While watching declination (DEC) drift only, if the guide star
moves UP, rotate the mount's AZIMUTH so that the guide star moves to the RIGHT in
your eyepiece. If the guide star moves DOWN, rotate the azimuth so that the star
moves LEFT. The reason you make these moves is described in the TUTORIAL. DO NOT
use the slew buttons on the hand controller to do this. Use the AZIMUTH
adjustment screws on the telescope mount.
The faster the guide star
drifts up or down, the more azimuth corrections you must make.
If the guide star drifts quickly, your mount is much further away
from the celestial pole, and will need to be moved a lot
further.. so far in fact that the guide star will move right out
of the field of view! No worry.. you can always grab another
star to finish the alignment process.
An excellent simulator that allows you to practice
this technique can be found at
Polar alignment (How to do Drift)
TO properly do the drift alignment you
should have an eyepiece with an illuminated reticule. If that's
not available you can find programs on the web that will
allow you to put a cross hair on your screen. One such is
STARTARG and can be found at
Andy's Shot Glass for a nominal fee.
Drift alignment do's and don'ts:
DO level the equatorial head before you
start. This isn't essential but it means that azimuth adjustments
won't affect elevation adjustments (or vice-versa).
DO make sure that the optical train is
not flexing. Mirror flop, bendy focusers etc will ruin your attempts
at polar alignment.
DO make the azimuth adjustment first.
It isn't significantly affected by refraction since stars are moving
parallel to the horizon as they cross the meridian, so you can take
time to get this as accurate as you wish.
DO use the polar drift method at least
for the elevation adjustment if you have a view of the pole. This is
quicker and more accurate than the standard declination drift method
above the horizon.
DO use a star in the eastern half of
the sky for the elevation adjustment if you can, when using the
standard method. The effects of refraction will decrease as it rises
higher in the sky. Avoid anything much below about 30 degrees
altitude where the effect of refraction becomes more severe.
DO use a star in the same region of sky
each time for the elevation adjustment when using the standard
method, if you are iterating between azimuth and elevation
adjustments. Otherwise you will find a different elevation
adjustment each time you do it.
DO approach the pole from below for the
elevation adjustment - you are raising the polar axis against
gravity which may give fewer problems with backlash.
DON'T expect perfection - there will
always be drift in hour angle and declination as a consequence of
atmospheric refraction - and we need the atmosphere to breathe!
DON'T fiddle on all night - an
important part of drift aligning is knowing when to stop. Remember
to take some images before the clouds roll in!
Another method that I have used is the
CCD drift method. Here you use you imaging camera to
get a picture of the drift angle. Basically you set up just
as in the Drift method above and take a 125 second image of a star in
the FOV. Hold that position for the first 5 seconds to get a
reference point then move the scope with the controller West for one
minute. Immediately after move the scope EAST for the remaining minute.
IF you scope is aligned properly the trace created by the East movement
should overlap the trace created by the WEST movement. If it's out of
alignment the image will create a horizontal "V" where the amount
of correction is shown by the angle created by the intersecting
lines. More details on this process can be found at
Drift alignment .
Drift Alignment Using PHD Guiding Software:
Just recently an acquaintance ( Ted Rafferty) put me on to doing
the drift alignment using the popular auto guiding program (PHD)
available for free at
I would recommend that you align your camera to the RA/DEC
axis of the telescope. I usually shoot images with the DEC axis being up
and down on the CCD chip. This also helps me orient the image later as I
prefer to show them with North being up.
The process begins as all the other drift alignments. To check the RA
alignment choose a star at 0° elevation close to
the meridian. To set the DEC alignment choose a star as close to
the E or W horizon as possible.
- Turn on the guiding program (PHD) , select the mount and camera
and run thru your calibration a usual.
- To Check the RA select a star near the Meridian at 0°
- Start Guiding on the selected star.
- Turn on the GRAPH and select DX/DY
instead of RA/DEC.
- Turn off the DEC guiding
- If your mount is perfectly aligned the DY
( red line) should track across the graph near the center line. You
may see some wiggle above and below the center line. If
it drift up or down you need to make a very small
adjustment to your AZIMUTH
screws to compensate for that. You will see the change
immediately. No need to wait 5 or 10 minutes for the visual
- Now set up the elevation.
- Stop the guiding and look for a star near the
western or eastern horizon at whatever elevation allows you to clear
any obstruction. The lower the better.
- Recalibrate the guiding on the new star.
- As in step 3,4 & 5 above start guiding and make sure that
the DEC guiding is off.
- Once again if your alignment is on then the
DY trace should not drift from the
horizontal graph line. If it does, carefully make small adjusts to
the ELEVATION screws to
compensate. Again you should see an immediate change in the graph.
This process is simple enough to allow you to check you
polar alignment whenever you feel it necessary. The only
requirement is that you get a copy of PHD and have a
camera that is supported by the program ( there are many).
As an update, the new PHD2 version 2.2.2 has a wizard that facilitates
the above process by turning off the guiding and prompting you what
to do next. I haven't used it yet but it sounds like a winner if you
use this type of alignment.
You can view the help file info for the wizard here. PHD2 is
available free HERE.
Here is an excellent
YouTube Video on how to use the PHD2 polar alignment process.
An update to the above. I tried the Drift alignment tool in PHD2 and
found it to be very easy to use and resulted in very good polar
alignment with little difficulty. Definitely something to keep in your
bag of tricks.
Using SBIG CCDops:
run across another method that uses the CCDops available from
SBIG. The details written up by Mike Malik can be found on the
HERE. The Polar alignment instructions can be found in Appendix P. It's a bit more time consuming but should also
yield excellent polar alignment.
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