Saturday, January 19, 2013

DIY Concrete Pier for a medium-sized SCT

What do you need for imaging with a Celestron 9 1/4" SCT operating at f/10 (FL 2350mm)? A mount with very little flex. In my case this works out to a Celestron CGEM mount. For a couple of years* now I've been working toward building a small observatory. I want to put the mount on a pier, and miracle of miracles I have actually gotten this part of the observatory done! Not without a lot of dithering about what sort of pier to install.

If you go to Cloudy Nights you'll see many people giving their ideas of what constitutes an adequate pier for imaging at a modestly long focal length. The largest telescope I anticipated using was a 9" SCT, so I felt much of the advice, though sound, was meant for larger loads. On the other hand, I decided that some suggestions were a little on the light side.

Some ideas I considered and rejected were for a pier made from 4x4" wood beams (possible polar realignment needed as the humidity changes), steel pipe piers (expensive), 10 or 12" concrete piers (overkill), 4-6" concrete piers (too much flex).

That left me with an 8" concrete pier as the ideal compromise. Cheap in cost, and easy to put together--even for me.

  1. Digging things, such as a hole auger and shovel
  2. A little small gravel to line the hole bottom
  3. Concrete forms
  4. Duct tape
  5. 2x4" lumber for holding the form level
  6. Bagged Concrete (Quikrete, for example) 
  7. Rebar
  8. Pier to mount adapter
  9. "J" bolts with hardware to match adapter
  10. Mixing tub such an old wheelbarrow
  11. Garden hoe for mixing concrete
  12. Bucket to measure water
  13. Water
Now, some lessons learned as applies to each of the above:

The frost line here is 42" down, so the hole must be at least that deep plus a few inches. Fortunately I had an old post hole manual auger that made digging a snap. It looks like this one, which can still be found for sale various places:

Manual Auger
Mine is actually an antique hand-me-down with a decayed wooden hand bar and several layers of rust on the metal parts, but it sliced into my mix of clay and gravel almost like the proverbial warm knife in butter. The only trouble I had was at 42" depth, where it ran into a larger rock (which now sits on a counter in front of me). The hole was deepened to 45" by using a long-handled shovel.

You just need enough gravel to put a few inches in the bottom of the hole for drainage. Pack it down with a 2x4".

Forms are usually available in 4 foot lengths (mine came from Home Depot). Because the total length of my pier was longer than this, I joined two forms by making a 1 foot long external sleeve and putting it all together with duct tape. The length is critical because the adapter will ride atop the form, which will be filled with cement to just below the top of the form. You need to plan things out so that the distance between the adapter bottom and the top of the cement is about 1.5" or so.

The form should fit nicely into the hole with a little wiggle room around it; level it and shore it up with 2x4"s:

Concrete form in place
Then I filled in the gap around the tube with the extracted dirt.  You can use the rebar to get that dirt packed in, and run some water into it to flush it down. (Don't add water unless you're ready to pour concrete; a wet form can lose its integrity in 24 hours).

You will need to know how much concrete to have on hand, calculated from the volume of the pier and the bag weight you're willing to handle. I was able to handle 60 pound bags, but smaller bags will spare you a possible blown back. I would suggest rounding up the amount you need by one bag, just in case. 

This is the fun part. You want everything ready before beginning, so cut the rebar to the right length (an inch or two shorter than the pier. I used one piece of rebar, centered in the pier, but some will say use three. Whatever your choice, make sure the rebar won't interfere with the J bolts.

You have, of course, already determined North, so that you know the way to orient the adapter!

Put together the pier adapter and J bolts so that the assembly is ready to put into the concrete. I used three 1/2" diameter 8" long J bolts (Home Depot) and the Starizona CGEM adapter. In my case the Adapter's diameter was smaller than that of the form tube, so I used shims to hold it in position and maintain reasonable level:

Pier Adapter in place with shims (after has been cement poured)
Now get your bags of quick-mix concrete, mixing tub, mixing tool, and water supply ready at hand. 

Note: Even if you're not using Quick-set concrete  you shouldn't let much time pass between one batch and the next. You won't take any breaks until you've embedded the adapter in the pier top. 

Dump a bag of concrete into the tub, add the manufacturer's suggested amount of water, and mix thoroughly. The common mistake (I've read) is to make the mix too wet, so try not to be tempted into adding more water. In my case I tried to avoid this error so diligently that I made the mix a little dry, but it seems to have worked out. Shovel the concrete into the form, minding the level.

After adding a second bag of concrete push the rebar into the poured concrete so that it's a the right height. You may need to hammer it to the right depth.  

Continue adding more batches of concrete, making sure your rebar remains centered in the form. You may also want to use a long rod from time to time to remove any trapped air pockets that form. When I made my pier I noticed that these pockets tended to form against the inside of the form.

The last batch of concrete should be large enough to complete the pier. Depending on the size of the shim(s), you will fill the form to about an 1.5" from the top. If needed mix in more concrete mix and water. You may want to make the last batch of concrete a little wet so that the J bolts will embed more completely. Shovel this into the form to the level you want and then push the adapter assembly into it. Make sure you have the adapter oriented to north correctly! Level the adapter and you're done!

Clean up your tools and the work site, then take a break. After 24 hours you can rip the form away. At this point inspect the J bolts to make sure they're solid--no wiggling allowed. If they're not, you have a problem. Post a comment if you do and I'll reply how I fixed this when it happened to me. 

Avoid the temptation to put a load on the adapter for at least several days (two weeks would be even better). If the weather is hot and sunny, moisten  the outside of the concrete from time to time and wrap it in plastic.

After a month you can apply a concrete sealer and paint the pier with an outdoor house paint. I went with white, but if fluorescent green is your thing, go for it.


*Why a couple of years on the observatory? The winter of 2011-2012 was so mild here that I found I hardly needed an observatory. And I was so busy during the summer and fall of 2012 that my talent of procrastination bloomed fully. Maybe next summer I'll get to it?


UPDATE, SPRING 2016: Still no observatory. There still seems little need for one; I'm doing more imaging at dark sites as the light pollution, neighbors' absurd "security" lights, and tree blockage all continue to increase. The wood planks on the platform have warped, but the pier continues to serve me well. Factoid: this blog entry is far and away the most viewed of all my posts (which isn't saying much).

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