Critical Focus Zone

I recently came across some interesting info related to the value of a CCD imaging system’s critical focus zone. Many imagers use the calcuator at the Wilmslow Astro website to compute the depth of the CFZ for their equipment. When I used that calculation, I got a value of 159 microns. With my Optec Gemini focuser, that translates into 1445 steps. So, if my camera was focused at the center of the CFZ, I should be able to move the focuser in or out by 700 steps and not see any change in the focus quality of a frame.

The reality was that I could move my focuser by as little as 300 steps and see a change in the focus quality of the frame. This was puzzling, so I began to do some research. It appears there is a lot of different opinions about the definition of the CFZ. I ran across a very detailed discussion of this subject conducted by Don Goldman (Astrodon) and Barry Megdal that provides a detailed discussion of the CFZ. I have put a copy of that document in a Dropbox for those interested:

Dropbox: ‘GetFocused.pdf’

Basically, this discussion supports the idea that the CFZ is actually only about 1/3 as deep as it is traditionally calculated – that is, for my system, only about 53 microns. This would help explain why I see a focus change at 300 steps and why temperature compensation is so critical.

So when setting up your SGP auto focus parameters, knowing what your “true” CFZ is could make a big difference in how successful your auto focus routines are.

Charlie

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That’s very interesting Charlie, and would also explain similar results I
observed a few years ago when I looked at this stuff. This is also the
reason why it is generally recommend to not worrying too much about all the
math and instead try to make the focus V curve such that the top two points
of the V are about 3-4x larger than the bottom point of the V. It is good
to know the CFZ though.

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Thank you for this interesting article !

Joel:

Unfortunately, I am imaging at 2440mm with a 4" central obstruction and the out-of-focus donuts confuse the focusing routines in SGP. So V curves won’t happen for me until the new focusing routines appear in a future v2.5.xxx release.

Charlie

Ah yes, that does make a huge difference. Can’t help you there!