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Never a silly question Rob.
There are basically 4 way to image through telescope, afocal (image 1), prime focus (2&3), positive projection (4&5) and negative projection (6&7). With my postive projection I use a variable projection adapter that allows me to vary the magnifying power.
Negative projection usually involves a barlow, powermate, teleconverter or some type of focal length multiplier. Positive projection involves using an eyepiece to project an image directly onto the recording surface (ccd, cmos or film).
For eyepiece projection there are formula you can use to work out effectivece focal length and f ratio etc. Attached is a zipped focal length calculator to help work these things out. You will need excel to use it. In the "Afocal Projection" worksheet fill out the details in the red squares for Telescope FL, eyepiece FLs, Camera lens FL and telescope aperture (Red boxes. Use numbers only no text, eg 50, not 50mm) All measurements are in millimeters.
Once you have done that if you check out the "EEP Positive Projection" sheet you will see the figures for FL, F/ratio, system magnification factor (Telescope FL/Eyepiece FL X magnification factor = final magnification)
Hope this helps
All images from Michael Covington's Astrophotography for the Amateur. Cambridge Press
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