SingAstro

the compass comes with the vixen polarie very small.
and i have ordered the polar meter.
but my image still drifted with lines for 20sec exposure

i suspect i align badly even with compass and the polar meter.
how to align better and more accurate?
i place iphone and the iphone app compass and level app.
but drifted also

help pls
thx

Hi liquidice,

using an iphone app compass could be quite inaccurate.
i have tried before, when using the compass on my iphone, the alignment go as far of as 10-15 degrees!
You might want to find a lensetic compass to do a better alignment.

as for the level app mine works on the iphone.
you might want to check the app on your iphone again.

In any case, do drop by any of our member's session or sidewalk session with your gear.
we would be more then happy to assist you.

Dave Ng

I find that using a Silva compass I can align to within 2-3deg on a bad night and within 1 deg on a good night. This is sufficient for me to do 1 min exposures at 540mm, unguided on my SXD. Above compass can be bought from Beach Road or more expensively from any camping store.
I find that sighting on a distant object is more sensitive than trying to align your mount with the needle. Stand behind your mount and sight along the needle to a distant object, ensuring that your mount is directly between you and the object, to minimise parallex error. The align your mount to the distant object. This will get you to about 1deg off since the Celestial Pole is not exactly co-incident with the Magnetic Pole.
Also, your mount contains Iron/Steel and/or your iPhone casing may contain magnets which distort the directional reading.
You should also ensure your mount is exactly level.
Hope it helps!

Ok thx
I have lensetic compass , was using polar meter compass all this while.
First time outing I was using polar meter n iPhone only. Maybe I align 2-3 times n taking my times.
This time I was rushing to start the shots, so only using polar meter n iPhone.
The low altitude cloud is very annoying here. Coming fast.

If I were staying in sg, I would love to join u all for side walk

I will try the lensetic compass next time. Thx for the tips, lensetic compass n distant object . Wasted good shooting chance last time, now have to wait for next one

Where do you live? The assumption of using a compass to align is because here in Singapore the Celestial Pole is just 1deg above the horizon so a compass will pretty much get you there.
On the other hand if you're living in higher or lower latitudes then a polar scope or some way of aligning on the Pole itself is necessary since a compass only helps you with the azimuth of your alignment, not its altitude.

i live in sarawak. across the south china sea . 1 hour flight from you.
also a 1 degree

Hi there,

I've two things to add:

First, regarding the deviation of magnetic North from true North (azimuth), use the following to ascertain what it is for your particular location:

The NGDC Geomagnetic Calculator:
http://www.ngdc.noaa.gov/geomag-web/?us ... eclination

Assuming you live in Kuching ( 1° 32' 56" N, 110° 20' 38" E), the result from that website is:

"Declination (+ E | - W)
2013-02-04 0° 36' 30" changing by -1.8' per year"

Here's what it says about interpreting the result:
"Declination: Declination is the angle of difference between true North and magnetic North. For instance, if the declination at a certain point were 10° W, then a compass at that location pointing north (magnetic) would actually align 10° W of true North. True North would be 10° E relative to the magnetic North direction given by the compass. Declination varies with location and slowly changes in time."

For Singapore, the result is:
"Latitude: 1° 16' 8" N
Longitude: 103° 49' 54" E

Date Declination (+ E | - W)
2013-02-04 0° 11' 49" changing by -1.1' per year"

For Telok Sari (Mersing), the result is:
" Latitude: 02° 36' N
Longitude: 103° 46' E

Date Declination (+ E | - W)
2013-02-04 0° 7' 46" changing by -1.2' per year"


Second, I think its better to use a physical bubble level, not an electronic device like a smartphone app, to ascertain a true horizontal (altitude) level position. Then since Kuching is about 1.5 degrees north, you would need to tilt the Vixen Polarie upwards very slightly, e.g. just enough so that the bubble reaches the end of the tube. I suppose you could experiment to see how much tilting yields the best result.


Hope the above helps.

Hi,

I've another two points to add, which are about using other methods besides compasses to find true north.

A. You could use a map to find a distant landmark that's exactly north of your observing site, then point the Polarie at it. Google Earth is I think very accurate; you could use the satellite image view, then use the ruler feature to draw a line stretching up and away from your site. If you're lucky enough, you might find a particular building, or part of a building, in the distance that's exactly north of your site to which you have a clear view of. You could compare the East-West (longitude) coordinates to double check that the distant object is indeed exactly north of your site.


B. I've heard a few times that when permanent observatories are built, the Sun is often used to find true North (or South). This is based on the principle that once every day, the Sun will cross the local meridian (which is the line running between the North and South Celestial Poles), i.e. it will 'transit' or 'culminate in' the sky. That is the moment of true local noon, which is almost certainly not the customary '12.00pm noon'.

Then use a vertical pole or plumb line to cast a shadow on the ground. At that one brief moment, the shadow runs exactly north to south.

To find the moment of true local noon, use a planetarium software and enter the exact coordinates for your site, and set it to a particular date. Then get the software to display the Sun's properties, and it will usually state its time of transit, which is also when its highest in the sky for that day.

Of course, your watch or time app must be very accurate.

I've not done the Sun-shadow method before, but the theory is correct and its also nice to know that it's based on astronomical concepts. The Sun, like other celestial objects viewed from the tropics, will transit the meridian once a day. There could be practical difficulties, e.g. in getting a sharp enough shadow on the ground. I think you could use a chalk or marker to quickly mark off the shadow on the ground.

These are some articles on using the Sun, searched from Google:
http://www.bisque.com/tom/transitfiles/transit.asp.

http://adventure.howstuffworks.com/surv ... north2.htm.


Of course, apart from all of the above, there is also the drift alignment method, but that's another story. That is used for very fine calibration of polar alignment after the initial step.