Re: 3600GTO - El Capitan unveiled at AIC

Joe Zeglinski


You can fill the pipe with concrete at ANY time you like, perhaps in a
year or two - if you think it might help.

The only problem is welding the top, which then commits you to the pier as
it is right now, with no adjustments, making height mods impossible. If you
could get a heavy 1/2" steel "trash can" (or top hat, if you prefer) made,
that could be slipped over the top and bolted sideways (mid can) to the pier
on 3 sides, then you could use the side bolts to adjust and shift the can to
level the top of the pier quite easily. As a bonus, next year you could remove
the mount, and unbolt the trash can top, to fill the pipe, if you need to. I
would fill it with beach sand rather than concrete which could crack or strain
the pipe with observatory environmental changes, and see if it helps or
hinders. Then you can always vacuum the sand back out, if it doesn't work.
Besides, if it were me, I would use the pier pipe for an inside PVC cable run,
for a tidier looking exterior. The PVC pipe would enter the pipe through a
small hole (reinforced by welding a steel plumbing nut or pipe collar, if you
like) below deck, and come out at the top below the mount.

Actually, a hollow cylinder is stronger than a solid pipe, so I don't see
the purpose of the discussion. DOB's don't use solid pipe, do they - for good
reason (solidity and vibration issues) ?
(Ref: The Dobsonian Telescope - by David Kriege and Richard Berry)


----- Original Message -----
From: "jsmiller58" <>
To: <>
Sent: Sunday, October 28, 2007 6:58 PM
Subject: [ap-gto] Re: 3600GTO - El Capitan unveiled at AIC

Great points Niels!

As to filling the steel pipe - only the bottom of the pipe had some
concrete go into it from below. However, there isn't much concrete in
the pipe. I purposely chose to not have much concrete put in the pipe
due to (a) desire to have the option to shorten up the pipe as needed,
and (b) a long article on this (I think on Cloudy Nights) where it was
mathematically demonstrated that for a sufficiently strong pipe
(meaning wall strength) what is most important is the pier diameter,
and not whether the pipe is full of concrete... Having said all of
this, if it makes a difference, until late November when the actual
observatory gets built around the pier, it will be easy for me to fill
the pipe with more concrete, so I am open to the idea!!

I had exactly the same worry about the flatness of the plate, and
exchanged some mail had a phone call as well as with Howard at AP
about this. He wasn't very worried about the level nature of the
plate at all - indicating that this is a very overrated concern and
that he was able to get very good polar alignment when purposely
putting the plate way off level in the field... Nonetheless, I have
already discussed this with my contractor and he assures me that the
welder can do a very good job of getting the plate welded quite flat.

So, combining the assurance that being perfectly level is no big deal
with the assurance that we can in fact get quite level, all in all,
not too worried, yet... Now, if I spend the next 2 months trying to
get a good polar alignment for astro-photography... well... :-)


--- In, "nfoldager" <nf@...> wrote:


- The steel pipe extends another 42" above the surface of the
observatory floor. This portion is just the 12" steel tube, and is
the "pier" on which the mount and OTA will be placed.
Did you fill anything into the tube?

As to attaching the mount to the pier, I will be welding a
flat plate to the top of the steel tube.
Are you sure it is wise to weld it instead of bolt it? There is a
danger that the plate will not be flat afterwards.

Best regards,

Niels Foldager

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