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1965healey
The Snarkster

Reged: 06/23/07

Loc: San Antonio, TX
Re: Pier for rooftop observatory [Re: Scott Horstman]
#5676452 - 02/12/13 07:16 PM

Musicos, is it possible for you to post some pictures of what you want to mount and where you want to mount it?

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*skyguy*
Carpal Tunnel

Reged: 12/31/08

Loc: Western New York
Re: Pier for rooftop observatory [Re: musicos]
#5676586 - 02/12/13 08:53 PM

Quote:

I have the situation that my original slab is only 4" thick.
I will therefore have to cut out the existing slab, dig in depth, etc....I did not know beforehands that I would one day construct a rooftop observatory at this place!

A 4" concrete slab should easily hold the weight of your proposed pier. Concrete has an extremely high compression strength ... on average, 3000 lbs/inČ... and it will rarely fail/crack from the excessive application of weight. For example, an automobile weighing approximately 3200 lbs. can remain parked on a garage floor for decades without cracking the concrete ... why? ... because the weight is spread across 4 tires (25 inČ contact area), each tire exerting only 32 lbs./inČ on the concrete pad. How do you get the 32 lbs./inČ answer ... well, you can do the math, or just measure the inflation pressure on each tire! They're both the same number ... neat!

Now, taking a look at your pier ... assuming 16"x16" cement blocks and a 14' height ... the weight of the pier filled with concrete (145lbs./ftł) will be about 3,600 pounds. Adding 800 pounds of equipment will make the total weight 4,400 pounds. Therefore, the weight of the pier and equipment on the 4" concrete pad will be only 17 lbs./inČ (4400/16x16)... a little more than 1/2 the weight per inČ of an average car on a concrete garage pad.

Of course, this is assuming the concrete pad is in good physical condition and underlying substrate material meets standard construction codes. Overbuilding a structure can be a serious waste of resources, time and money ... however, it will help you to sleep better at night!

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musicos
member

Reged: 06/16/10

Re: Pier for rooftop observatory [Re: *skyguy*]
#5676956 - 02/13/13 02:14 AM

I'm not worried that the slap cannot carry the weight of the pier, I'm worried that any mouvement of a person walking on this slab and close to the pier will translate 15' higher as strong vibrations...well, that's the reason people "disconnect" the pier from the rest of the building...
I'll try to integrate a photo later on...

cheers
Torsten

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Scott Horstman
Vendor - Backyard Observatories

Reged: 03/11/04

Loc: Here, There and Everywhere
Re: Pier for rooftop observatory [Re: musicos]
#5676967 - 02/13/13 02:35 AM

Once you have that whole mass in place, unless your having Irish Dance classes in the garage, I can't see it moving at all if someone came in, and I'd be surprised if your camera would see it too.

Just going by experience, I've never heard anyone complain about vibrations or movement by simply attaching to the slab.

Of course you do remove all doubt by doing an isolated footing but for the added expense.

At our old house we had a high speed railway about 2 miles away. Freight trains traveling up to 70mph.
I had a pretty good chunk of concrete in the ground for my pier in the observatory and Saturn would start shaking before I could hear the train whistle.

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*skyguy*
Carpal Tunnel

Reged: 12/31/08

Loc: Western New York
Re: Pier for rooftop observatory [Re: Scott Horstman]
#5677437 - 02/13/13 10:58 AM

My 14' chimney block pier is attached directly to the garage floor pad. There's approximately 46,000 pounds of concrete in that pad. People walking around the garage during imaging sessions have had absolutely no effect on the telescope's performance. From my experience ... from the past 12 years of imaging using this pier ... I would not be concerned at all about placing a tall pier directly on a large concrete pad.

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jrcrilly
Refractor wienie no more

Reged: 04/30/03

Loc: NE Ohio
Re: Pier for rooftop observatory [Re: *skyguy*]
#5677449 - 02/13/13 11:04 AM

Quote:

My 14' chimney block pier is attached directly to the garage floor pad. There's approximately 46,000 pounds of concrete in that pad. People walking around the garage during imaging sessions have had absolutely no effect on the telescope's performance. From my experience ... from the past 12 years of imaging using this pier ... I would not be concerned at all about placing a tall pier directly on a large concrete pad.

I've never experienced any problems with mine either (chimney blocks on slab floor). Of course, if someone WERE to be walking around in my garage at night, I'd want to know about it - but I believe I'd have to find out about it some other way.

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evhap
member

Reged: 11/09/10

Loc: California
Re: Pier for rooftop observatory [Re: jrcrilly]
#6071732 - 09/09/13 05:21 PM

Maybe this is a stupid question, but won't you pretty much need an elevating pier if you use a roll-off roof? And, if so, is the movement precise enough to retain alignment on a go-to system?

Evan

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1965healey
The Snarkster

Reged: 06/23/07

Loc: San Antonio, TX
Re: Pier for rooftop observatory [Re: evhap]
#6071829 - 09/09/13 06:12 PM

Quote:

Maybe this is a stupid question, but won't you pretty much need an elevating pier if you use a roll-off roof? And, if so, is the movement precise enough to retain alignment on a go-to system?

Evan

Not a stupid question at all! I would say that in most cases the answer would be no you don't need one. The floor of a rooftop ROR is like the floor of any observatory on the ground it's just that the concrete pier is built up to reach the floor height of the observatory. Once you've reached that height then a steel pier can be installed or you can simply continue the height of the concrete pier and attach a mounting plate for your telescope. A motorized pier is not a requirement unless you need to raise the scope far above the walls for some reason like trees or an adjacent structure that obstructs the view.

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evhap
member

Reged: 11/09/10

Loc: California
Re: Pier for rooftop observatory [Re: 1965healey]
#6072048 - 09/09/13 08:27 PM

Maybe I have the wrong picture in my head. It seems to me that if the OTA is mounted low enough for the roof to close, it will be too low for viewing near the horizon. I suppose I'm thinking in terms of refractors, where the problem may be especially severe.

Evan

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1965healey
The Snarkster

Reged: 06/23/07

Loc: San Antonio, TX
Re: Pier for rooftop observatory [Re: evhap]
#6072197 - 09/09/13 09:56 PM

Many ROR observatories have a drop down southern wall that aids in viewing closer to the horizon thus eliminating the need for a higher pier. In many areas views close to the horizon are impacted by atmospheric issues like pollution, heat induced air currents, humidity, lights etc. The more of the close to the earths surface issues you have to look thru the poorer the seeing is. Folks who live in really dark skies have constructed ROR observatories with three drop down walls that allow broader views. My ROR has a drop down southern wall, I rarely use it unless the nights are exceptionally clear, cold and dry. The walls of my observatory are 6' tall, I have both a pier mounted SCT and a CG5 tripod mounted refractor. When stowed the roof closes without difficulty.

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Megabusa
sage

Reged: 05/28/12

Loc: Page ,AZ
Re: Pier for rooftop observatory [Re: evhap]

Wow ! 800 Lbs , I can't answer how big around the pier has to be that would be a question for the concrete people and I know you need a lot of rebar , Frost line you need to check on the codes for that area , but I can answer about the vibrations , First you pure your pier and to hold 800Lbs it's got to be deep and big , then the foundation, the Pier does not touch the Foundation , If you look at mine you can see the outer concrete tube is 10" and the Pier is 8" That leaves a 1" gape and there is about about 6" to 8" of sand that separates The Pier Base from the Foundation with a 20" footing , My Frost line is I think 16", that way when you walk around on the floor it's almost no Vibrations to the Pier.

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Megabusa
sage

Reged: 05/28/12

Loc: Page ,AZ
Re: Pier for rooftop observatory [Re: Megabusa]
#6072298 - 09/09/13 10:58 PM

Well I can't make the photo bigger but if you go to Member Galleries and put my name Megabusa I think you can see the photo. Click on it to make it bigger . I hope this is some help for you and Congratulations I hope everything goes well . Bun Zhoor-nay

Nate

Edited by Megabusa (09/09/13 11:10 PM)

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Alex McConahay
Carpal Tunnel

Reged: 08/11/08

Loc: Moreno Valley, CA
Re: Pier for rooftop observatory [Re: Scott Horstman]
#6072319 - 09/09/13 11:08 PM

Generally deeper is better than wider. Put the big mass as low in the ground as you can, rather than high and wide.

Alex

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