Focuser Backlash Measurement

Overview#

Focusers can suffer from backlash when they reverse directions, which is basically the number of focuser steps that "slip by" before the focuser physically moves again during a change in directions.

It is possible to set that up in the Focuser Options under the Options -> Equipment tab. However, a tool is available to determine the current focuser backlash. It is available in the AutoFocus tool, in the imaging tab.

Launching Focuser Backlash

Pre-requisites#

For the focuser backlash measurement tool to be available and to work correctly, the following conditions need to be met:

  • Both a camera and focuser are connected
  • The autofocusing options have been set correctly, so that a decent autofocus run can already be accomplished (minus backlash). This is described in the Auto-Focus Documentation
  • Your overall auto focus curve profile should be a hyperbola - this should generally be the case
  • The system is currently well focused (using a Bahtinov mask for example)

Algorithm#

The following method will be used to measure the IN backlash, based on the assumption that far enough from focus, focus points should be on a line (e.g. the focus curve profile is a hyperbola).

  • Start at best focus (after an AF routine)
  • Move the focuser out by twice the amount moved by the autofocus routine (e.g. Auto Focus Initial Offset Steps multiplied by Auto Focus Step Size times 2) and measure HFR (HFR2 at position2)
  • Move in by the Auto Focus Step Size and measure HFR (HFR1 at position1). If HFR1 and HFR2 are very close, backlash is not cleared yet, and repeat (move in by Auto Focus Step Size, measure and replace HFR1 and position1 with the new position and measure). Remember the number of times the measurement is done, to a maximum of 3 times
  • Move in by Auto Focus Step Size again, get to position0, and measure HFR0
  • Compute the slope between position2 and position1 as measuredSlope = (HFR2 - HFR1) / (position2 - position1)
  • Compute the slope between position0 and position1 as idealSlope = (HFR1 - HFR0) / Auto Focus Step Size
  • If abs(measuredSlope) > abs(idealSlope), there is no significant backlash
  • Else, Backlash In is (1- measuredSlope / idealSlope) * (position2 - position1)

The same routine, inverted, will be used for the Backlash OUT.

As can be seen above, the routine relies a lot on the Auto Focus Step Size setting, which can be changed as necessary if issues are noted with the process

Running the tool#

Running the focuser backlash tool is very simple: simply click on the Measure Backlash button. This will run the routine automatically - note that for greater precision, the routine takes three exposures per focus points, so it may take longer than expected. Once it's over, the routine will output a result that can be automatically saved in the focuser options.