 Christopher Erickson

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Posted 11 December 2012 - 02:25 PM

A slab of concrete has a lot of thermal inertia and will negatively impact your local seeing.

If you can easily go either way, I would suggest some of that new composite plastic deck planking that looks like wood but never rots, is invulnerable to bugs and doesn't need painting.

#4 dwitek

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Posted 11 December 2012 - 02:43 PM

After two years of design and reading I've decided on Dek-Block and 2X6 joists to get air flow under the obs. I thought about a concrete slab but I'd feel better getting off the ground but keep it high enough up (clearance of about 10 inches to the subfloor) to keep critters from living underneath, hopefully.

#5 stmguy

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Posted 12 December 2012 - 10:06 AM

besides the thermal issues the wood floor will feel much better if you are standing any length of time

Norm

#6 DeanS

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Posted 12 December 2012 - 12:16 PM

I did a solid concrete slab. Perhaps a compromise but I wanted the flexibility to mount a couple piers, and be able to move them around if needed.

The slab is about 3' thick under the scope area, and around the perimeter to get well below our frost line so there is no heaving. Very solid, so much I can jump up and down next to a pier while imaging and there is no movement. Plus in its next life it can be used as a machine shop or similar ;)

The concrete is the same temp as the earth so does that really affect the seeing? I have carpet down as an insulator, plus the pad is in the shade so it is not absorbing the suns heat.

I do open windows and run a fan to help with cool down during the winter. Summer time I have a window AC unit on a timer so it is always cooler than ambient before imaging.

If you are going to do the wood, then you might as well get it up as high as you can to get all the benefits. I have no regrets, except I wish it was under some really dark skies ;)

Dean

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#7 Galaxyhunter

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Posted 12 December 2012 - 01:12 PM

-----The concrete is the same temp as the earth so does that really affect the seeing? I have carpet down as an insulator, plus the pad is in the shade so it is not absorbing the suns heat.

-----


I agree, How can there be a thermal issue when everbody that has a rooftop observatory claims that their shingles does NOT effect the seeing? And everybody knows that the shingles are soaking up the days heat.

#8 Footbag

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Posted 12 December 2012 - 01:32 PM

Well. I don't really think it would be any more expensive to pour the floor. I'd have to pay for a load of concrete, whether it's just a pier or the whole shebang. Initially I had planned to do it myself. I could probably dig a hole and mix up the pier but my back has gotten worse and my neighbor is a contractor. It's also in a better location then I had planned, so it will also have to look nice.

I'm inclined to just go with a slab. Animals are a primary concern. I had planned to put down a rubber mat in case of equip drops and just to soften my steps. If thermal issues effect it, maybe I'll get a solar powered fan to run across the floor.

It's a project for next spring, so now I'm planning and budgeting.

#9 Christopher Erickson

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Posted 12 December 2012 - 09:01 PM

Concrete has considerable thermal inertia and if it is unsealed, it can have a big impact on humidity in an observatory. And it is NEVER thermally-matched to the ground it is sitting on or in. All it takes is an evening with a thermal imaging camera to conclusively-prove that concrete should be avoided whenever alternatives exist.

In this picture you will note that at 5:48pm, just after sunset, the observatory is at 35F but the concrete pad is still at 50F. The surrounding ground (barren, rocky) is around 40F. The concrete pad finally stabilized with the ground around 1am.

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#10 Footbag

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Posted 12 December 2012 - 09:31 PM

Concrete has considerable thermal inertia and if it is unsealed, it can have a big impact on humidity in an observatory. And it is NEVER thermally-matched to the ground it is sitting on or in. All it takes is an evening with a thermal imaging camera to conclusively-prove that concrete should be avoided whenever alternatives exist.

In this picture you will note that at 5:48pm, just after sunset, the observatory is at 35F but the concrete pad is still at 50F. The surrounding ground (barren, rocky) is around 40F. The concrete pad finally stabilized with the ground around 1am.


Hmmm. I see your point from the image. I wonder if carpet or something else could insulate it. Maybe that with a solar cooling fan. Otherwise, it's probably easier and possibly cheaper to do a wood floor. It's keeping the critters out that I'm worried about.

#11 Tom and Beth

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Posted 12 December 2012 - 09:51 PM

I have a slab and considering a sub-floor for

1) standing
2) those times you drop an EP.

With a roll off roof, I've never encountered thermal problems from the slab. Maybe because it's shaded? YMMV.

#12 Christopher Erickson

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Posted 12 December 2012 - 10:49 PM

I have a slab and considering a sub-floor for

1) standing
2) those times you drop an EP.

With a roll off roof, I've never encountered thermal problems from the slab. Maybe because it's shaded? YMMV.


Most people don't recognize the thermal problems they have created with their observatory designs and unless you have an identical scope outside of the observatory and are set up to compare the seeing differences.

A thermal imaging camera can easily-show the temperature differences throughout the entire night.

One way to evaluate if there are big thermal problems is to compare seeing between, say, 10pm and 3am. If the 3am seeing is consistently better than your 10pm seeing then you are likely suffering from local seeing problems.

Many structural engineering firms and housing inspectors have thermal imaging cameras these days. You might also be able to rent one locally.

I hope this helps.

#13 Steve Drapak

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Posted 13 December 2012 - 12:56 AM

Raised platforms are a whole lot easier if you decide to move :)

#14 Starman27

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Posted 13 December 2012 - 07:58 AM

I have a raised deck under both of my observatories. It helps deal with local thermals along with appropriate landscaping.

#15 DeanS

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Posted 13 December 2012 - 08:23 AM

Everything is a compromise. I wanted flexibility and to do some visual out of mine.

If I was given the opportunity to build again in a dark sky site I would like to have a raised dome strictly for imaging, then a large roll off for visual.

I have a laser thermometer and will do some measurements and see what the slab is doing. I don't doubt it holds some heat since it is so thick.

We invest a lot in our equipment so it would be nice to get the most out of it, but at what point are we going overkill?

#16 csa/montana

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Posted 13 December 2012 - 10:47 AM

Wow! Dean, that slab will never move! Beautiful!

Since my Dobservatory is on a slope, wood was used, with a concrete pier (even with the wood floor) for my dob. The concrete is buffered from the wood floor with automotive foam. I then put indoor/outdoor carpet over the wood. It's worked out very nicely for me.

#17 Galaxyhunter

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Posted 13 December 2012 - 01:11 PM

Concrete has considerable thermal inertia and if it is unsealed, it can have a big impact on humidity in an observatory. And it is NEVER thermally-matched to the ground it is sitting on or in. All it takes is an evening with a thermal imaging camera to conclusively-prove that concrete should be avoided whenever alternatives exist.

In this picture you will note that at 5:48pm, just after sunset, the observatory is at 35F but the concrete pad is still at 50F. The surrounding ground (barren, rocky) is around 40F. The concrete pad finally stabilized with the ground around 1am.



That pic is all fine & Dandy. But there is a couple of questions:
1: Where is that Observatory
2: Is that a professional or a backyard observatory?
3: How much concrete is involved with that structure?
4: How much of the concrete is exposed to the Sun light during the day?


For the record, I am not opposed to a wood deck.

I just re-read you post, You counter-dick yourself.
"And it is NEVER thermally-matched to the ground it is sitting on or in." The next paragraph you said.

"The concrete pad finally stabilized with the ground around 1am."

#18 Tom and Beth

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Posted 13 December 2012 - 01:19 PM

I have a slab and considering a sub-floor for

1) standing
2) those times you drop an EP.

With a roll off roof, I've never encountered thermal problems from the slab. Maybe because it's shaded? YMMV.


<<SNIP>>
One way to evaluate if there are big thermal problems is to compare seeing between, say, 10pm and 3am. If the 3am seeing is consistently better than your 10pm seeing then you are likely suffering from local seeing problems.

<<SNIP>>

I hope this helps.


Yes, Erik...as I'm a visual observer who doesn't need to rise early I can/do this every clear night. Whatever OBS thermal issues are present are overwhelmed by the MC scope's thermal issue.

it's just a short scoot over to another scope that cools rapidly.

I attached a low resolution pic of the scopes from the Security

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#19 Christopher Erickson

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Posted 14 December 2012 - 04:19 PM

Concrete has considerable thermal inertia and if it is unsealed, it can have a big impact on humidity in an observatory. And it is NEVER thermally-matched to the ground it is sitting on or in. All it takes is an evening with a thermal imaging camera to conclusively-prove that concrete should be avoided whenever alternatives exist.

In this picture you will note that at 5:48pm, just after sunset, the observatory is at 35F but the concrete pad is still at 50F. The surrounding ground (barren, rocky) is around 40F. The concrete pad finally stabilized with the ground around 1am.



That pic is all fine & Dandy. But there is a couple of questions:
1: Where is that Observatory
2: Is that a professional or a backyard observatory?
3: How much concrete is involved with that structure?
4: How much of the concrete is exposed to the Sun light during the day?


For the record, I am not opposed to a wood deck.

I just re-read you post, You counter-dick yourself.
"And it is NEVER thermally-matched to the ground it is sitting on or in." The next paragraph you said.

"The concrete pad finally stabilized with the ground around 1am."


You are right. I could have phrased that a lot better than I did.

Answers to your questions:

1&2. That fully-robotic observatory is owned by the University of Hawaii and is called VYSOS-20. It has an uninsulated Ash dome and contains a 20" Planewave RC scope on an Astro-Physics 3600GTO mount. It is located at the Mauna Loa Weather Observatory, on the slopes of Mauna Loa on the Big Island of Hawaii. Its coordinates are 19.535948, -155.576140. It is a barren, volcanic, incredibly-dry place at 11,000'. It could be described as a professional observatory built with high-end, COTS amateur equipment.

3. There is an estimated three cubic yards of concrete involved in that observatory's pier and foundation/floor.

4. About 5% of the concrete is exposed to the Sun at any given time during the day.

The observatory's inside and outside temperatures are like two sine waves that are permanently out of phase with each other. The inside temps are always following after the outside temps. Because of the concrete mass, the local seeing is "bad" until about 1am, when it improves to "marginal." At about 3am, the local seeing improves to "acceptable." About one hour after sunrise, the outside temperature warms past the inside temperature and then the inside stays cooler until about sunset, when the outside temps once again cool past the inside temps.

One solution I have recommended to the university is to insulate the dome and install a remote-computer-controlled A/C unit. The plan would be to pre-cool the observatory interior to the predicted night-time temperature when night-time operations would commence. This will use a whole-lot more electricity (especially painful at $0.47/kWh) and still wouldn't give ideal local seeing but it would improve their local seeing considerably.

Local seeing is a cumulative issue. People who choose to believe that they can ignore one factor because it is "swamped by other factors" are overlooking the reality that all of the factors are cumulative. Every improvement that can be reasonably-made will give dividends.

I hope this helps.

#20 Galaxyhunter

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Posted 14 December 2012 - 04:24 PM

I just placed a temperature probe roughly 2" deep in the ground off of the North East corner of my Observatory so that it is in the shade most of the day. I placed the readout on the upper left hand corner of the attached pic. This pic was taked @ 15:03 Central Time.



Concrete has considerable thermal inertia and if it is unsealed, it can have a big impact on humidity in an observatory. And it is NEVER thermally-matched to the ground it is sitting on or in.



So much for THAT theory :tonofbricks:

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#21 Calypte

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Posted 14 December 2012 - 04:33 PM

I just placed a temperature probe roughly 2" deep in the ground off of the North East corner of my Observatory so that it is in the shade most of the day. I placed the readout on the upper left hand corner of the attached pic. This pic was taked @ 15:03 Central Time.



Concrete has considerable thermal inertia and if it is unsealed, it can have a big impact on humidity in an observatory. And it is NEVER thermally-matched to the ground it is sitting on or in.



So much for THAT theory :tonofbricks:

Sorry to sound ignorant, but where did you get those sensors? I have a new observatory, built on a thick concrete slab. Thus I have great interest in this discussion, although there isn't much I could do about it if the slab really has the alleged problems.

#22 Christopher Erickson

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Posted 14 December 2012 - 06:38 PM

I just placed a temperature probe roughly 2" deep in the ground off of the North East corner of my Observatory so that it is in the shade most of the day. I placed the readout on the upper left hand corner of the attached pic. This pic was taked @ 15:03 Central Time.



Concrete has considerable thermal inertia and if it is unsealed, it can have a big impact on humidity in an observatory. And it is NEVER thermally-matched to the ground it is sitting on or in.



So much for THAT theory :tonofbricks:


(Grin)

Physics is physics, mass is mass and thermal inertia is thermal inertia. The concrete-and-local-seeing guidelines originate from the professional observatories, however I have never seen anything happen in small observatories that is any different than the big ones in anything except scale.

A multi-probe, temperature data-logger would give more-interesting data than a bunch of autonomous temperature displays.

Another test would be to do un-binned, comparative star field images at different times of the night in the same Alt/Az direction and with the same camera settings. Compare star diameters. Note seeing issues (not local seeing issues) and do the tests on nights with the best seeing possible.

A time-lapse, thermal-image video of the observatory in question would be very interesting as well.

Minimizing large thermal masses in and around observatories whenever possible has been standard practice for about 125 years. Lowell Observatory was one of the first institutions to really take local seeing seriously and to make it into a science. It is only in the past 20 years or so that it seems to have become controversial in some corners of the amateur arena.

I hope this helps.

#23 Footbag

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Posted 14 December 2012 - 07:14 PM

I don't doubt that concrete has thermal inertia. At this point, I believe that an observatory that has airflow under the floor is probably a better option for the purpose of improving local seeing.

But, concrete still has some advantages when it comes to rot and rodents. Personally, I think it looks more professional.

So the question that I'm pondering is...
If thermal inertia is take into account during planning and construction(no concrete exposed to sun, fan blowing across floor, carpet covering concrete)can the effect be limited to the point where it is negligible?

#24 Christopher Erickson

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Posted 14 December 2012 - 09:17 PM

I don't doubt that concrete has thermal inertia. At this point, I believe that an observatory that has airflow under the floor is probably a better option for the purpose of improving local seeing.

But, concrete still has some advantages when it comes to rot and rodents. Personally, I think it looks more professional.

So the question that I'm pondering is...
If thermal inertia is take into account during planning and construction(no concrete exposed to sun, fan blowing across floor, carpet covering concrete)can the effect be limited to the point where it is negligible?


In my opinion the best way to make a concrete foundation/floor thermal-friendly would be to run "radiant floor heating" water tubing in it before you pour it. That would allow you to precisely-control the temperature of the concrete at night so it could always be exactly at ambient temperature. If the concrete is already poured then the next best option is probably insulation and an A/C unit. Concrete usually causes more disruption to local seeing during the summer months than the winter months.

#25 Galaxyhunter

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Posted 15 December 2012 - 12:09 AM

Chris,
I can understand where you are coming from if the thermal pic you posted is from a professional Observatory parked on the top of Mauna Kea & if that Observatory has maybe hundreds of yards of concrete with a large exposure to the Sun. Of course it is going to show hot in the thermal image, that’s just common sense. That is no different then looking down a road during the day & seeing the heat plumes rising.

But you must also understand where I'm coming from. My observatory has a total of 6.5 yards of concrete on a 14' x 16' footprint. The South face might have a inch & half exposed face & the West side about 3" exposed. So there isn't a lot for the Sun to heat up. I have nowhere near the mass of concrete that a pro observatory has to deal with. I am also from the Midwest at an elevation of 758'. On a good night, my FWHM is 2.5. It normally runs in the low 3's.

For 99% of the people on the forum, This is just a hobby. Not many people here are trying to do any research. You never answered my questions from a couple of posts ago. Another question is, If concrete is such a detriment, Why do pro Observatories use it? Why do people claim that a wood floor feels warmer in the Winter time?

It is true that thermal inertia is thermal inertia. But if it is cooler to start with, How is that a detriment?
Here is a pic from 10:00 tonight.



Edited for typo.

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