Polar Alignment for CCD Imaging
Polar alignment is the method by which you align a telescope mount's Right Ascension axis to the earth's axis of rotation. This allows the mount's RA (Right Ascension) drive motor to track objects in the sky to counteract the earth's rotation, allowing photography or CCD imaging of those objects. The more accurately the mount's RA axis is aligned with the earth's celestial pole, the more accurately the mount will track.
Telescope mounts that are suitable for photography/imaging have two axes of rotation at right angles to each other -- the RA axis, around which everything rotates when tracking, and the DEC (declination) axis. RA position specifies an object's current position in the sky along basically an east-west direction, while DEC specifies an object's position along a north-south direction.
Most mounts also have two other adjustments that can be made in order to align the mount with the celestial pole -- these are altitude (which adjusts the up-down position of the RA axis) and azimuth (which adjusts the left-right position of the RA axis). It is these two axes, altitude and azimuth, that you will be adjusting to align your mount for precise tracking.
First off, if you don't know this already you need to learn it quickly: Polaris, the North Star, does *NOT* coincide exactly with the center of the earth's axis of rotation. It's pretty close right now (the year 2004), but still not exactly on -- and it's getting further away as the earth precesses ("wobbles") on its axis of rotation. For quick viewing with many telescopes, it's usually good enough to point the RA axis of the mount at Polaris, and you'll be close enough to get decent tracking. For critical applications where you need very accurate tracking, like imaging, you have to do much better than point at Polaris. Let's look at why critical polar alignment is necessary for imaging, and some ways to get it very close to perfect.
If you have pointed your mount's RA axis roughly at the celestial pole, and the point the telescope that's on the mount to a position due south and at the celestial equator (that's where DEC is zero, and the scope points straight back along the mount's RA axis), you *should* be pointing right at the intersection of the celestial equator and the meridian (a line running north-south through both the north and south celestial poles). Put a crosshair eyepiece in your telescope, center a star on the crosshairs, and watch it for a while. What will happen to the star if your mount's RA axis is not pointing *right at* the celestial pole?
Refer to the image above. The black line represents the path stars actually take in the sky across the sky, with the center point of the curve being the intersection referred to above of the celstial equator and the meridian. Look at the blue line -- which represents the arc your mount will be making if its RA axis is too far LEFT of the celestial pole. After a while, the star will start to drift DOWNWARD in your eyepiece view (this depends on your telescope type, if you're using a star diagonal, etc. -- it might actually appear to drift UP). The further the mount tracks from the point where you first centered the star, the further away from the actual track of the star your mount will be. Now look at the red line -- which represents being too far to the right of the celestial pole. Now the star will appear to drift UP (remember, if could look like down), and again the further you get from the starting point the further up your mount will be from perfect tracking. Most of this drift appears to be in declination (up/down from this point of view), but because the sky and your mount are tracing different arcs, there will also be differences in the RA tracking as well. The further off you are from the pole, the faster this drift will show up, and the harder it will be to get good images. While you can use an autoguider to guide out some or all of this drift, since the sky is rotating along a different arc than your mount, you will get field rotation (the stars will appear to rotate around your guide star) even with perfect autoguiding if you're not properly aligned. Clearly, getting a good polar alignment is critical to making good images.
So how do you improve a rough polar alignment? One of the most popular, and tried-and-true methods, is called drift alignment. Drift alignment uses the geometry of the sky I outlined in the image above, and the fact that you can see drift in DEC fairly quickly when you're misaligned, to help you align your mount. Here's a brief description:
1. Point the telescope as I mentioned above -- close to the intersection of the celestial equator and the meridian.
2. Center a star in a crosshair eyepiece. Align the eyepiece so that the crosshairs match the axes, one line going N/S along the DEC axis, and the other going E/W along the RA axis. Put the star right on the E/W line vertically, and watch it for a few minutes.
3. Watch for drift up/down from the E/W crosshair line. You can ignore any RA drift. If it drifts up or down from that line, you need to adjust the azimuth (right/left) controls on your mount. Adjust it so that:
-- If the star drifts UP, move the azimuth so the star moves to the RIGHT in the eyepiece view (this works no matter what kind of eyepiece or diagonal you have).
-- if the star drifts DOWN, move the azimuth so the star moves to the LEFT in the eyepiece view.
4. After making an adjustment, re-center the star on the RA line, and watch for drift again. Repeat this process until you don't see any DEC drift for at least 5 minutes.
5. Now check for altitude alignment: leave the scope at DEC = zero, but swing the scope over to either the east or west horizon, as close to the horizon as you can. Find a star, and again center it on the RA crosshair in the eyepiece.
6. If you went to the Eastern horizon, imagine the image above rotated 90-degrees counter-clockwise; if you went to the western horizon, imagine the image rotated 90-degrees clockwise. Do you see why being on the horizon shows you whether you're too far up or down? ;-)
7. Watch for DEC drift as before. Follow the rules below to make adjustments to the altitude controls on your mount:
-- if the star drifts UP in the eyepice:
------ if looking EAST, move the mount so the star moves DOWN. If looking WEST, move the mount so the star moves UP.
-- if the star drifts DOWN in the eyepiece:
------ if looking EAST, move the mount so the star moves UP. If looking WEST, move the mount so the star moves DOWN.
8. As before, recenter the star and repeat until you see no drift for 5 minutes. If you made a large change in altitude, you should probably go back and re-do the azimuth drift test.
Drift alignment is simple to do, and will get you VERY close to perfect polar alignment if you use a fairly high-power eyepiece (so that you can see the drift quickly and accurately), and you do enough iterations. It's a simple and well-tested method that WORKS.
If you have a CCD camera, however, you can use a method similar to drift alignment to see how far you're off in polar alignment, but that doesn't require an eyepiece, and doesn't require you to stare at crosshairs waiting to see a little DEC drift! Here's a much faster method, with special thanks to J. Hall for writing it up!
Using a CCD Camera for Polar Alignment
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