Author Topic: Polar aligning EQ mount in Southern Hemisphere: can't I just use a compass?  (Read 321 times)

Steven Tolbert

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I'm fairly new to astronomy and recently bought an equatorial mount and reflector telescope. I live in the Southern Hemisphere (Australia) and I have been reading about all the techniques to polar align the telescope.

Why can't I just set the latitude on my telescope, and then use an app to find out the bearing over which the celestial south pole is sitting, then aim the telescope towards that bearing with a compass?

For example, if the celestial pole is directly over due south, can't I just set my latitude and then point the telescope exactly due south?



getneyprotges

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Well, for visual use, you can. If you have a GoTo scope, doing your star align will help keep you on target. The Southern Hempisphere is so much more difficult. It would be helpful to know what telescope you have, and if you have a Polar Alignment scope for it.

If you are planning on doing astrophotography, I would recommend downloading 'SharpCap', since it has a polar alignment feature. I've not tried it in the Southern Hemisphere, but it works well here in the North.

To use Sharpcap, you will need a guide scope, a guide camera, and a laptop.

Here is a set of instructions for aligning in the Southern Hemisphere.

http://www.ozscopes....hern-Hemisphere

I also agree with the Australian authorities on not using a laser for aiming. It's a great way to do it, but it's also a great way to disorient a pilot flying, if they accidentally fly into the beam, or you accidentally illuminate them.

Here's some more information.

https://www.assa.org...olar-alignment/

This YouTube video will give you a good way to set up for visual observing, but it's not good enough for photography.

If you want to do Astrophotography, I recommend you repost your request in the Beginner and Intermediate Imaging forum.

belmadeasus

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By rights you can use a compass and simply set the latitude. But each is dependant on other aspects.
The latitude is dependant on the scale being accurate and more relevantly on the mount being absolutly level in both directions. The (example only) value of 25 S is only 25 S with respect to the mount itself being level. Add in the you are likely to be at a non-integer number say 22 32 S then you likely cannot get that accurate.

A compass does not strictly point N-S it points along the lines of the local magnetic field - this may not be therefore at the 2 primary N and S poles we talk about. You need to look up your megnetic declination and you could find that your compass may point 5, 10 even 15 degrees out. I know one bit of the world where the N end of a compass points South - they do have warning or information signs along the road.

Historically a compass was for navigation at sea and I suspect that there the local magnetic fiels from the rocks is negliable therefore. Annoyingly I find that even a GPS compass on a tablet still uses a magnetometer, which I cannot see why. Mention this in case the idea is to use a tablet. Smart phone or similar.

Unfortunatly the solution is to work out how to set it up with the polar scope by the old fashioned method. There are no real short cuts, if there were people would be using them.

You may be best looking for a club in your area, if there is one, and getting advice on achieving good polar alignment from people there. It is a lot easier to get information from people that have direct experience. Bit like here - it is OK for visual to simply put Polaris in the middle and forget the rest if and only if you are manually viewing things, they remain in view for about long enough. No good for AP however.

poithegepur

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Quote
Why can't I just set the latitude on my telescope, and then use an app to find out the bearing over which the celestial south pole is sitting, then aim the telescope towards that bearing with a compass?
You don't need an app. The bearing of the south celestial pole is always 180 degrees -- precisely due south, by definition.As others have said, this is adequate in theory to achieve an absolutely perfect polar alignment, assuming that you take the magnetic declination into account. But in practice, you're likely to be off by a degree or two in altitude and probably more in azimuth.Being off by a degree or two in polar alignment has virtually no effect on visual observing. But it will cause a certain amount of trailing if you're doing long-exposure astrophotography. Drift alignment is recommended for photography, in both hemispheres. Sighting on Polaris in the Northern Hemisphere has its own set of problems.

elunmolunch

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It's easy to find the north-south direction: find the time when the Sun culminates at your location on a particular day and use a plumb line. The shadow of the plumb line shows the direction of your local meridian.

Niro Hardy

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I am looking to build one of these for my local area in the northern hemisphere: http://www.iceinspac...=63,499,0,0,1,0

calbeyrefrows

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According to my colleague who use to work on aircraft navigation software, magnet declination varies a lot within Australia due to strong local magnetic fields. Makes sense really seen as Australia exports something like 55% of the yearly global iron ore supply.

Not sure how accurate this is, but use this site to find the local magnetic declination for your observation site.
http://www.magnetic-...rne/121955.html

Dennis Collins

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nonbuysalcho

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Yes, level + compass.
Then drift alignment to get precise PA before I start PHD2 for star guiding.

Louis Sullivan

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You can without alignment at all if you like, learn star-hopping and go, but it seems useless have an equatorial mount if you don't align that imho

miswalltile

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Quote
Quote
Why can't I just set the latitude on my telescope, and then use an app to find out the bearing over which the celestial south pole is sitting, then aim the telescope towards that bearing with a compass?

You don't need an app. The bearing of the south celestial pole is always 180 degrees -- precisely due south, by definition.
As others have said, this is adequate in theory to achieve an absolutely perfect polar alignment, assuming that you take the magnetic declination into account. But in practice, you're likely to be off by a degree or two in altitude and probably more in azimuth.
Being off by a degree or two in polar alignment has virtually no effect on visual observing. But it will cause a certain amount of trailing if you're doing long-exposure astrophotography. Drift alignment is recommended for photography, in both hemispheres. Sighting on Polaris in the Northern Hemisphere has its own set of problems.
https://upload.wikim...nation_2015.pdf

About 11degrees where I live.
12-13 for where most people in Australia.
New Zealand is 20-26degree off

People living the 0-3 degree declination areas wouldn't deliberately setup there scope 10-20 degrees off ... why would we ?
South Africa looks to be over 20degree as well.
It might be a degree where you live. But even here at 11degree off it's quite noticeable.

Kunjan Blanco

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I'm in Oz, in Cairns. I seem to recall a latitude where I am of 16 degrees but that I actually need to point 6 degrees off due south I think.

I use a PoleMaster, the stars you need to align with are so dim that it's very difficult to see through the polar scope. But for viewing your theory is correct that you will get a good enough polar alignment so that your mount can track reasonably accurately. Not good enough for imaging though obviously.

Jerry Gilbert

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I live in Brazil and a compass with true North (you may need to know magnetic declination in your spot) and a level are ok for visual - but still tricky if you are using a goto mount
for astrophotography, you will definitely need to further work on it
I have been looking for easy, fast, accurate and cheap solutions - but these altogether seem to be unattainable in astronomy as of today

Lesego Dowdy

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It's been like eight days since the OP has posted this, and he's not posted anything since. I wonder if he even looks at this.