Do I need to isolate the pier from the floor?

[alcyone]

I am planning to install a 12'x8'(incl 4'x8' warm room) ROR observatory this year and I am wondering about whether I need to isolate the pier from the floor slab. I shall probably be the only user, I shall mainly be doing short-duration CCD imaging and I wonder if this merits the bother of separate foundations for the telescope. I should be grateful for the advice of anybody who has mounted their scope pillar directly on the floor slab.

[Bizibilder]

I would certainly isolate the pier. On my own observatory I have done just that and I can safely jump up and down and generally crash about with no transmitted vibration apparent at the eyepiece.

Before I had the pier installed I used the EQ5's tripod on the observatory floor (not the pier mount) and any movement - just moving my weight from one foot to the other - was clearly discernable at the eyepiece when using high power.

As for construction - I made the pier mount first and enclosed it in a foam made for making expansion joints in concrete (sorry, I don't know its official name!) in two layers arond the pier base. The main floor was then poured.

I did (deliberately) make the pier base about 3cm lower than the observatory floor level. This was specifically so that future owners of our property could easily cut off the pier mounting bolts and fill the shallow "hole" to make a level base for their own purposes - Bar-b-q base, seating area or greenhouse etc. If you want to see some pics have a look at m'blog (in the signature).

[steppenwolf]

I would definitely recommend isolating the two. Although my obs. is a dome type, my base will be the same as yours so you might find parts of this article on my website of some use

[Cornelius Varley]

The pier should be mounted on a separate deep concrete block. I would put a layer of non compressible board (trade name Flexcell) between the pier block and slab and seal this with a polysulphide sealant.

Peter

[RobH]

Yes, isolate it.

I used to set up my scope on the balcony, and at reasonably, but not stupidly, high power, you could see the image jump from the vibration transmitted from your heartbeat.

I have a suspended floor which doesn't toch the pier or pierbase.

Cheers

Rob

[alcyone]

Gentlemen,

Thank you for your comprehensive advice. It seems that there is no easy way to mount a stable observatory telescope so I shall just have to make my plans to do it the hard way.

[Tim]

Funnily enough, I have a seperate pier block and concrete floor around it, and sometimes, if the kids slam the obsy door, there is still a tiny bit of vibration evident, which must go through the soil!

So well worth the effort to isolate as much as you can.

[SteveP]

Definitely isolate the pier from the pad. You'll be surprised how well concrete transmits vibration ..even footfall. There is a guy on the CN site (or it maybe the Yahoo RoR group) who bolted his pier directly to the pad. He was so disappointed with his imaging results that he subsequently excavated part of the existing concrete pad, dug a new hole, installed a new footing for the pier and put everything back together. Based on his experience, I wouldn't recommend taking a shortcut at this stage

Steve

[alcyone]

Thanks again for the advice. It's even better that the opinions are unanimous about the benefits of isolation so I don't need to fret about making a decision.

I was thinking about asking for recommendations on the size of the pier foundations but I am guessing that the answer will be "as large and deep as possible".

Geoff

[martin_h]

About 2ft cube seems to be the norm, but yes the bigger the better

[Bizibilder]

A cube around 2ft to 3ft should work - it depends on your soil. The other advice that I have gleaned mainly from the web is to make the bottom of the block larger than the top. On my own the top is 2ft square and the bottom of the hole was around 2ft 6inches across before i poured the concrete..

[alcyone]

I was not comfortable with the idea of a tapered pier because I thought that vertical pressure from the overlying floor slab might be transmitted via the earth to the pier. For this reason I was planning vertical walls for the pier. Am I missing the point?

Geoff

[Bizibilder]

You want as much pressure acting downwards on the pier base as you can - the last thing you want is a block of concrete that is "floating" in the soil (Yes - I mean floating - this can be a real problem with very dense, wet, clay soil).