What do I need to know about mechanical backlash and guiding?
November 10, 2011
Mechanical backlash is caused by static friction (also called stiction). It happens when your mount makes minor adjustments to its position in the same direction, stopping between each adjustment. This situation occurs when guiding in declination, as signals are pulsed to the mount from manual guiding or an autoguider. It cause problems in RA because there is always tracking motion in RA when guiding.)
Mechanical backlash has been around for a long time. Here are some tips used by successful imagers to help combat mechanical backlash and make your guiding better:
1. Use gravity. By weighting or pre-loading in one direction or the other, you can dramatically reduce the mechanical friction. As one example, if you have drift to the north, add a small weight on your tube that will create a pull to the north. This puts tension on the drive in that direction, shrinking the backlash. (This trick does increase the backlash in the other direction, but since the mechanical backlash is much smaller than the gear backlash, it won’t be noticed.) Start with a small load and increase if needed, as you don’t want so much tension that the motion for a small correction in declination will overshoot.
2. If you can’t or won’t use this trick, then your best bet is to carefully balance your scope’s tube in declination. (This is recommended for remotely controlled observatories.) Rebalance after changing cameras or other accessories. Preserve your mount’s alignment while rebalancing by using the Set Mount Position function under the Utilities menu.
3. Use the autoguider settings. Many autoguiders/autoguider software programs allow you to set maximum pulse duration. The pulse duration must be long enough to overcome the mechanical backlash of your mount. Another setting you can vary is the Autoguide Rate in the NexStar hand control under Scope Setup.
If it’s available in your autoguider setup, try the minimum displacement control. For example, if your mount has mechanical backlash of 3 arc-seconds, set the minimum displacement to 3.5 arc-seconds. Now the autoguider will send a pulse long enough to overcome the backlash plus a bit more, leaving a net displacement of 0.5 arc-seconds. The drift will still be 3.0 arc-seconds, but once the error rises again to a value of 3.5, another pulse is sent resulting in another 0.5 arc-seconds net displacement. This will repeat and move your mount in tiny steps of 0.5 arc-seconds, always use 3.0 arc-seconds behind the object you are guiding on. As long as you start your exposure after the first correction from the autoguider, the star will not drift by more than 0.5 arc-seconds during your exposure.