How do I polar align my telescope if I'm in the southern hemisphere?
August 1, 2007
Polar alignment for southern hemisphere observers is very similar to techniques used for the northern hemisphere. The major difference is the lack of a bright pole star like Polaris near the south celestial pole (SCP) to aid orientation used in several alignment procedures.
Rough polar alignment using the latitude scale of your scope is identical to the procedure used in the northern hemisphere.
Either look on a map, use Google Earth, or consult 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.)
Latitude adjustment of the mount.
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 south on the horizon.
Another more accurate rough method is to point to Sigma Octantis. This star is located only one degree from the SCP, 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 south.
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° in this orientation. Clamp the declination lock.
Note: These last steps involve moving the entire mount. Don’t use either the RA or Dec motions to change the position of the tube.
Move the mount in altitude and azimuth until Sigma Octantis is in your finder’s field of view or centered in your finderscope. At magnitude 5.5, it is a fainter star--just slightly brighter than the unaided eye limit--so your finder will definitely be helpful here.
Sigma Octantis and the SCP
Tweak the position of the mount by again moving the mount, this time centering Sigma in the eyepiece field of view. Altitude can be adjusted using the latitude adjustment screw or shortening-lengthening tripod legs.
For greater accuracy, you can point directly at the SCP using an offset from Sigma Octantis. (This can be done after aligning the optical axis of your finderscope with the polar axis.) Use the chart above or these patterns of southern circumpolar stars to find the SCP. Draw an imaginary line toward the SCP through the Southern Cross stars Gamma Crucis and Alpha Crucis (down the long axis of the cross). Draw another imaginary line toward the SCP at a right angle to a line connecting Alpha Centauri and Beta Centauri. The intersection of these two imaginary lines will point you close to the south celestial pole.
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. Again, this is virtually identical to the northern hemisphere procedure.
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 south 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 north 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 north, 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 5° 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 north, the polar axis is too far east. If the star drifts south, 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° above the horizon and within five degrees of the celestial equator.
If the star drifts north, the polar axis is too low. If the star drifts south, 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.