Precise Polar Alignment – Declination Drift Technique

Precise Polar Alignment – Declination Drift Technique

For long exposure imaging, the alignment using Polaris will not be perfect and stars will slowly drift across the image. The declination drift method allows a much more accurate alignment by monitoring 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 time and patience to complete when first attempted. The declination drift method should be done after the above polar alignment steps have been completed.

You will need to choose two bright stars, one near the eastern or western horizon and one due south near the meridian. Both stars should be near the celestial equator. While monitoring a star on the southern meridian, any misalignment in the east-west direction is revealed. This is corrected by moving the mount in its azimuthal direction (i.e. east or west, or clockwise/anticlockwise). While monitoring a star near the east or west horizon, any misalignment in the north-south direction is revealed. This is corrected by moving the mount in the declination direction (i.e. up or down). The telescope mount manual will describe how to move the mount in these directions.

For drift aligning I use the software tool ‘drift align’ within PhD2. It is important to read the online guide to this process and follow the steps there: https://openphdguiding.org/tutorial-drift-alignment-with-phd2/

Here are my own notes:

After connecting the camera (either the guider or the main camera) to PhD2, you can start taking short exposures of about 1 second. You should see some stars in the image. If they are not in focus, then refocus. Then you can click the calibrate button – the software then moves the telescope in small amounts in different directions so the software can figure out automatically which way the image moves in the CCD frame (the software does not know in advance the orientation of the camera!). Then you can click the drift align tool button. The software will tell you to find a star in the South near the meridian and near the celestial equator. Within 20degrees of that point works fine. If you don’t know where the celestial equator is, then read aboutthat first. Click on your star so that a green box surrounds it. If the star is blurry then the camera or autoguiding camera are not in focus. After a minute or so the software shows a red line that track the alignment errors. Follow the steps in the manual to try to correct those by moving the azimuth screws on the mount to bring the red line horizontal. Its trial and error which way you move those.Keep a note of what works in the ‘notes’ section of the drift tool. If you are lucky, someone else hasalready done that! Once the red line is nearly horizontal, click done, and the software will tell you to point at a star near the East or West horizon. Then you repeat the steps to get the red line as flat as possible by moving the dec lever on the mount to raise or lower the angle at which the mount is pointed. Again, its trial and error which way the telescope mount should be moved. For extra precision, you can repeat the steps by returning to a star on the Southern meridian and then again on the East or West horizon.

Once the drift has been eliminated, the telescope is very accurately aligned. Try not to bump into the mount! You can now do prime focus deep-sky astrophotography for long exposure times. The laptop receives the signal from the autoguider via the USB cable, the software SIPS or PhD2 sends small corrections back to the mount via the USB2serial/rs232 cable. If the mount is not reasonably well aligned to begin with, the corrections may be too large to control the mount and the guiding software will give a lot of error warnings.