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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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