ok this is just an idea based on a theory based on reading various websites… so may be completely stupid….

In the C6 manual when it goes on about how to focus, it says to go counter clockwise as the last step (ie lifting the mirror). Then when you have got your target in sharp focus (I’m sort of assuming the target is in the middle here as I’ve not got the manual to hand) to give it an extra turn of 1/12th of a turn for visual use and 1/24th of a turn for photographic use.

What I believe the idea is, is that given the sensor is a flat plane but the image focus point is in a curve with the edges further ‘in’ , that moving it a bit further moves the central part towards the back of the cfz and in doing so brings more of the edge into it.

For my setup C6 with a Celestron focuser we are on about approx 30 microns or 40 steps. This sort of matches somethings I’ve read (specifically the article from sky and telescope in 2010 by Don Goldman and Barry Megdal - In perfect focus - link astrodonimaging.com/tutorials/ ) the cfz for an f/10 is around 40 micron and f/7 around 20 micron in one direction (so I assume 40 and 40 wide).

So once in focus on the target in the middle of the image a step out of 40 would bring more into the cfz.

Possibly this is a bit of a old idea and that doing the focusing using the whole or large part of the frame makes it irrelevant.

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