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
- 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