Astronomy Basics • 35
Declination Drift
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 guide stars. The drift of each guide star
tells you how far away the polar axis is pointing from the true celestial pole and
in what direction. Although declination drift is quite 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
will be revealed. While monitoring a star near the east/west horizon, any
misalignment in the north-south direction will be 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 with the scope on the side of the mount, insert the
diagonal so it points straight up. Insert a cross hair ocular and align the cross
hairs to be parallel to declination and right ascension motion. Use ± 16x guide
setting to check parallelism.
First choose your star near where the celestial equator and the meridian meet.
The star should be approximately ±1/2 hour of the meridian and ±5 degrees of
the celestial equator. Center the star in the field of your telescope and monitor
the drift in declination.
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If the star drifts south, the polar axis is too far east.
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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 managed to eliminate all drift, move to the star near the eastern
horizon. The star should be 20 degrees above the horizon and ± 5 degrees of
the celestial equator.
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If the star drifts south, the polar axis is too low.
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If the star drifts north, the polar axis is too high.
Once again, make the appropriate adjustments to the polar axis to eliminate
any drift. Unfortunately, the latter adjustments interact with the prior adjust-
ments 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 will now be able to 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. However, you will have to reverse the polar high/low error directions. If
using this method in the southern hemisphere, the procedure is the same as
described above. However, the direction of drift is reversed.
