[Amaranthus (Barry)]

The basic definition of back focus is this: "Back focus is the distance from some telescope component to the focal plane, or point where the telescope reaches focus."

This concept is simple enough to understand when considering a focal reducer. For instance, the back focus on my TeleVue 0.8 FF/FR is 56 +/-4 mm, which means that the camera chip needs to be this distance from the end of the FR in order to achieve focus (and the designed FR).

What I'd like to seek clarity on is the concept of camera back focus. How does this relate to the above, in terms of designing an imaging train? Let's say a CCD camera reports a back focus of 40 mm. Does this mean that the minimum back focus needed for this camera is 40 mm (i.e. the distance from the chip to where the T-thread connects to other equipment), and thus:

(i) In the above example with the TV FR back focus of 56 +/- 4mm and the camera with a 40 mm BF, you would need a spacer of 12 to 20 mm in order to achieve focus - correct?

(ii) If you had a FF with a BF of <40mm, it could not be used with the above camera - right?

Or am I missing something? It's important for me to get this concept of camera back focus crystal clear for when I purchase my next AP setup!

A related question - some equipment specifies extremely precise BF distances (e.g. 53.2 mm, whereas others give an integer and a tolerance range [e.g. the TV FF noted above]). Is it always the camera that requires the precise distance, and if you have a FF with a range, then as long as the range includes the camera BF distance you will be okay?