finding true north

puds55 (Peter) · #1 19-04-2009, 06:14 PM

Hi I have a mead lx90 and I have been told that I need to line it up to true north before I start so could anyone tell me how this is done I have a compass but I believe this only shows magnetic north so any info would be appreciated.
Cheers Peter

pmrid (Peter) · #2 19-04-2009, 06:42 PM

Well Peter, let's begin with this 'southern hemisphere' correction to the Meade manual. Down here, we need to find true south, not north. There are a few ways to do it some more precise than others.
The compass. These are generally not great, especially the cheap ones. But assuming you have access to a reasonably good one, remember that it points to MAGNETIC north and south. The south you need for astronomy work is different - and since I see you are at Morayfield near Cabooltue, you may work on the figure of 11 degrees for your area. Here's how it works. Use your compass to point out magnetic south. Then turn the compass until it is pointing out 169 degree (i.e. 180 minus 11). That will be your siderial south.
Anotgher way is to find out the solar noon time for your location and at precisely that time, hang a plumb-line from a metre or so above the ground and note carefully where the shadow of the string is on the ground at precisely that time. here's their web site:
http://www.srrb.noaa.gov/highlights/...e/sunrise.html
For example, solar noon at my place near Somerset Dam tomorrow will be at 11.48.59 local time. In the calculator, as a int, you'll need your latitude and longditude. If you can't find them quickly, try using the Weather Bureau's (http://www.bom.gov.au ) site and go to their rainfall map. If you enable the option to show the cursor position, and then move the pointer on the 128km rainfall map over your approximate home position, it'll give you the LAt and Long at least to accuracy to the degrees and minutes. HAving had a quick look, it seems likely that you will be somewhere near 27 degrees and 7 minutes EAST and 152 degrees and 58 minutes EAST. So in the solar calculator, enter these both as negative values. Once again, doing the calculation using those coordinates for tomorrow suggests that your solar noon on Monday the 20th April will be at 11.47 and 12 seconds. That'll certainly be pretty close. When you do the plumb-line, mark the shadow line on the ground and try to do it in a fairly permanent way - paint, pegs, post etc. You can then line that up with some object on a distant horizon - a few miles away preferably. You can then use that to get your scope's polar axis pretty close.
I will describe a third method using a program like Stellarium if you still need it but for now I hope the above 2 methids will get you started.

Peter

puds55 (Peter) · #3 19-04-2009, 07:48 PM

thanks very much for the quick reply and the info i am totally new to this stuff and i will do my best to follow your instructions

cheers peter

Barrykgerdes · #4 20-04-2009, 07:30 AM

Hi

The method intimated by Peter with Stellarium is probably the one I always suggest. It is extremely accurate and does not need a compass. Just set your telescope up on its levelled alt/azm mount power off. Turn stellarium on (set to your exact location) and locate a star that is easy to identify, Stellarium can also help here, and has an azimuth bearing of between 4 and 10 degrees (much less when you get the knack). Centre the star and track it manually and watch the azimuth in Stellarium as soon as it hits 0 degrees stop tracking. The telescope is now pointing due north. Wind the Altitude down to locate a landmark on the horizon. Next time you set up at the same place you can use the same land mark as your north reference.

A by product of being able to set north within a couple of arc minutes and find an accurate level also within a couple of arc minutes can bypass the alignment phase of a goto telescope for most purposes.

Barry

pmrid (Peter) · #5 20-04-2009, 10:32 PM

Hi Barry,
I was thinking about rewriting those Stellarium instructions for someone using say a LX200 or similar on an equatorial wedge.
Here's how it seems to me:
Set your scope into the Polar Home position in the usual way assuming you have the polar axis aligned as well as you can to the southassuming you have also set your local latitude and the reciptocal angle on the wedge - and you should be within a ball-park of southern alignment.
Start Stellarium and choose a star that will cross the 180 degree line within ten minutes or so - choose one that is within say 10 degrees of the SCP. For an example, Miaplacidus might be a good choice. Accelerate Stallarium and watch the star until it reaches the 180 degrees mark and note the declination angle. It'll be around 69 degrees 40 on the azimuth projection. Keeping your fork horizontal and adjusting only the declination, raise the tube until you are still on the meridian and pointing more or less where the star should be when it crosses the line. As it aproaches the line, use the azimuth adjustiment on your mount so that the star is on the line at the time it actually crosses it. Use a reticle or a program like StarTarg to give you a grid/scele if you want to do this using a webcam or the like.
When you get the azimuth adjustment just right, you are pointing due south.
You could also adapt the technique to adjust the altitude on your wedge I suppose. What do you think?
Peter

Barrykgerdes · #6 21-04-2009, 07:23 AM

Hi Peter

Make sure Stellarium is running at real time do not accelerate it when you want to read the star. Set your forks exactly paralell to the ground so that the meridian will be overhead. Don't use the SCP. swing the scope over and look for a star low in the north sky. Track this star using the the wedge azimuth and the normal declination control (telescope off). This will set the north/south line. Next set your scope to the 0,0 (azm) RA and 0,0 dec start up your scope and select a star in the north and press goto. All being well and if the scope was truly at 0,0 /0,0 the scope will slew to this star. centre it with the wedge controls (only) and you will be pretty close.

Barry

puds55 (Peter) · #7 22-04-2009, 06:43 PM

Hi Peter and Barry thanks for all the great help but I think as a beginner and totally unfamiliar with all the technical info I will have to join an astronomy group and get some hands on help. I do very much appreciate the effort and advice you both went to on my behalf
cheers Peter

astro_nutt · #8 24-04-2009, 01:23 AM

Hi Peter,
I have had the same problem so I looked up the AGRF Computation Geoscience Australia which will work out the degrees diffrence of your location..I gave your location Morayfield -27hrs 06mins 152.56 degrees
The diffrence value is 10.925 degrees..so if you're holding a compass and facing magnetic North you need to turn 10.925 degrees to the WEST for true North...hope this helps
Cheers!

AdrianF (Adrian) · #9 10-01-2010, 08:22 PM

If I set the points on the ground showing N-S & E-W using the solar noon method are they accurate all year round or do they have to be reset occasionally?

Adrian

V-Man · #10 10-01-2010, 08:28 PM

Quote:

Originally Posted by astro_nutt

Hi Peter,
I have had the same problem so I looked up the AGRF Computation Geoscience Australia which will work out the degrees diffrence of your location..I gave your location Morayfield -27hrs 06mins 152.56 degrees
The diffrence value is 10.925 degrees..so if you're holding a compass and facing magnetic North you need to turn 10.925 degrees to the WEST for true North...hope this helps
Cheers!

Errm SOUTH is a better idea

mithrandir (Andrew) · #11 10-01-2010, 10:13 PM

Quote:

Originally Posted by AdrianF

If I set the points on the ground showing N-S & E-W using the solar noon method are they accurate all year round or do they have to be reset occasionally?

Adrian

Not counting earthmoving machinery, landslides or earthquakes moving your markers, or continental drift moving the whole country it will remain correct.

You might repeat the process for a few days with semi-permanent marks and average out any discrepencies before making them permanent.

Winter gives longer shadows so might work better than summer.

It's not vital they be exact. They just make it easier to plonk the mount down in exactly the same place every time, to minimise the amount of fine adjustment you have to do.

AdrianF (Adrian) · #12 10-01-2010, 10:24 PM

Thanks I will go get a plumb bob and a square tomorrow and set it up. I dont have a permanent setup and use the grass but I will put some offcuts of coppers logs down to help plonk the scope down each time.
Ohh for a observatory.

Adrian

AndrewJ · #13 10-01-2010, 10:26 PM

Gday Peter

As you have an LX90, you can also cheat
Set the scope up alt/az and in home position, ie with OTA level and pointing north ( within 10deg will do )
Now select the "manual" one star align method.
When it asks for an align star, pick one really bright easy to see star
Sirius or Canopus are good early evening.
It will slew "near" to the target, so Just use the handbox to centre the star, and hit enter.
It will say align successful
Now hold the mode key on the Hbx down for say 5 seconds and release
The display will change to show a readout of position etc
Scroll down till it shows Alt and Az
Now use the Hbx slew keys to manually slew until the OTA is level and pointing north
( ie the AZ reads close as dammit 180deg ( or 0 deg, cant remember ) )
Turn off scope
The OTA is now pointing directly to True Nth.
Just make a mark and you have it.

Andrew

JimmyH155 · #14 12-01-2010, 01:43 PM

Half of you guys are talking about North

We are in the Southern hemisphere, and we point our scopes to SOUTH (180 deg)
The telescope instruction books are written for the American market and that is why they talk about north. My Meade instruction book does actually say that if you are in the Southern hemisphere, for North, read South!!
Remember that the bar on a magnetic compass has TWO ends to it - one is north-seeking, and the other is south-seeking, so just forget the north bit, and use the south seeking end.
North is 0 degrees

South is 180 degrees

Easy

mithrandir (Andrew) · #15 12-01-2010, 02:49 PM

Quote:

Originally Posted by JimmyH155

North is 0 degrees

South is 180 degrees

And largely immaterial. If you draw a line pointing North, what direction does the other end of it point?

If one end of the RA axis points S, what direction does the other point?

It's drift aligning that needs the most N/S instruction rewriting.

AndrewJ · #16 12-01-2010, 04:46 PM

Gday Jimmy

Quote:

Half of you guys are talking about North http://www.iceinspace.com.au/forum/..../shrugging.gif We are in the Southern hemisphere, and we point our scopes to SOUTH (180 deg)

Ummm, incorrect for AltAz.
He is quoting an LX90.
The correct home position for this ( in either hemisphere ),
is OTA level and corrector facing north.

Getting a true N/S line can be done many ways,
but the OTA must be pointing north to do an alt az align

Andrew

Outbackmanyep · #17 12-01-2010, 10:14 PM

Try facing East and take a sudden turn left.......

Liz · #18 13-01-2010, 08:41 AM

Quote:

Originally Posted by Outbackmanyep

Try facing East and take a sudden turn left.......

that_guy (Tony) · #19 14-01-2010, 03:07 AM

Is a compass facing north or south??

stephenb (Stephen) · #20 14-01-2010, 11:06 AM

Quote:

Originally Posted by AndrewJ

Gday Peter

As you have an LX90, you can also cheat
Set the scope up alt/az and in home position, ie with OTA level and pointing north ( within 10deg will do )
Now select the "manual" one star align method.
When it asks for an align star, pick one really bright easy to see star
Sirius or Canopus are good early evening.
It will slew "near" to the target, so Just use the handbox to centre the star, and hit enter.
It will say align successful
Now hold the mode key on the Hbx down for say 5 seconds and release
The display will change to show a readout of position etc
Scroll down till it shows Alt and Az
Now use the Hbx slew keys to manually slew until the OTA is level and pointing north
( ie the AZ reads close as dammit 180deg ( or 0 deg, cant remember ) )
Turn off scope
The OTA is now pointing directly to True Nth.
Just make a mark and you have it.

Andrew

Peter,

You do not have to be that accurate with true North. Do everything Andrew has suggested and it will work. No compasses, no bubble levels etc. Trust him. The LX90 has been the easiest alignment oof any telescope I have used. It aligns first time, every time.