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Reduce the R.A.
Aggressiveness in the Advanced Parameters menu
to better stabilize the guiding during bad seeing or wind gusts. You
may also want to decrease the
R.A. Aggressiveness if your guiding
exposures/corrections are set very quickly (less than 1 second). If the
guider sends several corrections per second to the mount, the mount’s
movement may osculate due to the response time of your mount.
Additionally the varying seeing conditions makes the star appear to
jump around rapidly and cause the guider to make unnecessary cor-
rections, sometimes referred to as “chasing the seeing”. If seeing is
poor, keep the guiding correction intervals/exposures to 1 second or
more. For most guide scopes, we recommend setting the autoguide
exposures between 1-4 seconds for best results.
Polar Alignment
A good polar alignment of your EQ mount is of critical importance for
long-exposure imaging. Inaccurate polar alignment leads to field rota-
tion, even with the autoguider tracking. If your equatorial mount uses a
polar axis finder scope, we highly recommend utilizing it for polar align-
ment. If not, a technique known as the “drift method” of polar alignment
has been used for many years, and can achieve an extremely accurate
polar alignment. Unfortunately it is very time consuming, since the
drift of a star over time must be observed. The basic idea is to let the
telescope mount track while watching a star to see which way the star
drifts. Note the direction of the drift, and correct by moving the mount
in the appropriate direction.
To perform the drift method of polar alignment:
1. Roughly polar align your mount by pointing the R.A. axis of the
mount at Polaris (the NorthStar).
2. Find a bright star near the meridian (the imaginary line running
north-to-south through zenith) and near the celestial equator (zero
degrees declination). Point the telescope at this star, and center it
in an illuminated reticle eyepiece (available from Orion). If you don’t
have an illuminated reticle eyepiece, use your highest- magnification
eyepiece.
3. Determine which way is north and south in the eyepiece by moving
the telescope tube slightly north and south.
4. Now, let the mount’s motor drive run for about five minutes. The star
will begin to drift north or south. Ignore any east-to-west movement.
5. If the star drifts north, the telescope mount is pointing too far west.
If the star drifts south, the telescope mount is pointing too far east.
Determine which way the star drifted and make the appropriate cor-
rection to the azimuth position of the mount. Rotate the entire mount
(and tripod) slightly east or west as needed or use the azimuth
adjustment knobs (if your mount has them) to make fine adjust-
ments to the mount’s position.
6. Next, point the telescope at a bright star near the eastern horizon
and near the celestial equator (Dec. = 0).
7. Let the telescope track for at least five minutes, and the star should
begin to drift north or south.
8. If the star drifts south, the telescope mount is pointed too low. If the
star drifts north the telescope mount is pointed too high. Observe
the drift and make the appropriate correction to the mount’s altitude
(or latitude); most mounts have some sort of fine adjustment for this.
Repeat the entire procedure until the star does not drift significantly
north or south in the eyepiece after a few minutes. When this is accom-
plished, you are very accurately polar aligned. Autoguiding on a well
polar aligned mount will yield the best results for your images. The
fewer tracking corrections that have to be made, the better your images
will be.
Differential Tube Flexure
A common obstacle in guiding with a separate guide scope and imag-
ing scope is differential tube flexure, the unwanted movement of a
guide scope in relation to the main imaging scope. If any part between
the autoguider and imager moves or flexes during the exposure, then
the resulting image will have oblong stars which gives the appearance
of poor tracking. There are a few basic measures to ensure your guide
scope is securely in place:
