basic back focus question

[graham.hobart]

If I know the cameras back focus- i.e 2.3cm, and I am considering using this on a TAK with an additional optec focuser- does this imply that the camera will come into focus at 2.3cm behind the point of most inward focus (on the tube with no focuser) & so if the focuser pushes the camera out more than 2.3cm does it imply it will not gain focus?
Sorry for dumb question but I am trying to get my head around this.
Graham

[Merlin66]

Graham,
It only means there's a "gap" of 23mm from the front of the camera and the CCD chip inside.
If you want to get the CCD to focus, then the front of the body will be 23mm inside the "normal" focus position.

[graham.hobart]

OK fair enough, so there is no way of predicting best focus position without actually trial and error? I mean I am concerned the OPtec focuser pushes the camera too far from the point of focus?
Cheers
Graham.

[Merlin66]

Graham,
You could use a reticule eyepiece where you know exactly where the eyepiece focus lies, or even project the moons image onto a translucent sheet behind the focuser....
At this "prime focus" position, there should be at least 23mm inward travel available.
( BTW I found the DMK which has a quoted 17.5mm backfocus [for the C lenses etc] only had 12.5mm when you remove the adaptor ring and use a std 1.25" nosepiece)

[graham.hobart]

can I use the position of the focus point for another camera -if I knew that back focus, to work it out- i.e that would be x mm in from prime (x= camera b's back focus?) then subtract x-2.3mm to get a rough and ready idea it where it would be?
Graz

[Merlin66]

Yes, that would work OK...(23mm not 2.3mm ;-) )