Confussion with polar star or latitude

[Gemini2544]

Hi I'm confused, If I set my RA to my latitude of 36 deg and viewing East stars are tracking in line, now if I view West am I still tracking as before and is the same said for viewing North or South.

Now the same question but using the polar star? or do I select differant star for differant directions.

As I said I'm confused about this, my logic suggest a change of 90,180,270 deg viewing direction changes the travel of the stars direction with respect to the observer.

Any help with this one would be apreciated. :question:

[dannat]

you're mount when set up should have one leg facing N and the other two e-W. that leaves the mount head to be pointing to the south celestial pole, or south pole ositiion in the sky..then you leave it as is when pointing to look at any object..the polar scope in the mount stays in place and does not move.

use a compass to get you roughly facing south, keep in mind the magnetic deviation though, melb is about 12 deg out, so you point to roughly 12 deg to the east of S..there is a map on this site somewhere

[Dennis]

Hi Doug

For a German Equatorial Mount, your polar axis should point to the South Celestial Pole (SCP). The SCP (“true” south) is the “imaginary” point on the celestial sphere around which the Earth appears to rotate. The altitude of the SCP above your local horizon is the same as your latitude.

So, if you are located at 36 deg S, then point your RA axis true south and elevate it to 36 deg and this should then point to the SCP.

Note that the SCP and Magnetic South as shown by a compass are not the same. You may have to add or subtract the magnetic declination to/from magnetic south (compass needle) for your location to find “true’ south. For your magnetic declination have a look here:

http://www.magnetic-declination.com/#

For me in Brisbane, the magnetic declination is 11 degrees E, so that “true” south (SCP) is 11 degrees E of magnetic south (compass needle), so I need to subtract 11 degrees from magnetic south and point my polar axis there (349 degrees).

Cheers

Dennis

PS – for a Fork Mounted Equatorial Mount the polar axis altitude is 90-Latitude.

[mithrandir]

If you want the exact value for your location, go to http://www.ga.gov.au/oracle/geomag/agrfform.jsp and fill in the boxes.

For here that gives D = 12.430 deg changing by -0.002 deg/yr

If near enough is good enough, there are links to JPEGs for all of Australia a bit further down the page.

For Marnoo (location from http://www.ga.gov.au/map/names/)
Requested: Latitude -36o 40' 0", Longitude 142o 52' 0", Elevation 0 km, Date 2009/12/15
Calculated: Latitude -36.6667o, Longitude +142.8667o, Elevation 0.00 km, Epoch 2009.9534

Magnetic Field Components

D = 10.290 deg
dD = 0.012 deg/yr

Andrew

[Gemini2544]

You guys are fantastic, you solved my problem..I forgot all about true north & magnetic north or south, too much home brew beer I guess...!

This explains why I can't track correctly..........BRILLANT !

[JimmyH155]

I ho-pe you are not thinking that south is at 360 degrees Dennis?? That would be a bit of a worry:lol::lol:

[Dennis]

Hi Jimmy

LOL! But thanks for asking!:lol:

All my compasses seem to only seek the North Magnetic Pole; I haven’t yet found one that seeks the SMP!;):lol:

I just point the red end of the needle to Magnetic North and then offset the inner, rotating scale (orange colour) to 349 degrees so that the parallel sides of the translucent base are parallel with 349 and thus are parallel with True South. I then "look back" along the compass due South to align my tripod.

It even works at night as the N seeking magnetic needle has a fluorescent blob on the end!:P

Cheers

Dennis

[JimmyH155]

AAAAGGGGHH All explained:D:D:D but remember, it's magnetic VARIATION, not declination, ;) and magnetic "deviation" is completely different:D

[Dennis]

No worries Jimmy – on reflection, my original explanation without the photos that I posted later was certainly open to (mis) interpretation!

Cheers

Dennis