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cloudnine

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About cloudnine

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    Ottery St. Mary, Devon
  1. Sad news indeed. I have 1962-2016. What will my family buy me for Christmas this year?
  2. I'm a bit late to the party, but my pulsar dome was installed just over one month ago on a DIY concrete base. I love it! It has transformed my observing as (a) I am naturally lazy, (b) the equipment is set-up and ready to go, and (c) being shielded from the breeze make for more comfortable and hence longer observing sessions! Based on the evidence of reading the temperature inside the dome at the start and end of observing sessions, where the temperature drop over the course of a couple of hours is only about 1 degree C, I reckon that the dome is thin enough and the concrete shielded enough from direct sunlight that reaching thermal equilibrium isn't a major issue... It could be just the time of year... maybe in the depths of winter it might be more noticeable. As others have mentioned, sealing the concrete base is important, primarily to avoid the surface dusting up resulting in concrete dust settling on your nice polished surfaces ... not a good combination when it comes to cleaning optics. Attached is an image showing my non-permanent cable routing ... The channel is back-filled with pea-shingle to avoid mice entering the building! The channel is also handy in case of flooding. Notice the differing levels of concrete... an example of my poor handiwork! The obsy copes well with this. Martin
  3. Final post, containing a pictorial evolution of the whole thing from start to finish... buckle up. Am happy to take any questions now or sometime in the far distant future.
  4. You won't regret using a mixer. I poured an approximately 60 x 60 x 60 cm cube a couple of weeks ago and the mixing took about an hour. I'm no expert, but curing can start as little as one hour after mixing depending upon conditions. I used mastercrete... the tech spec states that it reaches 25% final strength after one day, and something like 75% (from memory) after seven days. I would be wary about drilling and inserting expansion-style bolts too soon as I'd imagine the localised forces may be too much if it hasn't reached a reasonable strength. I'm sure others might have a better opinion.
  5. I haven't revealed what is going in there The scope that is initially going in the observatory is the type of scope you wouldn't put in an observatory. A 12" dobs-onion. Makes the central block pointless, as well as not being able to get near any horizon. However, sheltered observing is much better than exposed observing. The longer term plan is to install a 12" SCT-style scope on a monster mount and pier, but that will take at least a couple of years of savings to achieve, unless I win the lottery.
  6. What's a few inches between friends. I went for octagonal because I thought it was better to mix less concrete (i.e. saving having to mix corners). It made my shuttering a lot more challenging. Good luck!
  7. Looking good. I wasted a good year thinking about making a dome, and then making a roll-off-roof obsy, before deciding to buy one instead (primarily due to time-constraints and too much procrastination). Hats off to you for getting the project off the ground. Look forward to seeing how this progresses! Martin
  8. Pad minus 7 days 16th July 2016 : pouring the foundations and central block This is the first big day of concrete mixing. Everything I've read and watched becomes academic. Mixing consistently and quickly is key. Given that all the ballast is in 25 kg bags, just the thought of continuously shoveling from ground level made my back ache. So decided to make a platform out of an old composite worktop upon which to lift the 25 kg bags, and then slit open the bottom of the bag to produce a pile of ballast a couple of foot off the floor. This turned out to be my saving grace. Shoveling ballast into buckets below the platform hugely reduced the amount of effort, meaning that a) my back did not seize up (as it is wont to do), and I was able to mix a reasonable throughput sustained over a few hours. The other useful 'invention' was a couple of old shelves covered with chicken wire to make them non-slip. The location of the future observatory meant that access was difficult (but thankfully downhill) with two main approaches. Now the mixing begins. Firstly I filled the strip foundations by carefully shoveling in the concrete in order to avoid collapsing the vertical soil sides of the trench. Over two hours later, once the foundation were full, I realised I hadn't taken in enough electrolyte during the hot sunny morning. The result: dehydration, headache, and racing heart. Decided to take a break until early evening when the cube will be attempted. Three hours of rest and my heartbeat was still high … it's at times like this one wonders whether years sitting behind a desk have masked an underlying health problem! Thankfully after six hours and a great deal of hydration, was feeling fine again. Now the cube. It is 63 x 63 x 63 cm. A size based on informal surveys of the many great builds on SGL. In total it will weigh about 0.5 tonnes, which sounds a lot until you think about putting a pier, a big scope with large counterweights on top, which could easily come in at over 130 kg. 130 kg compared to 500 kg seems a reasonable ratio, especially as the scope set-up should be balanced and thus not imparting any tipping moment on the cube. Decided not to put in any steel reinforcement, nor steel bars partially submerged in the sub-soil. The latter because of risk of rust expansion. The former because, well, I reckoned that the block should be fine. No pier bolts were embedded either as a) I don't have a pier yet and when I do get one I'll drill into the cube with special concrete (epoxy?) bolts. The pour for the cube was superb. One hour of continuous concrete production, finished by leveling and floating resulted in a lovely shiny (slightly wet) cube surface. It is a bit of a disappointment examining the concrete the next day as it was no longer shiny. Was briefly tempted to varnish the whole thing to bring back the lustre, but that's plain silly! One other note. The concrete mix I was aiming for was 5:1 ballast:cement. Having counted the empty bags at the end, it appears I drifted towards 6:1 which is still a proper mix but allegedly less water-tight.
  9. Pad minus 4 days 19th-20th July 2016 – Gridwork! The concrete pad will be supported around the edge by the strip foundations. Why strip foundations? Firstly the garden is on a slope, and secondly a lot of landscaping had been performed by previous owners meaning that the top layer of soil wasn't natural. Having strip foundations is a lot more effort, a lot more soil to shift, but is necessary to provide a stable base given the weight of the concrete pad. Wouldn't want the eventual observatory to relign itself inexorably downslope! Despite the surface layers feeling quite firm to jump on, I decided to put in a steel grid for reinforcement, so that the pad will be much stronger, self-supporting and should float on the strip foundations. It may seem like overkill, but I only want to do this once. Armed with an angle grinder , I set to work on the original grid until it fitted within the frame. The presence of the two wooden strips that connect the outer frame to the central block meant that multiple 'sub'-grids were necessary to form a complete layer. Small concrete pacman-ghost spacers were used to elevate the grid above ground level, but I did not trust them not to move when pouring a 60kg barrow-full of concrete. Hence the cable ties! Having watched many youtube video of mesh bowing underneath the weight of poured concrete, I was expecting the worst. However the ghost spacers did their job admirably, holding the grid safely in the middle of the concrete. I've heard a number of times that steel rods should be embedded inside concrete with a minimum of 50mm distance from any side, top or bottom surface. The reason... when steel rusts (i.e. is in contact with the surrounding soil), it will expand two-fold thus threatening the integrity of the concrete... It sounds plausible! This is also why I didn't use any bars to anchor the central block in the sub-soil. I reckon that even if the concrete starts to break up due to rust, one can patch it up to fix it. What do I know? Reading is no substitute for experience More past to follow.
  10. Thanks The two wood strips provide four benefits. Firstly, they held the central block in the right position inside the octagonal frame. Secondly, when pouring the slab they proved useful for separating (at least on the surface) the four sections into easier-to-manage float-finishing jobs... There is no way I could have floated the entire pad in one go given how long it took me to mix the concrete. Thirdly, water-ingress is a common (apparently) issue with observatories, so chasing out the wood will provide drainage channels for the inevitable 'flood'. And finally, yes, they provide a raceway for wiring. The channels will be filled with pea-gravel to stop rodents making a home and to inhibit wind-driven water entering by the channel. The outside of the pad will also be pea-gravelled. I should point out I'm no expert, I think about things too much , and the wood strips may turn out to be counterproductive...
  11. Pad minus 1 day (yup, I'm going back into the past with this thread ) 22nd July 2016 – final preparations Like most things you haven't tried before, the time it takes to prepare is always a surprise. Today I thought that I could finish off some formwork in the morning and pour the pad in the afternoon. As mentioned in tomorrows entry, the pour was delayed due to a 10% chance of heavy-sustained downpours mid-late afternoon (slow moving air masses with unstable upper layers). Of course 9:1 chance of no rain, which is exactly what happened, but the risk wasn't worth taking. Anyhow, proceeded to bridge the gap between the formwork of the central cube and the bare ground. As can be inferred from the image above, the main frame is made of 4x2 which does not completely reach the surface most of the way round the cube. Hence cut up some 9mm exterior ply to form a rigid structure, and jigsawed each piece to connect with the ground. Attached polythene to each piece and screwed to the cube formwork. Before & after: The surface of the central cube was also protected with a chipboard square and polythene. Expected this 'carpentry' to take an hour max, in reality it took over twice as long to get right! Potential rain stopped play!
  12. Thanks :-) Concreting is hard work, and something I would have much preferred paying someone else to do, but the nature of the job put people off, as did the requirement to transport the 110+ ballast and cement 25kg bags from the front of the house, through the garage, and down a steep run of steps. The latter took me over four hours alone... Sensibly I had it all delivered one week prior to the first pour (foundations and cube) to give me a week to recover!
  13. I should be ashamed for not crediting my lovely wife for helping with the tamping down!
  14. Today is the 24th July 2016. Yesterday the long saga of the observatory base ended. I ache, but am not immobile, I slept long and my head is slightly fragile but not enough to take ibuprofen. I start this thread to document how I got to this point in the hope that others might take heed of the difficulties, uncertainties and the rewards of building your own concrete base. 23rd July 2016 - the final pour Today will be the culmination of months of planning. The weather is forecast to be cloudy with sunny intervals without the risk of heavy sustained downpours that delayed the concrete pour from yesterday. The foundation strip was poured one week ago as was the central block. The central block is protected with a chipboard square overlying a polythene square stapled to the surrounding formwork. The exterior ply skirting around the outside edge of the central block (with a polythene skirt) is in place, to maintain the separation between the block and the pad. The grid is raised on pacman ghosts. The duck tape is restuck around the edges to limit seepage through joins. All sides are level. How long will it take to pour a pad? Previous experience demonstrated my personal capability of producing two batches every 10 minutes, which included refilling buckets of ballast and cement (ready for next batch), mixing and pouring. This turns out to be about 500 kg concrete per hour. Measurement of the pad showed it to be about 0.6 cubic metres which is about 1.5 tonnes, hence should be done in 3 hours if non-stop. As it turned out, it took just over 5 hours as the tamping and floating took a lot longer than I envisaged. The mixing procedure (assuming one batch has just mixed): pour batch from mixer into barrow move barrow to one side add three ½ buckets of ballast 2 litres water then don dust mask ½ bucket cement in mixer refill ½ cement bucket remove dust mask add final two ½ buckets of ballast and 0.5 litres of water to taste refill the five ½ buckets with ballast take barrow down to site and pour on return go back to step one Five hours later it was difficult to stem the exhaustion that starts to arise when the end is in sight. However, the result was pleasing enough to make our mark (30 minutes after the final smoothing). 6 hours later, wetted the surface and covered. Back to bed, Martin
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