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Polar Alignment

Precise 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° 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 R.A. and DEC to check parallelism.

First, choose your star near where the celestial equator (i.e. at or about 0º in declination) and the meridian meet. The star should be approximately 1/2 hour 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.
I
f 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.

Polar Alignment
As the earth rotates around its axis, the stars appear to move across the sky. If you are observing them using an altitude-azimuth (alt-az) mount, they will quickly drift out of view. Readjustments to get them back in view are awkward and frequent or require computerized tracking.

In order to avoid these problems for either visual astronomy or astrophotography, you need a different type of mount that’s oriented or aligned to make following the apparent motions of the stars much easier than with an alt-az mount.

A telescope on an equatorial mount can be aimed at a celestial object and easily track the daily motion, keeping it in your eyepiece. It works by first polar aligning or inclining it at an angle equal to your latitude and pointing one axis (called either the polar axis or right ascension (RA) axis) in the same direction as the earth’s rotational axis (towards the celestial pole). Once the polar axis is parallel to the earth’s axis and turned at the same rate of speed as the earth, but in the opposite direction, objects will appear to stand still when viewed through your scope. There is no rotation of the field of view and tracking can be extremely accurate, making the equatorial mount perfect for astrophotography.  It has two motions: in RA (east-west) and in declination (dec, north-south). With the use of setting circles, a polar-aligned equatorial mount can quickly find celestial objects.


Polar alignment

The north celestial pole (NCP) is the point in the sky
around which all the stars appear to rotate.
The star Polaris lies less than a degree from
the NCP and it can be used to roughly polar align
a telescope. However, for accurate polar alignment,
the polar axis of the telescope's mount needs
to be aligned to the true NCP.


Aligning the telescope to the earth's rotational axis can be a simple or rather involved procedure depending on the level of precision needed for what you want to do. For casual observing, only a rough polar alignment is needed. Better alignment is needed for tracking objects across the sky (either manually or with a motor drive) at high magnifications. Still greater precision is needed in order to use setting circles to locate those hard-to-find objects. Finally, astrophotography will require the most accurate polar alignment of all.

Rough 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 equals north celestial pole altitude
Your latitude will equal the altitude of the north
celestial pole.

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.

First, set up the mount so that the polar axis is pointing north.

Second, 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. Don’t use either the RA or Dec motions to change the position of the tube.

Third, move the mount in altitude and azimuth until Polaris is in your finder’s field of view or centered in your finderscope.

Fourth, tweak 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, get 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.

Accurate polar alignment

You can use your finderscope (finder) to accurately polar-align your equatorial mount. This is more accurate than a rough alignment using the mechanical scales on your mount or roughly pointing the whole mount towards Polaris.  

To use your finder as a polar finder is a three-step process. First, the optical axis of the finder must be aligned to the mechanical polar axis of your equatorial mount. Second, Polaris will be centered in the finder (polar axis pointing towards Polaris). Since Polaris is nearly one degree away from the true north celestial pole (NCP), the last step will offset the finder’s view and the polar axis to get true polar alignment.

The alignment of the optical axis of the finderscope with the mechanical polar axis of the mount can be done either at night with Polaris or (perhaps more easily) during the daytime on a distant building or landmark. If you do align during the day, use the latitude adjustment screws and the tripod to level the polar axis to make it easier to do.

NOTE: Any star can be used to get the finder optical axis and polar axis aligned. Polaris is chosen for convenience and also because it will be used in the second step.

You’ll flip the mount several times and recenter Polaris (or the landmark) in the finderscope each time, successively getting closer to alignment of the finderscope and polar axis.

Working with Polaris, start by setting up your mount as you would for polar alignment. The Dec setting circle should read 90 degrees. Unclamp the RA and rotate the mount until the telescope and finder are all the way to the left or right, Dec axis horizontal. Get Polaris in the field of view of the finder and centered in the crosshairs by moving the mount (using fine adjustment screws). Now move (flip) the mount all the way to the opposite side (180 degrees or 12 hours RA away from the original position). Note the shift of Polaris off the crosshairs. Actually the finderscope and the crosshairs themselves have rotated in a small semicircle around where the polar axis points. You can see where that is by looking through the finderscope as you rotate the mount, watching for the center of motion. Clamp the mount and turn the three setscrews around the finderscope to move the crosshairs over this pivot point. Recenter Polaris by only moving the mount.


Flipping the tube
Even with the telescope positioned 180 degrees away around
the mount, the telescope (and finderscope) should still be
pointing at the same object in the sky. 

Repeat the flipping-setscrew-recenter Polaris procedure. Each time you go from one side to the other, the off-center distance of the crosshairs from the pivot point will be smaller. After three or four repeats, the crosshairs won’t move when you flip the mount. You will be pointing at Polaris. Your polar finderscope optical axis is now pointing in the same direction as the polar axis. 

(If you did this during the day with a landmark, now wait until dark. Set up your mount as you would for polar alignment. The Dec setting circle should read 90 degrees. Use the latitude scale and adjustment screws to center Polaris in the finder.)

The pivot point
When rotating the scope and finder 180 degrees around the polar axis,
the crosshairs will rotate around where the polar axis is pointing
(this pivot point is the "X" in the right-hand figure). Adjusting the
finder and the equatorial mount until an object remains centered in the
crosshairs during the rotation aligns the finderscope with the
telescope's polar axis. 

Steps one and two are done and the polar axis of the telescope is aligned with Polaris, but as any star atlas will reveal, the true pole lies about ¾-degree away towards the last star in the Big Dipper’s handle (Alkaid). To make this final adjustment, the telescope mount will need to be offset from Polaris towards the actual NCP.

Since Polaris makes a complete rotation around the NCP once a day, how far should the mount be moved and in what direction? One easy way is approximation. Guesstimate the direction by using Alkaid. Guesstimate the amount by knowing the field of your finderscope and dividing it by the ¾-degree distance of Polaris from the pole.

Example: On August 1, at 8PM, Alkaid is above and to the left of Polaris in the 10 o’clock position. You have a 6-degree field-of-view finderscope. Starting with Polaris on the crosshairs, use the fine adjustment screws to shift the mount in altitude (latitude) and azimuth up and left by one-eighth finder field (6 divided by 0.75).

Offset of NCP from Polaris
The true North Celestial Pole (NCP) lies less than a degree away from Polaris in the direction of Alkaid, the last star in the handle of the Big Dipper (Ursa Major). 

Now use the setting circles to check how close the polar axis alignment is to the NCP. Unclamp the axes and swing the scope’s tube to a bright star of known RA and Dec near the celestial equator. Turn (set) the RA setting circle to this star’s RA. Now move the tube until the RA setting circle reads 2 hours 31 minutes and the Dec circle reads +89 degrees 15 minutes. These are the coordinates of Polaris and the Pole Star should now be in the finder’s crosshairs. If it’s off, once again move the mount in latitude (altitude) and azimuth to center Polaris.

Now you have a polar alignment for your scope within a fraction of a degree of the NCP. This is excellent for visual purposes and short-exposure photographs piggybacking on the main tube. However, guiding corrections and field rotation will still be problems for long-exposure astrophotography, which demands the most precise polar alignment.

NOTE: At the completion of this process, Polaris may very well not appear in the center of your main scope's eyepeice field. This is because the optical axis of the finder and the polar axis are now parallel. But the finder's optical axis and the main tube's optical axis may not be parallel. So centering a star in your finder won't necessarily center it in your eyepiece. To overcome this, once you've achieved polar alignment, you can realign the finder with the main tube optics to return the finder to normal operation with the main scope.


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