Compass woes

[John Green (John)]

Hello all: Thanks to everyone who responded to my introductory message a while back, even found a local who is a font of information on all things astró. I have cast 3 concrete pads for my eq5pro to sit on, whilst playing around with my 4 compasses, I have found that all 4 give different versions of magnetic North, there is approx' 3 degrees between them, plus the sighting slits do not align!. Can anyone tell me what constitutes a good compass?, I know what doesnt!. and how do you know it is good?. Any suggestions on what to buy would be gratefully received. For eg'; Should I go for a marine version?, or is it simply a case of you get what you pay for!. John.

[cventer]

You can check which one is working easily.
Puit a broom handle into ground vertically use a bubble level to make sure it's straight up. Or use a plumb bob line.

Use some software like the sky. Make sure time and lat long are accurate for your location. Click On the sun. It will give you info like rise set and transit time. Transit is what you want. At that exact time of transit look at the shadow cast by your plumb bob or stick. That is true south. Mark it with tape or ground marking spray.

Now you can check your compass against that. Bearing in mind magnetic north south will differ by the magnetic declination for your location.

[ZeroID (Brent)]

If you are trying to use a compass to get a polar alignment started, forget it. Too many things to upset them.

Easiest way is to take a solar noon alignment using a plumb bob. Find Solar noon time for your location ( it will be slightly diferent from the clock time ) and just mark the shadow of the plumb bob on the ground where you will set up. That will be exactly Nth Sth. You can get local Solar noon off the interweb, google is your friend.

Have fun and don't waste your money on magnetic compasses. A decent GPS is another option of course and you can use a star transit on the Nrth/Sth line as well but it's bit harder at night. Stellarium will tell you when a star crosses the line.

[acropolite (Phil)]

Brent's on the money with Solar Noon. Some useful info and links in this thread.

[mithrandir (Andrew)]

Some GPSs have a builtin magnetic compass and that will suffer form all the same problems as an ordinary compass.

If you were to record the lat/lon of your desired mount point, then move north or south maintaining the longitude you could set out an n/s baseline, but consumer grade GPS coordinates are only accurate to within 5 metres at best, so you would have to make this line as long as possible. A surveyor I know said his surveying quality GPS cost him quite a few thousand dollars. One of mine has a sun/moon icon at the right location on the dial for the current time, but the baseline is so short it would not give a good result.

Better may be to get your location and those of some visible and notable object from something like Google maps, derive the azimuth and use that to calculate the sides of a right triangle that would give you one side pointing n/s. For example, from one of our viewing locations the Kurrajong Hotel is 41km away on a bearing of 300 deg, so a suitable triangle is 200cm for the bearing, 173cm for e/w and 100cm for n/s.

All in all it's easier to use the plumb bob and local solar noon.

Andrew

[Paduan (Brett)]

compasses need "swinging" calibrating every year as they do not all point the same. large amount of iron in the rocks or a seemingly harmless metal like aluminium can change the lines of isogonic flux around an object not to mention the magnets in the motor drives of the mount so use another method like the ones mentioned eariler also do not forget that you need to be around 13-14 degrees on the east side of south to account for local deviation

[COSMOPARADISE (Nick)]

All well and great if your EQ is on ground level with an open view of the sky. As i live in a unit and have only my balcony to plant my HEQ5PRO on I also have the same compass problem when im trying to find south as my view of the sky is limited to SE to SW. Really frustrating not being able to have a clear view whenever i want.

[Paduan (Brett)]

there is also a more crude way to get close
1 locate crux and the pointers
2 draw imaginary line vertically through crux Gamma crux through Alpha crux
3 drop another imaginary line perpendicular to the line of the pointers
4 where the two lines intersect then drop another line vertical to the ground and that will be close enough to due south that you can start aligning, you may need to turn a little east or left but good to know if out at a dark site without technology