No Focus Crossing in VCurve

I encountered some issue with focusing that maybe someone can help me with. I set autofocus in setup and added VCurve for my Light filter. I can perfectly focus on Light using Localfield. Now when I switch my filter wheel to L-Extreme narrowband filter I am getting this error “No Focus Crossing VCurve” every time I switch from Light to L-Extreme or vice versa. What am I doing wrong or missing?

BTW, I haven’t encountered the same issue in my other RC 8" scope.

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Localfield focus, like all focus on all field, needs to be near the real focus position, you are too far from this point. Focus on a single star doesn’t need this and works in any case.

In your case probably filter is not parafocal or you have moved the focuser.
L-Extreme is a narrow band filter so is better to use focus on single star get fastness focus instead of the localfield (exposure for narrow must be more longer).

You need to add at least another 3 points of the sample in LocalField setting to crossing focus but this will slow the focus.

If for you is ok you can add this 3 point changing the field Sample in LocalField AI Engine box:

Better solution is to use the single star focus.

All the best

Thank you very much! Yes, single star works like a sharm. The problem with narrow band is also that stars are much dimmer and it is often finds less than 5 stars for the local field. Single star workaround works in that case.

You must increase the exposure time for narrowband for localfield ( Star Magnitude Interval / Exp for Filter Autofocus):

The set narrow band button uses a default setting that usually works

Al the best

Thank you! This is what I did. I used default NB settings. Localfield btw, worked well for a target like M42 even with NB filter.

However, I am imaging a difficult target Christmas Tree nebula from the city (Bortle 9). Maybe this is one of the reasons that I have difficulty. Anyway single star focus worked very well and I am imaging now.

BTW, should I also play with HFD Parameters like Start Focus and/or Vcurve Limit?

You must configure the time for narrow based localfield exposure on your telescope/filter/sky. NB setting from default is just a point where to start.

Dont touch the vcurve parameters, localfield is related to VCurve just for the “Focuser Step for sample” , anything else.

All the best

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