Buying a compass

[DJVege]

Hi guys,

I was going to buy a compass today to help with polar aligning my scope. Anything I should look for in particular? Anything to watch out for? Or can i just get any old thing?

Cheers.

[erick (Eric)]

A good oil-filled Silva would do the trick. Don't need any fancy stuff, sights and lids to close etc. About $30?

The only point to mention is that it will likely be affected by big lumps of metal near it (read telescopes and mounts!), so use it to align an aluminium or wooden marker on the ground, say, then align the mount/scope to that marker.

Now, the experts can correct me.....

[DJVege]

Thanks Erick! Any suggestions as where to buy them from? I've got no idea! Never had to buy a compass before.

[erick (Eric)]

Bought mine from the Silva outlet (factory?) here in Melbourne!

Try your yellow pages?

• Great Outdoors Centre
  18- 24 Townshend St Phillip ACT 2606
  ph: (02) 6282 3424
  • map
• Kathmandu
  20 Allara St Canberra ACT 2601
  ph: (02) 6257 5926
  • map
• Paddy Pallin
  11 Lonsdale St Braddon ACT 2612
  ph: (02) 6257 3883
  • map
• Snowgum
  9 Lonsdale St Braddon ACT 2612
  ph: (02) 6257 2250
  • map

[erick (Eric)]

Or a bit of googling?

http://www.macson.com.au/Silva/Silvacomp.html

[erick (Eric)]

Here is all you need:-

[DJVege]

Thanks Erick! LOL, I'm in Brisbane. Will have a look now.

[erick (Eric)]

I'm also messaging an IIS member in Canberra. I'm lost!!

[DJVege]

haha. Ah, there's a Silva here in BNE too. I'll go there on Saturday and snag one!

Thanks again, Erick!

[wasyoungonce (Brendan)]

Yeah ericks spot on. The compass will deviate with influences from iron sources, which change the magnetic flux densities around them.

In other words putting a compass near your scope is practically worthless unless you have done magnetic compass swing (on the whole setup) and corrected the compass deviations (from the setup)..This is not possible on small cheap compasses they do not have NS and EW flux variation correctors.

You best be is to use the iceinspace procedure:

http://www.iceinspace.com.au/index.p...63,405,0,0,1,0

But be aware that magnetic compass can be out by a few degrees because of what i mentioned.

[DJVege]

Thanks guys! I bought the SUUNTO A-20...which I was told was the same as Silva. =) $39.95. Will be up all night polar aligning and checking out this motor.

[Lee]

You do know that you don't align to magnetic south don't you? A compass gets you part the way there - you need the magnetic deviation for your area too.....

[MarkN]

Last post is very true. True also is that a compass will not necessarily point to the magnetic poles. This is dependant on the earth's magnetic flux at your location.

Where I live it just so happens that aligning the mount to the compass gets me so close to the celestial pole for visual observingthat it doesn't matter .

Mark.

[Bassnut (Fred)]

Stand at least 1m away from the pier whilst taking readings. Magnetic interference from the scope/pier can be very embarrising;-). I drew a line to true north (from the UBD directory compass diagram) on the compass face.

[dukesisking]

I actually bought one 2 days ago from the great outdoors centre in fyshwick. I was a bit disappointed in all honesty with the selection available. The $5 ones didn't have north and south the $30 ones didn't have mag deviation, the $40 ones didn't have luminescence, the $50 ones had all I needed but were sold out. So I am the proud owner of a whiz bang compass worth $60 that can tell you south, mag deviation lines, can be seen at 30 paces in a cave but won't ruin night sight, can be used to traverse the arctic circle (trust me, it had a picture of a polar explorer on the packaging, is that a slight case of misleading advertising?) and can cook me up a coffee in 5 seconds flat. The true value of the compass (it seems so wrong to call it a compass, it is so much more..) comes from the fact that we have had 3 nights of cloud since the purchase. Why oh why do I refuse to shop around?

[Ric]

Great sounding compass for $60 dukes. You should have bought the $80 model it comes with a fan to blow away the clouds

Seriously though, thanks for the tips guys it's proving to be a very helpful thread.

Cheers

[Gargoyle_Steve (Steve)]

DJ magnetic variation for us around here in SE QLD is 11° - ie magnetic south and true south are 11° apart. Find magnetic south with your compass, then (facing that direction) align your mount 11° to the left / East of magnetic south.

(Those who actually have polar aligned mounts please feel free to correct me here - I'm just a poor dob owner who needs nothing except clear skies and flat[-ish] ground to get my scope working.)

[DJVege]

Hi guys! Yep. Yep. I've been 'a readin!! I know about mag. south/north, and the declination needed (I was reading up on it at work yesterday ). However...and don't laugh!!! I need to read up on how to read this compass!

haha. That's what I'll be doing today! That and collimating my laser pointer, but that's going in another thread.

[glenc (Glen)]

I bought a watch recently that has a compass on it. You can put in a correction for the local magnetic deviation. It also measures max & min temperature, has a stopwatch etc. It cost $40 and was delivered without any problems and it is accurate too.
Atech Navigator Digital Multifunctional Sports Watch
http://www.gtmall.com.au/catalogue/a....html?shopping

[schmourghenne]

Here is a site that gives you the offset for true north/south based on location and date:

http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

Apparently, they take a survey every five years and extrapolate the data they get to closely approximate any magnetic field drift that may occur.