Consistent-Approach Slewing

Some low-end amateur mounts have a certain degree of backlash in their drive systems. This causes reduced go-to accuracy, specifically, the repeatability of position is reduced. Depending on the direction of the slew, the drive gearing and bearings are loaded in a particular direction. When the direction is reversed, some slack is taken up before motion in the reverse direction occurs. The objective of consistent-approach slewing is to always approach a target from the same direction in RA and Dec, making the repeatability of slewing as good as possible. You can turn it on in the Telescope Preferences tab. For more info read How Does CA Work? below.

Consistent Approach is something you should use only if you really need it. It slows things down, robbing you of observing efficiency, and puts extra motor/gear cycles on your mount.

ACP's built-in Telescope Pointing Corrector will perform only as well as the mount's repeatability allows. Since the pointing corrector learns as it goes, errors that appear random (due to variations in the direction of approach to the slew destination) can nullify the effect of the corrector. In extreme cases, the corrector's learning curve will climb instead of descend, and errors will actually be magnified.

What is Consistent-Approach?

Consistent-approach (CA) is a technique for assuring that a slew will approach its final destination from the same direction at all times. By doing this, the effects of backlash will be consistent, and thus the mount will have the best possible slewing repeatability (essential for getting the best out of the pointing corrector).

How Does CA Work?

Since the right ascension motor drives the scope toward the west during sidereal tracking, the best approach for right ascension is toward the west. This assures that the slack in the RA drive is always taken up.

For declination, there is no "right" direction. I determined that CCD-equipped scopes are more likely to be weighted toward the rear and thus would want to tilt-up if the clutch were released. Thus I chose to approach from the direction of the celestial pole, towards the equator. In the southern hemisphere, the approach is from the south, and in the northern hemisphere, the approach is from the north.

German Equatorial Mounts

German equatorial mounts have additional issues due to their meridian flipping. ACP attempts to track the mount's motion and estimate when meridian flips occur. The Flip Settings in the Telescope Preferences tab are critical to this operating properly. Click here for more information on this.

For right ascension, if the final approach from the east happens to cross the mount's flip angle, the mount will flip and then complete the slew to the final destination, negating the effect of consistent approach. In this case, ACP does a three-phase consistent approach: (1) Slew to the final destination, forcing the flip, then (2) slew back to the initial approach location, and (3) complete the consistent-approach slew to the final destination.

For declination, the direction of approach is reversed depending on which side of the pier the mount is positioned. ACP handles this by reversing the normal declination approach direction when the mount is on the east side of the pier looking west.