[AZ-Observing] Focusing input requested

Greetings all, some time ago I read an article (don't recall the 
reference) that addressed the use of diffraction spikes as a means to 
achieve a higher level of focus versus when astroimaging versus using a 
Hartman mask.  The approach was touted as being easier and much more 
accurate.  String or some other thin material, positioned to bisect the 
objective lens, creates the spike effect.  When the width of the spikes 
has been minimized - excellent focus has been achieved.  Has anyone used 
this technique?  Is a single "string" used, or two at 90° apart?  And, 
what is the thinness/thickness of material to use?  Guidance will be 
much appreciated.

Thanks,

Bob Christ
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