Declination Drift - Celestron C4.5 Instruction Manual

NOTE:
Declination Drift
This method of polar alignment allows you to get the most accurate align-
ment 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 merid-
ian. 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. As for hardware,
you will need a motor drive, an illuminated reticle ocular to help you recog-
nize any drift. For very close alignment, a Barlow lens is also recommended
since it increases the magnification and reveals any drift faster.
Insert the cross hair ocular 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 C4.5 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.
* Ifthe star drifts south, the polar axis is too far east.
* Ifthe 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.
e Ifthe star drifts south, the polar axis is too low.
* Ifthe 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 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 can now do prime focus deep-sky
astrophotography for long periods.
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.
Astronomy Basics œ 31