Pier construction

[Anthonyexmouth]

Ok, so i've got myself a pier head for my future pier. i'd like to keep connections to a minimum so with that i'd like to set 3 threaded bars in the concrete to attach to the head. whats the easiest way to align this when setting to ensure its pointing north. ideally as accurate as possible to the az bolts are as close to centre as possible. is this easily doable or do i need an intermediate plate for adjustment after the bolt have been set and concrete cured?

[Davey-T]

I set mine pointing North using a compass, there should then be enough adjustment in the mount to PA.

Make sure there's no steel about to give a false reading.

Dave

[Anthonyexmouth]

and there is the problem, i'll be using a steel tube filled with concrete. im worried about setting them and finding the az bolts are either just out of range or right on the limits. 

[Davey-T]

Just tried to check if the phone compass is affected but it seems to have disappeared 

Dave

[Oddsocks]

When I drilled the concrete pad for my steel pier I used the solar transit method to find true North, the local magnetic variance here is around 8deg and a compass would have been too far off.

To use the solar transit method use a planetarium program with your observatory coordinates / observing location entered correctly, or an online solar calculator program such as this one:

http://www.jgiesen.de/astro/suncalc/index.htm

(Note: In the linked solar calculator program West of Greenwich is entered as a negative value, Exmouth is approx 3.41 West so it is entered in the calculator as -3.41)

Suspend a plumb line above the centre of the pier pad with a temporary tripod or other means of support, as in the picture below, and with an accurate timepiece mark the shadow of the plumb line as it falls across the pad at local noon solar transit given in the program then extend the line across the pad from your mark and the centre of the plumb bob above the pad, the line will be true North.

Things to be aware of, obviously, choose a calm day so the plumb line is not swinging about, In the planetarium program, or on-line calculator, verify what the noon local solar transit time is quoted in, i.e.  GMT, UT or BST and adjust or allow for the difference between your timepiece and the quoted time otherwise you will be an hour out with the markings on pad.

Have a dry run-through in a different location to your observatory, away from any metal or local geological features that can disturb the compass bearing and check that the solar transit bearing and the compass bearing coincide, allowing for local magnetic declination which you can find for your location at various mapping services, then carry out the same procedure at your observatory.

Using the linked program for today, 20th August 2018 and Exmouth coordinates of 50.62N and -3.41W gives a local noon transit time of 13:17 BST. The shadow cast on the ground by a vertical plumb line would be pointing exactly North at that exact time, 13.17 BST.

 

 

[Davey-T]

Updated the phone and extras has reappeared so don't know what was going on there.

It does true north or magnetic north.

I'm thinking a nice straight piece of timber could be used to get the phone away from the steel and use as a straight edge to align the pier.

Dave

[Anthonyexmouth]

thanks, i'll have a dry run with the solar method.