Drift Alignment

Having a good polar alignment is necessary for astro-photography and there are several methods of achieving this. Polar alignment scopes and software routines make it simple to achieve anywhere from reasonably close to almost dead on in polar alignment but not everyone has access to these. In this case the method to use is called 'drift' alignment; so called because it uses how a star drifts in a reticule eyepiece to adjust the polar axis precisely.

A good starting point is very helpful so start with a compass (properly set for magnetic deviation) and a level to get the mount close to being aligned.

Fig. 105

85 Fig. 106

Now select a star near the meridian close to the celestial equator and put the star in the reticule eyepiece and watch it for several minutes to watch for any drift. If it drifts north then the polar axis is pointing too far east if looking south. If the star drifts south then the axis is pointing too far west if looking south. Adjust the axis and re-centre the star. Continue until the star 'rides' a cross hair for 15 minutes.

Now select a star in the east near the horizon and around the celestial equator. Put the cross hair on it and again watch it for drift. If it drifts north then the RA axis is above the pole (between the pole and the zenith). If it drifts south then the axis is below the pole (between the pole and the north horizon). Adjust until the star again rides the cross hair for 15 minutes. Then go back to the meridian because things there will have gotten out of kilter.

Fig. 107

86 Fig. 108

The drift alignment method has the ability to put a mount dead-on the celestial pole. It is more useful for permanently mounted telescopes but I've done it for scopes that won't move for a few days. Other than that I've found that the alignment routine in my CGE's firmware can get me very close in a short time.

Notice that I said in the beginning that a 'good' polar alignment is necessary not a 'perfect' polar alignment is necessary. For the short subs that many people take good is fine as there won't be any noticeable field rotation in that short time. Also, remembering the mis-alignment mentioned in the previous section to allow easier guiding one may not want a perfect alignment.

Image of M27 taken with an unmodified Canon Rebel XT mounted on a Celestron C8 at f/6.3. Ten 5-minute images at IS800.

87