Why does my NexStar GPS telescope jump when I press or release a direction button? Is this backlash?
Your scope is experiencing backlash–play in the drive gears. All telescopes have some backlash, as gears cannot be too tight, preventing the motors from turning. Backlash causes the delay in scope movement when using the direction arrows on the hand control. It’s especially a problem when moving in the direction opposite the scope’s tracking, when the lag before the scope moves can be on the order of 10 seconds if the backlash is bad. The goal is to minimize backlash and your telescope’s motor control firmware can rewind the motors enough to eliminate gear play when the arrow is pressed and winding it back the other direction when the arrow is released–to make the gears re-engage and go back to smooth tracking. Positive backlash compensation is applied when the mount changes its direction of movement from backwards to forwards. Negative backlash compensation is applied when the mount changes its direction of movement from forwards to backwards. When tracking is enabled, the mount will be moving in one or both axes in either the positive or negative direction, so backlash compensation will always be applied when a direction button is released and the direction moved is opposite to the direction of travel. The amount needed will depend on the slew rate: slower slews will be more sluggish and will need higher values. Visual work will be less demanding and won’t need as high values as guiding for astroimaging. The procedure is as follows: Align the scope. Go to Menu – Scope Setup – Anti-Backlash, where you can input positive and negative values to control the winding compensation for both azimuth (left-right arrows) and altitude (up-down arrows). Zero all four values. Now look at an object in your eyepiece and pay attention to the responsiveness of each arrow button. Work separately in azimuth and altitude, adjusting one axis at a time so that the positive and negative settings are high enough to move the scope immediately but don’t cause the scope to jump when pressing or releasing the button. Start with identical values for positive and negative settings. If the scope jumps upon button release, then pauses if you lower the settings, try a higher positive value and a lower negative value. Try values between 20 and 50 for visual work. For the GPS mount, reasonable responsiveness is motion within 3 seconds at rate 3. Your scope will remember and apply these settings each time it’s turned on until you change them. Updated 12/21/13
Created On: Oct 30 2008 02:00 PM