Magnetic variations question

[mangrovedutch (Dutch)]

G'day all, I know that there is a difference between between Magnetic North (the way the compass points) and True North (where North is actually positioned). When I was in the Army some 20 years ago, it used to be 1 Mils a year difference, and we would add that to our bearings by looking at the print year of that map (say 2008), that makes 3 years difference = 3 Mils. I know that 2 Mils = 0.1 degree, and that it moves in an Easterly direction
In Astronomy we use degrees, minutes and seconds. Can someone tell me what the difference is between True North and Magnetic North? The reason I ask this question is that I want to start Astro Imaging (and I keep reading that it is vital to find the true SCP),and I use a compass to locate South to set up. Am I being to anal? Am I making this harder than I need to, and should just set up Magnetic south and do a 3 point triangulation? If I am, I would still like to know the variation, if someone could tell me, that would be muchly appreciated. Should I just use a GPS, and will that automatically calculate the variation?

NB. A bit of trivia :- 360 Degrees = 6400 Mils. It was (back then) a more accurate way of finding your way in the bush. If you were 1 Mil out over 1 Km, you would be 10 metres of the mark.

In need of a cuppacoffee, Dutch

[mithrandir (Andrew)]

The variation changes across the country. Go to this GA page, fill in your location and get the exact figures. There are links towards the bottom of the page to JPEGs for the whole country to get an overall view.

Using the place name search for North Mclean Qld gives this page.

Feeding those coords back into the page gets:
Australian Geomagnetic Reference Field Computation

Requested: Latitude -27o 45' 00", Longitude 152o 59' 00", Elevation 0 km, Date 2011/01/1
Calculated: Latitude -27.7500o, Longitude +152.9833o, Elevation 0.00 km, Epoch 2011.0000

Magnetic Field Components

D = 11.059 deg
dD = -0.015 deg/yr

So your local variation is about 11 deg E, reducing by 0.015 deg/year.

As for how accurate you need to be, it all depends on what you are trying to achieve.

An equatorial mount has to be reasonably aligned or it will drive you batty.

An alt/az mainly needs the base to be level. The orientation can be set by pointing the scope at a known object and adjusting the setting circles so they match the target. Scopes like Celestron's and Meade's GPS models can work it out by asking you to point at a given star, you tell the program when the star is centered, and it does all the calculations by itself.

Mounts with smarts like GoTo or Argo Navis requires the guiding software and the scope to agree on where they are pointing. The better the scope is aligned, the better they work.

If you want to do photography, you have to be more accurate in your orientation. That would be a discussion for another day.

[GrampianStars (Rob)]

No wonder we stuffed up vietnam in the 70's
with such hocus pocus the army gives it's solders
the rate of magnetic deviation is not a fixed variable! and is different in all locations based on the geology of the area.
i.e. magnetic mountains will throw a compas out and even make it spin!
Ring your closest local airport they will give you the current variance
i.e. 11° E (01/04) Toowoomba
http://www.airports-worldwide.com/australia.html

[mangrovedutch (Dutch)]

Thank you Robert and Andrew, for that information. I'll work on 11 degrees East and see how I go. As you say Andrew, it is an entire new subject to cover the Astrophotography, I guess I've got part 1 out of the way. That subject has been researched over and over again (it was my main objective in buying a scope etc), and the more I read, the less I am getting how to go about the finer details on photography. There seems to be so many conflicting ideas out there, it makes me wonder how some people end up with the images they do get. I've purchased the obligatory books on Astrophotography (still waiting on arrival), I've stopped Googling the subject, just to save my sanity. I have seen some wonderful images on this forum, so there are people out there that are in the know. Is there a link that someone could direct me to, I've searched this forum in a couple of places, but still haven't found what I am looking for. I know this is a steep learning curve, but I'm enjoying it so far. I figure if I can set up my scope correctly, then all I have to worry about is the settings on my camera and processing the data captured - and that too is a seperate subject as well LOL

Thanks for the help and information so far, and Robert - I think the diggers did a fantastic job in Vietnam - something every Aussie should be proud of. (no, this IS NOT an invite on a political view point, so don't even bother with that)

Regards, Dutch

[floyd_2 (Dean)]

Hi Dutch,
the other thing you could do is mark your north-south line on the ground at solar noon, if you observe in the same spot every time. Just string up a plumb bob and wait for solar noon. When it's solar noon, trace the shadow of the string (attached to the plumb bob) onto the ground. That will be your true north-south line. The shadow may be a little on the short side so be as accurate as you can - use a 1m or longer ruler if you can.

Dean

[rcheshire (Rowland)]

I guess it depends on the size of your property and overall set-up as to whether surveying true north with a string line and marking the tripod position is a practical solution. But not very helpful if your moving around. And not very helpful if your in an observatory.

Google map is oriented TN/TS up/down the page. Working off a printout, laying off TS from a straight line feature on the property is not too difficult.

That provides the location for the south facing tripod leg. The other two are equidistant either side of the string line. Mark the 3 spots and it's permanent.

This put Sigma Octans in the FOV of the polar scope making precise alignment easier. Setup is trivial these days and pretty much aligned each time.

No magnetic anomalies to worry about.

[ZeroID (Brent)]

For my home first alignment I got onto Google Earth, zoomed in on my house, fortunately a hi-res pic and checked the alignment of a feature ( a long straight path ) against True North. (22 degrees ) and then went out and marked up a spot. levelled the tripod with dec angle set (37.5 Degrees) and then did a drift alignment to refine when Sirius was visible.
Was pretty close from start and once I got the image inversion sorted out in my brain tweaked dec and azimuth to refine the tracking. I need to make a reticule eyepiece though to make it easier. Might add cross hairs to my 25 mm plossl, make it useful as a tracking EP.

[ZeroID (Brent)]

[EDIT]
Ooops, didn't read the post above, very similar. But it does work.

[GrampianStars (Rob)]

Originally Posted by ZeroID

......I need to make a reticule eyepiece though to make it easier. Might add cross hairs to my 25 mm plossl, make it useful as a tracking EP.

here's a cool one
22mm wide-view 70 degree crosshair eyepiece with variable illumination
http://www.myastroshop.com.au/produc...sp?id=MAS-059B

[solissydney (Ken)]

Have a look inside any street directory. There you will find the information
Ken

[mangrovedutch (Dutch)]

Thanks Ken, it's always the simple answer that make life easier. I never knew that the good ol' refedex had that information in it.

Regards, Dutch