Skip to content


Autofocus options

General settings#

Use FilterWheel offset#

Determines whether the focuser should move per the defined offset in the list at the bottom when the filter wheel changes filters. See below for more details.

Autofocus Step Size#

The number of focuser steps that the autofocus routine will move by between autofocus points. This setting is highly dependent on the telescope and focuser.

Autofocus Initial Offset Steps#

The number of focus points that will be used on each side of perfect focus by the autofocus routine.
The default value should work for most cases

Default Autofocus Exposure Time#

The exposure time in seconds that will be used by autofocus, if filter specific times in the list below are not set

AF Method#

Method used to detect data points for autofocus.

  • Star HFR: This is the default method that will work reliably with images containing stars
  • Contrast Detection: An alternative method to focus on terrestrial objects or planets

AF disable Guiding#

Activate to pause guiding during AF routines. Enable this setting when using an Off-Axis Guider. For Guidescopes it is recommended to be left off.

AF Curve Fitting#

Fitting that should be used to determine ideal focus position out of the measured data points. A traditional autofocus curve follows a hyperbolic shape, so Hyperbolic method is recommended.

Focuser Settle Time#

The amount of time, in seconds, that should be awaited after a focuser move before starting a new exposure. Some focusers will require a small settle time to not lose steps and to prevent vibrations.

AF Number of Attempts#

The number of attempts the autofocus routine should be retried in case of unsuccessful focusing. Be careful when increasing this, as this can unnecessarily lengthen the AF routine when no focus can be achieved anyways, due to for example clouds. Better to be handled with the advanced sequencer

AF Number of Frames per Point#

The number of frames whose HFR or contrast will be averaged per focus points. For most of the time only one point per focus point is enough.

Use brightest n stars#

The number of top brightest stars that the autofocus routine will use - 0 means there is no limit and all stars are considered.

AF Inner Crop Ratio#

Inner ratio that will determine a centered region of interest for autofocus

AF Outer Crop Ratio#

Outer ratio that will determine a centered region of interest for autofocus

Backlash Compensation Method#

This controls the backlash compensation method used. The method can only be changed when the focuser is not connected!

  • Absolute: When the focuser changes directions, an absolute value will be added to the focuser movement. Backlash IN: when the focuser changes from moving outwards to moving inwards the Backlash IN value will be added Backlash OUT: when the focuser changes from moving inwards to moving outwards the Backlash OUT value will be added
  • Overshoot (recommended): This method will compensate for Backlash by overshooting the target position by a large amount and then moving the focuser back to the initially requested position. Due to this compensation the last movement of the focuser will always be in the same direction (either always inwards or always outwards)

Backlash IN/OUT#

The focuser backlash in the IN (decreasing position) and OUT (increasing position) directions, expressed in focuser steps.


When Overshoot is chosen, only ONE value between Backlash IN and OUT must be set! When setting IN, the amount will be applied on each inward movement, so the final movement will always be outwards. For Backlash OUT, it will be the other way around. The recommended direction is OUT, as then the autofocus routine will need less compensation. Use IN only when your equipment requires this direction to work properly.


The binning to be used for Autofocus exposures, if filter specific binning in the list below is not set

R² Threshold#

This setting refers to the coefficient of determination which is used to grade the calculated fitting of an autofocus run to the actual data points. When an autofocus run leads to an R² value that is below this threshold, the autofocus run will be deemed as failed. This can happen due to bad parameters, clouds rolling in and other problems during an autofocus run. An ideal autofocus run will have an R² value that is easily beyond 0.95.

Autofocus filter settings#

Filter offsets#

Most filters are not exactly par focal, meaning that when changing filters, the ideal focus distance changes slightly. This will cause an imaging system that was in perfect focus with one filter to be slightly out of focus with another filter. This can be a big problem for precise imaging, requiring an additional autofocus run each time the filter is changed.

To avoid this, it is possible to set filter offsets, which are the amount of focuser steps that the focuser should move by when switching from one filter to another.

For example, I could run the autofocus routine on each of my filters one after the other (with hopefully very little temperature change in between), with the following results:

  • L filter achieves perfect focus at focuser position 5000
  • R filter achieves perfect focus at focuser position 4990 (10 steps fewer than L filter)
  • G filter achieves perfect focus at focuser position 5030 (30 steps more than L filter)
  • B filter achieves perfect focus at focuser position 5045 (45 steps more than L filter)
  • Ha filter achieves perfect focus at focuser position 4988 (12 steps fewer than L filter)

If we take the L filter as the reference filter, we can set up all the filter offsets relative to the L filter, as below:

  • L filter offset 0 (reference filter)
  • R filter offset -10 (10 steps fewer than L)
  • G filter offset 30 (30 steps more than L)
  • B filter offset 45 (45 more steps than L)
  • HA filter offset -12 (12 steps fewer than L)

This is what has been done in the above screenshot.

Note that for this to work, the Use FilterWheel Offsets parameter under the Focuser Options needs to be set to On.

Autofocus Exposure Time#

The ideal auto-focus time can change per filter, particularly between broadband and narrowband filters (in the above example, the narrowband filter requires an exposure time 5 times longer than the broadband filters). This can easily be set up here.

Finding a good exposure time for autofocus is further explained in the Auto-Focus section

Autofocus Filter#

From the filter list, it is possible to set (or unset) an autofocus filter, which will be used by the autofocus routine (if the Use FilterWheel Offsets setting is enabled). This can be done by simply selecting a filter in the list, and clicking on the Set as Default AF Filter button. The same button can be used to unset the autofocus Filter.