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Having a tough time trying to polar align your telescope for astrophotography ? Read on to see if we can help shed some light for you !! 



Polar Alignment Guide for Singaporeans

Date: 29/07/2009

For those of us living in Singapore, we are, for all practical purposes, on the equator. And as such, there is no way we can ever "spot" the north star to help us with polar alignment. So drift alignment is what most of us rely on. Here's a little tutorial on what this is all about.

Hope this helps !! And if you have suggestions to improve this tutorial, please let me know !!

Joo Beng



Here are the steps to understanding this :

1) Understanding Polar Alignment

2) What's different about being in Singapore

3) How does Drift Alignment work

4) What equipment is needed

5) What are the steps to Drift Alignment

Advanced topics :

6) Other methods of alignment

7) Mathematical approach to alignment


Step 1 Understanding Polar Alignment
Polar alignment is primarily for astrophotography.

Light from deep sky objects are very faint, so to take a picture of them through a telescope, you need to keep your shutter open for a long time. Instead of the usual fraction of a second with normal cameras, think of 10 seconds to 10 hours !
While looking at a star through a telescope for 10 seconds, you'll notice that the star will "move" or drift.

Here's a photo of the night sky that is exposed for several minutes - you'll notice that the star forms "trails" of light.

Actually, it's not the star that is moving, but instead, it's you that is moving. You are on the Earth, which is always rotating. (By the way, this is why the Sun "rises" - the Sun doesn't really "rise", it just looks like it does.)
So Polar Alignment is all about cancelling, or counter-rotating, the Earth's rotation.


As the Earth rotates one way, your telescope + camera must rotate in the opposite direction, so that the target you are trying to photograph, stays in the exact same place.
You do this by lining up the axis of rotation of your equipment with that of the Earth. Both axes must be exactly parallel.

Since stars or other deep sky objects are very very far away, they can be considered at infinity. So as long as the axes are parallel, a deep sky target will appear stationary.

When the axes are parallel, your equipment is polar aligned.

When your axis is aligned with the Earth, one end of the axis points north, and the other points south. Aligned with the North and South Pole. Hence the term Polar Alignment.

To get your axis parallel, basically, you will need to point the axis north, and incline it above the horizontal position by the lattitude of your location. Singapore's lattitude is

If you look at the geometry involved, it just so happens that your lattitude is the same as the number of degrees up that is needed.

Now all you need to do, is to turn, or drive your axis at the exact same angular speed as the Earth's rotation, but in the opposite direction.

For a computerised mount, the computer will control the speed. For non-computerised mounts, this can be accomplished using either single or dual-axis motor drives.


Step 2 What's the problem in Singapore
We are practically on the Equator. So we can't use the "normal" method of polar aligning on the North Star. The North Star just so happens to be almost on the North-South axis so it's a great aid in aligning telescopes. Polar alignment in the Northern lattitudes (e.g. USA) can align their equipment using the North Star. They can see the the North Star from where they are. Equipment that we buy are typically made for the northern hemisphere (US, Europe, China).

But...near the equator, we can't see the North Star ! It's "hidden" by the horizon - we are only able to "see" stars that are greater than 5 above the horizon due to constraints like atmosphere and geometry.

You can do a "rough" polar alignment using a compass. This is normally accurate enough for low power magniciation and for visual (as opposed to photographic) use. The compass points North/South, but this isn't always the exact same direction as the true north/south axis of rotation. The difference is called the magnetic declination, and varies by location on the earth.

For Singapore, this "offset" is practically negligible, so using the magnetic north/south will do.

For higher powers of magnification, the rough alignment can be frustrating since the target object will "drift" out of view relatively quickly. Field of View (FOV) is the angular amount that you can "see" at any given instant. FoV of normal unassisted human vision is about 60°. With higher magnification, things further away appear larger, and the effective FoV is also correspondingly decreased.

With high magnification (say about 150x), the FOV is reduced to about 0.5° or less.

The Earth rotates 15° per hour, which is 0.25° per minute. So if you are not correctly aligned, your target can easily "drift" out of your FOV in a minute or less !

Also for photographic use, any "drifting" of the target object (especially dim objects which require long exposures) is unacceptable as it will create a streak of light. With digital photography, a CCD sensor comes in varying "resolutions" - e.g. the Meade DSI II has a resolution of 752x582. For convenience let's use 1000x1000.

If the FOV of the telescope+CCD is 0.5°, then each pixel of the CCD "sees" only 0.0005° !! If you aren't able to hold your image steady enough during your exposure you will get "blurry" or less-than-sharp images.

So without the pole star, the only other way to accurately align your telescope is via a method called Drift Alignment  


Step 3 How does Drift Alignment work








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