Poor Goto Accuracy

The "Alignment Successful" message appeared on the HC, and the scope slews to where it thinks the object of choice should be. But, there is nothing in the eyepiece. The root of this problem is usually insufficient care taken in alignment star centering. Center the stars in the main scope's eyepiece as accurately as possible, preferably using a medium-power crosshair reticle eyepiece.

Another potential cause of poor go-to is backlash. The gears on most telescopes have some slack in them. That can be a problem for go-to accuracy if the scope does not know about it during alignment. Meade telescopes inform the computer about backlash via a procedure called drive training, which is found in the Autostar utility menu. This procedure has the user point the scope at a distant terrestrial object (Polaris works even better). The CAT then slews away from the target and has the operator recenter it with the HC direction buttons, enabling the computer to determine the exact amount of backlash present. Be sure to do both azimuth and altitude training; they are separate procedures in the hand control. Celestron scopes use a simpler method to take into account backlash effects during alignment. When centering alignment stars with the NexStar controller, only use the up and right keys (down and left on some models; see the manual) for final centering. Down and left can be used to position the star in the eyepiece field initially.

There is one other major reason nothing is in the CAT's eyepiece after the slew stops: The object selected is too dim for the scope or conditions. Just because the Autostar's object library includes M74 that does not mean that terrifyingly dim face-on Messier galaxy will be visible from a bright backyard with an ETX90.

Finally, Meade and Celestron go-to CATs work well, but they are not dead-on accurate all the time across the entire sky. Objects in the east, for example, may be near the center of the eyepiece field, while those in the west are not visible at all. One way to fix that is to "sync" the scope on a bright star in the area of interest. Go to a prominent star in the "off" area, center it in the scope, and, following HC/manual instructions, sync it. That alters the scope's model of the sky and ensures objects in the area of the sync star wind up in the eyepiece. The only problem with syncing is that when the scope is moved very far from the sync star, go-to accuracy will degrade rapidly. Celestron scopes include an "unsync" feature that returns the sky model to its original condition.

As a final suggestion, If you are having go-to problems in a particular part of the sky, just switch to your lowest-power, widest-field eyepiece to give the scope the best chance of landing on its targets; most of the time that works fine. Some scopes feature a "Precise Go-to" mode that also works well. When Precise Go-to is turned on, the scope will stop at a bright star in the area of the target object. The observer centers this star, pushes a button, and the telescope continues on to the requested object, which will usually wind up in the field thanks to the "auxiliary centering" of the bright star.

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