Ah, so the focal length from the primary mirror to the focal point behind the secondary mirror. There's probably a way to tell by measuring the distance between the primary and secondary and then measuring the shorter diameter of the secondary and then doing some math.
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Steve
'08 Boxster RS60 Spyder #0099/1960
- never named a car before, but this is Charlotte.
'88 targa
SOLD 2004 - gone but not forgotten