NickMorris

Well said Mr Steppenwolf,

Perhaps that should be the new acronym: CDR - critical distance requirement.  I like it and less confusing than back focus distance.......

Thanks John,

The part I don't understand is what to choose between geometric back focus or optical back focus.  The distance between the ccd chip and reducer or reducer/flattener is the only thing of importance to me to get the spacing correct and therefore have perfect stars in the corners.  Where the focuser needs to be in order to bring this into focus is irrelevant to me as long as it is within the focus travel range.  It seems to me that back focus is a poor choice of terminology and manufacturers should just call chip/corrector distance!

cheers

Nick

Hi John,

I think you are right in that Astrodon and QSI are saying the same thing, just worded differently.....  This means that behind the CA-35 Tak adaptor, it is expecting 55mm to CCD plane therefore the spacer needs to be 55-(50.2-1.0) = 5.8mm ?  Do you concur?