What is rough/ precise polar alignment?
February 20, 2005
Polar alignment of your equatorial mount can be done in several ways. The easiest way is to use your latitude.
The polar axis of an equatorially mounted scope must point at or be polar-aligned to the north celestial pole, the point in the sky around which all the other stars appear to rotate. The pole is directly above the north point on the horizon. So your axis must point both north and also be tilted up at an angle. Since the altitude of the north celestial pole is always equal to your latitude on the Earth, you can just use the scope’s latitude adjustment to raise the polar axis to the right angle.
Your latitude will equal the altitude of the north
Either look on a map, use Google Earth or an almanac to find your observing site’s latitude. Unlock any latitude adjustment screws on the sides of the mount and turn the latitude adjustment screws until the index on the polar axis reads your latitude. Tighten the adjustment screws if needed to secure the latitude setting. (You may also need to loosen the center pivot bolt by turning the hex nut to allow the equatorial mount head to be tilted.)
Now complete the polar alignment by turning the entire mount (not either axis – both should be clamped tightly) to align the upwards end of the polar axis with north on the horizon. If doing this at night, north is located directly below Polaris, the Pole Star.
Another frequently used method is to point to Polaris. This star is located only one degree from the north celestial pole, the point in the sky around which all the other stars appear to rotate, and where the polar axis of a properly aligned equatorial mount should point. To do this:
1. Set up the mount so that the polar axis is pointing north.
2. Unlock the declination clamp and move the scope in declination so that the tube is parallel to the polar axis. Your declination setting circles should read 90 degrees in this orientation. Clamp the declination lock.
The last steps involve moving the entire mount. Do not use either the RA or Dec motions to change the position of the tube.
3. Move the mount in altitude and azimuth until Polaris is in your finder’s field of view or centered in your finderscope.
4. Adjust the position of the mount by again moving the mount, this time centering Polaris in the eyepiece field of view. Altitude can be adjusted using the latitude adjustment screw or shortening-lengthening tripod legs.
The alignment is now good enough for visual purposes. To refine this alignment, obtain a chart showing the offset of Polaris from the pole and move the mount so that this point (in the sky) is centered in the eyepiece field of view. You now have an excellent polar alignment well within one degree of the true north celestial pole.
The Precise method is another way that you would be able to do a polar alignment.Polar alignment of your equatorial mount can be done in several ways. The most precise way is to use the drift of a star in declination (north or south on the sky) in your field of view.
The declination drift method requires that you monitor the drift of selected stars. The drift of each star tells you how far away the polar axis is pointing from the true celestial pole and in what direction. Although declination drift is simple and straight-forward, it requires a great deal of time and patience to complete when first attempted.
The declination drift method should be done after using latitude and north celestial pole alignment techniques.
To perform the declination drift method, you need to choose two bright stars. One should be near the eastern horizon and one due south near the meridian. Both stars should be near the celestial equator (i.e., 0 degree declination). You will monitor the drift of each star one at a time and in declination only. While monitoring a star on the meridian, any misalignment in the east-west direction is revealed. While monitoring a star near the east horizon, any misalignment in the north-south direction is revealed. As for hardware, you will need an illuminated reticle ocular to help you recognize any drift. For very close alignment, a Barlow lens is also recommended since it increases the magnification and reveals any drift faster. When looking due south, insert the diagonal so the eyepiece points straight up. Insert the cross hair ocular and rotate the cross hairs so that one is parallel to the declination axis and the other is parallel to the right ascension axis. Move your telescope manually in RA and DEC to check parallelism.
First, choose your star near where the celestial equator (i.e. at or about 0 degree in declination) and the meridian meet. The star should be approximately .5 hours of right ascension from the meridian and within five degrees in declination of the celestial equator. Center the star in the field of your telescope and monitor the drift in declination.
If the star drifts south, the polar axis is too far east.
If the star drifts north, the polar axis is too far west.
Make the appropriate adjustments to the azimuth of the polar axis to eliminate any drift. Once you have eliminated all the drift, move to the star near the eastern horizon. The star should be 20 degrees above the horizon and within five degrees of the celestial equator.
If the star drifts south, the polar axis is too low.
If the star drifts north, the polar axis is too high.
This time, make the appropriate adjustments to the polar axis in altitude to eliminate any drift. Unfortunately, the latter adjustments interact with the prior adjustments ever so slightly. So, repeat the process again to improve the accuracy, checking both axes for minimal drift. Once the drift has been eliminated, the telescope is very accurately aligned.
Note: If the eastern horizon is blocked, you may choose a star near the western horizon, but you must reverse the polar high/low error directions. If this is done in the southern hemisphere, swap south and north in the above instructions.