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Basic question:Do I need to have my pier/plate set up so that the mount is facing true north?

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#1 SimonIRE

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Posted 27 June 2019 - 05:55 AM

Hi All, 

Thanks for reading. I have a pretty basic mount question; this is discussed all over the place on the internet but its hard to get a crystal clear answer and I want to make sure I get this absolutely right. I am having my home observatory pier installed in the next couple of weeks and I will be putting an iOptron CEM 120 on it. Very simply, do I need to have the pier and plate set up so that the mount is facing true north? If so, what is the most accurate way to do this. I apologise for what I know must seem like an obviously basic question but sometimes things are so basic, they aren't explicitly stated in other threads or elsewhere on the net. 

Thanks. 

Simon



#2 Ishtim

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Posted 27 June 2019 - 06:22 AM

Yes. The mount needs to be facing north within the azimuth adjustment tolerance values, as when you complete your polar alignment the mount will be facing true north. 

 

Some pier designs, as well as some mount base plates also incorporate azimuth "slots" so that the pier/plate can be rotated in azimuth allowing for additional means of adjustment.


Edited by Ishtim, 27 June 2019 - 06:33 AM.

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#3 Umasscrew39

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Posted 27 June 2019 - 07:27 AM

 "If so, what is the most accurate way to do this." 

There are lots of designs and pics on the internet.  My pier is buried 4 ft below the ground and cemented like the image below using rebar.  Make sure the pier is level.  There are many plate designs based on your mount.  Once it is facing north (most pier plates have it marked as "N"), ensure it is cemented in well and also leveled.  Most of the plates like what I used have J-bolts to do this properly.

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#4 BobT

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Posted 27 June 2019 - 07:37 AM

Simon,

 

As far as method goes, I used the solar noon method.  At solar noon, the sun is due south so a vertical pole casts a north/south shadow.  You can get the local time for solar noon here:

 

http://www.srrb.noaa...se/sunrise.html

 

I drove stakes in the ground on the north/south line prior to pouring the pier and built a small plywood template, drilled with the bolt pattern of my pier plate, and drove two nail on the north/south line of it to use for aiming.   I mounted the J-bolts to the plywood plate and, while sighting along the north/south line, pushed the J-bolts into the wet concrete.  Both my original G11 and my current CEM120-EC were nearly dead center on the azimuth adjustment.

 

It doesn't need to be super accurate but it does need to be close.

 

BobT


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#5 Ishtim

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Posted 27 June 2019 - 07:41 AM

If so, what is the most accurate way to do this.

The method I used:

  • Suspend a string with a weight (think plumb bob) over the spot that would be my pier location.  I used a tall "A" frame ladder to attach the string.
  • Determine the Sun's transit time for a particular day(s).
  • At the time of transit the string will cast a shadow pointing due north, mark this line paint, another string, etc. along the ground.
  • Align the front edge of your plate, mount, rebar geometry, etc NORMAL to this.

 

^^^ Yes, what BobT said... :)


Edited by Ishtim, 27 June 2019 - 07:42 AM.

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#6 SimonIRE

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Posted 27 June 2019 - 08:30 AM

Thanks for this advice All...

 

Umasscrew39: what I was asking was what method was considered best for determining true north. I've seen many ways - google, phone apps, gps compass and by calculation but I was keen to understand what others did and if there were any pitfalls (seeing as its a permanent pier). Thanks for the pics - that looks like it is designed to mount the Hubble!



#7 denny-o

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Posted 29 June 2019 - 07:42 AM

Well, my double check method was the shadow line at Sun transit-noon for my longitude and driving a stake in the ground on the shadow line. That was followed by being out there after dark with a carpenters square clamped to a bit of wood so it would stand up on  top of the pier - and drawing a line when it was aligned to Polaris. Next day in the light I compared the two and it appeared to be less than 1 degree difference. Close enough for government work I muttered and went on from there.


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#8 macdonjh

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Posted 29 June 2019 - 08:11 AM

I simply used a compass...  As Ishtim said, you only have to be accurate to within a couple of degrees.  You can use your mount's azimuth adjustment to polar align then.

 

Wish I'd thought about the solar noon method.  I had plans to make a jig and align with Polaris, like denny-o did.  But the compass was quick, easy, and accurate enough.



#9 OldManSky

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Posted 29 June 2019 - 02:12 PM

I did something similar to Dennyo.

The night before setting the concrete, I went out with two pieces of rebar. Pounded one into the ground about three feet behind the pier. Handed my daughter the other one, and had her go north of the pier. I sighted along the back one to Polaris, had my daughter move the other one until it lined up, then pounded it in the ground.

The next morning I tied a string between the two, across the top of the pier, and used that to line things up.

It wound up just under a degree off, well within tha az adjustment of the mount.



#10 Chucke

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Posted 29 June 2019 - 02:39 PM

Here is a link to a useful site for determining solar noon.

 

https://www.esrl.noa...d/grad/solcalc/




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