When you set up your equatorial platform to track the heavens, you already know that you need to align the polar axis of the mount to the celestial pole, and for visual use you can simply use the star Polaris if you live in the Northern Hemisphere. Any basic, rough alignment to the pole will allow the mount to track the sky with enough precision for visual use, GOTO computer systems, and very short photographic exposures.
However, for long exposure astrophotography, you need your mount to be very accurately aligned to the celestial pole. A simple rough alignment or just using Polaris will not do. In these simple alignments you are certain to be either too far North or too far South (altitude error) or too far East or too far West (azimuth error). Some mounts come with excellent polar alignment scopes built into the RA axis of the mount and enable quite accurate alignment ... once you learn how to calibrate and use them. But for very long exposures even this may not be accurate enough.
One way to be absolutely sure you have precise polar alignment is with the DRIFT TECHNIQUE to accurately check the polar alignment of the mount and correct any errors. This technique will give you a very high degree of precision, enough for very long exposures with tracking error due to misalignment.
Drift Method
The technique is basically simple. You monitor a star in an illuminated eyepiece and watch for any tracking error in DECLINATION. If the mount is not accurately aligned you will see the star drift. You then correct the position of the mount and recheck the star for continued drift. You will use two stars to check the alignment. The two stars are located in a specific area of the sky that will show the greatest amount of error in either altitude (mount too high or too low) or azimuth (mount too far East or West).
STEP ONE: Level the mount: To begin, you should make sure your mount is level. You can skip this step, but every time you correct either the azimuth or altitude you will mess up the other! This will mean you will have to re-do the alignment many times before you have it very accurate. Level the mount.. it will save you a LOT of time!
STEP TWO: Rough Align:Next you want to get a rough polar alignment by using Polaris or your polar alignment scope. This will also speed things up for you.
STEP THREE: Aligning the Reticle:
During each step below, you need to make sure your Illuminated Reticle Eyepiece lines are lined up with the East/West axis of your mount. This step may be confusing, but it really is a simple step. First, locate a bright guidestar and watch it in your illuminated reticle eyepiece. As you move the telescope in RA forwards and back (using the hand controller) you will see the star glide left and right. Most likely your eyepiece is not orientated properly, so the star will not line up with the reticle lines.
Now move the star to one side and rotate the eyepiece until the star glides along the lines when you move the scope's RA axis.
STEP FOUR: Adjusting the Azimuth: To begin, you need to locate a star near the intersection of the meridian and the celestial equator.
The MERIDIAN is the imaginary line that runs from the Northern horizon, across directly overhead, and then down to the Southern horizon.
The CELESTIAL EQUATOR is zero degrees Declination. It is the imaginary line that runs East to West, 90 degrees from the celestial poles. It is the imaginary extension of our equator, if you extend it out into the sky, and would be directly over your head if you lived at the equator!
You need to locate a star that lies near the intersection of both those lines.
Once you have found your star, center it in your illuminated reticle. Rotate your eyepiece so that the reticle lines run East/West as described in step 3 above.
Guiding and Watching:This will make sure that any movement of your star in right ascension (RA) will not be confused with any movement in declination (DEC). Remember, we are only going to be concerned with the star’s declination drift, not RA. Many astrophotographers find it better to align the guidestar along a reticle line, instead of between the lines.
Watch the star, and keep it centered with slight RA controls (East/West) as needed to correct for periodic error (PE). You will notice PE as a repeated movement back and forth, East and West. This cycle is due to irregularities in your gears.
DO NOT make any adjustments in DEC. You are just watching for this declination movement, not correcting it.
Now it's time to make the adjustments to correct your Polar Misalignment in AZIMUTH.
AziMuth is MAD! Your mount may pointing too far east or West of the Celestial Pole. Once to note some declination drift while watching a star near the celestial equator and meridian, you are seeing errors in AZIMUTH, or East/West. The azimuth adjustment is maddening because as you note the star drifting in declination, you rotate the mount in azimuth to shift the star PERPENDICULAR to the direction it just drifted. If the star moves UP you want to rotate the mount’s azimuth to make the star move to the RIGHT in your field of view. If the star drifts DOWN you want to rotate the mount to make the star move to the LEFT. That's just MAD!!!!
Just remember… you are almost never standing upright as an astrophotographer anyway.. UP - RIGHT (you can then reverse that if the star drifts down). This is why azimuth alignment is mad! Then re-center the star and check again. Keep doing this until you no longer see any drift within 10 minutes!
NOTE: If you using a NEWTONIAN telescope, it's even more maddening! You must REVERSE the directions above. If you see the star drift UP, you must move the mount to make the star move to the LEFT. If the star drifts DOWN you must move the star RIGHT. This is just the opposite from a Refractor or Schmidt Cassegrain.
STEP FIVE: Adjusting the ALTITUDE:Your mount may pointing too far North or South of the Celestial Pole too. Once to note some declination drift while watching a star near the Eastern horizon, you are seeing errors in ALTITUDE, or North/South. To begin, you need to locate a star that is about 15 - 20 degrees above the Eastern horizon. Do not locate a star too far near the horizon or atmospheric refraction will cause errors in tracking by itself, and cause some confusion when trying to gain polar alignment. Once you have found your star, center it in your illuminated reticle. Rotate your eyepiece so that the reticle lines run East/West as described in step three above.
Guide and Watch again: As before, make sure that any movement of your star in right ascension (RA) will not be confused with any movement in declination (DEC). Remember, we are only going to be concerned with the star’s declination drift, not RA. Watch the star, and keep it centered with slight RA controls (East/West) as needed to correct for periodic error (PE). DO NOT make any adjustments in DEC. You are just watching for this declination movement, not correcting it.
Now it's time to make the adjustments to correct your Polar Misalignment in ALTITUDE.
AltitudE is Easy!If you see the star moving in declination, that means your mount is not accurately aligned and you must now SHIFT THE MOUNT slightly to bring the mount closer to accurate polar alignment. The question is which way do you move it?
Altitude adjustment is easy to remember ... raise or lower the mount so the star moves back towards the center of the field. That is, whichever way you see the star drifting, adjust the mount to relocate the star back towards the center of the eyepiece. That's EASY! Keep doing this until the star no longer drifts! (Just remember – you are only watching for declination drift only, not RA!)
If the star is drifting quickly, move it a lot further than just the center. Move it as far as you think it would go if you let it drift for 5 minutes.
The more inaccurate your mount, the faster the stars will drift. This means you should make larger mount adjustments when the star moves quickly, than when the star moves only slightly. If you see DEC drift within the first few seconds, estimate how far the star would drift in 5 minutes, and move the star (mount) that far. This will speed up your alignment process.
STEP SIX: Repeat. Now you simply repeat the whole process as needed to refine the accuracy. Once you can go 10 minutes without seeing any declination drift, you will have very accurate polar alignment, suitable for very long exposures!
This now opens up the entire cosmos to your camera!
To try your hand at the Drift Method in the comfort of your home, you may wish to use my free Guiding Simulator. The simulator will let you practice polar alignment with the drift method or practice manual long exposure Guiding. Click on the image of the simulator after you finish with this tutorial to try it.
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UNDERSTANDING HOW IT WORKS
The Drift Method is used by thousands of amateur astrophotographers and it works perfectly. But very few really understand WHY the guidestar drifts the way it does, or why you must guide on a star in a certain part of the sky. Why do you need to look at the Celestial Equator and Meridian for Azimuth drift? Does it really make a difference? I'm a visual kind of guy, and think nothing explains things better then a good image. So here, for your viewing pleasure, I have created a simple tool to show you just what is going on in the sky, and why the drift method works!
Let's look at the sky, from the point of view of your MOUNT. A guidestar, and indeed the whole sky, circles around it in a perfect circle centered around the Celestial Pole. If your mount is perfectly aligned with the celestial pole, your guidestar will stay within the Guiding Reticle. Look at the diagram to the right.
This shows the guidestar centered within an Illuminated Reticle eyepiece. The blue line represents the path the star takes around the sky, with the Celestial Pole at the center.
The reticle, riding on your mount, is rotating around the Polar Axis at the same speed as the Earth. This causes the Guidestar to stay centered, or at the most, it will move back and forth along the E/W line of the reticle with Periodic Error of your mount. The Guidestar will NOT move up or down the eyepiece.
The repeated movement you see here represents several hours of exposure, far more than you would ever need, but it shows that the Guidestar will stay centered with ease.
Now lets consider what would happen if our telescope's Polar Axis was NOT lined up properly. If we offset it a bit, it will travel around a circle too, but not the same PATH as the sky. The MOUNT is pointing too far EAST of the pole.
In this example, the guidestar's path is grey, and the telescope's path is blue. The scope is pointing at a star on the Celestial Equator and the Meridian.
Both sky and telescope are moving at the same rate, and it starts with the Guidestar centered within the Illuminated Reticle.
As you can see, from the view of the Reticle, the guidestar moves UP the eyepiece as the Earth rotates and the telescope moves along in Right Ascension. It is the SCOPE that is actually moving down in relation to the true path the star is taking, but from our perspective, it appears the star is moving up.
This is EXACTLY what you see with the DRIFT METHOD of polar alignment. For a scope pointing at the Celestial Equator and Meridian, a star will drift UP when the MOUNT is too far EAST. This is why you must adjust the MOUNT to the WEST to correct for an upward drift!
So what will happen if we look at the Eastern Sky, instead of the Celestial Equator / Meridian intersection?
Well, you may indeed see error if the mount is too far above or below the pole, but not any EAST / WEST misalignment! Look at the illustration to the right.
This mount is still pointing too far East of the celestial pole, but it is PERFECTLY aligned North and South. If you look ONLY at a guidestar on the Eastern or Western horizon, you will NOT spot any declination error due to being too far east or West of the pole for several hours!
The guidestar tracks along just fine, with almost no drift up or down. This is why it is CRITICAL that you look at the Celestial Equator / Meridian area to spot any AZIMUTH misalignment.
The SAME logic holds for any misalignment above or below the celestial pole, except in this case, you must look at an Eastern or Western star to clearly show it. If you only look at the celatial equator / meridian area you will NOT spot any misalignment above or below the pole.
Here's a visual demo of moving the mount. If you remember from the tutorial, you can simply watch the guidestar and make it move a certain direction and that will make sure you are making the appropriate adjustment. In this animation, you can see how that works.
This is the same Eastern misalignment shown above. If you remember the madness, when you see the guidestar move up - you adjust the mount to make the guidestar move to the RIGHT (with a refractor or SCT). Watch the guidestar in this example when the mount is being moved to the WEST.
Press the demo to begin.
I hope this clear up the whole Polar Alignment issue, and makes the Drift Alignment method a bit easier to understand! Please email me if I can improve this discussion or if I have made any errors! Thanks!!!
Now try your hand at the Drift Method with the Guiding Simulator. The simulator will let you practice polar alignment or manual long exposure Guiding. Click on the image of the simulator after you finish with this tutorial to try it.
