Pier

Design

The design of the pier for the observatory was a particularly challenging effort.  When the initial facility design was that of a ROR (Roll-Off-Roof) the pier design was fairly simply.  Your basic flanged 3′ pipe with a nice flat surface on top was all that was required.  The concrete base was around 6000lbs and there was a 2′ x 2′ round concrete pier that came to the floor level of the building.  This was more than sufficient for anything that was originally planned.  When the design changed to a dome, it becomes necessary to lift the instrument up to approximately level with the slot opening of the shutter.  In this case, when all various mount configurations where taken into consideration, this resulted in a steel pier that would be at a minimum of 8′ above the current raised 2′ concrete pier so the the top of the pier was 10′ above ground level.  When you add the requirement that the load on top of that pier could be upwards of 500lbs you now have some issues that must be considered.  First is heat.  A dome design by it’s very nature does not open up to the elements like a roll off roof.  The pier could be made of additional concrete but this poses a significant heat issue.  The building is neither cooled nor heated.  A concrete pier of the size required would build up a significant amount of heat during the day, only to spend most of the night releasing it into the building.  This would most likely render the optics of the observatory useless.  A steel pier costs more but looses heat very rapidly.  An open design, such as what was finally used allows for a lot of airflow around the structure to assist in cooling.  Steel costs more than concrete but thermally it is a far better solution.  The next issue is rigidity.  Here again, a concrete pier would provide a very rigid solution but the heat issue trumps that solution.  To resolve this there were a number of design considerations that were considered.

pier design 1
One of the early contenders.

The slotted design on the base allowed for rotation on the placement to properly align the pier to north.  At the time it was assumed that the top plate would be drilled and tapped to accommodate a mount.

second pier design
This is a second iteration.

In the second iteration there are more struts and internal support plates.  One particular problem with a design like this is it’s symmetry.  Symmetric design lends itself to resonance.  This can actually enhance vibrations instead of dampening them.

third design
This is the third iteration.

On this third iteration a number of improvements were made.  A particular problem of the previous two designs was that the base was simply too narrow to support an 8′ beam with 500lbs on the other end.  The new design widens the base and instead of attaching to the top of the concrete pier that is at floor level, this design would attach to the concrete base at ground level.  This more than doubled the effective size of the base thus significantly increasing the stability.

final pier design
This is the final design.

Here you see the final design of the pier.  There are a number of interesting features that are built into this design.  As can be seen, the base attaches to the floor level concrete platform.  This is bolted down in 12 locations.  What can’t be seen in this render is that there are 8 bolt holes on the first metal plate up from the bottom.  This plate actually sits a few inches above the top of the raised round concrete pier.  There are 8 bolts on this plate and there are gussets that tie this plate to each of the rising support beams.  When this second plate is bolted down it puts stress on each of the rising beams thus placing them under tension.  It is the tension, coupled with the tapered sides that provides a particularly strong and rigid frame for the pier.  While this picture may not show it the interstitial supports going up the frame are not equal distances apart.  This is to defeat any symmetric resonances that might be in the structure.  The top plate itself holds the structure but it is not where the mount is attached.  There is an adapter plate that was designed to bolt on to the top plate.  This adapter plate is made from 1/2″ plate steel and is drilled and tapped to allow it to be bolted to the top and then have one of two types of mounts bolted to the plate.  This provides a very flat, rigid attachment of the mount to the pier.

Leveling is really not a consideration nor is it necessary.  There are many pier designs where the top plate is, for lack of a better name, a “rat cage”.  It is a system of what is usually 3 long bolts that are bolted to the top of the pier and then to another plate that the mount is attached.  This top plate is “floated” above the top of the pier with the intent that using double nuts, one above and one below, can be used to level the final top plate.  The reality is, leveling is not necessary to achieve correct polar alignment of a telescope.  It certainly aids this process and in portable “in the field” set ups leveling your tripod makes polar alignment much faster.  That being said, it is not a requirement.  Putting a rat cage on my pier would simply weaken the design and is unnecessary so it was not done.  A point to note.  After all was assembled and bolted down the top of the pier is actually only a few degrees if any off true level.  Using a standard concentric ring bubble level, the central bubble was only slightly off true center.  Not bad for 10′ and 1000lbs of steel.

Construction

With the design complete next came building the pier.  As I mentioned my son Matt has his own metal fabrication shop.  This design could have only been done because of this.  I don’t want to even consider what the cost of this pier would have been without his help.  But I did have his help and that is what made all of this possible.

cnc plasma cutting table
Here is the 10′ CNC Plasma cutting table.

It’s fun to have access to the right tools.  All of the plates used in the pier from the bottom to the top and adapter plate were cut from a single 4′ x 8′ x 1/2″ sheet of plate steel.  The design of the pier was done completely using Solidworks.  Having a 3D CAD program to design the pier then allowed us to use CNC equipment to cut out all of the basic pieces required to build the pier.  Above you can see the 10′ CNC plasma cutting table that was used to cut the steel.

drop piece
Here is the “drop” or scrap leftover from the cut.

You can see the size of the base ring as well as make out all of the other intermediate pieces used to support the pier.

plates
The intermediate plates prepped and ready to weld.
uprights
Here are the uprights and gussets.
initial setup
This is the initial setup.
welding
The welding begins.
more welding
More welding.
finished pier
Here is the finished pier.

Here is the finished pier ready to head to the powdercoat shop.  The top plate needed to be drilled with a very high degree of precision.  For this reason the top plate was actually sent out to a laserjet cutting facility where they cut all of the holes using the output from Solidworks.  Again, having access to 3D CAD equipment made very quick work out of that would have been a very time consuming and error prone task.  The final taping of the holes necessary to hold the mounts was done by hand.  The top plate was designed to hold either the wedge mount for my LX200 or the flat plate adapter used to hold an Astro-Physics AP1600 GTO mount.  These were the two mounts that were to be used for the telescope.

top plate
The finished machined top plate adapter.
top of pier
Here is a top down view of the top of the pier.

You can see from the top view of the pier that the adapter plate bolts on to the top of the pier through the large counter sunk holes in the adapter plate to the holes in the pier top.  Large countersunk stainless steel bolts were used to attach the two plates.

broken bolt
Example of why you don’t use SS nuts on SS bolt.

This is where I learned you don’t use SS nuts on SS bolts. At least without using anti-cease grease or compound.  The similar material causes the threads to gall and heat up so much that they literally weld themselves together.  I was trying to get the nut off with a 2′ breaker bar when I snapped the bolt.  The bolt was replaced and all of the rest were attached correctly.

lx200 mounted
Pier finished with LX200 wedge attached.

This is a little bit better view of the pier with the adapter plate attached when my LX200 was wedge mounted.

ap1600 gto mount
Current setup with AP1600 GTO mount.

Current setup with my Astro-Physics AP1600 GTO mount.

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