Observatory base / elbow pier foundation advice

[tooth_dr]

In both my past observatories (ROR sheds) I dug out a 600mm cube and poured a large concrete base, to which I inserted metal rods and then poured up a concrete pier within a plastic pipe.  I then packed 25mm of foam around this and poured up the rest of the foor. I brought the floor a cm above the pier foundation so my wooden shed base wouldn’t contact that concrete.

Im looking at buying an offset elbow pier. I’m also relocating my observatory so will be digging out new founds again.

My question is - can I avoid the need to dig out a separate pier foundation and just pour a deeper single floor. This time I’m going for a 2.2m pulsar dome.  I understand that vibrations etc will be more pronounced but I don’t intend to be in it whilst imaging.  I plan on using a metal pier and bolt it to the floor.

If a single floor is suffice how deep?

And how would I go about calculating the offset of the bent pier to plan for ducting?

 

Thanks in advance

Adam.  

[tooth_dr]

I was thinking of 300mm deep floor.  This would mean about 1.7m3 of concrete. Would there be a chance it would crack on setting?  Do you think there would be a sufficient base?

[pete_l]

For a large area, as in the photo, you will put a steel mesh in the concrete. If you are really worried about cracks you can add polypropylene fibres to the concrete as you mix it.
That reduces the chance of cracking due to shrinkage as the concrete sets and also increases its load bearing (although that won't be an issue for you).

[tooth_dr]

13 minutes ago, pete_l said:

For a large area, as in the photo, you will put a steel mesh in the concrete. If you are really worried about cracks you can add polypropylene fibres to the concrete as you mix it.
That reduces the chance of cracking due to shrinkage as the concrete sets and also increases its load bearing (although that won't be an issue for you).

Thanks. The large area in the photo is my actual current base, I poured it myself and I didn’t add anything. Thankfully it didn’t crack but tbh I never considered it at the time.

Im now wondering specifically about a 2.7m circle, as a new base for a dome.  If I pour it deeper will it be more likely to crack? How deep would be enough?

 

Thanks again

Adam. 

[WanderingEye]

15 minutes ago, tooth_dr said:

Thanks. The large area in the photo is my actual current base, I poured it myself and I didn’t add anything. Thankfully it didn’t crack but tbh I never considered it at the time.

Im now wondering specifically about a 2.7m circle, as a new base for a dome.  If I pour it deeper will it be more likely to crack? How deep would be enough?

 

Thanks again

Adam. 

Why are you concerned about cracking...as long as the ground under is solid, then the should be fine as long as over 8” deep, we just had that patterned coloured printed concrete done for our drive, and that’s just 6” deep at most, and it’s 90 square metres, but to stop any chance of cracks they cut 2” deep cut lines into it, and fill with colour matching silicon, this means that any cracks will now follow these lines as they will be the weakest point, and so will not be seen, but they only put those cuts approx every 3M , so each section would be bigger than you need....

but if you are concerned then you could do the same... 👍

 

Edited January 5, 2020 by WanderingEye

[tooth_dr]

33 minutes ago, WanderingEye said:

Why are you concerned about cracking.

It’s a base for a dome, it needs to not crack.

[Physopto]

I know the usual thing/thinking is to isolate the mount from the observatory base, but is it really necessary ?

At star parties I image with the mount on grass or on hard standing. People walk all around at various times and I have never had a problem with any vibrations affecting images.  At home I have built a solid concrete base without any isolation between the mount pier base and the observatory base. It is an unfinished project as we are trying to sell up and move house. I have tried imaging on the tripod base on top of the concrete and not seen any problems in the images. 

It may be different if you are using a long focal length scope. We do have a very solid sub base around our house so any lorry passing can be felt and heard. But no visual problems seem to occur. 

I am including a photo of my observatory base. It was a Koi pond and is about 5 feet deep. It was filled with chinmey breast/rubble from our house renovations. Then a base of 8" concrete with fibre and steel reinforcement poured on top. The outside wall is the original pond wall with a cut out for the door. Under the wall is a 15" deep by 24" ring of the same concrete mix with reinforcement as in the base. In the centre is buried a 5 foot deep by 2.5 foot upturned water barrel filled with the same reinforced concrete mix and a plastic water pipe for electrics just under the surface. 

So in all a very substantial/heavy base. It was what I did at the time knowing I suffered from no vibrational problems. 

It is your choice what you do but be aware it could be hell to fix if you make an error and costly. Using reinforced concrete properly mixed and of the correct proportions in the mix, you can expect no cracks so I would forget that worry.

Derek

 

[tooth_dr]

5 minutes ago, Physopto said:

I know the usual thing/thinking is to isolate the mount from the observatory base, but is it really necessary ?

At star parties I image with the mount on grass or on hard standing. People walk all around at various times and I have never had a problem with any vibrations affecting images.  At home I have built a solid concrete base without any isolation between the mount pier base and the observatory base. It is an unfinished project as we are trying to sell up and move house. I have tried imaging on the tripod base on top of the concrete and not seen any problems in the images. 

It may be different if you are using a long focal length scope. We do have a very solid sub base around our house so any lorry passing can be felt and heard. But no visual problems seem to occur. 

I am including a photo of my observatory base. It was a Koi pond and is about 5 feet deep. It was filled with chinmey breast/rubble from our house renovations. Then a base of 8" concrete with fibre and steel reinforcement poured on top. The outside wall is the original pond wall with a cut out for the door. Under the wall is a 15" deep by 24" ring of the same concrete mix with reinforcement as in the base. In the centre is buried a 5 foot deep by 2.5 foot upturned water barrel filled with the same reinforced concrete mix and a plastic water pipe for electrics just under the surface. 

So in all a very substantial/heavy base. It was what I did at the time knowing I suffered from no vibrational problems. 

It is your choice what you do but be aware it could be hell to fix if you make an error and costly. Using reinforced concrete properly mixed and of the correct proportions in the mix, you can expect no cracks so I would forget that worry.

Derek

 

Thanks Derek. That looks interesting.  The ground I’m using was compacted by machinery 3-4 years ago, and is naturally very solid, and we have a lot of blue granite.  I just order concrete from a local supplier to my spec so there aren’t any issues there.

 

[Physopto]

If you are not mixing the concrete make sure they actually put in enough fibres. Problem with the fibres and premix is they don't like doing it. The fibre mix clings to there machinery causing a headache to  clean out. So some "suppliers" will either refuse, or if they do it, will basically put in short of the minimum, just so it looks like there are fibres in the mix. It is a job for self mix and pour really. The base I did took me two days to mix and pour. I also forgot to mention I used a waterproofing additive as well, called Sika 1. It is not cheap at around £50 for the 4/5 litres but very effective. If you use it be careful as I believe now they do a cheaper one but not as good. 

Derek

[WanderingEye]

39 minutes ago, tooth_dr said:

It’s a base for a dome, it needs to not crack.

Sorry, I did not mean like that, I meant, why are you concerned, as it won’t crack.... be positive... 😀👍

[Benjam]

I poured my dome base in one hit during last summer’s heatwave. Six inches of concrete + pier hole which was 800 wide and 600 deep. I’ve had no issues with cracking, however I did water the concrete 3 times a day to stop it from drying out to quickly. 

Edited January 6, 2020 by Benjam

[JamesF]

If what's under the concrete is stable then I see no reason for it to crack without the need for fibres or any other additive.  If it isn't stable then the concrete will probably crack (or at least shift) eventually regardless of what you put in it.

Here the soil is quite shallow and I can get down onto rock very quickly.  Even my 7.5m x 2.5m brewery barely has 150mm of concrete in the base and that isn't showing any sign of failure.  If you have deep topsoil though, I'd probably think about putting a load of hardcore into the hole first, tamping it down and then just lay concrete for the top 100mm to 150mm.

If you don't intend to be in the observatory bumping into things whilst imaging then I'd not worry about a large block for the pier.  The entire base will effectively become your large block.

James

[tooth_dr]

3 minutes ago, Benjam said:

I poured my dome base in one hit during last summer’s heatwave. 6 inches of concrete + the pier hole which was 800 wide and 600 deep. I’ve had no issues with cracking, however I did water the concrete 3 times a day to stop it drying out to quickly. 

This is pretty much how I pictured mine. Thanks for sharing 

[Physopto]

Just an afterthought. I think there is a possibility of a vibrational movement in an image if a door to the observatory is shut hard or slammed as in by wind. This can be transferred to the scope and camera. But in general just be careful. You can always fill the pier if metal with sand and lead shot, if you do notice a problem.

Derek

[tooth_dr]

1 minute ago, JamesF said:

If what's under the concrete is stable then I see no reason for it to crack without the need for fibres or any other additive.  If it isn't stable then the concrete will probably crack (or at least shift) eventually regardless of what you put in it.

Here the soil is quite shallow and I can get down onto rock very quickly.  Even my 7.5m x 2.5m brewery barely has 150mm of concrete in the base and that isn't showing any sign of failure.  If you have deep topsoil though, I'd probably think about putting a load of hardcore into the hole first, tamping it down and then just lay concrete for the top 100mm to 150mm.

If you don't intend to be in the observatory bumping into things whilst imaging then I'd not worry about a large block for the pier.  The entire base will effectively become your large block.

James

Thanks James. There is a few inches of topsoil, then into a solid base.

 

The reason for this base is that I’m looking at a Mesu with a bent pier, and although I’ll still have to work out ducting accurately it means I’m not concerned about where the  pier is going to bolted down and also can revert to a straight centre pier in the future.

[tooth_dr]

1 minute ago, Physopto said:

Just an afterthought. I think there is a possibility of a vibrational movement in an image if a door to the observatory is shut hard or slammed as in by wind. This can be transferred to the scope and camera. But in general just be careful. You can always fill the pier if metal with sand and lead shot, if you do notice a problem.

Derek

What if the dome structure itself vibrates a bit in the wind. Could that be transferred to the scope too? That would be a big concern 

[Physopto]

I would suppose it would depend upon the dome manufacturers quality. If the wind is that bad then you will not be imaging anyway!

Derek

[tooth_dr]

16 minutes ago, Physopto said:

I would suppose it would depend upon the dome manufacturers quality. If the wind is that bad then you will not be imaging anyway!

Derek

This is true! It’s a 2.2m pulsar dome I have. Where I’m at it can be windy but also clear, part of the reason I am moving to a dome - hoping it will shield the scope.

I’ve been in touch with Bern at MA regarding the dimensions of the pier.  This should help me a lot in deciding how to go about this. 

[Physopto]

I have never had a dome, it is a wish for when moved. But the most I manage to image in due to scope being wind blown is about 7/8 mph. After that I do see blurring in my images. But I would guess with a dome you may get up to 20 ish mph capability. It will depend upon seeing conditions. I.e. Moisture and particle content of the atmosphere. It would be interesting to hear from others who image using a dome. 

[Craney]

Just to add a point of consideration .... how good are domes if the (strongish ) wind is coming from the direction that the opening is pointed ??   

This may funnel the wind into the inner of the dome. ie this actually might accentuate the problem !!!!

Maybe the real advantage of the dome is so that you can 'aim' away from the prevailing wind,  something a Roll Off design cannot.... for the UK this makes you look at the NW to SE skies for targets on nights like these.   Not having owned a dome or a RO design this is all Sunday afternoon speculation on my part.

 

[tooth_dr]

4 minutes ago, Craney said:

Just to add a point of consideration .... how good are domes if the (strongish ) wind is coming from the direction that the opening is pointed ??   

This may funnel the wind into the inner of the dome. ie this actually might accentuate the problem !!!!

Maybe the real advantage of the dome is so that you can 'aim' away from the prevailing wind,  something a Roll Off design cannot.... for the UK this makes you look at the NW to SE skies for targets on nights like these.   Not having owned a dome or a RO design this is all Sunday afternoon speculation on my part.

 

Hi Sean. That was my plan. To choose targets so that the opening is on the opposite side of the wind. I usually didn’t lose many subs with my ED80 rig, but the reflector is much more susceptible to wind. 

[Laurin Dave]

Hi Adam

My 2.7m Pulsar dome is on a 3m square slab 20cm thick and with a 1m x 1m x1m block under the pier, all poured in one go.  I've never noticed any effect on the guiding of me being in there and moving about, nor since I moved to a Mesu (from an AZEQ6) of wind ,opening shutting or slamming doors affecting guiding.  The only thing in there that does seem to affect it is forgetting to switch the dehumidifier off!  Also are you sure you want the offset pier,  which looks like to me as if it will take a lot of space,  as automated meridian flips with SGPro (or alternatives) take very little time, a few minutes or so...   seems even less when I'm asleep!

Dave 

 

[tooth_dr]

2 hours ago, Laurin Dave said:

Hi Adam

My 2.7m Pulsar dome is on a 3m square slab 20cm thick and with a 1m x 1m x1m block under the pier, all poured in one go.  I've never noticed any effect on the guiding of me being in there and moving about, nor since I moved to a Mesu (from an AZEQ6) of wind ,opening shutting or slamming doors affecting guiding.  The only thing in there that does seem to affect it is forgetting to switch the dehumidifier off!  Also are you sure you want the offset pier,  which looks like to me as if it will take a lot of space,  as automated meridian flips with SGPro (or alternatives) take very little time, a few minutes or so...   seems even less when I'm asleep!

Dave 

 

Hi Dave

Thanks for your reply.  I have just started using SGPro - literally set it up not fully used it in anger.  I have no issues with flips. Currently what I do is plate solve after the meridian crossing and it repositions itself.  But it all happens 100% with no cable issues so no reason to go a step further. I only thought that using a bent pier might be worth the initial hassle of planning and building.  But I don’t know his much offsetting it needs as I don’t know the dimensions. A guy that I know has offered to make my pier, I just need to send him the specs.

I guess the bottom line is - do I need or want a bent pier.  If not then it would certainly make my life easier, and cheaper. The only issue is ‘FOMO’

 

[Peter Drew]

I would have reservations about an "elbow" pier, even when fairly massive, ones that I have seen "ring" at the slightest touch.  The very large stock one made for the Meade 16" SCT was particularly poor in this respect, too much weight hanging off axis.     🙂

[tooth_dr]

5 minutes ago, Peter Drew said:

I would have reservations about an "elbow" pier, even when fairly massive, ones that I have seen "ring" at the slightest touch.  The very large stock one made for the Meade 16" SCT was particularly poor in this respect, too much weight hanging off axis.     🙂

Thanks Peter. This mount is an investment for the future, and I can hope to increase the payload by adding a bigger scope in a few years. Sounds like something to consider