Celestron C8-S - Manual (Page 34)

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Declination Drift Method of Polar Alignment
This method of polar alignment allows you to get the most accurate alignment on the celestial pole and is
required if you want to do long exposure deep-sky astrophotography through the telescope. 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 any one of the previously
mentioned methods has been completed.
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/west horizon, any misalignment in the north-south direction is revealed. It is
helpful to have an illuminated reticle eyepiece 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 eyepiece and
align 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 and the meridian meet. The star should be
approximately within 1/2 an hour of the meridian and within five degrees 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 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.
Again, make the appropriate adjustments to the polar axis 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. You can now do prime focus deep-sky astrophotography for long periods.
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. Also, if using this method in the southern hemisphere,
the direction of drift is reversed for both R.A. and DEC.