Auto Adjust Focus Per Filter

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Using this option will attempt to adjust the position of the (motorized) focuser based on information of the newly requested filter position (and possibly the previous filter position).  The observed behavior will vary in one of three ways depending on frame type.


Auto adjust focus per filter can be turned on via the Focus Control Panel, it can also be set within a profile using the Profile Manager.


To use Auto adjust focus per filter you will need:




Light Frame Behavior


When operating on light frames, checking this option will auto-adjust focus per filter using the difference in pre-recorded (absolute) positions.  


In addition to the above requirements, you will need:



Explanation:


Focus tends to shift throughout the night so there's no possible way that the "Focus Pt", as specified in the Filter Setup Dialog, will be correct 100% of the time.  These values have two purposes:

  1. To give you a rough starting point of where your focus point lies.
  2. To give you offsets relative to the other filters.  


The Auto adjust focus per filter relies on the offsets relative to the other filters to adjust focus.  To do this accurately the "Focus Pt" values all need to be setup around the same relative time span when the temperature is fairly constant.  Ideally when setting these up you would run auto focus for each filter and set these values using the Focus Point Settings (then save the sequence as a profile or translate the values over to a profile).  This will give you the most accurate offset positions relative to your filters.  Alternatively, you can use a Bahtinov (or similar) mask and determine the focus point for each filter this way.


When a filter is changed, the "Focus pt" of your current filter is subtracted from the "Focus pt" of the target filter.  This offset is then subtracted from your current position and the focuser is moved to that location before running an auto focus routine.  Assuming that the "Focus pts" were setup correctly this should get the focus extremely close, if not dead on.


Example:

Assume you have a focus point for a Red filter set to 7010 and a focus point for a Green filter set to 7030.  You are imaging at a temperature much colder than the one used to set these focus points.  You are currently at focus with your Red filter at 6940.  If you are using this option and switch to your Green filter, your focuser will move out to 6960 (not 7030) to obtain focus.  This is because, at the same temperature, the filters differ in focus position by 20 steps.


Flat Frame Behavior


When operating on flat frames, checking this option will auto-adjust focus per filter using the mean focus position of that filter (focus position is recorded throughout the life of the sequence (after successful auto focus... over days or however long it takes to run the sequence).  If insufficient data exists to set the flat focus position, the focuser will not be adjusted.  Critical focus is not necessary when collecting flat-frames and this method will will get you close enough to capture useful calibration data.


In addition to the above requirements, you will need:



Note:

Resetting the sequence will clear a filter's focus data (the data collected during auto focus).


Other Frame-Type Behavior


When operating on dark or bias frames, checking this option will do nothing.