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PHD2 Drift Align: ( excerpt from Phd2 Help file)

Drift alignment is a well-known technique for achieving polar alignment and is considered by many to be the "gold standard".
The Drift Alignment tool is a "wizard-like" sequence of dialogs that can help you work through the drift alignment process and get quantifiable results. Once you've calibrated your guider:

  • Click on 'Drift Align' under the 'Tools' menu. The first Drift Align dialog will appear to help you adjust the azimuth on your mount. If you are using an ASCOM mount, you'll have the option of slewing to an area near the celestial equator and the celestial meridian. If you're not using an ASCOM mount, you'll need to slew to that location manually.
  •  Once the scope is positioned and you have a suitable star in the field of view, click on the 'Drift' button to begin collecting data. You'll see the graph window with a display of star deflections and corrections and, more importantly, two trend lines. Let the exposures continue until the declination trend line has stabilized and is no longer jumping around with each new exposure. At the bottom of the graph window, you'll see a measurement for the polar alignment error in azimuth.
  •  Now click on the 'Adjust' button to halt guiding, then make a mechanical adjustment in azimuth.
  • Once done, click on the 'drift' button again to repeat the measurement. Continue iterating in this way until you are satisfied with your azimuth accuracy.

You can use the 'notes' field to record which way the drift line moves depending on how you make the adjustment. For example, you might note that a counter-clockwise turn of the mount altitude moves the drift line "up." Since these notes are retained across PHD2 sessions, subsequent drift alignments will probably proceed more quickly.

Until you are experience with drift aligning your particular mount, the 'adjustment' part of the process can be a bit tedious. At first, you'll have to determine how to adjust a knob on the mount to achieve the desired effect: "how much" and "what direction." To help with this, the PHD2 drift align tool supports "bookmarks".
 These are a handy way to record the positions of the guide star before and after you've made an adjustment. Bookmarks are accessed using keyboard shortcuts, as follows:  

  • b : toggle/show bookmarks
  • Shift-b : set a bookmark at the current guide star position (the "lock position")
  • Ctrl-b : clear all bookmarks
  • Ctrl-click somewhere on the image : set a bookmark at that position, or remove the bookmark that's already there

By setting a bookmark before you make a mount adjustment, you can get a clear view of how the adjustment has moved the star on the guide frame.

  • Next, click on the 'Altitude' button.
  • Then slew the scope to a position near the celestial equator and 25-30 degrees above the horizon.
  •  Click on the 'drift' button to begin collecting data for the altitude part of the alignment process.

 As before, you will iterate between making adjustments and measuring your alignment until you are satisfied with the result, using the declination trend line as a guide as for the AZ adjustment  and   keeping notes as you go about how mount adjustments affect the behavior of the declination  drift line. If you make substantial adjustments in altitude, you'll need to go back to the 'azimuth' measurement and repeat that procedure. If you work through these procedures systematically, you'll converge on a good polar alignment with a known degree of accuracy. A good polar alignment will help your guiding performance, especially in declination.



Page last changed: 10/14/2014

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