Understanding Homing vs Syncing

There are primarily two types of focuser in the astronomy world, ASCOM calls them Absolute and Relative. Absolute focusers can repeatably move to specific locations, while relative can only be told to move in or out, not to a specific location. Absolute focuser positions are normally measured in steps, where a step is the smallest discreet distance that a focuser can move.

All Optec focusers fully support the ASCOM requirements to be Absolute focusers. Optec further divides our focuser models into two separate groups, Homing and Syncing. These groups are based on how the focuser finds and maintains the zero-point.

Optec’s Homing focusers automatically find the zero point and all moves are made referencing that location. They have a well-defined zero-point, maximum position, and step size. Because the focuser always knows the zero-point Homing focusers require very little setup. All of Optec’s complete focuser units are homing.

The Optec QuickSync and DirectSync lines are designed to motorize an existing focuser. Because of this they do not support automatic homing. Instead they allow the user to sync the focuser, telling it where the zero-point is. All moves are then made referencing that location.

A strength of the QuickSync and DirectSync focusers is the ability to disengage the motor and move the focuser manually. One of the keys to success with a syncing focuser is using a consistent sync position. After a manual move the focuser should be resynced so the controller knows where the focuser is. This helps autofocus and automation software as the zero-point remains consistent, any stored positions and offsets will remain valid.

By selecting a repeatable location for syncing the focuser can be quickly and consistently used for both manual (using the focus knobs) and computerized (motorized) focusing. One of the best ways to accomplish this is to disengage the motor, manually move the focuser all the way in and then reengage the motor. Finally, this position should be synced to zero in the driver. This tells the focuser that this is as far in as it can travel.

Finding the Maximum position requires a few extra steps. By default the Focuser starts with a Maximum position of 65,535 steps. However the actual maximum travel may be more, or even sometimes less for a very short focuser.

Once the focuser is Synced to zero you can begin moving it out with FocusLynx Commander. Move a little bit (10,000 steps is often a good value) to get a sense as to the total travel. Then move out until you approach either the physical limit of the focuser or the 65535 step limit. If you reach the physical limit first (try to avoid actually reaching it as that will throw the step count off) you can set that position as the maximum in settings. If you reach the 65535 limit you can increase the maximum in settings until you approach the end of travel. Then set the limit to the correct value. This will limit the focuser to it’s full range of travel.

For some applications it can be useful to be able to convert steps into more common units. The Step Size allows you to convert from units of steps to physical distance. In ASCOM this is reported in units of microns per step. This is for informational use only, changing the step size does not change how the focuser moves. Because the QuickSync and DirectSync motors work with different focuser bodies the default values are only approximately correct.